1

GPS Space Service VolumeIncreasing the Utility of GPS for Space Users

Michael C. Moreau, Ph.D.Flight Dynamics Analysis Branch

NASA Goddard Space Flight Center

October 16, 2008

2

Outline

• Background on use of GPS in High Earth Orbits• Space Service Volume Definition and

Characteristics• NASA flight experiment (AO-40 satellite)

– Results and observations

• Evolving Space User Requirements • Updated GPS III Space User Requirements

– Pseudorange accuracy– Received power– Signal availability

• Closing Remarks

3

Background

• GPS availability and signal strength requirements for PVT services originally specified for users on or near surface of Earth– Primarily Land, Air, & Maritime users

– Transmitted power levels specified at edge-of-Earth, 14.3 degrees

• NASA and DoD space programs increasingly rely on GPS for spacecraft navigation– Most space users in Low-Earth Orbits (below 3000 km)

– Strong interest in the use of GPS in high altitude orbits.

• NASA “high altitude” GPS activities have included:– Conducting flight experiments to characterize GPS performance

– Development of new GPS receivers for spacecraft in Geostationary or highly elliptical orbits

– Working with the GPS Wing to formally document GPS requirements for space users

4

GeosyncAltitude:

35,887 km

GPS Altitude:20,183 km

Main Lobe(~47° for GPS L1 signal)

First SideLobe

First SideLobes

LEO Altitudes< 3,000 km

3,000 km

HEOSpacecraft

Reception Geometry for GPS Signals in Space

5

Terrestrial and Space Service Volumes

Space Service Volume(High/Geosynchronous Altitudes)

8,000 to 36,000 km

SpaceService Volume

(Medium Altitudes)3,000 to 8,000 km

TerrestrialService Volume

Surface to 3,000 km

6

Terrestrial Service VolumeLEO (≤ 3,000 km) Characteristics

• PVT performance consistent with that enjoyed by terrestrial users

• Uniform received power levels• Fully overlapping coverage of GPS main beams• Nearly 100% GPS coverage• Instantaneous navigation solutions

7

Space Service VolumeMedium Altitudes (3,000 – 8,000 km) Characteristics

• Four GPS signals available simultaneously a majority of the time

• Conventional space GPS receivers will have difficulty:– GPS signals over the limb of the earth become increasingly

important– Wide range of received GPS signal strength

• One-meter orbit accuracies feasible

8

Space Service VolumeHEO/GEO (8,000 – 36,000 km) Characteristics

• Nearly all GPS signals received over the limb of the Earth

• Users will experience periods when no GPS satellites are available

• Received power levels will be weaker than those in TSV or MEO SSV

• Properly designed receiver should be capable of accuracies ranging from 10s of meters to 100s of meters, depending on receiver sensitivity and local oscillator stability.

9

High Earth Orbit GPS Timeline

20001990 2010

EQUATOR-S, TEAMSAT, Falcon Gold flight experiments

Kronman paper published on DoD mission using GPS in GEO orbit

STRV1-D mission lost to launch vehicle failure

NASA/AMSAT AO-40 flight experiment

STENTOR (GEO) mission lost to launch vehicle failure

Feb, 2000 version of GPS Operational Requirements Document (ORD) includes first requirements for Space Service Volume

Capability Description Document for GPS III includes updated Space Service Volume definition and requirements

Many civil and military missions with plans for operational use of GPS in high altitude orbits…

GIOVE A

10

GPS

GEO

spin axis & high gain antennas are co-alligned

AMSAT-OSCAR 40 (AO-40) Experiment

High Gain Antenna (1 of 4)

TANS Vector Receiver

AO-40 Spacecraft

11

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70-15

-10

-5

0

5

10

15

20

gain

[dB

]

GPS off-nadir angle [deg]

AO-40 Measurements -vs- Mean Block II/IIA Pattern

AO-40 Measured -vs- Predicted Antenna Gain

• Comparison of AO-40 measurements and predicted GPS satellite antenna gain (mean Block II/IIA)

• AO-40 data detected significant differences in gain patterns on new Block IIR satellites - side lobes were significantly higher than expected

data from II/IIA satellites follows predictions

IIR data has steeper drop-off of main lobe signals, but higher side lobes

12

Main lobes only (within 23.5 degrees) All signals above –182 dBW 4 or more SVs visible: 2% 2 or more SVs visible: 31% no SVs visible : 39%

4 or more SVs visible: 100% 2 or more SVs visible: 100% no SVs visible : 0%

AO-40 data affirmed GPS side lobe signals significantly improve signal availability

Simulated GPS L1 C/A Availability for GEO user–182 dBW threshold, IIR antenna

13

“High Altitude GPS” Observations

• On-orbit performance of GPS varied from block build to block build (IIA, IIRM, expected IIF) due to antenna gain variances

• Side-lobe signals, although not specified, can significantly boost GPS signal availability for users above the constellation

• During GPS III Phase A, NASA noted significant discrepancies in power levels specified in GPS III specification documents, and measured on-orbit performance

• To stabilize signal for high altitude space users, created new Space Service Volume (SSV) definition and specifications – Guarantee backward compatibility

– Identify areas for improved performance through objective requirements

14

SSV Requirements for GPS III

• Users in the SSV cannot typically rely on conventional, instantaneous GPS solutions

• Performance requirements established via three parameters– Pseudorange accuracy– Received power– Signal availability

15

Evolving Space User Requirements

• Established two operational volumes– Terrestrial Service Volume (TSV)

• Earth surface to 3,000 km altitude– Space Service Volume (SSV)

• 3,000 km to 36,000 km (~GEO) altitude• Signal availability and power defined only for geostationary

equatorial users• Minimum performance specified corresponding to a 23.5º GPS

transmitter half angle

• Shortcomings of ORD space user requirements:– Did not cover mid-altitude users (above LEO but below

GPS)– Did not cover users outside of the equatorial plane– Only specified reqts on L1 signals (L2 and L5 have wider

beam-width and therefore, better coverage)

GPS IIF Operational Requirements Document (ORD)(ca. 2000)

16

GPS III Capability Development Document (CDD)

• Threshold requirements specifically document current system performance– Divided Space Service Volume into two regions

• Medium Earth Orbit (MEO) SSV– 3,000 km to 8,000 km altitude

• High Earth Orbit / Geostationary Earth Orbit (HEO/GEO) SSV– 8,000 km to 36,000 km altitude

– Minimum performance specified at 23.5° (L1) and 26° (L2/L5) GPS transmitter antenna half-angles

• Objective requirements also defined– Objective signal availability consistent with current

performance if side-lobe signals are considered.

Evolving Space User Requirementscontinued

17

SSV Pseudorange Accuracy

• Also known as User Range Error (URE)• Error bound on GPS range measurement• Function of

– Accuracy of GPS orbit and clock solutions from Control Segment– Age of Data– Uncertainty in GPS physical and modeling parameters

• Antenna group delay and phase errors vary as a function of off-nadir angle

• Current performance ≈ 1 meter• GPS III requirement: ≤ 0.8 meter (rms)• GPS III objective: ≤ 0.2 meter (rms)

18

Received Power Levels for Block IIA SV

19

GPS III Minimum Received Signal Power (dBW) Requirement

• SSV minimum power levels were specified based on the worst-case (minimum) gain across the Block IIA, IIR, IIR-M, and IIF satellites

• Some signals have several dB margin with respect to these requirements at reference half-beamwidth point

Signal

Terrestrial Minimum

Power (dBW)SSV Minimum Power (dBW)

Reference Half-beamwidth

L1 P(Y) -161.5 -187.0 23.5L1 C/A -158.5 -184.0 23.5L1 M -158.0 -183.5 23.5L1C -157.0 -182.5 23.5L1 composite -151.2L2 P(Y) -161.5 -186.0 26L2 C/A or L2C -158.5 -183.0 26L2 M -158.0 -182.5 26L2 composite -151.5L5 I5 -157.0 -182.0 26L5 Q5 -157.0 -182.0 26L5 composite -154.0

20

GPS III Minimum Availability Requirement

• Assuming a nominal, optimized GPS constellation and no GPS spacecraft failures, signal availability at 95% of the areas at a specific altitude within the specified SSV are planned as:

• Objective Requirements:– MEO SSV: 4 GPS satellites always in view– HEO/GEO SSV: at least 1 GPS satellite always in view

MEO SSV HEO/GEO SSV at least 1

signal 4 or more

signals at least 1

signal 4 or more

signals L1 100% ≥ 97% ≥ 80% 1 ≥ 1% L2, L5 100% 100% ≥ 92% 2 ≥ 6.5%

1. With less than 108 minutes of continuous outage time. 2. With less than 84 minutes of continuous outage time.

21

Example NASA Application: GPS Tracking for Lunar Missions

GPS altitudeEI – 1.2 hrs

Periods 2 or more GPS available

25 dB-Hz sensitivityEI – 12 hrs

Periods or 2 or more GPS available

35 dB-Hz sensitivityEI – 2 hrs

Final Correction Burn, EI-5 hrs

Ground Updates

Correction BurnEI-16 hrs

• Weak GPS signal tracking technology enables tracking of GPS signals well beyond the GPS constellation sphere

• GPS can potentially improve navigation accuracy in the 12-24 hours preceding Earth entry interface

22

Closing Remarks

• NASA and other space GPS users rely on GPS as critical component of space navigation infrastructure over expanding range of orbital applications

– NASA’s Space Communications and Navigation Architecture relies heavily on GPS

• Space user community was vulnerable to design changes because requirements were not explicitly stated

• Space user requirements identified by volumes based on altitude– Terrestrial Service Volume (TSV): surface to 3,000 km– Space Service Volume (SSV)

• Medium Earth Orbit (MEO): 3,000 to 8,000 km• High Earth Orbit / Geostationary Earth Orbit (HEO/GEO): 8,000 to 36,000 km

• New requirements baselined as part of GPS III:– Maintains backward compatibility with current constellation– Identifies potential areas for improved performance through objective

requirements– Provides a green-light for civil and military space missions considering

operational use of GPS beyond LEO• Interoperability for all space users will be enhanced if other PNT

service providers such as Galileo also implement similar requirements/operational capabilities.

23

References

F.H. Bauer, M.C. Moreau, M.E. Dahle-Melsaether, W.P. Petrofski, B.J. Stanton, S. Thomason, G.A Harris, R.P. Sena, L. Parker Temple III, The GPS Space Service Volume, ION GNSS, September 2006.

M.Moreau, E.Davis, J.R.Carpenter, G.Davis, L.Jackson, P.Axelrad, “Results from the GPS Flight Experiment on the High Earth Orbit AMSAT AO-40 Spacecraft," Proceedings of the ION GPS 2002 Conference, Portland, OR, 2002. 

Kronman, J.D., "Experience Using GPS For Orbit Determination of a Geosynchronous Satellite,"  Proceedings of the Institute of Navigation GPS 2000 Conference, Salt Lake City, UT, September 2000.

These and other NASA References can be found here:

http://www.emergentspace.com/related_works.html