Dale A. Force

Glenn Research Center, Cleveland, Ohio

Individual Global Navigation Satellite Systems 

in the Space Service Volume

NASA/TM—2015-218465

April 2015
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Dale A. Force

Glenn Research Center, Cleveland, Ohio

Individual Global Navigation Satellite Systems 

in the Space Service Volume

NASA/TM—2015-218465

April 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

International Technical Meeting 2013

sponsored by the Institute of Navigation

San Diego, California, January 28–30, 2013
Acknowledgments

The Policy and Strategic Communications Division, NASA Space Communications and Navigation (ScaN) Program funded this

work. I want to thank the NASA PNT Team for helpful comments, and David Bittner of NASA Glenn Research Center and 

Ted Driver of AGI for advice on using STK. This paper was also published in the Proceedings of the 2013 International 

Technical Meeting of the Institute of Navigation, pages 604 to 607, http://www.ion.org/publications/.

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NASA/TM—2015-218465 1

Individual Global Navigation Satellite Systems in the Space Service Volume

Dale A. Force

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract 

Besides providing position, navigation, and timing (PNT) to 

terrestrial users, GPS is currently used to provide for precision 

orbit determination, precise time synchronization, real-time 

spacecraft navigation, and three-axis control of Earth orbiting 

satellites. With additional Global Navigation Satellite Systems 

(GNSS) coming into service (GLONASS, Beidou, and 

Galileo), it will be possible to provide these services by using 

other GNSS constellations.

The paper, “GPS in the Space Service Volume,” presented at 

the ION GNSS 19th International Technical Meeting in 2006

(Ref. 1), defined the Space Service Volume, and analyzed the 

performance of GPS out to 70,000 km. This paper will report a 

similar analysis of the performance of each of the additional 

GNSS and compare them with GPS alone. 

The Space Service Volume, defined as the volume between 

3,000 km altitude and geosynchronous altitude, as compared 

with the Terrestrial Service Volume between the surface and 

3,000 km. In the Terrestrial Service Volume, GNSS 

performance will be similar to performance on the Earth’s 

surface. The GPS system has established signal requirements 

for the Space Service Volume. 

A separate paper presented at the conference covers the use 

of multiple GNSS in the Space Service Volume.

Acronyms

AGI Analytical Graphics, Inc.

GEO geosynchronous Earth orbit

GNSS Global Navigation Satellite System

GPS Global Positioning System

HEO highly elliptical orbit

IOV in orbit validation

LEO low Earth orbit

MEO medium Earth orbit

NASA National Aeronautics and Space Administration

PNT Position, Navigation, and Timing

PVT Position, Velocity, and Timing

SCaN Space Communication and Navigation

SSV Space Service Volume

STK Systems Tool Kit (formerly Satellite Tool Kit, 

Analytical Graphics, Inc.)

TSV terrestrial service volume

Introduction 

Although GNSS were created to provide position, velocity 

and timing (PVT) for terrestrial applications, they can also 

provide PVT to Earth orbiting satellites. GPS is already being 

widely used for this purpose, having first flown onboard 

Landsat 4 in 1982 (Ref. 2).

For satellites operating in low Earth orbit (LEO), GNSS 

receivers will see signal that are similar to those seen by 

terrestrial users, apart from the high dynamic effects due to 

orbital velocity. They can receive signals through a zenithpointing antenna because they are within the primary 

transmitted beam of the GNSS satellites. For GNSS use, LEO 

extends to beyond 3,000 km.

For satellites in higher orbits, a zenith-pointing antenna can 

receive fewer signals, since the satellite will be outside the main 

beam of many satellites. Above GNSS altitude, of course, the 

number of signals received from above the satellite will be zero. 

However, GNSS can still be used for PVT at these altitudes by 

taking advantage of tracking GNSS signals crossing the Earth’s 

limb using a nadir-pointing antenna, as shown in Figure 1,

while satellites at intermediate altitudes can use a combination 

of zenith- and nadir-pointing antennas. Due to the increased 

range and reduced transmitter gain at the larger off-nadir 

angles, the signals will be much weaker than those available at 

the Earth’s surface. However, specialized GPS receivers have 

demonstrated the increased acquisition and tracking sensitivity 

and integrated a navigation filter for state estimation when less 

than four satellites are available. Multi-constellation GNSS 

receivers for satellites are currently under development.

Space Service Volume

Based on the unique nature of GNSS signals as a function of 

altitude, requirements for GNSS spacecraft services can be 

allocated to two service volumes. The terrestrial service volume 

(TSV) includes all terrestrial and space GNSS users up to an 

altitude of 3,000 km, and the space service volume from

3,000 km to the approximate geostationary altitude of 

36,000 km.

Terrestrial Service Volume

Users in the TSV enjoy uniform received powers and have 

fully overlapping coverage. The use of multiple GNSS will
NASA/TM—2015-218465 2

Figure 1.—Geometry for reception of GNSS signals by a HEO 

satellite (Ref. 1).

Figure 2.—Terrestrial and Space Service Volumes (Ref. 1).

have some benefit due to the increased number of pseudoranges 

available although individual GNSS already provide very good 

coverage.

Space Service Volume

The SSV can be divided into two regions: (1) the medium 

Earth orbit (MEO) SSV (3,000 to 8,000 km), and (2) the 

HEO/GEO SSV (8,000 to 36,000 km). Figure 2 illustrates the 

relationship between the three altitude regions described above. 

In the TSV, adequate coverage can be supplied by a zenithpointing antenna; in the MEO region, a combination of zenithand nadir-pointing antennas are needed, but even a single 

GNSS will often provide four-satellite coverage; in the highly 

elliptical orbit (HEO)/geosynchronous Earth orbit (GEO)

region nearly all GNSS signal will come from satellites 

transmitting across the Earth’s limb. 

Availability of GNSS Services

Currently GPS provides promises of future signal strength 

and quality within guaranteed beamwidth out to GEO altitude

(Ref. 3).

Signal Availability

This section summarizes the analyses of multi-constellation 

GNSS availability for spacecraft navigation.

STK simulations were used to evaluate the availability of 

GNSS signals for each of the new systems (GLONASS, 

Galileo, and Beidou) as well as GPS.

The GPS constellation used in the simulations is the 24+3 

constellation currently planned for future availability. The 

GLONASS constellation is the current 24-satellite

constellation. The Galileo constellation is the planned 24-

satellite constellation. The Beidou constellation is the 24 MEO 

satellite portion.

The beamwidths used in the simulation are the committed

GPS half-beamwidths of 23.5° for GPS L1 and 26° for GPS L2 

and L5 (Ref. 3). Since beam data for Beidou and GLONASS 

are not available, we assume half-beamwidths of 23.5° for 

GLONASS L1 and Beidou B1; and 26° for GLONASS L2 and 

L3, Beidou B2 and B3, similar to GPS beamwidths. The 

published Galileo IOV antenna data show somewhat narrower 

beams (appropriate for their higher altitude) so we used 24° for 

the half-beamwidth of E5 and E6 and 20° for the halfbeamwidth of E1 (Ref. 4).

An Earth atmosphere mask was applied requiring signals to 

pass at least 50 km above WGS84. While ionospheric effects 

can be important for signals passing less than 1,000 km above 

the Earth’s surface, all GNSS satellites transmit multiple 

frequency signals through this region allowing multi-frequency 

receivers to correct for ionospheric effects.
NASA/TM—2015-218465 3

At each altitude, a grid of approximately 2,000 evenly spaced

points was generated covering all latitudes and longitudes. For 

each grid point, the GNSS constellations were propagated 

forward in time 48 hr (in 60-sec steps) and line of sight vectors 

were evaluated for each step in time. The products of a run were 

time histories of GNSS satellites visible. From this availability,

statistics were calculated, giving the following metrics listed in

Table 1 to Table 8:

• Availability of at least one and of at least four GNSS 

satellites, both for an average point and for the worst point

• Duration of longest single-fold outages (intervals when 

no satellite visible)

• Duration of longest four-fold outages (intervals when less 

than four satellites were visible)

• Minimum, average, and maximum number of satellites 

visible

TABLE 1.—SIMULATED ALTITUDES

Altitude Comment

300 km Typical LEO Altitude

3,000 km Border between TSV and SSV

8,000 km Border between medium and high orbit service

15,000 km Within high orbit service, below GNSS altitude

25,000 km Within high orbit service, above GNSS altitude

36,500 km Approximate GEO altitude

70,000 km Approximately twice GEO altitude

TABLE 2.—A 300 km ALTITUDE

GPS GLONASS Galileo Beidou

1+ (%) 100 100 100 100

4+ (%) 100 100 100 100

<1 (s) 0 0 0 0

<4 (s) 0 0 0 0

Min. (#) 10 14 12 12

Ave. (#) 14.3 15.2 12.7 12.5

Max.(#) 19 18 17 17

TABLE 3.—A 3,000 km ALTITUDE 

GPS GLONASS Galileo Beidou

1+ (%) 100 100 100 100

4+ (%) 100 100 100 100

<1 (s) 0 0 0 0

<4 (s) 0 0 0 0

Min. (#) 16 15 16 18

Ave. (#) 20.9 18.0 18.7 18.7

Max. (#) 24 22 20 20

TABLE 4.—A 8,000 km ALTITUDE

GPS GLONASS Galileo Beidou

1+ (%) 100 100 100 100

4+ (%) 99.9+ 99.9 100 100

<1 (s) 0 0 0 0

<4 (s) 595 706 0 0

Min. (#) 3 3 4 5

Ave. (#) 9.6 7.5 9034 9.9

Max. (#) 15 12 12 12

TABLE 5.—A 15,000 km ALTITUDE

GPS GLONASS Galileo Beidou

1+ (%) 99.9+ 97.3 98.8 98.6

4+ (%) 80.0 61.6 68.7 75.2

<1 (s) 604 2,395 2,502 2,083

<4 (s) 8,289 7,247 7,581 5,405

Min. (#) 0 0 0 0

Ave. (#) 4.7 3.8 3.6 4.6

Max. (#) 10 9 8 9

TABLE 6.—A 25,000 km ALTITUDE

GPS GLONASS Galileo Beidou

1+ (%) 99.4 98.7 99.9 99.98

4+ (%) 36.0 19.6 15.8 24.6

<1 (s) 3,158 1,069 414 262

<4 (s) 30,166 28,692 79,839 25,283

Min. (#) 0 0 0 0

Ave. (#) 3.1 2.6 2.7 3.0

Max. (#) 8 6 6 6

TABLE 7.—A 36,500 km ALTITUDE

LOW FREQUENCY SIGNALS

GPS GLONASS Galileo Beidou

1+ (%) 97.0 94.0 94.9 90.2

4+ (%) 15.6 9.6 5.4 8.0

<1 (s) 9,763 2,194 3,024 1,649

<4 (s) 72,272 Never Never Never

Min. (#) 0 0 0 0

Ave. (#) 2.4 2.1 2.0 2.3

Max. (#) 6 6 4 5

TABLE 8.—A 70,000 km ALTITUDE

GPS GLONASS Galileo Beidou

1+ (%) 89.6 79.9 79.2 85.1

4+ (%) 2.8 4.7 2.4 3.2

<1 (s) Never 4,013 Never 4,763

<4 (s) Never Never Never Never

Min. (#) 0 0 0 0

Ave. (#) 1.7 1.4 1.4 1.6

Max. (#) 6 4 4 4
NASA/TM—2015-218465 4

Conclusions and Summary

The results clearly show the increased availability and 

reduced outage durations from using multiple GNSS for 

spacecraft PVT at the higher altitudes. At lower altitudes, the 

increase in number of GNSS satellites in view will allow more 

accurate PVT, even though the use of multiple GNSS 

constellations is not necessary to prevent outages.

References

1. Bauer, F. H., et al., “The GPS Space Service Volume,”

Proceedings of the ION GNSS-2006, Fort Worth, TX, 

Sept. 2006.

2. Bisnath, S., Spaceborne GPS Information Site, Geodetic 

Research Laboratory, Department of Geodesy and 

Geomatics Engineering, University of New Brunswick, 

http://gauss.gge.unb.ca/grads/sunil/sgps.htm

3. Anon., Global Positioning System Directorate Systems 

Engineering & Integration Interface Specification, ISGPS-200 Revision F, Sept. 21, 2011.

4. Monjas, Fernando, et al., “Test Campaign of the IOV (In 

Orbit Validation) Galileo System Navigation Antenna for 

Global Positioning,” 2010 Proceedings of the Fourth 

European Conference on Antennas and Propagation 

(EuCAP).

Biography

Dale A. Force is an electronics engineer at NASA Glenn 

Research Center studying the use of Global Satellite Navigation 

Systems for navigating satellites. He received the B.S. and M.S. 

degrees in Physics from Michigan State University and an M.E.

degree in electrical engineering from the University of Utah.