RapidScat Instrument Overview
RapidScat is a scatterometer that will use the International Space Station as a platform to measure surface winds over the Ocean to provide valuable scientific information and act as a calibration standard for other space-based scatterometer instruments, taking over that role from the SeaWinds instrument on the QuikScat spacecraft that has shown degraded performance.
Scatterometers are instruments that allow the precise measurement of the normalized radar cross section of the surface by emitting a microwave pulse aimed at the surface and measuring the reflected energy. The signal power of the backscatter is determined by subtracting the noise-only power and measurements from different azimuth angles are combined to allow the determination of the near-surface wind vector using a geophysical model function.
Winds over the ocean are normally in equilibrium with wind-generated capillary-gravity waves that can be in resonance with the microwaves emitted by the satellite, causing Bragg scattering. The power of the backscatter depends on wind speed and direction which can be extracted through data processing of measurements made at different azimuth angles.
The importance of scatterometer measurements is severalfold – measurements are used to study sea-air interactions, climate evolution, hurricane monitoring, the assessment and tracking of vegetation, soil moisture, polar ice, and mobile icebergs. Data from scatterometers is applied in operational meteorological models and microwave instruments have been flown on solar system mission to extract valuable information on atmospheres other than Earth’s.
The need for quickly deploying a scatterometer arose in late 2009 when the SeaWinds instrument on the QuikScat satellite suffered an unexpected degradation in data quality that limit the instrument to acquire measurements from only a small swath width. In addition to leaving a hole in the global constellation of scatterometers, it also eliminated the normal cross-calibration capability for the international scatterometer constellation of NASA, NOAA, ISRO and Eumetsat.
Since a follow-on spacecraft could not be launched for several years, NASA decided to deploy the QuikScat engineering model to the International Space Station to restore full calibration capability and make use the the unique orbital characteristics of ISS.
RapidScat uses the engineering model of the QuikScat scatterometer with the exception of the antenna module that was changed to comply with ISS and Dragon payload envelopes. Also, modifications were made to make the engineering model fit for space flight and the system was modified to fit ISS data and electrical interfaces provided through its attachment system.
The RapidScat instrument is a conically scanning pencil-beam scatterometer using a two-beam design with an inner beam at a look angle of 45 degrees and the outer beam at a 50.5-degree look angle – the two beams have incidence angles of 49 and 56° respectively. The instrument uses a conical scan geometry, rotating a 0.75-meter diameter reflector at 18rpm. This means that the two beams sweep out a circle around the instrument nadir vector. The diameter of that circle will depend on the altitude of the Space Station which can vary from 375 to 435 Kilometers for RapidScat to achieve its planned accuracy. At the present ISS altitude (~415km), the inner beam will scan a swath of 900 while the outer beam scans a swath of 1,100 Kilometers.
The scanning geometry has been chosen to allow the instrument to view each point on Earth that is within the inner beam four times as the instrument passes over (twice by the inner beam looking forward and then aft, and twice by the outer beam also looking forward and then aft as the spacecraft passes over).
The instrument transmits microwave pulses at a frequency of 13.402 GHz (Ku-Band) at a pulse width (duration) between 0.5 to 1.5 milliseconds. Compared to QuikScat, the pulse width has been shortened for timing margin due to the lower orbit of ISS. Pulses are repeated every 6 milliseconds, in between pulses is a 1.4-millisecond data window during which the echo is recorded. Timing has been modified for RapidScat to transmit and receive on the same beam.
The instrument achieves a peak transmit power of 80 Watts. RapidScat generates 40kbit/s of data, requires 250W of average power and operates at temperatures of 5 to 40°C. In charge of spinning the reflector is an activator assembly that provides a very accurate spin rate and also measures the precise position of each scan to allow the instrument to calculate the scanned area on the ground based on orbital and pointing information from ISS.
The instrument takes measurements of the radar cross section at several different azimuth angles for vertical and horizontal polarization.
The wind vector is retrieved by fitting the measurements to the NSCAT-2 geophysical model function that ties together the expected radar cross section as a function of wind speed and direction relative to the look and incidence angles. This requires extensive calibration of the scanning geometry that is completed pre-flight.
Overall, RapidScat can measure winds from 3 to 20 meters per second with an accuracy of 2m/s and direction with an accuracy of 20 degrees. The ground range resolution achieved by the instrument is 0.79 Kilometers while the maximum azimuth resolution is 16 Kilometers.
RapidScat will be installed on the Columbus External Payload Facility. Launching with RapidScat is a 90-degree adapter frame that will be installed on the Columbus External Platform Adapter to provide a nadir-facing mounting platform for RapidScat that uses a Flight Releasable Attachment Mechanism. The nadir adapter includes power and data interfaces to connect RapidScat to the associated interfaces of Columbus.
Columbus provides RapidScat with two power buses – one 120 Vdc bus for survival heaters only and one 120 Vdc bus for the operation of the instrument. Commands to the instrument are transmitted via a two-way analog and a 1553B data bus which are also used for the transfer of housekeeping data. An Ethernet connection delivers instrument data to Columbus for storage and eventual downlink via ISS assets.
The Nadir Adapter of RapidScat houses the Command and Data Subsystems that deliver power-switching and distribution services and it includes the Instrument Data Processors. A Power Converter Unit and Digital Interface Bridge also resides within the adapter. From the nadir adapter, a 120Vdc power supply for survival heaters is provided by the Scatterometer Electronics Unit to the Scatterometer Antenna Subsystem. 48 Vdc power buses are used for power the various electronics.
The Scatterometer Electronics Subsystem generates the transmit signals using an Exciter and Synthesizer system to provide the chirp signals that are amplified using a TWTA (Traveling Wave Tube Amplifier) that then provides the signals to the Transmit/Receive Switch assembly for transfer to the Dual Pencil Beam Feeds. The Receive Signal is routed to the Receiver via the Transmit/Receive Switch Assembly operating on timed commands by the processor unit. The receiver converts the signals into a digital format and passes the data to the SES Command Processor for transfer over to the Nadir Adapter that then sends it to ISS.
Commanding of the Spin Mechanism and RF Rotary Joint is provided by the CDS in the Nadir Adapter that also processes the precise position information provided by the activator assembly to properly locate the position of each scan on the surface of Earth.
Having ISS as a platform for the RapidScat instrument (not originally designed to fly on a Space Station) requires a number of measures taken by the instrument to protect ISS and by ISS to fully exploit the instrument’s capabilities in terms of accuracy.
Residing on the Columbus EF, RapidScat will face the issue of having ISS hardware within its field of view, particularly the solar arrays and USOS visiting vehicles. While the Solar Arrays can handle the blast of microwave radiation from RapidScat, they represent a blockage that cuts the instrument field of view by up to 8%. Visiting vehicles can not tolerate the radiation emitted by RapidScat since the electrical field it generates exceeds given limits by up to 9.5dB. A reduction of the transmission level by 9.5dB when visiting vehicles are present (~50% of the time) is not feasible, therefore, RapidScat will use a 60° sector blanker, reducing the field of view but keeping visiting vehicles safe.
Another issue for RapidScat is the attitude of ISS that is not driven by a single instrument’s requirements. Variations up to -10° on pitch, 1° of roll and –6° in yaw could be tolerated by RapidScat.
Attitude maneuvers for visiting vehicle operations will lead to a loss of science on RapidScat and potential additions of modules (MLM, Russian Node) have to be anticipated over the lifetime of the instrument leading to permanent changes in pitch attitude. For a good prediction of ground scanning areas, the orbit and attitude of ISS has to be known precisely to be implemented in data processing (ISS shows a daily pitch variation of 0.2° which lead to timing errors of 0.02ms that can be accommodated easily along with manageable doppler errors). It is also required for RapidScat’s scanning tables to be updated frequently to adjust scanning parameters as a function of ISS altitude.
The characteristics of the ISS orbit are very different to those of Sun-Synchronous Orbits employed by most scatterometers which can provide valuable additional insights obtained via RapidScat. Having different scatterometers in different Sun-Synchronous Orbits means that each satellite visits each position on Earth at approximately the same local time meaning that the various satellites have different observation times. This presents a challenge to data calibration and stitching of data products from different instruments, especially when attempting to monitor subtle changes over long time periods.
The orbit of ISS is not synchronized and drifts so that locations on Earth can be viewed at different times of day. Also, the orbit of ISS intersects all Sun-Synchronous Orbits every hour, allowing winds to be determined by RapidScat and other scatterometers for a cross-calibration.
RapidScat will act as golden standard for calibration of the international scatterometer instruments in way in which RapidScat itself uses QuikScat for calibration to eliminate any effects of the special operational environment of ISS. This will allow all satellites to use a common reference frame for producing consistent wind data.
RapidScat on its own will be extremely valuable for the study of diurnal variations in winds which are poorly understood and hard to study from SSO using different spacecraft due to the high accuracy of relative calibration required by this type of study. The ISS orbit visits all points of Earth (between 51.6° south and north) at all times of day over a two-month period. This will allow RapidScat to estimate semi-diurnal wind components over its two-year primary mission.
There is high demand for the study of diurnal and semi-diurnal wind variations due to their impact on cycle of cloud formation and precipitation in the tropics, a key component of the Earth’s water and energy cycles.
Furthermore, RapidScat will be used to assess the diurnal evapo-transpiration cycle of vegetation in the tropics which influences the water cycle and has great importance for the calibration of scatterometers that use the rain forest as calibration target. Data products from RapidScat will also find application in weather forecasting of marine storms.
RapidScat will use the ground system established for the QuikScat mission that has been refined over the years to ensure a rapid data availability after acquisition via NOAA.