GPS Space Service Volume Increasing the Utility of GPS for ...
1. GPS Space Service Volume Increasing the Utility of GPS for Space Users Michael C. Moreau, Ph.D. Flight Dynamics Analysis Branch NASA Goddard Space Flight Center October 16, 2008
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4. Reception Geometry for GPS Signals in Space Geosync Altitude: 35,887 km GPS Altitude: 20,183 km Main Lobe (~47 ° for GPS L1 signal) LEO Altitudes < 3,000 km 3,000 km HEO Spacecraft First Side Lobe First Side Lobes
5. Terrestrial and Space Service Volumes Space Service Volume (High/Geosynchronous Altitudes) 8,000 to 36,000 km Space Service Volume (Medium Altitudes) 3,000 to 8,000 km Terrestrial Service Volume Surface to 3,000 km GEO Altitude - 36,000 km GPS Altitude - 20,183 km
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9. High Earth Orbit GPS Timeline 2000 1990 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. AMSAT-OSCAR 40 (AO-40) Experiment High Gain Antenna (1 of 4) TANS Vector Receiver AO-40 Spacecraft
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12. Simulated GPS L1 C/A Availability for GEO user –182 dBW threshold, IIR antenna 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
Here is a 2-dimensional representation of the main beam of GPS in relation to the various volumes, also shown “to scale.” GPS is unique in its ability to support space users as far as GEO over the limb of the earth. This shows the first side lobe of the GPS Block II/IIA signal. Side lobe signals are generally weaker than the main lobe signals, but for a sufficiently sensitive receiver, may contribute significantly to availability and help to fill in gaps when no main-lobe signals are available. It should be noted that the beamwith of the “main lobe”, as well as the amplitude and variability of the side lobe signals varies significantly between the various blocks of GPS satellites.
This slide shows a “to scale” visualization of the volumes defined in the previous two slides. The TSV from 0-3000 km has performance and availability consistent with that of terrestrial users. The vast majority of existing space applications of GPS are in the that volume. In the the Medium Earth Orbit Space Service Volume, from 3000-8000km a user will still typically have a minimum of 4 satellites available nearly continuously, although the receiver will require omni-directional receiving antenna coverage, and some GPS signals may have reduced power levels In the HEO/GEO Space Service Volume, from 8000 to 36000 km, most GPS satellites will be available at reduced signals levels and must be tracked through a nadir (Earth pointing) GPS antenna. A receiver will rarely have four satellites present simultaneously. Finally, the relationship to the GPS orbital altitude is shown . The Space Service Volume originally defined in the 2000 ORD is the combination of the two blue areas. [NOTE: The material I added is somewhat repetitious with charts 9-11. But this allows you to spend a little more time on this chart and let people absorb the differences between the various service volumes.]
Low Earth Orbit satellites fly completely within the bounds of the Terrestrial Service Volume. Some satellites in highly eccentric orbits spend part of each orbit within the TSV. With a suitably designed GPS receiver, users in LEO receive even more signals than those on the surface of the earth, while receiving generally the same signal performance. As you will hear about in the next paper in this session, the state of the art in GPS navigation performance for space users in the TSV is at the level of one centimeter (post-processed), although typical real-time performance can vary from hundreds of meters to under one meter depending on the receiver/implementation.
Users in Medium Earth Orbit can receive a high availability of 4 or more satellites simultaneously in view, allowing real-time navigation solutions. As you’ll see in a few slides, however, these users must be capable of receiving GPS signals originating from GPS satellites visible over the limb of the earth, in combination with those from satellites overhead. One meter orbit accuracies should be feasible in this region.
Users in High Earth Orbits, including Geostationary satellites, are severely restricted in both the visibility of GPS signals and the signal strength received over the limb of the earth. Users in the HEO/GEO Space Service Volume cannot reliably solve for an instantaneous position solution; rather, the receiver must utilize a kalman filter that includes orbit and clock models and is capable of processing the sparsely available GPS measurements over an extended time period to support the necessary accuracy. The achievable navigation accuracy will vary based on the capabilities/design of the GPS receiver and onboard software. Probably the most important factor is the sensitivity of the GPS receiver with regards to the capability to track very weak main lobe and side lobe signals. Another important factor is the stability of the reference oscillator, which directly affects the accuracy of the solution when the receiver must propage through signal outages. Nevertheless, a well designed receiver should be able to achieve steady-state position accuracies approaching 10 meters.
This is a picture of the business end of the spacecraft, illustrates the layout of the GPS antennas Trimble TANS Vector receiver was flown. 6 channel, L1, C/A code used for orbit and attitude determination in LEO. Receiver connected to 4 high gain antennas and custom designed low noise amplifiers. 10 dB peak gain. 12 cm. Spacecraft is 2 meters top to bottom. Spin stabilized about z-axis (refer to model?) Various communications antennas, 400 n motor A second GPS receiver, with antennas mounted on the opposite side of the vehicle, won’t discuss further
Originally GPS minimum performance was spec’d at 13.8 degrees off the transmitter boresite (the limb of the Earth), so although there was significant GPS signal spillover available to space users beyond the limb of the Earth, there was not specification on the availability or power levels of these signals. The first explicit statement of expected performance characteristics for space users was captured in the 2000 GPS Operational Requirements Document. A “Terrestrial” and “Space Service Volume” were explicitly defined, and signal strength and availability were specified for the SSV based on a user in a geostationary equatorial orbit. Unfortunately, the ORD was published after both the Block IIR and IIF contracts were in place, so it had no effect on defining requirements for those programs. I’ll show a graphical representation in a couple slides that provides a visualization of these volumes. Block IIA L1 transmitter half angle of 23.5 deg (called out in paper)
For the GPS III CDD, the intent was to develop requirements that would adequately document the levels of GPS services provided to space users by the existing GPS constellation, while identifying performance objectives that would highlight potential areas for improved performance in the future. T hree parameters were used to characterize the requirements – pseudorange accuracy, received signal strength, and availability of the signal.
Originally GPS minimum performance was spec’d at 13.8 degrees off the transmitter boresite (the limb of the Earth), so although there was significant GPS signal spillover available to space users beyond the limb of the Earth, there was not specification on the availability or power levels of these signals. The first explicit statement of expected performance characteristics for space users was captured in the 2000 GPS Operational Requirements Document. A “Terrestrial” and “Space Service Volume” were explicitly defined, and signal strength and availability were specified for the SSV based on a user in a geostationary equatorial orbit. Unfortunately, the ORD was published after both the Block IIR and IIF contracts were in place, so it had no effect on defining requirements for those programs. I’ll show a graphical representation in a couple slides that provides a visualization of these volumes. Block IIA L1 transmitter half angle of 23.5 deg (called out in paper)
In the intervening 6 years, we’ve learned a lot more about the needs of space users. In the GPS III Capability Development Document, which documents GPS III requirements, the Space Service Volume was divided into two separate regions: one extending from the top of the Terrestrial Service Volume to 8,000 km altitude, and the other extending from 8,000 km to geostationary earth orbit. This approach makes it possible to capture the current performance and ensure backwards compatability within three separate regions of space. Sort of “bottom line up front”
[Note: Is 0.8 meter URE correct? It is listed as 0.25 meter in the Sept 2005 SS-SYS-800] Current broadcast URE is approaching 1 meter. GPS III will provide improved User Range Error through the use of improved clocks and intra-constellation crosslinks, which will help keep the age of data to near zero. The ranging error of the broadcast GPS signals is an important parameter for high altitude space users because uncertainty in the GPS transmitter phase characteristics can be larger for signals transmitted beyond the limb of the Earth.
Based on our analysis, the highlighted column shows the minimum received power for any receiver in the Space Service Volume, for each of the planned signals for GPS III. These minimum power levels correspond to a user a GEO altitude (worst case point in the SSV). The terrestrial minimum received power is spec’d at the edge of the earth. Note that for L1 the SSV power is spec’d at 23.5 deg, and for L2 and L5 the half beamwidth angle is 26 deg, resulting in improved availability. Discrepancy between L1M and L2M power levels and ICD-GPS-700A; if asked, note that a 1 Aug 06 TOR from Aerospace updated the values to those shown here.
This shows the minimum, or Threshold, signal availability requirements for the MEO and HEO/GEO Space Service Volumes. In addition to an availability requirement, the total and continuous outage duration times are also specified. It is important to note that these availability numbers only consider signals that are within the half-beamwidth angles specified on the previous charts; meaning side lobe signals that exceed the specified minimum power levels are NOT included in these availability numbers. [If someone asks, the objective requirements are closer to the performance that can be met if side lobes signals are included, or if minor modifications are made to assumed half-beamwidths…]
I talked about the rationale for developing requirements for space users and the need to break the Space Service Volume into two separate regions. I defined the three service volumes and specific requirements for availability and received signal strength within the space service volume. The paper goes into much greater detail, and I encourage you to read it.
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