Water height is measured by a wide-swath radar interferometer operating in Ka-band. KaRIn (Ka-band Radar Interferometer) has been developed by JPL on the legacy of the American Shuttle Radar Topography Mission (SRTM) and Wide Swath Ocean Altimeter (WSOA) instruments. It has two transmitting/receiving antennas operating simultaneously and located at either end of a 10 m boom. It is this exact configuration that enables the interferometric measurement needed for the prescribed accuracy, spatial resolution and wide swath of the SWOT mission.
Dual-swath Ka-band radar interferometry system.
The KaRIn instrument operates on the principle of a Synthetic Aperture Radar (SAR), illuminating two swaths of approximately 50 km on either side of the ground track. The echoes are received by two antennas 10 m apart when deployed (flight configuration) and are then processed to yield an interferometry measurement.
Characteristics of the KaRIn instrument
- Carrier frequency: 35.75 GHz
- Transmission bandwidth: 200 MHz
- Duration of transmitted pulse: 4.5 µs
- Pulse Repetition Frequency (PRF): 2 x 4,420 Hz
- Boom length: 10 m
- Antenna size: 5 m x 0.25 m
- Viewing angle: +/- 2.7 deg
CNES is responsible for providing the RFU for the KaRIn instrument whose development is being supervised by JPL. The figure below illustrates the main links (functional links shown by dotted lines and radar signals shown by solid lines) between the RFU and other KaRIn subsystems, which are as follows:
- The KaRIn digital electronics subsystem (KDES) developed by JPL is in charge of the instrument’s telemetry/telecommand interfaces with the spacecraft bus, instrument sequencing, analogue/digital conversion of signals from the RFU and onboard digital processing.
- The antenna subsystem developed by JPL. Each antenna (right/left) consists of two sources (one mono-polarised source per swath), a reflector and a deployable mast which bears the reflector.
- The high power amplifier (HPA), which completes the RFU to form the KaRIn instrument’s analogue subsystem. The HPA subsystem delivers a peak power of about 1.5 KW in Ka-band. It is assembled by JPL and consists of:
- an RF power amplifier (EIK) provided by the Canadian Space Agency (CSA) and developed by CPI (Communications & Power Industries Canada Inc.)
- a high-voltage power supply (HVPS) developed by JPL
The RFU forming part of KaRIn.
The RFU comprises all the interferometer’s RF electronics (excluding the HPA). It includes the whole transmission system (including the digital chirp generation part), the Ka-band ferrite switching matrix and the pair of interferometry receivers delivering the baseband echoes. The RFU, whose development has been entrusted to TAS-F, itself consists of two items of equipment:
- the HyperBox developed by TAS-F in Toulouse
- the duplexer whose development has been subcontracted by TAS-F to COMDEV UK
RFU (HyperBox + duplexer) (Thales Alenia Space).
Part of the data processing (low-throughput ocean data) will be carried out by the KDES unit in order to limit satellite-ground transmissions. This processing function, developed by JPL, will be carried out by the KaRIn instrument calculator when flying over the oceans.
KaRIn also includes the KaRIn Nadir Channel (KNC), a complementary, experimental nadir echo measurement payload intended to fill in the measurement gap at the nadir while providing an extra calibration channel.