Dale A. Force

Glenn Research Center, Cleveland, Ohio

James J. Miller

National Aeronautics and Space Administration, Washington, D.C.

Combined Global Navigation Satellite Systems 

in the Space Service Volume

NASA/TM—2015-218469

April 2015
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Dale A. Force

Glenn Research Center, Cleveland, Ohio

James J. Miller

National Aeronautics and Space Administration, Washington, D.C.

Combined Global Navigation Satellite Systems 

in the Space Service Volume

NASA/TM—2015-218469

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 & 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 803 to 807, http://www.ion.org/publications/.

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NASA/TM—2015-218469 1

Combined Global Navigation Satellite Systems in the Space Service Volume

Dale A. Force

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

James J. Miller

National Aeronautics and Space Administration

Washington, D.C. 20546

Abstract 

Besides providing position, navigation, and timing (PNT) 

services to traditional terrestrial and airborne users, GPS is also 

being increasingly used as a tool to enable precision orbit 

determination, precise time synchronization, real-time 

spacecraft navigation, and three-axis attitude control of Earth 

orbiting satellites. With additional Global Navigation Satellite 

System (GNSS) constellations being replenished and coming 

into service (GLONASS, Beidou, and Galileo), it will become

possible to benefit from greater signal availability and 

robustness by using evolving multi-constellation receivers.

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 seventy thousand kilometers. This 

paper will report a similar analysis of the signal coverage of 

GPS in the space domain; however, the analyses will also 

consider signal coverage from each of the additional GNSS 

constellations noted earlier to specifically demonstrate the 

expected benefits to be derived from using GPS in conjunction 

with other foreign systems. 

The Space Service Volume is formally defined as the volume 

of space between three thousand kilometers altitude and 

geosynchronous altitude circa 36,000 km, as compared with the 

Terrestrial Service Volume between 3,000 km and the surface

of the Earth. In the Terrestrial Service Volume, GNSS 

performance is the same as on or near the Earth’s surface due 

to satellite vehicle availability and geometry similarities. The 

core GPS system has thereby established signal requirements 

for the Space Service Volume as part of technical Capability 

Development Documentation (CDD) that specifies system 

performance. Besides the technical discussion, we also present 

diplomatic efforts to extend the GPS Space Service Volume 

concept to other PNT service providers in an effort to assure 

that all space users will benefit from the enhanced 

interoperability of GNSS services in the space domain.

A separate paper presented at the conference covers the 

individual GNSS performance parameters for respective Space 

Service Volumes.

Acronyms

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 constellations were created to provide 

position, navigation, and timing (PNT) services for terrestrial and 

airborne applications, they can also provide PNT functionality to 

space borne applications such as Earth orbiting satellites. GPS is 

already being widely used for this purpose, having first flown 

onboard Landsat 4 decades ago in 1982 (Ref. 2).

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

receivers will see signals that are similar to those seen by 

terrestrial and airborne users, apart from the high dynamic 

effects due to orbital velocity. They can receive signals through 

a zenith-pointing 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 medium Earth orbit 

(MEO) altitude, of course, the number of signals received from 

above the satellite will be zero. However, GNSS signals can 

still be used for PNT at these altitudes by taking advantage of 

tracking GNSS signals crossing the Earth’s limb using a nadirpointing antenna, as shown in Figure 1, while satellites at 
NASA/TM—2015-218469 2

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

satellite (Ref. 1).

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

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, and are 

already a fixture in terrestrial signal monitoring systems such as 

the International GNSS Service (IGS) managed by the NASA 

Jet Propulsion Laboratory (JPL).

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 

have some benefit due to the increased number of pseudoranges 

available although individual GNSS constellations already 

provide very good coverage.

Space Service Volume

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

SSV (3,000 to 8,000 km), and (2) the highly elliptical orbit

(HEO)/geosynchronous Earth orbit (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 zenith pointing antenna; in the MEO region, 

a combination of zenith- and nadir- pointing antennas are needed, 

but even a single GNSS constellation will often allow for satellite 

coverage from at least four satellite vehicles; in the HEO/GEO 

region, nearly all GNSS signal will come from satellites 

transmitting across the Earth’s limb; it is in this realm where the 

use of multiple GNSS constellations in combination will greatly 

increase availability and service performance. 

Availability of GNSS Services

Currently GPS provides promises of future signal strength 

and quality within a 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, 
NASA/TM—2015-218469 3

Galileo, and Beidou) in combination with GPS; and for all four 

systems in combination. The availability for GPS alone is 

included in the tables for reference.

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.

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-s steps) and line-of-sight vectors 

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

time histories of GNSS satellite visibilities. From this 

availability, statistics were calculated, giving the following 

metrics listed in Table 1 to Table 8:

• Availability of at least 1 and of at least 4 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 only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 100 100 100 100 100

4+ (%) 100 100 100 100 100

<1 (s) 0 0 0 0 0

<4 (s) 0 0 0 0 0

Min. (#) 10 20 22 22 44

Ave. (#) 14.3 26.9 27.3 27.1 52.7

Max. (#) 19 36 36 34 65

TABLE 3.—A 3,000 km ALTITUDE 

GPS only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 100 100 100 100 100

4+ (%) 100 100 100 100 100

<1 (s) 0 0 0 0 0

<4 (s) 0 0 0 0 0

Min. (#) 16 29 33 33 64

Ave. (#) 20.9 39.4 39.8 39.6 76.9

Max. (#) 24 46 44 44 85

TABLE 4.—A 8,000 km ALTITUDE

GPS only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 100 100 100 100 100

4+ (%) 99.9+ 100 100 100 100

<1 (s) 0 0 0 0 0

<4 (s) 595 0 0 0 0

Min. (#) 3 5 9 10 21

Ave. (#) 9.6 17.1 19.0 19.5 36.4

Max. (#) 15 26 27 27 49

TABLE 5.—A 15, 000 km ALTITUDE

GPS only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 99.9+ 99.9+ 100 100 100

4+ (%) 80.0 99.0 99.4 99.4 99.9+

<1 (s) 604 159 0 0 0

<4 (s) 8289 2895 8289 6619 1259

Min. (#) 0 0 1 1 2

Ave. (#) 4.7 8.5 9.0 9.3 17.5

Max. (#) 10 20 17 18 34

TABLE 6.—A 25,000 km ALTITUDE

GPS only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 99.4 99.9+ 100 99.9+ 100

4+ (%) 36.0 90.0 95.5 96.7 99.9+

<1 (s) 3158 2132 0 138 0

<4 (s) 30166 7452 8701 10119 672

Min. (#) 0 0 1 0 2

Ave. (#) 3.1 5.7 5.8 6.1 11.4

Max. (#) 8 15 14 14 25
NASA/TM—2015-218469 4

TABLE 7.—A 36,500 km ALTITUDE 

GPS only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 97.0 99.8 99.8 99.9+ 99.9+

4+ (%) 15.6 70.6 74.9 80.2 99.4

<1 (s) 9673 2697 4698 4232 528

<4 (s) 72272 13753 51634 44385 5397

Min. (#) 0 0 0 0 0

Ave. (#) 2.4 4.4 4.5 4.7 8.8

Max. (#) 6 13 10 10 19

TABLE 8.—A 70,000 km ALTITUDE

GPS only GPS +

GLONASS

GPS + 

Galileo

GPS + 

Beidou

All

1+ (%) 89.6 97.8 97.8 98.3 99.9

4+ (%) 2.8 37.7 38.1 44.2 88.8

<1 (s) Never 5347 Never 86556 3732

<4 (s) Never 34696 Never Never 22018

Min. (#) 0 0 0 0 0

Ave. (#) 1.7 3.1 3.1 3.3 6.2

Max. (#) 6 11 10 10 18

Conclusions and Summary

The results clearly show the increased availability and 

reduced outage durations from using multiple GNSS 

constellations for spacecraft PNT at the higher altitudes. At 

lower altitudes, the increase in number of GNSS satellites in 

view will allow even more accurate PNT, even though the use 

of multiple GNSS constellations is not necessary to prevent 

outages since more than four satellites are always within view.

The practical implications of this analysis clearly shows that 

there is definitive benefit for future space missions to take a 

multi-GNSS approach to meet PNT requirements out to the 

GEO boundary (36,000 km) in the space domain. At NASA, 

this has prompted investments in spacecraft multi-GNSS signal 

processing, as well as antennae configurations to optimize 

“over the limb of the Earth” signal processing. 

A key point of this work also shows that onboard signal 

processing complexity can be significantly reduced when four 

or more satellite signals are always in view from various GNSS 

constellations. More signal availability naturally translates into 

greater real-time point positioning, thus negating the need for 

specialized predictive orbit software such as the Goddard 

Enhanced Onboard Navigation System (GEONS) required 

when fewer signals are in view (Ref. 5).

Since the technical success of using GPS, and other multiGNSS solutions for space navigation in the Space Service 

Volume will become standard to enhance spacecraft autonomy 

and reduce network tracking burdens, it is paramount that the 

world’s space agencies work with their respective global PNT 

service providers to ensure that their own sovereign systems 

provide for some guaranteed level of service performance out 

to GEO. Otherwise satellite vehicle power levels, pseudoranges, and beam widths may continue to diverge away from 

meeting space user requirements unless a stated need is 

documented and adhered to as constellations continue to

modernize.

To this end, NASA has been very active in the national and 

international community coordinating with the United Nations’ 

International Committee on GNSS (ICG), as well as the space 

agency standards body known as the Interagency Operations 

Advisory Group (IOAG), to secure recognition of the need for 

a formal, fully interoperable, Space Service Volume (SSV)

based on the use of all emerging constellation signals. NASA 

does this under authority granted by the 2004 President’s 

National Space-based PNT Policy, where, “The Administrator 

of the National Aeronautics and Space Administration, in 

cooperation with the Secretary of Commerce, shall develop and 

provide to the Secretary of Transportation requirements for the 

use of the Global Positioning System and its augmentations to 

support civil space systems” (Refs. 6, 7, and 8).

In conclusion, it is obvious that the use of multi-GNSS 

solutions for future spacecraft positioning, navigation, and 

timing is a technical option that derives numerous benefits as 

indicated in the analyses conducted thus far. To make this 

scenario a true reality on a worldwide scale however, will 

require far more than a robust technical assessment and 

rationale; it will rather require the concentrated efforts of the 

very space agencies that stand to use their spacecraft to provide 

societal benefit in the most cost effective manner possible. 

The GNSS constellation satellite vehiclesthemselves must be 

adapted for the emerging space user, which in turn will require 

further diplomatic efforts to ensure transparency amongst the 

respective bureaucratic organization of Russia’s GLONASS, 

Europe’s Galileo, and China’s Beidou constellations. In some 

cases a Space Service Volume capability may already exist; 

however this functionality must be formally recognized if it is 

to succeed over the long runs of modernization change. At the 

last UN ICG in Beijing, China, November 2012, the U.S., the 

Europeans, and even the Japanese with the augmentation 

satellite Quasi-Zenith Satellite System (QZSS) recognized the 

need to adopt the SSV as a future contributor of the multi-GNSS 

environment. NASA looks forward to continuing this dialogue, 

supported by analysis such as documented in this paper to 

continue the argument for SSV worldwide adoption.

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 
NASA/TM—2015-218469 5

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).

5. http://techtransfer.gsfc.nasa.gov/ft_tech_geons.shtm

6. http://www.oosa.unvienna.org/oosa/SAP/gnss/icg.html

7. https://www.ioag.org/default.aspx

8. http://www.gps.gov/policy/docs/2004/

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. 

James J. Miller is Deputy Director of the Policy & Strategic 

Communications Division with the Space Communications and 

Navigation (SCaN) Program at NASA Headquarters. He is a 

commercial pilot with degrees in Aviation Flight, Aviation 

Management, Master of Public Administration degree from 

Southern Illinois University and Master of International Policy 

and Practice from The George Washington University.