The Modernization of GPS: Plans, New Capabilities and the Future Relationship to Galileo

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Journal of Global Positioning Systems (2002)

Vol. 1, No. 1: 1-17

The Modernization of GPS: Plans, New Capabilities and

the Future Relationship to Galileo

Keith D. McDonald

Navtech Consulting, Alexandria, VA USA

Received: 18 July 2002 / Accepted: 18 July 2002

Abstract. This paper reviews the development, status

and current capabilities of GPS. The modernization

improvements planned for GPS are then discussed

and summarized, including brief descriptions of the

additional features planned for the spacecraft, the

control segment and the user equipment. A

discussion is presented of the impact of the system

modernization plans and activities in improving the

performance of the four principal operating modes

of GPS. The implications of the GPS modernization

and enhancement activities and their relationship to

the analogous European Galileo program activities

and other GNSS efforts are covered. Several

technical, policy and implementation concerns

relating to the timely deployment of the

improvements to GPS are briefly addressed.

Key words: GPS, Modernization, GNSS, GALILEO

1 Introduction

During the past three decades, the Global Positioning

System (GPS) has grown from a navigation concept

through development and implementation to an

operational system of 28 spacecraft currently serving

millions of users. Its use has increased such that over a

million GPS receivers a year have been produced since

1997. The rapidly growing GPS market, including

equipment and applications, has been reliably estimated

(USGAO, 1998) at about $8.5B in 2000, to about $17B

by 2003 and is expected to be in excess of $60B in 2010.

The GPS has performed extremely well but a number of

desired and needed improvements have been identified

that could be implemented with new generations of GPS

replenishment and follow-on spacecraft. This paper

addresses the concerns, options, issues and plans during

the next fifteen years and beyond for improvements to

GPS and the significant performance benefits that will be

available to users. The European Community is planning

to deploy a navigation satellite system with similar

performance characteristics to GPS in the 2008 time

frame. We will briefly investigate the benefits and

features of the combined capabilities of GPS and Galileo

that may impact international users in the future.

GPS has become an essential part of the navigation,

positioning, surveillance and timing aspects of ground,

marine, aviation and space applications. The current uses,

with new ones, will continue to grow resulting in a need

for even more demanding capabilities.

2 Background

2.1 Development and Implementation

The US Department of Defense (DOD) developed the

concept and general configuration for GPS in the early

1970's as a joint program involving all three military

departments. The program was initially directed by the

Joint Services Navigation Satellite Executive Steering

Group (NAVSEG) formed in the Pentagon in 1968. This

group was chartered to determine the feasibility and

practicality of a space-based navigation system for

improving military navigation and positioning. The group

met regularly for over three years. It was to prepare, if

appropriate, a Development Concept Paper (DCP)

describing the technology, the performance capabilities,

the principal development areas, the cost, the benefits and

the overall funding requirements for the system. The DCP

would then be presented to the Defense Systems

2 Journal of Global Positioning Systems

Acquisition Review Committee for consideration as a

DoD development program.

It was recognized early that the use of satellite systems,

solid state electronics, evolving digital computers and

related technologies could possibly provide significant

enhancements to the performance capabilities of

navigation users. The NAVSEG was supported by the

DoD Navigation Satellite Management Office and the

laboratories of the military departments. The author was

Scientific Director of the DoD Navsat Program during

this period that included serving as Executive Director of

the NAVSEG and Chairman of the Navsat Management

Office.

In 1973, after over three years of intensive technology

investigations, concept development, requirements

analyses and program definition efforts by the Steering

Group and others, the system was approved by the

DSARC (representing the military departments) and the

Director of Defense Research and Engineering for

advanced development and test.

With the approval of the GPS program, the US Air Force

was designated the Executive Agent for managing the

implementation, and the GPS Joint Program Office (JPO)

was established at the USAF Space and Missile Systems

Organization in Los Angeles. Ten GPS Block I

developmental spacecraft were built under contract by

Rockwell Space Division (now Boeing) and successfully

launched from Vandenberg AFB between early 1978 and

1985. Tests with these spacecraft demonstrated the

capabilities of the system and resulted in the approval by

DOD for the implementation of an operational system,

with the first operational (Block II) spacecraft launched

on February 22, 1989.

The basic signal characteristics for GPS that we have

today and for which millions of receivers have been

designed and produced, were basically established in the

early to mid-1970's. Fortunately, the GPS system concept

parameters developed by the DoD Navsat Steering Group

and investigated in detail, as well as developed further

and implemented, by the GPS JPO and their contractors,

have performed very well. However, it is clear that nearly

all users, both civil and military, can now benefit

substantially from various GPS system enhancements,

modifications and additions.

After the tremendous technology advances during the last

quarter century, it appears reasonable to assess the current

and future capabilities of GPS in the context of these

developments. This has become apparent to many. Both

civil and military Committees and other activities in the

past several years have strongly recommended

modernizing GPS. Support for this effort has also come

from the highest levels of government (Gore, 1999).

2.2 Current Status

Figure 1, entitled GPS Today illustrates the main

elements of GPS as well as the frequencies, signals and

signal spectrum currently used by GPS. Table 1, entitled

GPS Operational System Parameters and

Characteristics summarizes the principal current GPS

system characteristics.

1 ON3

men u

Rockwell

456

WPT

POS

NAV

CLR

MARK

OFF

NUM

LOCK

FIX FOM 1

N 42* 01” 46.12”

W 091* 38’ 54.36”

EL + 00862 ft

ZERO IZE

Ground

Antennas

(4)

Monitor

Stations

(5)

Master Control

Station

(Shriever AFB)

Colorado Springs, CO

Receiver

calculates

3-D location,

velocity & time

C/A Codes

Time, orbit

position and health

•24+ satellite constellation

•~ Half-geosynchronous orbits (10,900 nmi)

1227.6 MHz

±12 MHz

+Ascension

Diego Garcia

Kwajalein

Hawaii

* Vandenberg

1575.42 MHz

±12 MHz

P(Y)-Codes

Fig. 1 GPS Today

McDonald: The Modernization of GPS 3

The general characteristics of the ground segment are

given in Figure 2 entitled GPS Ground Control System

and similar information for the GPS spacecraft (S/C)

deployment is given in Figure 3 entitled GPS Space

Segment.

Monitor

Stations

Ground

Antennas

Master Control

Station

(Schriever AFB)

HAWAII

CAPE CANAVERAL

COLORADO SPRINGS

ASCENSION DIEGO

GARCIA

KWAJALEIN

VANDENBERG

GPS Constellaton

Master Control Station (MCS):Satellite control, system operations

Alternate Master Control Station:Training, back-up

zMonitor Station (MS):L-band; Collect range data, monitor nav signals

▲Ground Antenna (GA):S-band; Transmit data/commands, collect telemetry

Fig. 2 GPS Operational Control System (OCS)

Tab. 1 GPS Operational System Parameters and Characteristics

2.22.99kdm

Space SegmentControl SegmentUser Segment

24 GPS spacecraft (S/C) in orbit

is the baseline constellation

6 orbit planes spaced 60°apart (LAN)

4 S/C per orbit plane; >4 for > 24 constel

10,898 n.mi. (20,180 km) orbit altitude

55° S/C orbit plane inclination

Near circular orbits (e ≈ 0)

11 hr 58 min S/C orbital period

Provides PRN-coded ranging signals

Circularly polarized transmissions

Frequencies and PRN codes:

1575.42 ± 12 MHz: L1 band

1227.6 ± 12 MHz: L2 band

1176.45 ± 12 MHz: L5 band

1.023 Mbps = C/A-code bit rate

10.23 Mbps = P/Y-code bit rate

1msec = period of C/A-codes

7 days = period of P/Y-codes (each S/C)

1 GPS S/C per launch (Delta II)

C/A-codes in the clear (SA removed)

P/Y and M-codes, secure (encrypted)

Data message: 50 bps (mod 2 encoded)

S/C lifetimes: II, IIA -7.5 years;

IIR -7.5 +years; IIF: 12-15 years

S/C: Multiple Rb and Cs atomic clocks

Civil GPS standalone accuracy: 5 - 10 m. -

Military GPS accuracy: 2 – 9 m.-

14 -180 day S/C nav. message storage

Headquarters & Master Control Station

(MCS) at Schriever AFB, Col Spr, CO

Uplink and monitor station locations:

Schriever AFB

Hawaii (monitor only)

Ascension Island

Kwajalein

Diego Garcia

Kennedy Ctr., FL (up-link backup)--

New monitor sites:

8 NIMA tracking stations

MCS receives monitor station measure--

ments, 2f iono corrections, UTC time

Large Kalman estimator at MCS

MCS computes and schedules uplink

transmissions that include:

S/C ephemeris (orbital elements)

S/C atomic clock corrections

S/C almanac data

Ionospheric delay model terms

S/C health and status information

Monitor stations linked to MCS

Up-link antennas: 10 m. dishes

Near continuous ant. visibility of GPS S/C

Unified S-band tracking, telemetry & -

command (TT&C) links

Orbit and ephemeris accuracy (OCS):

Passive receivers; no user transmissions

required for navigation reception

Receivers normally process signals from

multiple (5-12) S/C simultaneously

Number of S/C in view of unobstructed

GPS receiver: 6-12 S/C (average 8)

Coarse acquisition (C/A)-codes, 1.023 Mbps

used normally by civil community and

by military for acquisition of P/Y-code

New M-code to provide direct access

Spread spectrum signal; pseudorandom-

noise coded: 1.023 Mbps & 10.23 Mbps

37 PRN C/A Gold codes available

Direct sequence P/Y-code: 37+ weeks long;

1 week segment used for each S/C

GPS receiver antennas normally are for

L-band upper hemisphere coverage

Signal power from S/C at receiver:

-160 dBwfor L1 C/A-code signal

-163dBwfor L1 P/Y- code signal

-166dBwfor L2 P/Y- code signal

Modernized L1, L2 and L5 signals are

planned for current L1 C/A pwror more

Differential GPS accuracy: 1 mm - 3 m.

Digital correlation receivers are small,

light and reliable. 8M+ in service (1/02)

24 GPS spacecraft (S/C) in orbit

is the baseline constellation

6 orbit planes spaced 60°apart (LAN)

4 S/C per orbit plane; >4 for > 24 constel

10,898 n.mi. (20,180 km) orbit altitude

55° S/C orbit plane inclination

Near circular orbits (e ≈ 0)

11 hr 58 min S/C orbital period

Provides PRN-coded ranging signals

Circularly polarized transmissions

Frequencies and PRN codes:

1575.42 ± 12 MHz: L1 band

1227.6 ± 12 MHz: L2 band

1176.45 ± 12 MHz: L5 band

1.023 Mbps = C/A-code bit rate

10.23 Mbps = P/Y-code bit rate

1msec = period of C/A-codes

7 days = period of P/Y-codes (each S/C)

1 GPS S/C per launch (Delta II)

C/A-codes in the clear (SA removed)

P/Y and M-codes, secure (encrypted)

Data message: 50 bps (mod 2 encoded)

S/C lifetimes: II, IIA -7.5 years;

IIR -7.5 +years; IIF: 12-15 years

S/C: Multiple Rb and Cs atomic clocks

Civil GPS standalone accuracy: 5 - 10 m. -

Military GPS accuracy: 2 – 9 m.-

14 -180 day S/C nav. message storage

Headquarters & Master Control Station

(MCS) at Schriever AFB, Col Spr, CO

Uplink and monitor station locations:

Schriever AFB

Hawaii (monitor only)

Ascension Island

Kwajalein

Diego Garcia

Kennedy Ctr., FL (up-link backup)--

New monitor sites:

8 NIMA tracking stations

MCS receives monitor station measure--

ments, 2f iono corrections, UTC time

Large Kalman estimator at MCS

MCS computes and schedules uplink

transmissions that include:

S/C ephemeris (orbital elements)

S/C atomic clock corrections

S/C almanac data

Ionospheric delay model terms

S/C health and status information

Monitor stations linked to MCS

Up-link antennas: 10 m. dishes

Near continuous ant. visibility of GPS S/C

Unified S-band tracking, telemetry & -

command (TT&C) links

Orbit and ephemeris accuracy (OCS):

Passive receivers; no user transmissions

required for navigation reception

Receivers normally process signals from

multiple (5-12) S/C simultaneously

Number of S/C in view of unobstructed

GPS receiver: 6-12 S/C (average 8)

Coarse acquisition (C/A)-codes, 1.023 Mbps

used normally by civil community and

by military for acquisition of P/Y-code

New M-code to provide direct access

Spread spectrum signal; pseudorandom-

noise coded: 1.023 Mbps & 10.23 Mbps

37 PRN C/A Gold codes available

Direct sequence P/Y-code: 37+ weeks long;

1 week segment used for each S/C

GPS receiver antennas normally are for

L-band upper hemisphere coverage

Signal power from S/C at receiver:

-160dBwfor L1 C/A-code signal

-163dBwfor L1 P/Y- code signal

-166dBwfor L2 P/Y- code signal

Modernized L1, L2 and L5 signals are

planned for current L1 C/A pwror more

Differential GPS accuracy: 1 mm - 3 m.

Digital correlation receivers are small,

light and reliable. 8M+ in service (1/02)

Space SegmentControl SegmentUser Segment

4 Journal of Global Positioning Systems

•24-satellite (nominal) constellation

•Six orbital planes, four satellites per

plane

•Semi-synchronous, circular orbits

(~10,900 n. mi., 20,200 km.)

Block II/IIA

Block IIR

Block IIF

Fig. 3 GPS Space Segment

The Block II operational spacecraft launches which began

in early 1989 later incorporated slightly modified (Block

IIA) spacecraft. These included some improvements

including additional on-board data memory, providing an

extended period (several months in lieu of 14 days) over

which data could be transmitted from the S/C to users

without ground uploads.

The DOD contracted with Rockwell Space Division for

28 Block II and IIA spacecraft at a cost of about $1.2B.

In July 1995, the full operational capability of GPS was

achieved, consisting of a ground segment, a constellation

of 24 operational GPS spacecraft providing navigation

services worldwide and a variety of user equipment.

Since that time, the GPS space segment has performed

continuously with between 24 and 28 operational

spacecraft. User equipment development and

manufacture has increased dramatically, especially in the

civil community.

2.3 The GPS Constellation

The Block II and IIA spacecraft's limited lifetime in orbit,

nominally about 7.5 years, establishes the need and

schedule for replenishment spacecraft. The DOD

contracted with Lockheed-Martin Astro-Space for

twenty-one GPS third generation replenishment (Block

IIR) spacecraft.

In January 1997, the first of the Block IIR spacecraft was

launched but did not achieve orbit when a strap-on

booster failed on its newly configured Delta II launch

vehicle. A later launch in July of 1998 and all subsequent

launches have been successful. As of 1998, all 28 of the

initial Block II (and IIA) operational spacecraft have been

launched. The remaining Block IIR spacecraft, including

nine to twelve S/C planned for modernization, will phase

in to provide the principal operational signals and

capabilities for the system during the next ten years or

more.

The DOD has contracted with the Boeing Space Division

for a fourth generation follow-on GPS (Block IIF)

spacecraft. Although a buy of 30-33 spacecraft was

originally planned, modernization requirements have

resulted in the need for a new generation of spacecraft

beyond the IIF. To this end, the GPS Block III’s are in

development. The Block III’s will replace all but twelve

of the originally planned Block IIF spacecraft.

The first six IIF spacecraft are on contract and an

additional six are planned for GPS mid-term constellation

sustainment purposes. Beyond that, the new Block III

spacecraft will be deployed. The Block IIR, IIF and III

spacecraft provide the principal space vehicles available

for the enhancement and modernization of GPS.

It is the periodic replenishment of the spacecraft

constellation that provides the opportunity for

implementing upgrades and modernization features to the

system. Since the lead time for acquiring a new

generation of GPS spacecraft is long (typically 5-8 years),

and the full deployment of a new constellation requires a

number of years (typically 7-10 years), the introduction

McDonald: The Modernization of GPS 5

of improvements to GPS is normally a slow and gradual

process.

2.4 The Navigation Signals

GPS spacecraft currently transmit navigation signals to

the earth at two frequencies, designated L1 and L2 (refer

to Figure 1). L1 is the principal GPS carrier signal, at a

frequency of 1575.42 MHz. This signal is modulated by

two types of pseudo-random noise (PRN) codes, termed

the coarse/acquisition (C/A) codes, at a bit rate (or

"chipping" rate) of 1.023 Mcps, and the precision/secure

(P/Y) codes, at a chipping rate of 10.23 Mcps.

The GPS L2 signal, transmitted by the spacecraft at

1227.6 MHz, was established to provide a second

frequency for ionospheric group delay corrections to GPS

user receivers. Since the ionosphere is a dispersive

medium with close to a 1/f2 dependence, the combination

of the L1 and L2 frequencies provides an excellent real

time technique for determining the group delay effects on

the GPS signal paths caused by the refraction of the

signal paths by the free electron content in the

ionosphere.

Ionospheric errors, if uncorrected, can contribute the

largest single propagation error to GPS operation, causing

ranging errors of up to 40 meters. The GPS L2 signal was

originally implemented primarily for correction of the

slowly changing ionospheric delay. For this reason, it is

transmitted at a lower power level (at about one fourth, or

-6dB) relative to the L1 C/A-code signal. The P/Y-code

signals on L1 and L2 are normally secure, so the full

capabilities of these signals are accessible only to DOD

or other authorized users.

(No data message on

L5 quadrature signal)

Current

Modernized

Current

Modernized

Frequency in MHz

Aeronautical Radionavigation

Services (ARNS) Band

Radionavigation

Satellite Services

(RNSS)Band

1215 1227.6 1575.42

1575.42

P/Y-codesP/Y-codes

C/A-codes

(for civil &

military use)

C/A-codes

C-codes

(new)

1176.45

1164.45 1188.45

1575.42

1227.6

P/Y-codes

Civil Signals

1563.42 1587.42

1575.42 (L1)

1215.6 1239.6

1227.6 (L2)

RNSS Bands

1215-1240 1560-1610 MHz

960-1215 MHz

Note 1:Military M-codes are in definition.

L5 codes (I& Q-codes) are specified

Note 2: Modulation envelopes for only the

code principal signal lobes are shown.

Note 3:Phase quadrature signals are shown

below the origin lines.

I5, Q5-codes

(new)

Freque ncy

Military Signals

M-codes

Fig. 4 GPS Signal Evolution and Spectrum Occupancy

signals transmitted by GPS and the planned evolution to

additional codes and signal frequencies.

2.5 GPS Performance

GPS has a number of different modes of operation, each

with its own set of performance capabilities. First, GPS

can be used autonomously (on a stand-alone basis), i.e.

without any augmentations. In this case, the user

equipment receives and uses only the signals received

from the constellation of spacecraft to determine user

position, velocity, time (PVT) and related parameters.

GPS can also be used in a differential mode in which

known navigation data or ranging data received at a

reference location and time are compared (or differenced)

with similar GPS measured data at the same point and

time. The corrections from this process are then applied

at the same time (or with minimum latency) to the GPS

measured data taken at a remote point.

For real-time operation, a data link between the reference

receiver and the remote receiver is normally used to

communicate the corrections. This process has the great

advantage of canceling the fixed, or slowly varying,

(bias) errors in measurement that have the same effect at

both locations. The differential correction technique,

6 Journal of Global Positioning Systems

which many other navigation systems have used to

improve performance, performs well with GPS since

most errors appear as slowly varying biases.

These differential corrections can be applied directly in

real time or they can be stored, normally synchronized to

a common time source (such as GPS time) and employed

later using post processing methods. The corrections are

usually established at the reference location and applied

at the remote (rover) receiver. Corrections can be based

either on the GPS differential code measurements, or for

greater precision, the differential measurements of the

GPS carrier phase.

The various modes of operation for GPS user equipment

and the corresponding nominal current performance

capabilities are summarized in Table 2. The change in

GPS performance characteristics for various GPS stand-

alone and differential modes of operation for 2001 and

2011 are discussed later. Performance accuracy values in

position, velocity, time and angle measurement (attitude)

are provided.

Tab. 2 GPS Performance and Modes of Operation With a Summary of Nominal Current Capabilities

Differential (DGPS) Code Receivers

•Environmental and other systematic effects: greatly reduced; limited by spatial decorrelation

•Differential corrections: code measurement based; some latency effects

•Low values of receiver code noise advantageous for improving measurement precision

•Overall nominal accuracy: C/A-code DGPS receivers ~0.7 - 3 m., with narrow correlator ~0.5 - 2.0 m.

P/Y-code DGPS receivers ~0.5 - 2.0 m.

Stand-alone Receivers

•Autonomous with unaided code Exception (typical): carrier (Doppler) aiding of code to reduce code noise

•Pseudorange performance: C/A-code ~0.5 - 2.0 m.; with narrow correlator ~10 - 40 cm.

P/Y-code ~10 - 50 cm.

•Environmental effects: ionosphere and troposphere delays

•Other errors: ephemeris, multipath, receiver noise, S/C clock, receiver and S/C unmodeled delay errors

•Overall nominal accuracy: C/A-code receivers ~ 5 -10 m.

P/Y-code receivers ~ 2 -9 m.

Real Time Kinematic (RTK) Carrier Phase Measurement (CPM) Receivers (DGPS)

•Low noise measurement observable; differential phase error ~ 0.5 - 2 mm.

•Differencing techniques employed (1,2,3∆) to remove unmodeled delay errors

•Overall nominal accuracy: ~2 mm -20 cm.; depends on baseline separation, observation period, corrections …

C/A (or P/Y) – codes typically used but not essential; measurement observable is relative carrier phase

Interferometric Measurement (carrier phase) Attitude Determination Receivers

•Same general characteristics as CPM and RTK receivers

•Nominal accuracy: ~1 mRadian; normally employs C/A (or P/Y) –codes but relative carrier phase is observable

2.6 Local Area and Wide Area Enhancements

To improve the performance of GPS, a number of local

area and wide area differential GPS (DGPS) systems and

networks have been established or planned. These

include:

• Local post processing and real time (kinematic)

systems for the GPS surveying community.

• FM radio sub-carrier transmissions of DGPS

corrections for vehicular applications in many North

American and European cities.

• Satellite-based transmissions of DGPS corrections

worldwide (by RACAL, Fugro and others) for use by

surveyors, geodesists and geographic information

system (GIS) users.

• The U.S.Coast Guard Differential Network, for

continental US coastal regions, Gulf of Mexico, Alaska

and Hawaii, for accurate coastal, river, harbor and

harbor entrance navigation for marine users.

• The Wide Area Augmentation System (WAAS) of the

Federal Aviation Administration (FAA) planned for

initial operation in 2002 to provide code corrections,

ionospheric delay and integrity data.

• The FAA’s Local Area Augmentation System (LAAS),

for use in precision landing of aircraft. Planned for

implementation by about 2003, this provides code

corrections, integrity and other data.

• The European Geostationary Navigation Overlay

System (EGNOS), similar to WAAS and planned for

initial implementation in Europe for aviation uses by

about 2002.

• The Mobile Transportation Satellite System (MTSAT),

planned by Japan for operation in a large region of the

Pacific, providing code corrections and other data, for

operation in about 2002.

McDonald: The Modernization of GPS 7

3 The Modernization of GPS

Although GPS has performed extremely well and has

generally exceeded expectations, some significant

improvements are needed. A number of committees,

representing both government and civil communities,

have investigated the system's needs and deficiencies

over the past decade in order to determine what

capabilities and features should be incorporated into a

future GPS to satisfy both military and civil users.

The modernization of GPS is a difficult and complex

enterprise. It involves not only addressing civil and

military needs and costs for performance improvements,

but also issues with far-reaching implications in other

areas. These issues include spectrum needs and use,

security, civil and military performance, system integrity,

signal availability, institutional concerns on GPS

financing and management, and the future operation of

GPS as a national and international resource.

Fortunately, many of the critical issues have been

identified and are resolved or appear to be near final

resolution. If all goes as planned, or as hoped, it becomes

reasonably clear what can be expected in the next decade.

However, this assumes an optimistic view of the

commitment of the U.S. government to provide the

decisions, the institutional arrangements and the funding

necessary to meet the generally agreed upon needs in a

timely manner.

3.1 Signals and Signal Separation

Initially, the military GPS planners wanted to separate

their GPS frequencies from those used for civil

applications. This separation was intended to avoid signal

interference and interactions and to provide maximum

flexibility for both user groups.

An intensive search, however, failed to find any new

spectrum that would satisfy the military requirements.

Therefore, the military decided that their signals will

remain within the current 24 MHz bands authorized by

the ITU for GPS in the Radionavigation Satellite Services

(RNSS) band at L1 (1563.42-1587.42 MHz) and L2

(1215.6-1239.6 MHz).

Additional Signal Needs

It has been agreed for several years that additional GPS

signals are needed for civil applications. These signals are

required for a) reducing the ionospheric errors by use of

the two frequency correction technique, b) for increased

signal robustness, especially in aviation safety operations,

and c) for improved acquisition and accuracy.

Concerns on L2 and a New L5

The FAA for some time has opposed the use of L2 for

aviation safety applications. Their concern is that since

the International Telecommunications Union (ITU) has

authorized this band for use on a co-primary basis with

radiolocation services (including high power radars), that

aircraft using the band may be subject to unacceptable

levels of interference. Since this may compromise

aviation safety operations, the FAA considers the L2

band unacceptable. The FAA requested a GPS aviation

frequency in the Aeronautical Radionavigation Services

(ARNS) band, which is located directly below the GPS

L2 band. As we will discuss, this has occurred.

The Presidential Decision Directive (PDD) on GPS of

March 29, 1996 (Gore and Pena, 1996) stated that both

the L1 and L2 frequency bands would be available for

civil use and that a third civil signal would also be

authorized. Although the L1 and L2 frequency bands can

satisfy most civil users, aviation users need a third civil

frequency to replace the L2 band and its limitations in

safety-of-life applications.

After an intensive search for new frequencies by the

Department of Transportation (DOT), the Department of

Defense (DOD) and other agencies, Vice President Gore

announced on January 25, 1999 that a region in the

ARNS band had been agreed upon as the new (third) civil

frequency. This frequency is referred to as L5 and is

centered at 1176.45 MHz. This selection appears to

satisfy aviation safety uses.

Figure 5 illustrates the arrangement in the spectrum now

planned for the modernized signals, showing the ARNS

band below the GPS L2 band.

Prior to operational use of L5, coordination with other

systems in the band as well as approval by the

International Telecommunications Union (ITU) is

required. The ARNS band is currently established by the

ITU for ground-to-air services. For part of this band to be

used by GPS requires an ITU satellite-to-earth

transmission classification. This matter was favorably

considered at the ITU World Radio Conference held in

April, 2000 in Istanbul. It was expected that the

international aviation community would support this

change.

3.2 New and Retained GPS Signals

For backward compatibility (or “legacy”) purposes, the

existing C/A-codes on L1 and the P/Y-codes on L1 and

L2 are to be retained. Continuation of these codes is

necessary until modernized GPS spacecraft transmitting

the new GPS signals for both civil and military users are

deployed. New GPS user equipment also needs to be

produced to operate with the modernized signals.

8 Journal of Global Positioning Systems

Band Assignments:

1559 1610

1563.42

GPS L1

1605

1595

1587.42

MSS

GLONASS

1215

GPS L5

1188.45

1164.45 1240

GPS L2

1260

GLONASS

1239.6

JTIDS…

ARNS BandRNSS Band

RNSS Band

L5 L2

1250

RNSS - Radionavigation Satellite

Services Band

ARNS - Aeronautical Radionavigation

Services Band

JTIDS - Joint Tactical Information

Distribution System (DoD)

GLONASS - Glonass Navigation Satellite

System (Russian Federation)

MSS – Mobile Satellite Services Band

1575.42

1176.45 1227.6

Fig. 5 GPS and GLONASS Signal Placement Arrangement: L1, L2 and L5

Signal Codes

The current plans are to continue providing the civil

community with the C/A-codes on L1, and to transmit

newly configured C-codes (or sometimes referred to as

CS-codes) on L2 from subsequent spacecraft when

feasible. Plans for the L5-codes are for a higher bit (chip)

rate and longer codes than the current C/A-codes or the

new C-codes. The proposed codes on L5 consist of a

10.23 Mcps code rate with a code length of 10,230 bits

(in one millisecond). This is the same code period as the

C/A-codes. Compared to the C/A-codes, the longer, high

rate code sequences provides improved ranging accuracy,

a lower code noise floor, acceptable acquisition times,

•Safety-of-life applications(e.g., civil aviation)

•In Aeronautical Radio Navigation Service (ARNS) band

•Two signals:(a) In-phase signal (I5) with data message

(b) Quadrature signal (Q5); no data message for improved tracking

•Higher accuracy when used with civil codes on L1 or L2

–Similar accuracy as military signals today (P/Y-codes)

–More robust than current C/A-codes on L1

–Greater resistance to interference than C/A-codes

–Higher chipping rate improves multipath performance

–Plans are to have about four times greater power (needed to operate

in ARNS band that has higher noise and interference levels)

–Improved data message (and shortened)

1176.45 MHz

1186.68 MHz

1166.22 MHz

Fig. 6 L5 - New Civil Signal

McDonald: The Modernization of GPS 9

better isolation (cross-correlation properties) between

codes, and substantially reduced multipath interference

susceptibility. Figure 6 entitled L5 – New Civil Signal

shows the main spectral characteristics of the L5 in-phase

and quadrature codes referred to as the I5 and Q5-codes.

To take full advantage of L5, it is planned for one of the

two quadrature signals to be transmitted without data

modulation. The “data free” signal provides advantages

for accurate phase tracking and more precise carrier

phase measurements, of special interest to the survey and

scientific communities. Similarly, the new C-codes on L2

are at the C/A-code chipping rate (1.023 Mcps) and are

time multiplexed to provide a data free signal as well as a

signal with data.

The current P/Y-coded signals on L1 and L2 perform

very well. However, because of the extremely long

sequence length (~1013 bits), and the corresponding

period of this code (7 days), acquisition of the P/Y-code

is very difficult unless some precise knowledge of the

system timing is known. The P/Y-code acquisition

normally involves first, the acquisition of the short

sequence (1 ms) C/A-codes on L1. The C/A-code

message contains system timing data (in the hand-over, or

HOW, word) that provides an authorized user with

information for acquiring the P/Y-code.

However, the military believes it essential in the future to

acquire their secure signals without first accessing the

C/A-codes. The C/A-codes are available to civil users and

others. The military requirement for direct acquisition of

their secure signal appears to require the transmission of a

set of new military codes, called the M-codes.

Code Options Considered

Initially, the DoD indicated that it planned to place its

new signals in the center of the L1 and L2 bands, similar

to their C/A and P/Y-codes. To minimize interactions

with the main part of the DoD signals, it appeared

advantageous for the civil signals to avoid the center of

the bands. The author proposed on several occasions

during 1997-98 the use of pairs of coded signals (offset

from each band center) in the L1 and L2 bands

(McDonald, 1998a,b). The use of civil “separated

carriers” or “split spectrum” signals in the outer regions

of the bands where the military signals would be at a low

level provided separation between the military and civil

signals. It also provided some significant advantages for

precise carrier phase measurements and more precise

code processing.

These split spectrum signals were then found to have

excellent performance properties, especially compared to

the current military P/Y-code signals, and to be easily

implemented (Spilker et al., 1998). During the past two

years the DoD has sponsored a number of investigations

of split spectrum signals and has selected this signal as

their new M-code signal for their use at both L1 and L2.

The final arrangement for the M-code signals appear to

use a secure M-code with a bit rate of about five Mbps

modulated on dual "split spectrum" carriers that are

spaced about 10 MHz above and below the centers of the

GPS L1 and L2 bands [at the first nulls in the current

P(Y)-code structure]. The GPS JPO and their contractors

have investigated the military signal alternatives and are

responsible for the final selection.

4 Security Issues

Selective Availability

A number of Committees have investigated the need and

effectiveness of SA. Almost all of these, both civil and

military, have found that SA is not effective, is easily

mitigated (for example, by the use of differential

techniques) and that it is costly to continue, especially to

civil users. Several of the groups strongly recommended

that SA be removed immediately (NRC, 1995; NAPA,

1995).

The Presidential Decision Directive on GPS released on

March 29, 1996 stated that the continuance of SA would

be reviewed annually by the President starting in 2000

and that SA would be discontinued no later than 2006.

The civil community has consistently recommended that

SA be removed. However, users adapted quickly to the

use of SA mitigation techniques, principally by the use of

differential corrections.

GPS Accuracy Improvements and Performance

Augmentations

On May 1, 2000, the White House announced that the

Selective Availability degradation would be removed

starting at midnight, May 1, and that there were no plans

or intentions to restore it in the future. The substantial

improvement in stand-alone GPS accuracy to civil users

has become apparent. Even at the time of the solar

maximum effects on the ionosphere (in 2000-2001),

performance of SPS receivers is now typically at the 5-10

meter level. The removal of SA combined with the

availability of a second (and third) civil signal frequency

can improve GPS stand-alone civil horizontal accuracy to

the 1-3 meter range (at a 95% confidence level). The

removal of SA also has a substantial impact on reducing

DGPS data link capacity requirements because of the

reduced need for frequent SA corrections.

Military Signal Security

Current military users normally access the C/A-coded

signals but their security system corrects for the effects of

the SA degradation. The DoD encryption of the P-code to

form the Y-code not only limits access to the most

accurate codes to military (and other authorized) users

but also provides anti-spoof (A-S) protection. The current

plan is for the P/Y-coded signals to be retained until the

10 Journal of Global Positioning Systems

new M-code signals are generally available. The phase-

over interval can be expected to last until 2015 and

possibly longer.

The new M-code signals will also be secure. They differ

from the P/Y-code signals in that they provide improved

performance and are planned to be directly accessed. The

military M-code signal structure is in final development.

Figure 7 entitled New Military Signals : M-Code shows

the general characteristics of the military P(Y) and the

planned M-code signals.

1563 1575 1587

1215 1227 1239

•Anti-jam through higher power

•Robust and autonomous acquisition

•Spectral isolation from civil signals

•Improved security (exclusivity, authenticity, confidentiality)

•Better performance

•Flexibility

•Compatibility with C/A-code and P(Y)- code receivers

•Operation within existing L1 and L2 bands

Frequency in MHz

Fig. 7 New Military Signals: M-code

5 GPS Accuracy Improvements and Performance

Augmentations

Ionospheric Errors

Ionospheric propagation group delay effects on GPS

signals cause most of the residual receiver error. These

delay effects vary considerably depending on random

effects, the time of day, season of the year and the

activity state of the 11-year period solar (sunspot) cycle.

Figure 8 entitled Solar Cycle 23 Sunspot Number

Prediction provides a graphic indication of the cyclic

character of the solar emissions that affect the earth’s

ionosphere.

The ionospheric propagation delay error effects can be

effectively removed by using the two frequency

ionospheric correction technique that the military now

uses. For this reason, there has been interest for many

years in a second frequency for civil GPS users. As

indicated earlier, agreement has been reached within the

government to incorporate a second civil signal (at L2)

and a third civil signal (L5, in the ARNS band at 1176.45

MHz) in future Block IIF spacecraft. The excellent

capabilities of these signals for ionospheric correction can

improve the civil accuracy for stand-alone GPS users to

the 1-3 meter level, or better.

Unfortunately, the current schedule for the deployment of

enough Block IIF spacecraft (about 18 are required) to

allow confident access to the L5 signals is over a decade

away. A reasonable estimate for the date at which full

operational reception of L5 signals would be about 2015,

depending on the lifetime of the IIA, IIR and IIF

spacecraft.

Receiver Noise Errors

Other factors that influence GPS user equipment accuracy

include the number of parallel receive channels available

to a receiver, the receiver’s code noise performance, its

susceptibility to multipath, and the errors associated with

implementing, or mechanizing, the solutions. The number

of GPS receiver channels corresponds to the maximum

number of spacecraft that can be simultaneously tracked.

Ideally, the number of channels is large enough to

provide continuous all-in-view tracking of the visible

GPS spacecraft, since this provides the best geometric

performance.

Although the receiver code noise for a conventional C/A-

code receiver (with carrier aiding) is equivalent to a

ranging error of about one meter, code noise errors for

McDonald: The Modernization of GPS 11

Fig. 8 Cycle 23 Sunspot Number Prediction (September 200)

"narrow correlator" C/A-code receivers can be as low as

10-20 cm. Implementation errors are usually small or

negligible. Receiver thermal noise performance at the 10

cm. to 1 m. level is considered good, however GPS

precise carrier phase measurements are typically

considerably better, nominally at the 0.5-2 mm.

equivalent noise level.

Control Segment Errors

The GPS control segment determines the quality of the

GPS spacecraft orbital elements and timing data. These

are uploaded to the GPS spacecraft memory and then

periodically transmitted to the users in the GPS data

message. This spacecraft position and other data directly

affect user accuracy. Moreover, since it degrades with

time, the data is influenced by the update rate of the

uploads to the GPS spacecraft. Recent improvements in

the control segment provide spacecraft ephemeris

accuracy at the 1-2 meter level.

The planned addition of six ground stations of the

National Imagery and Mapping Administration (NIMA)

to the GPS tracking network will substantially improve

the quality and timeliness of the GPS tracking

measurements and the computed parameters. More

frequent uploads to the spacecraft are also planned. In the

2000-2010 period, sub-meter ephemeris accuracy, which

will improve to the decimeter range, is expected for the

GPS tracking network and the Operational Control

System.

Autonav Operation

The GPS constellation may be required to operate

without the GPS ground segment for an extended period.

By accurately ranging to other spacecraft using the GPS

inter-satellite link the Block IIR, IIF and III spacecraft

can operate in an autonomous navigation (autonav) mode.

The autonav ranging data is cross-linked to other

spacecraft in the constellation to provide continuous on-

board information that is used to autonomously compute

accurate new ephemeris data for each spacecraft. This

data, incorporating the measured GPS spacecraft orbital

perturbations, can provide excellent system accuracy over

an extended period (several months).

Position Accuracy Estimates for 2002 and 2012

Figure 9, entitled Position Accuracy Estimates for Civil

and Military GPS Receivers for 2000 to 2010 illustrates

the anticipated system performance improvements in

GPS as reflected in GPS receivers operating in their

various modes. This chart addresses position accuracy

only, however there are many other measurements that

will be affected as well. A brief summary of the

additional measurement parameters and their estimated

accuracy is given in Table 3, entitled GPS Performance

in Various Modes of Operation for 2000 and 2010.

12 Journal of Global Positioning Systems

2000 2010200506 07 08 09

1 m

10 cm

1 cm

1 mm

10 m

100 m

1 m

10 cm

1 cm

1 mm

10 m

100 m

1 m

10 cm

1 cm

1 mm

10 m

100 m

Current Civil GPS Systems

Standard Positioning Svc. (SPS)

C/A-code (1.023 Mcps)on

L1 (1575.42 MHz)

L2 (1227.6 MHz) carrier phase

ionospheric delay correction

*Note:Selective Availability degradation

of SPS at ~60 m. level until 1 May 2000

60 m

6 m

2 m

50 cmDGPS receiver using C/A-code

(C2)

Carrier aided; separation of ~20-200km

Stand-alone C/A-code receiver (Current 1)

30 cm

7 cm

RTK, Real Time Kinematic(C3) ; carrier

phase measurement receiver;L1 only

2 cm

5 mmSurvey receiver w. post processing (C4)

carrier fmeas’dat L1 & L2 for iono-correction

Future SPS Systems

(Using New Civil GPS Signals)

C/A-codes (1.023 Mcps) on

L1 (1575.42 MHz) and

L2 (1227.6 MHz

L5-codes (I5,Q5 at 10.23 Mcps)

on L5 (1176.45 MHz)

Note: L5-codes: I5 on L5 with data msg.

Q5 (quad. signal) on L5, no data msg.

Current 1

Degrades with solar cycle max at ~2011

Stand-alone SPS receiver (Future 1)

C/A- codes on L1, L2

F2 2.0 m

Precision SPS stand-alone receiver (F2)

C/A-codes on L1, L2 and I,Q -codes on L5

3.0 m

3 cmReal Time Kinematic receiver (F3); carrier φ

C/A-codes on L1, L2; I5, Q5-codes on L5

5 mmSurvey receiver/post process’g (F4); carr. φ

C/A-codes on L1, L2 ; I5, Q5-codes on L5

6 m5 mMilitary PLGR stand-alone receiver (Mil. 1)

C/A and P/Y- codes at L1 only

2 m

0.8 mMilitary (2f) stand-alone receiver (M2)

PPS: C/A +P/Y or M-codes; on L1 & L2

50 cmMilitary DGPS receiver (M3)

PPS: C/A +P/Y or M-codes; on L1 & L2

M 3

Year

Note: IOC for M-codes and C/A-codes on L2 ~08+;

current plans call for L5 signals IOC at 2012+

Future 1

2 m

Position Accuracy in meters

Position Accuracy = Horizontal position

accuracy at 95% confidence level.

Note: SPS = Standard Positioning Service

PPS = Precise Positioning Service

Current and Future

Military Systems (PPS)

(Using existing or new signals)

C/A-codes on L1 (& L2) and

P/Y-codes on L1 and L2

and/or (evolving to)

M-codes on L1 and L2

Note: M-codes are the new Military codes

planned as L1, L2 split spectrum signals

M 2

Military 1

Note: Current plans call for M-codes IOC at ~2008

2 - 4 m.

7 m3-5 m.

Fig. 9 Position Accuracy Estimates for Civil and Military GPS Receivers in Various Modes of

Operation for the 2000 to ~2010+ Time Period

Tab. 3 GPS Performance in Various Modes of Operation - for 2000 and 2010

Accuracy estimates for 95% confidence in horizontal; vertical accuracy is about 1.4 x horizontal dimension

Position Velocity Time

Comments

Mode of Operation

Conventional civil stand-alone

SPS: C/A-code

E.g., PLGR (P/Y)

Iono dependent

C/A-code differential

Standard Positioning Service (SPS)

L1 L2 L5

GPS Bands

Code differential

*SPS: C/A-codes at L1 and L2



Precision stand-alone w 3f’s

*SPS: C/A &L5 I,Q-codes

2000

2010

2000

2010

2000

2010

Real time kinematic (RTK)

SPS: C/A-code, carrier phase me as.

Survey: Post-processing; long b

SPS: C/A & carrier phase (with 2f)

Diff.

GPS

Real time kinematic (RTK); 3f

*SPS: C/A, L5 I,Q-codes, carrier φ

Military receiver (1f)

Precision Positioning Svc. (PPS)

Military receiver (2f)

PPS: C/A+P/Y or Mil (M) codes

Military DGPS receiver

PPS: C/A+P/Y or M-codes

Precision attitude meas.; 3f’s

Std. 2f rec:(P/Y)

Future 2f rec:(M)

5-10 m

3-8 m

1-3 m

20 cm-1 m

10-50 cm

3-10 cm

0.5-10 cm

0.1-3 cm

30 cm-1 m

1 m-3 m

Conventional civil stand-alone

SPS: C/A-codes at L1 and L2

2-5 m

L2 carr φin 00

Baseline (b) dep.

No L2c in 2000

1-10 cm

15-30 cm/s

10-20 cm/s

3-10 cm/s

10-20 cm/s

5-10 cm/s

2-10 cm/s

1 mradia n

0.1 mradAttitude, angle θ

2-10 ns

20 ns

10 ns

40-100 ns

20-40 ns

30-60 ns

20-30 ns

5-15 ns

40 ns

4-8 m

3-6 m

10 cm/s

5 cm/s

100 ns

40 ns

2-4 m

0.5-1 m

1-2 m

20-80 cm

10 cm/s

5 cm/s

5-10 cm/s

1-5 cm/s

80 ns

25 ns

50 ns

10 ns

Iono dependent

5-10 cm/s

1-5 cm/s

0.5-3 cm/s







Civil C/A-codes at L1 and L2

Civil I5, Q5-codes (10.23Mcps) at L5

-RNSS -ARNS







Military M-codes P/Y-codes







Civil Current Sy st ems

Civil Future (2010+)

Signals

Military Current

and Future (2010+)

Off in

2000



Off in

2000

*SPS: C/A, L5 I,Q-codes, carrier φ

Off in

2000

6 Integrity, Availability, Constellation Size and Power

Concerns

System Integrity

Another civil application of concern is the monitoring of

the data obtained by GPS airborne receivers to determine

autonomously (without the use of ground-based data) the

validity, or integrity, of the navigation information

received. In particular, this involves establishing the

credibility of the GPS user equipment measurements and

determining if any of the spacecraft are causing an error,

or are out of tolerance, to the extent that they might

provide hazardous or misleading information.

McDonald: The Modernization of GPS 13

Although ranges to four spacecraft are sufficient to

determine position, as many as six or more spacecraft

ranges may be required to determine signal integrity to

the acceptable high confidence level needed to meet

aviation integrity safety requirements. This fault detection

and elimination (FDE) process, frequently termed

receiver autonomous integrity monitoring (RAIM), is an

especially difficult problem. The removal of SA improves

the viability of the FDE process very significantly. Prior

to its removal, SA could result in position errors of over

100 meters for five percent of the time according to its

specification.

Signal Availability

The baseline GPS constellation of 24 spacecraft was

established for military applications. Most observers

consider the availability of the spacecraft signals for

some civil applications, especially those relating to

safety-of-life applications, marginal or in some cases

unacceptable. Also, in limited visibility conditions, such

as the “urban canyon” situation where a GPS receiver is

near ground level with tall buildings on either side, the

number of spacecraft in view can be reduced

substantially. At times, this results in an insufficient

number of signals to provide a navigation solution. For

these and other reasons, there has been interest in

increasing or augmenting the GPS constellation, possibly

by 6 to 12 spacecraft, to provide a total of 30 to 36

operational spacecraft.

Increased Constellation Size

As mentioned, proposals have been made to add GPS (or

similar) spacecraft as needed to provide the additional

robustness desired for the GPS constellation.

Investigations accomplished for the FAA and others

indicate that 30 to 36 spacecraft may be necessary to

meet integrity and other safety-of-life requirements. To

date, however, the high cost of placing payloads into orbit

has precluded serious consideration of a substantial

increase in the GPS constellation size. As discussed

earlier, augmentations have been planned to provide

additional signal availability as well as the differential

correction, integrity and ionospheric correction data

needed in marine and aviation applications.

Power Level Improvements

Since the modernized GPS spacecraft will provide the

civil community with a C-code signal on the L2

frequency as a second primary signal, the power

requirements for this signal are comparable to those of

the C/A-code signal on L1 (-160dBw). Also, for similar

reasons, the P/Y-code power on L2 requires about a four-

fold increase (6 dB) in all of the modernized GPS

spacecraft since the L2 signal was previously used only

for ionospheric delay correction. The new L5 signal in

the ARNS band will require a power level about 6 dB

higher than that of the C/A-code signal on L1 to

compensate for the higher levels of interference and noise

in this band. A additional increase of 3-6 dB in the power

levels of the civil signals has been promoted by many for

a variety of safety, cost and performance reasons. This

now appears to be planned for the new generations of

GPS spacecraft, or at least for the Block III spacecraft.

The military signals at L1 and L2 are planned for

transmission at higher power levels (by 6-10 dB) than

current levels. Further, a substantial increase in power

beyond this is desired for moderate operational intervals

and in selected tactical areas. This would both improve

performance in an electronic countermeasures (ECM)

environment and provide additional signal robustness.

This capability may require a collapsible large aperture

antenna system. This would provide a steerable spot

beam to cover selected tactical areas. The military needs

tend to absorb the substantial prime power requirements

of the new spacecraft and other planned functions may

have some difficulties in the competition. Civil

improvements may be at risk.

Figure 10 entitled GPS Signal Power Spectra shows the

relative signal power spectra for the various existing and

planned GPS signals to be transmitted by the spacecraft.

-10-8-6-4-20 24 6810

-100

-95

-90

-85

-80

-75

-70

-65

-60

Frequency (MHz)

Power Spectrum (dBW/Hz)

C/A

BOC(10,5)

Fig. 10 Power Spectrum for the GPS M-Codes, P/Y-Codes, C/A-

Codes, C-Codes and L5 I- and Q-Codes

Implementation Schedule and Timeliness

A principal concern in the modernization and system

performance improvement area is the ability of the

executive and legislative parts of the government

organizations involved to recognize the importance

associated with GPS modernization, as well as the risk

involved in delaying or discarding various aspects of the

program.

Planned Schedule for GPS S/C Launches

An example of this concern is the currently planned

schedule for GPS spacecraft launches. From the

announced launch information provided by the GPS Joint

Program Office and other sources, Figure 11 was

prepared, entitled Modernized GPS Capability Dates for

Planned GPS Spacecraft Phase-in. This indicates that, at

14 Journal of Global Positioning Systems

II, IIA

1 - 28

(28 SVs)

Current capab.

Lifetm: 7.6 yrs.

IIR 1 - 9

(8 SVs, 1 Fail)

Current capab.

Lifetm: 10 yrs.

III 1 -?

SV quan. TBD

Acquisition

in planning

Capab’s to 2030

Life: ~15 yrs.

L5 (1176.45)

L2 (1227.6)

C/A CI5 Q5

MM1990 2000 2010 2020

GPS

Space

Vehicle (SV)

Block No’s

GPS Frequency Bands and

Signal Codes within Bands*Calendar Year

26 SVs

89 05

8 SVs

Phase-in / Phase-out of GPS Spacecraft (All Blocks)

2030

12 SVs

SV-14

18SVs (IOC)

9-12 IIR-M’s ; 6 IIF-Lites

95 97

* C/A: The C/A-codes, or coarse/acquisition codes (at 1.023Mcps), used currently by both military and civil receivers. ** or alternative civil C signals

P/Y: The P/Y-codes, or precise/secure codes (at 10.23Mcps), used by military and other authorized users.

M: The M-codesare the new military split spectrum signals with codes of 5.11Mcps at the P/Y-code nulls codes of L1 and L2.

L1: The GPS band centered at 1575.42 MHz. L2: The GPS band centered at 1227.6 MHz. L5: The planned civil GPS band centered at 1176.45 MHz.

I5: A PRN code (at 10.23 Mcps, 1 ms length, w/data message) for use by the civil community. Signals are in the new civil L5 frequency band .

Q5: A PRN code (at 10.23 Mcps, 1 ms, no data) in phase quadrature to I5on L5. I5and Q5 are defined by RTCA SC-159 Working Group-1(6/2000).

Estimates based

upon published

S/C lifetimes

IIR-M

10 - 21

(12 SVs)

Modified

Lifetm: 10 yrs.

2010 2020

? ?

Optimistic

SV decay

projection

L1 (1575.42)

IIF-Lite

1 - 12

(6 SVs firm &

6 SVs planned)

Modified

Life:12-15 yrs.

Optimistic

SV decay

projection

12 SVs

18 SVs (IOC)

12 IIF-Lites and 6 III’s

FOC

2014

FOC

2010

IOC

2012

24+SVs

2030

IOC

2008

+M-codes and L2 CS-codes**

S/C modified for

Military M-codes

C-codes on L2**

Civil L5 signals

S/C modified for

Military M-codes

C-codes on L2**

S/C built for

All signals

4.30.99; Rev. a) 1.14.00; b) 8.29.00 c) 11.4.00; d) 2.28.01 e) 6.01

Existing P/Y- and C/A-codes

II, IIA

IIR-M 10-21

IIF-Lite 1 - 12

III 1 -?

L5 desired but

not planned on

IIR-Mod’s

IIR’s have a

power limitation

New SVs: GPS III - All signals

+L5; I &

Q-codes

2000

1990

P/Y P/Y

IIR 1-9

Fig. 11 GPS Modernization: Estimated Availability Dates for Planned Spacecraft (IIR, IIF, III)

the current launch rates for planned GPS Block IIR and

IIF spacecraft, the new modernized capabilities may not

be available to civil and military GPS users until about

2015-1018. It is clear that significant modifications to the

GPS spacecraft are needed soon to move the availability

date for modernized capabilities to an earlier time. This

date could be moved to 2010, as shown in the figure, or

possibly earlier, if appropriate modifications (the new

civil and military signals) to the IIR and IIF spacecraft

can be implemented and if an accelerated schedule for the

production and launch of these spacecraft can be

arranged.

7 International Implications

European Concerns and Galileo

For some time, the European community (and other

nations worldwide) has had concerns about committing

their navigation services to GPS. Their concerns are first,

they have no control or role in GPS operation; second,

there are no assurances that GPS civil signals will be

available in times of conflict involving the US, and third,

Europe would like to maintain its technological

competence and participate economically in the

navigation satellite field. The first concern reflects their

strong interest in the commercial, strategic and political

implications of having European control of the

positioning, navigation and precision timing services in

the European region.

The European Commission (EC) has sponsored

investigations of the introduction of satellite systems over

Europe during the past several years. These indicate a

world market for GNSS equipment and services of about

$40B in a few years.

On February 10, 1999, the European Commission (EC)

requested the governments of the 15 states in the

European Union (EU) to give their political and financial

backing to develop a state-of-the-art Global Navigation

Satellite System (GNSS), called the Galileo project (EC,

1999). The EC appears convinced that the development

of a European system would avoid the problems now

caused by their dependence on the US GPS and provide

substantial economic benefits to the European

community.

Since navigation satellite signals are currently directing

air and sea movements, the EC states that there is a clear

market potential for routing cars and trucks more

efficiently as well as for farming, fishing, timing and

synchronization; and for infrastructure planning, mineral

exploration and land surveys.

Signal and User Equipment Compatibility

There have been proposals in the US (McDonald, 1998c)

and Europe for independently fielded navigation satellite

systems (e.g., GPS and a GNSS) that would use the same

civil frequencies and signal structures, as a common base.

If successful international coordination on this principle

could be achieved, then the signals from the independent

systems would be compatible and allow worldwide

equipment inter-operability. The US DOT has

McDonald: The Modernization of GPS 15

Spacecraft in Orbit30 +3 58 +6 User view: 14–25

Spacecraft Availability (aver.) 8 – 9+16 – 18 Excellent,+geom.

Integrity (autonomous) Fair Excellent FDE/ RAIM oper.

Coverage (worldwide) Good Excellent Espec. high lat's

Dilution of Precision 1 – 3 0.7 – 2 Improved accur.

Interference Susceptibility Low Very LowAdaptive f select

Safety Services Protection 4+signals 6+signalsLg. range ofsig's

Frequencies Available (civil) 1 - 5 2 – 8User flexibility

E911, Related Capabilities Fair Very GoodAlso greater pwr.

Receiver Cost (relative) 1C 1.2C-Production fcn.

Accuracy (Autonomous, code)*1-2-m.0.6-1.3 m. Multi-f, estimated

Galileo

+ EGNOS S/C

Combined

Capability Notes

Characteristic

(nominal HDOP-VDOP)

See included charts for accuracy estimates for all classes of GN services

28 +3

8 – 9

Fair

Good

1 – 3

Low

2 signal

1 - 3

Fair

GPS

+ WAAS S/

1-2-m.

acknowledged the potential benefits of a European

system that is compatible with GPS. Also, the Russian

Federation has indicated its willingness to consider

participating in a joint approach. Russia has access to a

set of frequencies (for Glonass) that provide some

desirable features for use in a global navigation satellite

system. The EC appears willing to proceed constructively

on an international basis to develop Galileo and has stated

that failure for the EU to act now means "missing a huge

and probably unrepeatable opportunity".

Cost and Funding

The EU estimates the cost of the GALILEO project at

between $2-3B, which would be funded in part from the

EU budget and in part from private and national

government sources. The Galileo Project may also

provide a significant role for the Russian Federation. The

GNSS orbital arrangement is not fully defined but current

planning indicates the use of spacecraft in medium Earth

orbit somewhat above GPS that will provide good

coverage to Europe, especially to the high latitude regions

of the European continent.

Combined Performance

Table 4 illustrates the significant advantages of having

both GPS and Galileo available to users worldwide.

Substantial benefits are obtained including improvements

in system accuracy, autonomous integrity, interference

mitigation, urban canyon operation and in kinematic

precision measurements. These advantages are tabulated

in greater detail on the figure with brief notes provided.

The improvement in quality of the combined system

operation is impressive.

Tab. 4 The GPS-Galileo GNSS Characteristics, Features and Combined System Capabilities for existing and Planned Civil Operational Systems

Figure 12, entitled Comparison of GPS and Galileo

Programs illustrates a number of the system

improvement considerations relating to GPS and Galileo

and how they compare in some of the principal areas of

concern. The current delay in the implementation of GPS

modernization improvements indicates that the Galileo

program could become the future navigation satellite

system of choice for a period of several years. Urgent and

dedicated action needs to be continued on behalf of GPS.

8 Conclusions

8.1 The Impact of GPS Modernization

The modernization of GPS will provide many new

capabilities, such as two new civil frequency bands (L2

and L5), new civil and military signals, higher signal

power levels, more extensive ground tracking and more

frequent and accurate spacecraft position updates. All of

these dramatically improve accuracy, integrity and other

aspects of system performance.

Additionally, the management of GPS has changed, and

now involves coordinated civil and military funding and

oversight. Hopefully, other institutional changes will

occur to provide a centralized, coordinated management

for GPS that will allow the program to compete

successfully for future federal funding.

These factors, combined with the increasing worldwide

importance of navigation systems and services, provide a

strong basis for integrating GPS into an international

Global Navigation Satellite System consisting of a

number of independent requirement but coordinated

system elements. The modernized GPS will continue to

play an important military role, as well as a central role in

providing position, velocity, attitude and time services for

16 Journal of Global Positioning Systems

...

System Improvement ConsiderationGPS

GPS Galileo

Galileo

Centralized management and funding

Primarily DoD management,

funding and priorities

Some IGEB involvement

European Union

centralized

management

Guaranteed levels of civil service: Now

Later

2-3

Schedule for new user full capabilities 2012 - 2014+ (~2016)*

Assumes launch on demand

2007- 2008+ (~2010)*

Poss 5-7yr safety svc lead

Civil capabilities in next 10 years

(from 1/02, by announced plans, policies)Current capabilities;

some spacecraft improved

Improved operational

performance

Funding commitment (current)~ $1B+ 2B Euros

+1- 2BEuros to complete

Increased civil signal operational power

levels by 2010? (Rel. to current GPS)

On some spacecraft

(by ~6 dB)

Yes

(by ~6 dB)

Autonomous capabilities – Now

After implementation (system/enhancements)

L1(C/A): 5-10 m.

L1 (C/A), L2(C); L5: 0.3-3 m.

None

2-5 Frequencies: 0.3- 3 m.

Note: SA removed 1 May 2000

* Indicates normal (realistic) delay of schedule 135.2-3/122.2-5B 99.6, Rev a:00.1.14; Rev.b:00.10.6

Fig. 12 A Comparison of Civil GPS and Galileo Programs - The Galileo Opportunity

civil safety, security, science, engineering and related

applications in an economically sensible manner.

In the next decade, GPS modernization will significantly

increase the navigation capabilities of both civil and

military users by incorporating the following

improvements:

New civil frequencies at L2 (1227.6 MHz) and at L5

(1176.45 MHz), as well as retention of the long-standing

civil signal at L1 (1575.42 MHz). Military capabilities

will remain in the L1 and L2 bands. The new

arrangement provides capabilities to civil users for

ionospheric correction, improved signal robustness,

increased interference rejection and improved dynamic

precision through the use of techniques for resolving the

ambiguities associated with precision carrier phase

measurements.

New signal structures for both civil and military users.

The new civil signals at L5 are projected to support a

code rate 10 times that of the C/A-code. This will

improve code measurement accuracy, reduce code noise,

reduce cross-correlation concerns, and provide improved

multipath mitigation. The new military signal structure

will provide improved code accuracy, desirable power

distribution in the spectrum, and direct access to the

military secure codes.

Removal of Selective Availability, which had been

planned for between 2000 and 2006 (by the March, 1996

Presidential Decision Directive on GPS), has been

accomplished. GPS now provides full, undegraded

accuracy to the civil signals. This, with the additional

civil frequencies (for ionospheric correction), will

improve civil GPS performance by a factor of about ten

(compared to that with SA). For example, GPS has

progressed from a 50-60 m. accuracy (at a 95%

confidence level) with SA on to a current accuracy

(without SA) on an unaided basis of about 5-10 m.

Reduction in Systematic Error Sources includes not only

SA removal and ionospheric error correction, but

substantial improvements in GPS receivers, in the control

segment redundancy (with the added NIMA monitor

stations), and improved statistical estimation techniques

providing substantially better capabilities for minimizing

spacecraft position prediction (ephemeris) errors.

Increased Signal Availability and Power from GPS

Spacecraft which have greater reliability and lifetimes.

Power in the new civil L2 signal is to be consistent with

the current L1 civil signal for greater system robustness.

Power in the military M-code signals is to be flexible and

substantially greater than the current P/Y-code power

levels. While a larger GPS spacecraft constellation cannot

be guaranteed, there is strong interest in this expansion.

Improved Performance with Augmentations, such as the

very substantial performance enhancement achieved by

the removal of Selective Availability degradation, as well

as the planned augmentations. These include the USCG

Differential Network, the National DGPS, the FAA's

WAAS and LAAS, the European EGNOS, the Japanese

MSAS, and a large number of other DGPS systems that

can provide highly precise position, velocity and time

measurements for a great variety of applications.

International Implications - The GPS has become the de

facto standard for navigation satellite system

performance, but there have been long-standing concerns

internationally because of the US military origin and

control of the system. However, systematic and

institutional changes have occurred, and are occurring

McDonald: The Modernization of GPS 17

such that GPS now has some new features and some

excellent opportunities. GPS now has or provides: a) a

joint civil/military management structure, b) an important

national resource with worldwide applications and

implications, c) independent civil and military

capabilities, both of which are being significantly

improved, d) a substantial economic “engine” for U.S.

industry and worldwide users, and e) the capabilities for

taking a leading role, and substantially influencing the

formation of an international GNSS.

8.2 Final Comment

For the modernization of GPS to occur in a timely and

useful manner. it is clear that strong funding support, a

strong national commitment, possibly White House

leadership, new institutional arrangements and a variety

of other factors are necessary. The technical and

implementation issues appear straightforward; however,

the institutional, funding, international and national

priority concerns appear critical. The modernized GPS

with the introduction of the European Galileo system and

augmentations to both provides a tremendous combined

capability that can benefit civil users worldwide.

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