ISS Utilization: ISS-RapidScat
ISS Utilization: ISS-RapidScat
The ISS-RapidScat instrument is a speedy and cost-effective replacement for NASA's QuikSCAT Earth satellite, which monitored ocean winds to provide essential measurements used in weather predictions, including hurricane monitoring.
Background: After ten years of successful operations, in late 2009 the NASA SeaWinds instrument on the QuikScat satellite suffered a degradation that significantly decreased the amount of wind data it could collect over the oceans, leaving a hole in the global constellation of wind scatterometers. The QuikSCAT instrument is still able to operate collecting a small swath, and has been used successfully by NASA to provide a cross-calibration standard for the international scatterometer constellation of ISRO’s OSCAT (OceanSat-2 Scatterometer) and EUMETSAT’s ASCAT(Advanced Scatterometer). Although next-generation replacements to this satellite have been under study by NASA and NOAA, these instruments will not be readily available to mitigate the degradation of QuikScat in the near term. 1)
To meet this challenge, the Jet Propulsion Laboratory (JPL), in partnership with NASA’s International Space Station Program Office, will deploy the QuikScat engineering model, which had been used to test the basic functionality and performance of the instrument, on the ISS to continue and improve QuikSCAT’s calibration standard across the present scatterometer constellation and demonstrate NASA’s capability for fast response to science challenges in a cost constrained environment.
ISS-RapidScat will also exploit the special characteristics of the ISS orbit to advance our understanding of the Earth’s winds. Current scatterometers are in polar sun-synchronous orbits, visiting each point on the Earth at approximately the same local time. Consequently, satellites in the scatterometer constellation have different local observation times, and products significant challenges in stitching the data from different satellites into a data record appropriate for monitoring subtle changes in the wind field across satellite records and over long periods. The ISS orbit, on the other hand, is not synchronized with the Earth’s rotation and has a lower inclination than polar sun-synchronous satellites. This will cause the ISS orbit to intersect the orbits of every one of these sun-synchronous satellites approximately every hour, allowing winds to be estimated simultaneously by ISS-RapidScat and the other scatterometers. These simultaneous views will allow ISS-RapidScat to serve as the calibration golden standard that will enable improved calibration of the international scatterometer constellation.
The primary goal of this investigation is to provide a gap-filler ocean vector winds measurement capability to mitigate the loss of the NASA QuikSCAT scatterometer. Scatterometers are radar instruments that can measure near-surface wind speed and direction over the ocean, and have proved to be extremely valuable for weather forecasting, including hurricane monitoring, and for monitoring large-scale changes in the Earth’s climate, such as El Niño. The ISS RapidScat instrument will provide wind measurements that will enhance the international scatterometer constellation, provide unique cross-calibration capabilities to extend the climate data record initiated by the QuikSCAT satellite. In addition, because of the unique orbit characteristics of the ISS, RapidSCAT will be enable the first measurements of the systematic diurnal changes of winds over the ocean. 2)
In 2013, NASA's ESD (Earth Science Division) is planning to use the ISS (International Space Station) more than ever as a platform for observing the Earth. In addition to those missions solely funded by the ESD, like SAGE III and OCO-3 (Orbiting Carbon Observatory-3), the CATS (Cloud Aerosol Transport System) and RapidSCAT were selected as part of an ISS Program science utilization solicitation. CATS for the ISS, is a cloud aerosol LIDAR that is a possible precursor for the upcoming ACE (Aerosol/Cloud/Ecosystems) Decadal Survey Mission. 3)
Figure 1: Planned NASA Earth Science Instruments for the International Space Station(image credit: NASA)
Launch: A launch of the RapidScat instrument to the ISS on a Falcon-9/Dragon spacecraft of SpaceX (CRS-4) is planned for the summer 2014. 4)
Orbit: Near-circular orbit of ISS, altitude range of 375-435 km, inclination = 51.6º. The prograde orbit of the ISS means that there will be an intersection with the orbits of every scatterometer in the constellation (ASCAT, OSCAT, QuikSCAT) once every revolution, and the likelihood of having nearly coincident temporal coverage (within 0.5 to 1 hours) is guaranteed. Furthermore, as the orbit moves over the year, the loci of these intersections will shift in latitude, yielding over time a global estimate of the relative wind and geographical biases between RapidScat and any other system in the constellation (Ref. 2).
For the sun synchronous orbits, it is impossible to obtain this type of global collocation, since orbital overlaps tend to occur at very high latitudes, limiting coverage to either land or the Southern Ocean, where special conditions apply in terms of wind speed and stability that are not globally representative.
Figure 2 presents estimates of the standard error on the relative bias between RapidScat and ASCAT as a function of latitude. This type of cm/s accuracy is what is required to enable climate studies of wind variability.
Figure 2: Estimated standard error in the estimated bias between RapidScat and ASCAT as a function of latitude, and for a mission duration of one (dashed) and two (solid) years (image credit: NASA/JPL)
The proposed plan for the joint use of RapidScat and QuikSCAT will consist of using the QuikSCAT-RapidScat collocations to achieve cross-calibration between the two instruments on an ongoing basis during the RapidScat mission. The continuous calibration will alleviate any issues that might arise in RapidScat due to the special environment on the ISS. The calibrated RapidScat will then be used as the golden standard to develop bias corrections (as a function of wind speed, direction, and geographical location) so that all instruments on the constellation (ASCAT, OSCAT, QuikSCAT, and, potentially, OSCAT2) have a common reference frame for producing a consistent winds data set.
Diurnal studies: A hurdle in estimating the semi-diurnal observations from sun-synchronous scatterometers is due to the fact that biases between two scatterometers will alias into the semi-diurnal component, so that good relative calibration is required.
The ISS orbit, on the other hand, visits all points at latitudes smaller than 51.6o at all times of day over a period of roughly 2 months (Figure 3). This will allow, over a period of two years, the estimation of the semi-diurnal wind components from the RapidScat data alone (see Figure 4 for estimated accuracies). Furthermore, since RapidScat enables a consistent set of biases, other scatterometers in the constellation can also be used in obtaining this estimate, which will lead to improved precision in the estimates.
Figure 3: The local time sampling characteristics of the ISS are to revisit the same latitude at slightly different local times each orbit. To fully sample the diurnal and semi-diurnal cycles once globally requires at least 2 months of data. To estimate diurnal and semi-diurnal cycles accurately, on the order of 10 sets of observations (~2 years) will be required (image credit: NASA/JPL)
Figure 4: Upper bound on the estimated semi-diurnal cosine component of zonal and meridional wind components assuming inversion using RapidScat data alone (image credit: NASA/JPL)
The ISS-RapidScat instrument will utilize the spare engineering model used to test the QuikSCAT scatterometer, modified to operate from the ISS to provide a low-cost mission to mitigate the significant loss of measurement capability by QuikSCAT in 2009. The resulting instrument package will produce ocean vector winds similar in accuracy to QuikSCAT, but with a measurement swath on the ground smaller by a factor of two due to the lower ISS orbit. This swath width will be similar to the EUMETSAT ASCAT (Advanced Scatterometer) flown on MetOp, and the two data sets will complement each other to achieve coverage similar to QuikSCAT. Using engineering models in space, like on ISS-RapidSCAT, represents a low cost approach to acquiring valuable wind vector data. It does come with technical and programmatic risks. The hardware was not directly fabricated for space and will require rework in order to prepare for the rigors of space travel and operation. Meeting the cost commitment will require new and innovative approaches to development.
The specific objectives of the ISS-RapidScat mission are: 5)
1) To provide ocean vector wind data for a period of two years to mitigate the loss of QuikSCAT to scientists and weather forecasters. -The ISS orbit will enable coincident measurements in space and time with each of the satellites in the constellation ASCAT, OSCAT (OceanSat-2 Scatterometer).
2) To serve as a cross-calibration standard to the international scatterometer constellation, enabling the continuation of the QuikSCAT data record, and enabling monitoring of climate variability and change over multiple decades.
3) To study the systematic variation of ocean winds as a function of time of day. These variations are important in understanding the dynamics and interactions of the ocean and atmosphere in the tropics, where current climate models still exhibit shortcomings, and which play a significant role in governing the Earth’s energy and water budgets.
Figure 5: Artist's view of the ISS-RapidScat observation geometry and accommodation on the Columbus module (image credit: NASA) 6)
ISS-RapidScat will have measurement accuracy similar to QuikScat's and will survey all regions of Earth accessible from the ISS orbit. The instrument will be launched to the space station aboard a SpaceX Dragon cargo spacecraft. It will be installed on the end of the station's Columbus laboratory of ESA as an autonomous payload requiring no interaction by station crew members. It is expected to operate aboard the station for two years.
ISS-RapidScat will take advantage of the space station's unique characteristics to advance understanding of Earth's winds. Current scatterometer orbits pass the same point on Earth at approximately the same time every day. Since the space station's orbit intersects the orbits of each of these satellites about once every hour, ISS-RapidScat can serve as a calibration standard and help scientists stitch together the data from multiple sources into a long-term record.
Figure 6: Flight system overview (image credit: NASA/JPL, Ref. #
Figure 7: ISS-RapidScat versus ASCAT coverage (image credit: NASA, Ref. #
Inherited hardware constraints (Ref. 5):
• Pulse Repetition Interval (PRI) commandable between 5 and 6 ms
• Fit within the dynamic range of the receiver
- Roughly 45 dB instantaneous
- Attenuator can be set over 20 dB range
• Pulse width commandable between 0.5 and 1.5 ms
- Bandwidth is tied to pulse width via constant chirp rate of 250 kHz/ms. Reducing the pulse width from the 1.5 ms used with QuikSCAT reduces the bandwidth and range resolution
- Frequency resolution of processed slices is tied to length of the data collection, increasing range gate width decreases resolution.
• Antenna spin rate 18 rpm or 19.8 rpm
• Range delay and Doppler frequency offset are commanded via tables
- The variation of these parameters over a 360º scan has to be represented as A + B cos(θ+phase)
- Maximum Doppler 600 kHz, max range delay 12.75 ms
• SeaWinds EM is being used as is with the exception of the antenna system, thus the design space consists of:
- Selection of operating parameters within existing constraints (ISS and existing hardware)
- Design of antenna characteristics for desired performance
• Timing is a central concern in our analyses due to ISS altitude and attitude constraints
• SeaWinds was designed to transmit on one beam and then receive on the other (interleaved), operating at 800 km altitude
• At 400 km altitude the roundtrip time to match the inner OceanSat-2 angle is 4.0 ms; to match outer is 4.7 ms
• These are within the commandable PRI range of 5-6 ms
• To match Seawinds interleaved operational design, the PRI would need to be reduced to less than 3 ms – not feasible
• Hence, timing must be modified to transmit and receive on the same beam.
ISS constraints on instrument design and performance:
• EMC requirements
- Fields from RapidScat must on all ISS equipment and on ISS solar panels must be below required limits
• ISS altitude and attitude variations
- Expected altitude range: near 410 km mean altitude, plan for 375 km to 435 km (variation over an orbit ~ 20 km)
- Pitch can be roughly 0º to -10º, depending on visiting vehicles and presence of MLM (Multipurpose Laboratory Module)
- Roll should generally be less than 1º
- Yaw around -6º
- Control is to LVLH system; adds max of about 0.2º error to project desired geodetic nadir (based on project calculations).
- Visiting vehicles and solar arrays are within the RapidScat main beam (E-field limits exceed specs by up to 9.5 dB)
• Impact (if not mitigated)
- While ISS solar arrays can handle the radiation level, they do create a blockage in RapidScat’s FOV (worst case = 8%)
- Reducing tx level by 9.5 dB for visiting vehicle safety not feasible; would need to turn off instrument during all visits
- Estimate about 300 days of visiting vehicles at Node 2 during RapidScat’s 2-year on-orbit period; about half time not operating.
- Implementation of a sector blanker; the RF energy will be blanked over a sector of up to 60º on every rotation of the antenna
- The sector blanker will affect performance of the radar by reducing the swath up to approximately 50 km.
ISS attitude and altitude variability:
• The ISS can have significant changes in attitude (primarily pitch) due to docking of vehicles
• Additionally, the MLM (Multipurpose Laboratory Module) is scheduled for launch and to be installed on ISS in December 2014: will shift station’s pitch bias by -4º
• The future Russian module will shift station’s pitch bias by another -2º; however it is not likely to be installed till the middle or end of the RapidScat mission
• Previous history of attitude variation has been evaluated from ISS data and from HICO RAIDS data
- Study found good agreement between the two sources
- Attitude mean difference is typically less than 0.1º
• Future attitude variations are predicted by JSC (Johnson Space Center).
ISS attitude (pitch) change effects:
• Excessive ISS attitude changes throughout the RapidScat ops period can lead to RapidScat performance loss (echo overlap with nadir return or next transmitted event)
- The radar can compensate for small changes by adjusting the pulse width and data range window
- Excessive change is beyond radar’s capability to compensate and will lead to loss of signal.
Figure 8: Schematic view of ISS pitch angle change effects (image credit: NASA, Ref. 5)
ISS attitude variability: mitigation
• PRI will be set to 6 ms; this gives maximum timing margin between end of received pulse and start of new transmit event
• Pulse width is reduced to 1 ms (or less) for timing margin
• Rx gate width to 1.4 ms for 10 km slice width (based on SeaWinds processor operation); maximized to allow for pitch variations
• Frequent update (daily-weekly) of range delay and Doppler tables (which tell the radar when to start recording data and what frequency offset to use for compensating Doppler shift)
- Tables are designed to hold parameters for an orbit
- The largest short-term variation in attitude is due to orbital motion, so this must be predicted with sufficient accuracy
- Due to on-board implementation of Doppler shift update a maximum of ±12.5 kHz error can be expected in the return echo in addition to errors from attitude variation.
• Mount radar with forward pitch offset (+2.5º or +5.5º) to reduce pitch bias over mission
• Roll bias is expected to be small; yaw bias of 6º will not have significant impact on conically scanning instrument
• Simulations indicate that for expected altitude and attitude range, data loss should be small (less than a few %).
• Initial assessment of system performance is based on simple spreadsheet calculations of the the noise equivalent sigma0
• For parameters shown here it was assumed that the station’s orbit and attitude were perfectly known and stable i.e. no variability in pitch or altitude.
- Direct assessment of the scatterometer’s wind retrieval ability relies on a simulation that uses a wind field as input.
Table 1: System performance parameter comparison of RapidScat and QuikSCAT
- Measure antenna gain, antenna pointing relative to alignment feature (e.g., cube or tooling balls)
- Calibrate antenna encoder (measure actual antenna position versus digital readout)
- Measure spin axis orientation relative to CEPA (Columbus External Platform Adapter)
- Measure receiver gain and noise over expected temperature range
- Measure transmit power over expected temperature range
- Update tables used by ground processor with these measurements.
- Absolute calibration, slice balance, sigma0 from all slices match the sigma0 from the whole footprint, sigma0 vary appropriately along the scan (Amazon rain forest to determine scan bias)
- Use data to estimate pointing and update Doppler and range tables
- Use data to update gain and update calibration tables
- Solar array positions used to discard affected data (<8%).
• RapidScat can be accommodated at the ISS SDX (Starboard Attach Point) site
- Requires two FRAM-based (Flight Releasable Attachment Mechanism) units
- Becomes an oversized payload once installed on orbit
• Partial FOV blockage leads to some degradation in performance
- In worst case, when solar arrays are in FOV, up to 8% of the scan is blocked
• Radiation level exceeding ISS subsystem safe level requires sector blanking (up to 60º)
- Radiation on ISS solar arrays not a concern
- Mitigation to allow ops when visiting vehicles are present will reduce swath and may impact calibration
• RapidScat measurements will provide vector winds retrieval to the accuracies comparable to those from QuikSCAT
- Requires pitch pointing offset to counter ISS attitude bias variations
- Design provision to set pitching pointing offset as late as feasible during the integration period
- If ISS pitch bias is incorrectly predicted performance is degraded
1) Meet stated performance 99% of the time if pitch bias predicted correctly
2) If assumed presence of MLM is incorrect, performance met 32% of the time.
Figure 9: Measurement geometry of RapidScat (image credit: NASA)
Table 2: RapidScat instrument parameters
Table 3: ISS accommodation of RapidScat
Figure 10: Flight system configuration, block diagram, instrument constraints (image credit: NASA, Ref. 5)
Figure 11: Illustration of the flight system (image credit: NASA, Ref. 5)
1) Ernesto Rodriguez, “ISS-RapidScat,” January 23, 2013, URL: http://www.nasa.gov/mission_pages/station/research/experiments/ISSRapidScat.html
3) Steven P. Neeck, Stephen Volz, “NASA’s Earth Science Flight Program and Small Satellites,” Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013
5) Dragana Perkovic-Martin, Stephen Durden, Alexander Fore, Bryan W. Stiles, Gregory A. Sadowy, Simon A. Collins, Howard J. Eisen, Yuhsyen Shen, “ISS-RapidScat Mission, Instrument and Expected Performance,” IOVWST (International Ocean Vector Wind Science Team) Meeting, Kona, Hawaii, May 5-8, 2013, URL: http://coaps.fsu.edu/scatterometry/meeting/docs/2013/Future%20Missions/-Rodriguez_2_IOVWST_RapidScat_engineering_final.pdf
6) Alan Buis, Trent J. Perrotto, Josh Byerly, “NASA to Launch Ocean Wind Monitor to Space Station,” Jan. 29, 2013, URL: http://www.jpl.nasa.gov/news/news.php?release=2013-037
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.