August 15, 2002
Progress Report on the SAC-C GOLPE and Magnetic Mapping Package
With contributions from
Tom Yunck, George Hajj- JPL, John LaBrecque- NASA HQ
Michael Purucker-GSFC, Carol Raymond- JPL
A:
GOLPE: GPS Occultation and Passive Reflection Experiment:
There has been considerable progress in the GOLPE experiment over the past year.
1.
Autonomous Navigation Experiment: As outlined in the AM constellation prospectus by LaBrecque, Ungar, and
Colomb,(1998-attached) we have begun
Phase 1 of the formation flying experiment also now know as the autonomous
navigation experiment. Real time navigation software has been uploaded to the BlackJack and testing of
this software is underway with a week worth
of real time tracking data. We are now in the process of analyzing the data. We
expect some software adjustment will be necessary. After the satisfactory
operation of the orbit determination software we expect to upload the
navigation code for the maneuver planning in the September timeframe. NASA has established
a global real time GPS signal broadcast in partnership with John Deere to
provide 10-20 cm real time ground and air navigation. We expect that using
reduced dynamic positioning aboard spacecraft could provide cm level
positioning and nanosecond time accuracy. This would enable formation flying of
satellites such as bistatic SARs.
2. GPS Reflection: The reflection antenna on SAC-C is very marginal in its gain however experiments continue to search for reflection signals. It is believed that with sufficient integration time, a reflection signal will be observed. Tom Meehan expects to have new reflection s/w ready for testing within a few weeks. Both SAC-C and CHAMP have observed grazing angle GPS reflections through the limb sounding antennas. Grazing angle reflections do not induce a polarization change. High angle reflections do induce polarization changes and will be observed through the nadir reflection antenna. A paper on the first space-borne observation of high angle GPS reflections has been reported by Lowe, S. T., J.L. LaBrecque, C. Zuffada, L.J. Romans, L. Young and G. A. Hajj, 2002 " First Spaceborne Observation of an Earth-Reflected GPS Signal ", Radio Science, Vol 37, no. 1, pp. 1-28. We expect significant scientific return from the operational measurement of GPS reflections in the areas of ionospheric and atmospheric tomography, sea surface winds, altimetry, and possibly salinity and soil moisture. The techniques requires antennas gains of at least 15-24 dB. The SAC-C antenna is 6 dB. Discussions concerning the inclusion of a BlackJack and larger reflections and limb sounding antenna on Acquarius should be pursued as a follow-on to SAC-C capabilities.
GPS limb sounding: A paper reporting major new technical and scientific results from the GPS occultation experiments on CHAMP and SAC-C has been submitted to JGR-Atmosphere. Results described include:
-- 60% of all profiles reach the bottom 0.5 km of the atmosphere, a number that should soon reach 90% or more -- with GPS anti-spoofing (encryption) turned on
-- Clear demonstration of single-profile precision of 0.5 K between 5 and 20 km
-- Clear demonstration of averaged profile precision/stability approaching 0.05 K
-- Detection of the top of the planetary boundary layer with an accuracy of ~100 m
-- Comparable detection, resolution, and mapping of the tropopause
-- Demonstration of significant errors in current weather analyses in regions of sharp atmospheric gradients
-- Geopotential height accuracy of <10 m -- perhaps approaching 2 m
No other measurements from space approach these levels of accuracy. Earlier studies predicted averaged temperature precision of ~0.1K (10 times better than anything else available), but now 0.05 K or better appears attainable. This is revolutionary from a climate detection standpoint. We will be able to see real climate signals within just a few years -- within the lifetime of a single ESSP mission. These results are obtained with a system that is:
-- self-calibrating and absolute for all instruments and all times
-- all-weather
-- almost instantaneously global (with a small constellation)
-- low-cost
The GOLPE data products, levels 0-2, are made available through NASA’s GENESIS data system http://www-genesis.jpl.nasa.gov/html/index.html with just a few days delay and are being continuously accessed by several dozen research groups around the world. A fair number of papers are beginning to appear.
SAC-C is the only satellite other than GRACE equipped with follow looking limb sounding antennas. Software to activate this capability is underdevelopment. We expect this capability to be enabled on SAC-C in 2003.
How useful is SAC-C? It is a unique and invaluable pioneering test bed for space-borne GPS remote sensing. No previous flight GPS instrument has allowed such unfettered access and freedom to experiment with new configurations and techniques or delivered such immediate feedback. It is an indispensable and priceless developmental tool. The efficiency and versatility of the SAC-C GOLPE experiment will save huge sums over a comparable development in a more restricted project environment. In that sense it will pay for itself many times over. All of our new uploads are first fully tested on SAC-C before being sent to CHAMP or GRACE, providing the ultimate in mission assurance. Together with CHAMP, SAC-C it has already provided clear proof of incomparable measurement accuracy that could not have been shown any other way. We could not have hoped for a more ideal developmental tool, and soon we can expect important new science to emerge from it as well. (Thomas Yunck is the point of contact for GOLPE- tpy@mail2.jpl.nasa.gov.
B.
Magnetic Mapping Payload-the Scalar Helium Magnetometer:
The loss of the star imager and therefore the attitude information on the MMP has caused considerable difficulty in the analysis of the data set. We are still resolving the accuracy of the Scalar Helium Magnetometer measurements which will be critical to the interpretation of the data sets for solid Earth science. Space physics applications can continue to use both the fluxgate and the SHM measurements with minimum concern for the calibration issues.
Recent comparisons between close encounters of SAC-C and Oersted indicate that the SHM may have achieved an accuracy of better than 2.5 nT with 1 nT being the goal of the SHM calibration. A calibration of the SAC-C scalar and vector magnetometers against the Oersted scalar magnetometer was performed during the close approach of Oersted and SAC-C in December, 2001. The figure showing the results is included below. The correction for the Scalar Helium magnetometer (consisting of both spacecraft fields and any magnetometer anisotropies) varies from +2.5 nT at 60 degrees South latitude to +1 nT at 60 degrees North latitude. This calibration data set was assembled from 17 passes on December 28 and 29 when Oersted and SAC-C were in the same orbit and within a few minutes of one another. These results, and hopefully a similar calibration from the dayside close approaches, will be presented at the field modeling workshop to take place immediately after the OIST-4 meeting. Details on this workshop are available at www.dsri.dk/Oersted/Meetings/ NASA has placed a high priority on the estimate of the SHM calibration and has allocated sufficient funding at both JPL and GSFC to complete this analysis.
The August 20th issue of EOS will carry an article 'Highlights from AGU's virtual session on new magnetic field satellites'. The article is available at www.dsri.dk/multimagsatellites/eos_article.html. The article will appear in the 'About AGU' section of EOS, because AGU is trying to get other AGU members to try this meeting approach. We count eight mentions of SAC-C in the article. Fifteen days of SAC-C data were made available to participants in the virtual session. The
virtual session included a significant
Argentine contribution, from Ghidella (Instituto Antartico Argentino), Kohn and
Gianibelli (Universidad Nacional de la Plata). The title was 'Low-altitude
magnetic anomaly complation in Argentina and its comparison with satellite data. The presentations are
available at www.dsri.dk/multimagsatellites. We have also included a CD
containing the results of a virtual session at the Spring, 02 American
Geophysical Union meeting in which several papers utilized data from the SAC-C,
Oersted, and CHAMP magnetic field satellites. Additional copies of the CD are
available from Michael Purucker. Michael Purucker
(purucker@geomag.gsfc.nasa.gov) and Carol Raymond
(Carol.A.Raymond@jpl.nasa.gov) are the points of contact for the SAC-C NASA MMP
component.
The AM Constellation:
Landsat, EO-1, SAC-C, Terra
John LaBrecque (JPL), Stephen Ungar (GSFC), Raul
Colomb (CONAE)
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SAC-C, the Argentine Earth Observing satellite, will adopt an orbit to coordinate Earth observation with NASA’s Landsat 7, EO-1 and Terra satellites. The constellation will include Argentine, Danish, French, Italian, Japanese, and US environmental measurement systems. The coordination of orbits by these satellites will increase the synergy between observing instruments, provide new Earth observing capabilities, explore the utility of coordinated synoptic observational capability proposed for future satellite systems, and compare new observational technology. The coordinated observation strategy of the new constellation will encourage ground studies between Argentine and U.S. scientists to verify the space-based observations.
The decision to coordinate orbits will significantly enhance the importance of SAC-C observations in weather, climate and natural hazards applications and improve the collaboration between CONAE and NASA. Furthermore the Argentine science community will benefit from a greater access to NASA’s Earth Observing data sets and participation on EOS science teams.
The strategy to coordinate the SAC-C orbit with the three NASA EOS satellites involves a minor 2 km change in orbit altitude. Although the SAC-C exact repeat orbit interval will increase from nine days to sixteen days, the wide swath width of SAC-C (360 km) coupled with SAC-C’s ability to point toward desired targets will allow SAC-C to meet previously defined natural hazards requirements. Furthermore, the data exchange agreements with NASA which come from the constellation agreement will bring additional instrumental capabilities to support CONAE’s objectives for the mitigation of natural hazards and the study of the environment for the Cono Sur.
Advantages
for SAC-C formation flying within the AM Constellation
(1). The AM Constellation will provide imaging for coordinated Earth scenes from the visible to the thermal infrared with a spatial resolution from 15 meters to 1 km. SAC-C participation in the AM Constellation will enhance the return of SAC-C science objectives through the combined force of multiple imaging instruments aboard the constellation satellites including Landsat 7, ALI, Hyperion, MODIS, MMRS, ASTER, MISR. The multiple and contemporaneous imaging of important targets with extraordinarily broad hyper-spectral coverage will provide a very significant advance in high resolution time sequence imaging of natural hazards, agriculture, fisheries, weather, and climate.
(2). Opportunities for collaborative science and mission operations will enhance the development of the CONAE space program and the international profile of the Argentine space science community. The Argentine scientific community will enjoy increased membership opportunities within the EOS scientific community with the availability of EOS data sets and enhanced collaboration in EOS environmental studies and natural hazards applications.
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(3). SAC-C’s 360 km swath width and 175m spatial resolution with optional pointing capability compliments the Terra satellite imaging capability which includes MODIS (wide swath low resolution), MISR (low spectral resolution, low spatial resolution) and ASTER (high spatial resolution, thermal imaging). Formation flying with Terra will provide continuous coverage of all South America including Argentina at a 500 m resolution or better with the optional pointing of SAC-C and ASTER for high resolution imaging of important targets such as fires, floods, pollution plumes.
(4). Collaborative calibration programs will enhance instrument utility. International ground truth campaigns will circumvent seasonal and budgetary constraints of the individual experiments and provide rapid inter-calibration of imaging instruments. The planned December launch of EO-1 and SAC-C will place a premium on southern hemisphere calibration sites such as the Argentine land and coastal zones in order to conduct the calibration efforts during growing seasons.
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(5). The large fuel contingency of SAC-C which could be as large as 5 kg of hydrazine (SAC-C CDR) will provide the opportunity to close orbit separation on EO-1, Landsat 7 or Terra during the lifetime of the constellation in order to improve inter-calibration, experiment with formation flying, or develop new imaging capability. SAC-C MMRS imagery will benefit from the aerosol measurements of MISR as well as ALI/ LEISA atmospheric corrector aboard EO-1.
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Figure
2 a, b: The GOLPE GPS limbsounding
signal path is shown in red with respect to the Landsat 7, EO-1
(upper) and Terra MISR (lower) optical sensor swaths. The SAC-C GOLPE limb
sounding experiment coupled with Landsat 7, EO-1, and Terra optical measurements
will enhance the studies of atmospheric temperature, water vapor, and
aerosols. Other instrument synergies
between Terra, EO-1, Landsat 7, and SAC-C measurements will result in strong
data inter-relationships to improve imaging resolution and accuracy, through better atmospheric parameterization.
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(6). SAC-C carries a number of unique observational capabilities which will extend the observational science opportunities of the AM Constellation.
(6a). The GOLPE GPS Occultation and reflection experiment will provide the opportunity to compare and integrate the atmospheric water vapor and temperature measurements with the MODIS, Landsat 7 and EO-1 optical measurements of these parameters. The suggested orbit for SAC-C will provide simultaneous fore and aft occultation measurements beneath the observation tracks of both Landsat 7, EO-1 and Terra (Figure 2A,B). These simultaneous measurements will permit the data assimilation of GPS occultation with optical measurements. This instrumental interoperability provides the opportunity to explore combined data sets for improved instrument calibration and will enhance atmospheric measurements including aerosol estimation and the study of land, sea, and air interactions.
(6b). Short revisit times by the constellation imaging systems provides for the observation and mapping of transient phenomena. The slant range imaging capability of MISR or ASTER could be combined with the pointing capability of the SAC-C MMRS or HRTC to provide high resolution time lapse images. Closure of orbital separation might be used to develop stereoscopic images of atmospheric and land surface phenomena.
(6c). Intensive high resolution imaging of the Earth’s surface. The spatial and spectral imaging resolution of SAC-C complements the capabilities of the other constellation imaging instrumentation.
(7) The coordinated orbit of SAC-C will provide and opportunity to build upon and extend the EO-1/Landsat 7 Enhanced Formation Flying demonstration (EFF). The primary goal of formation flying is to overfly observational swaths of EO-1, Landsat 7, SAC-C and Terra. EO-1 and SAC-C both are equipped with GPS receivers and therefore will provide optimum opportunity for real time formation flying. GPS controlled formation flying is an important new technology and SAC-C with it’s NASA supplied TurboRogue “Blackjack” receiver is capable of providing a significant opportunity to test and extend formation flying technology.
Because the SAC-C
ground swath width is a moderately broad 360 km, only along track maneuvers
will be routinely required to compensate for atmospheric drag. Approximately 5
kg of hydrazine should be available for maneuvers beyond these orbit
maintenance requirements, this is
equivalent to about 32 m/s delta-V.
The traditional
approach to formation flying would combine
the SAC-C and EO-1 GPS observations within SAC-C ground operations to produce
maneuver sequences. The traditional approach does not take advantage of the
near real-time navigation capability of the SAC-C TurboRogue GPS receiver. Both EO-1 and SAC-C could formation fly with TERRA and/or Landsat-7
through their onboard GPS capability. The formation flying experiment could be
conducted in three distinct phases through the lifetime of SAC-C and EO-1 and
will require close collaboration and participation of the CONAE SAC-C project team.
Phase 1 Onboard Semi-Autonomous: Demonstrate onboard maneuver decision and design
functions. Onboard orbit determination and maneuver decision and design will be
made in the SAC-C "Blackjack" GPS receiver/processor. Onboard orbit
determination solutions are of sufficient accuracy (<100m position, 1-sigma)
to allow for the determination of drag compensation maneuver events. SAC-C
maneuvers designed onboard are downlinked for ground verification and embedded
in the ground developed maneuver sequence for subsequent execution. There is no
on-board control of the satellite.
Phase 2 Onboard Nearly-Autonomous:
Compute entire maneuver
sequence in "Blackjack" GPS receiver/processor. Maneuver sequence
downlinked for ground validation then uplinked for execution. This requires
working with the SAC-C spacecraft team to ensure a properly formatted maneuver
sequence.
Phase 3 Precision onboard
autonomous navigation: This
ultimate phase is accomplished by linking EO-1 GPS pseudorange observations and
maneuver plans through a ground station for retransmission to SAC-C as it comes into view a few
minutes later. This accomplishes the crosslink function without modification to
either EO-1 or SAC-C spacecraft tracking systems. Given sufficient EO-1
telemetry and SAC-C command capacity a combined orbit determination solution
onboard SAC-C could yield sub-meter relative position accuracy in near-real
time. Control of the spacecraft will require modification of the SAC-C system
software and will require strong interaction with the SAC-C project team.
Items
for attention in the development of the AM Constellation:
(1). Ground data links may become saturated with close formation flying but this seems to be surmountable because the nominal orbit has a 15 minute orbital separation between satellites.
(2). Ground repeat pattern by SAC-C will be reduced from the desired nine day repeat pattern but the four satellite constellation will increase the reliability of measurements and could provide needed emergency coverage on a contingency basis.
Orbit:
SAC-C and EO-1 will be
launched on the same Delta II launch vehicle on December 15, 1999. The SAC-C
project has determined that a sun synchronous 10:15 AM descending node is
required for its observational requirements. EO-1 will orbit in formation at
the 10:01 AM node one minute behind Landsat 7 while Terra will orbit at the
10:30 AM sun synchronous node. Landsat 7 and EO-1 and Terra will follow the
Worldwide Reference System (WRS) Grid with a 233 revolution 16 day repeat
period. We propose that SAC-C adopt an orbit which will coordinate its
observations within this constellation with the following mean orbit
parameters:
Landsat 7 EO-1 SAC-C Terra
Semi-major Axis(km) 7077.732 7077.732 7077.732 7077.732
Eccentricity 0.001175 0.001175 0.001175
0.001175
Inclination (deg) 98.21 98.21 98.21 98.21
Lon. of Descending
Node (deg) 295.4 LS7+0.25 LS7+3.76 LS7+7.52
Argument of Periapsis (deg) 90.0 90.0 90.0 90.0
Mean Anomaly (deg) 0.0 LS7-3.6450
LS7-54.6755 LS7-109.3510
Descending equatorial
crossing times (local) 10:00 AM
10:01AM 10:15AM 10:30 AM
Assumes LS7 is at first
descending equator crossing and at periapsis.
To achieve the proper
SAC-C node at launch, a coast phase may be required (after EO-1 separation) to
get to min or max latitude - the location in orbit to perform a node change. A
delta V maneuver of about 450m/s is required to change the node 3.51 deg
(3.76-.25 deg). Because this maneuver is
large it must be performed by the launch vehicle final stage or by
modifying the SAC-C orbital altitude to induce precession of the node until the
appropriate orbital plane is achieved.
Acknowledgements: Special thanks to Joe Guinn, Larry Young and Gil Ousley for help in preparing this report.