Chapter 5 Constellation Spacecraft System Performance
5.4 GOX Payload Science Performance Results
5.4.1 GOX Payload On-Orbit Performance
Table 5-4 shows the GOX firmware build (FB) change history since launch. Figure 5-3 shows the RF Signal-to-Noise Ratio (SNR) performances of four GOX antennas (POD1, POD2, OCC1, and OCC2) on each GOX payload instrument in all six spacecraft after one year in orbit. In these figures, only data received after July 13 (Day 194), 2006, where FB4.2.1 was uploaded, are shown. The definition of the daily SNR value shown in Figs. 6 (A) to 6 (B) is the bottom limit of the top 90% SNR value of all the tracked GPS satellites＇
signal SNR values received by that particular antenna either in Coarse/Acquisition or Precision (P2) signal code. Following the uploading of FB version 4.3 (FB4.3) of the GOX payload to all the six spacecraft, from December 2006 onward, the trends of the GOX payload’s SNR data did not show any sign of degradation at all from the available GPS RO science data. The SNR value of OCC1 on spacecraft FM3 shown in Figure 5-3(C) did show a decreasing tendency; the value drops very rapidly when the spacecraft is at a high beta angle.
We observe that the SNR value returns to its normal value when GOX temperature is below 40oC and spacecraft FM3 leaves the high beta angle. The decreasing of GOX SNR on FM6 as shown in Figure 5-3(F) is related to the reboot loop issue and will be addressed later [6].
The FB version 4.4 (FB 4.4) was provided to fix GOX reboot loop issue (see Section 4.4.2) even only the fore navigation antenna (POD) is working and to improved L2 tracking and
produced the tracking data of the new L2C GPS signal.
5.4.2 GPS RO Profile Statistics
In Figure 5-4 we show the number of daily atmospheric profiles (atmprf) and ionospheric profiles (ionprf) retrieved for two years since launch. The term “atmphs” in the figure indicates the number of excess phase files that are generated and also represent the atmospheric RO profiles that can be observed by F3 satellites in the neutral atmosphere (stratosphere and troposphere). The “ionphs” in the figure indicates ionosphere. The new open loop FB version 4.2.1 (FB4.2.1) was uploaded to the GOX payload in July 2006, which caused a large jump in the daily RO profile numbers for August 2006. From Figure 5-4 it is clear that ~37% of the total events cannot be retrieved to neutral vertical atmosphere profiles.
This is true for ~25% of ionospheric profiles. It also shows that the F3 mission has processed 1800 to 2200 high-quality neutral and ionospheric atmospheric sounding profiles per day, which is more than the total number of worldwide radiosondes launched (~900 mostly over land) per day [31]-[32], [61]. .
5.4.3 Lowest Altitude Penetration of GPS RO Retrievals
We studied the global distribution statistics of the lowest height of the retrieved profiles for F3 and CHAMP satellites for the period from January 1 to May 10, 2007 [31], [33]. Figure 5-6 shows the comparison of the lowest altitude penetration of RO profiles versus latitude for F3 and CHAMP mission. The solid lines above and below the median value are respectively the 75% and 25% statistical average value of the distributed data for F3. The bold dashed line is the median value of the lowest altitude penetration for CHAMP. The dashed lines above and below the median value are the 75% and 25% statistical average value of the distributed data for F3. The gray area plot is the water vapor specific humidity distribution with respect to altitude and latitude. The specific humidity data are obtained from a NCEP (National Centers for Environmental Prediction) analysis averaged from March 1968 to 1996
[31], [33].
We observe that the lowest height of the tangent point of the RO signals is limited by high terrain. The retrieved profiles were separated into two groups: one over the ocean and the other over the land. The lowest heights reached by the profiles of the land group for F3 and CHAMP were analyzed. It was noted that they are mostly below 0.5 km over the surface in the southern polar region. In most other land regions, the lowest heights reached are all below 1 km. Those with lowest heights reached above 1 km are mostly located in the mountainous areas such as Himalaya mountains, the Tibetan plateau, and the Andes Mountain because high mountains prevent RO signals with lower tangent point heights from being tracked [31], [33].
5.5 Conclusion
We have summarized the satellite constellation system performance after two years in orbit. With the development and application of the open loop tracking technique by JPL and UCAR, the quality, accuracy and lowest penetration altitude of the RO sounding profiles have been improved in comparison to previous RO missions. After two years in orbit about 1800 to 2200 high-quality soundings were being retrieved daily on a global basis. It is anticipated that an increasing number of global operational centers will use F3 data operationally for the years to come.
TABLE 5-1 CONSTELLATION SPACECRAFT PERFORMANCE SUMMARY (AFTER TWO YEARS IN
ORBIT)
SC ID Summary
FM1 Bus GPSR GPS Non-Fixed -> Operation Solution
GOX Reboot Loop -> Auto Recovery
FM2
Stay in Phoenix -> Operation Solution
GOX Reboot Loop -> Auto Recovery
Solar Array Power Shortage -> Reduced GOX Operation
BCR dMdC Charge Algorithm Issue-> FSW Update
Battery Pressure Difference Anomaly -> FSW Update
PCM DC Converter Abnormally Off -> TBB & TIP Off
FM3
Lost of Communication -> Auto Recovery
Solar Array Driver Lockout -> Reduced GOX operation
Bus GPSR GPS Non-Fixed -> Operation Solution
OCC2 (ANT03) SNR Decreasing -> Recovery after High Beta Angle FM4 Bus GPSR GPS Non-Fixed -> Operation Solution
FM5 GOX Reboot Loop -> Auto Recovery
GOX RF1 Lower SNR -> Auto Recovery
FM6
Lost of Communication -> Auto Recovery
GOX Reboot Loop -> GOX FB 4.4 Update
Bus GPSR GPS Non-Fixed -> Operation Solution
TABLE 5-2 SPACECRAFT OPERATION STATUS OF EACH SUBSYSTEM IN ALL SIX SPACECRAFT
(AFTER 2 YEARS IN ORBIT)
Spacecraft Operational
Mode SC State ACS Mode EPS Mode C&DH
Mode GOX TIP TBB
FM1 Normal Normal Fixed-Yaw Normal High Rate Operating Operating Plan IX
FM2 Normal Normal (Power
Shortage) Fixed-Yaw Variable
Power High Rate Reduced
Operating Off Off
FM3 Normal
SAD Abnormal (Power Shortage)
Fixed-Yaw Variable
Power High Rate Reduced
Operating Off Off
FM4 Normal Normal Fixed-Yaw Normal High Rate Operating Operating Plan IX
FM5 Normal Normal Fixed-Yaw Normal High Rate Operating Operating Plan IX
FM6 Normal Normal (Resume Contact)
Fixed-Yaw Normal High Rate Operating Operating Plan IX
TABLE 5-3 SPACECRAFT SUBSYSTEM PERFORMANCE (AFTER 2 YEARS IN ORBIT)
Unit Major Function Two-Year Performance
Payload (PL) GPS RO primary mission
Trends on low SNR data on FM3, FM5 and FM6 after FB4.3 uploaded did not show any sign of degradation at all from the available data.
FM1, FM3, FM5 and FM6 had reboot loop issues.
All RF trending data meet specified criteria.
Command and Data Handling Subsystem (C&DH)
Command handling and telemetry gathering, health and maintenance, GPSR management
The GPS Non-fixed on FM1, FM3, FM4 & FM6 Bus GPSRs impacted onboard time maintenance, ACS performance and TIP payload time stamping.
Operation Solution by upload State vector using GOX PVT data was performed to eliminate all impacts.
The suspected space weather correlated onboard computer reboot and spacecraft reset events have no performance impact on C&DH and Spacecraft
Flight Software
FSW status on all satellites is normal; SC is automatically recovered from abnormal conditions.
Under normal FSW condition, the error count increased rate is smaller than 10/day.
Attitude Control Subsystem (ACS)
Control of nadir pointing and sun pointing, GPS data processing
Correct ACS mode transition was observed.
All six spacecraft performed their ACS functions on orbit as expected.
Reaction Control Subsystem (RCS)
Orbital transfer and raising
FM2, FM5, FM6 and FM4 have arrived at the mission orbits, and the remaining propellant masses for these three satellites are around 2.0 kg (~30% of full capacity)
RCS functions are all healthy and ready for any planned orbit maneuvers in the future.
Thermal Control Subsystem (TCS)
Maintain avionics and battery at operating temperatures
Thermal behavior of all six satellites is normal and in good shape.
Electrical Power Subsystem (EPS)
No sensible degradation on all six satellites except FM2 and FM3.
Solar power reduced on FM2 & FM3and Reduced GOX operation plan was modified.
Pressure difference on FM1~FM4 reduced to safe range (<650 psi) and stable now.
Power margin is estimated at 40% on solar power except FM2.
Battery High Pressure Sensors on FM2 is fixed by FSW 6.2
TABLE 5-4 GOXFIRMWARE BUILD (FB)CHANGE HISTORY SINCE LAUNCH
Version Upload date Objective
FB4.1 5/18/2006 An improved atmospheric model for open loop tracking.
FB4.2 5/30/2006
1. Double precision P2 Phase.
2. To facilitate ionospheric occultation.
3. Bookkeeping.
FB4.2.1 6/29/200
1. To avoid logging unnecessary data and to get more occultation events.
2. To make sure that occulting satellites do not get used in the Navigation solution.
FB4.3 12/27/2006
1. Fix bugs such as: azimuth window, rising occultation to end earlier than at the commanded height, integer cycle slips during transition from open to closed loop tracking of rising occultation, halt acquisition and tracking of a particular PRN
2. Insertion of S4 scintillation parameter for ionosphere study.
FB4.4 6/2007
1. Fixed GOX reboot loop issue even only the fore navigation antenna (POD) is working
2. Improved L2 tracking and produced the tracking data of the new L2C GPS signal
Figure 5-1. The six satellites attitude on-orbit performance with respect to the sun beta angle for one-year data since launch.
Figure 5-2. Trending plots of the tank pressures and temperatures for FM2, FM4, FM5, and FM6 (from 15 April 2006 to 15 April 2007)
(A)FM1 (B)FM2
(C)FM3 (D)FM4
(E)FM5 (F)FM6
Figure 5-3. F3 Payload POD & OCC CA and P2 SNR for all six spacecraft.
Figure 5-4. Two Years Statistics of the Number of Daily Occultation Events for Atmosphere Profiles since Launch.
Figure 5-5. Two Years Statistics of the Number of Daily Occultation Events for Ionosphere Profiles of Electron Density since Launch.
Figure 5-6. Comparison of the lowest altitude penetration of RO event versus latitude for F3/COSMIC and CHAMP.