Water height is measured by a wide-swath radar interferometer operating in Ka-band. This innovative instrument called KaRIn (Ka-band Radar Interferometer) has been developed by JPL, drawing on the heritage of the Shuttle Radar Topography Mission (SRTM) and Wide Swath Ocean Altimeter (WSOA) instruments. It has two transmit/receive antennas operating simultaneously and located at either end of a 10-metre boom. It is this specific configuration that enables the interferometric measurement needed to achieve 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 kilometres on either side of the ground track. The signals are received by two antennas 10 metres apart when deployed in flight configuration and are then processed to yield interferometry measurements.
KaRIn’s radio frequency unit (RFU) is being supplied by Thales Alenia Space.
KaRIn instrument features
- 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.
KaRIn comprises the following subsystems:
- The KaRIn digital electronics subsystem (KDES) developed by JPL handles telemetry/telecommand interfaces with the spacecraft bus, instrument sequencing, analogue-to-digital conversion of signals from the RFU and onboard digital processing. KDES will process low-throughput ocean data to reduce data volumes downlinked to ground. This processing function developed by JPL will be performed by KaRIn’s processing unit when over oceans.
- The RFU contains KaRIn’s RF electronics (excluding the HPA), including the entire RF transmission system, the Ka-band ferrite switching matrix and the pair of interferometry receivers delivering the baseband signals. The RFU, developed by Thales Alenia Space France, itself consists of two units:
- the HyperBox developed by Thales Alenia Space France in Toulouse
- the duplexer developed by COMDEV UK
KaRIn RFU (CNES/Thales Alenia Space)
- The high-power amplifier (HPA) subsystem, which complements the RFU to form KaRIn’s analogue subsystem. The HPA delivers a peak power of about 1.5 kW in Ka-band. It is being assembled by JPL and consists of:
- a 1.5-kW RF power amplifier (EIK) supplied by the Canadian Space Agency (CSA) and developed by CPI (Communications & Power Industries Canada Inc.)
- a 15-kV high-voltage power supply (HVPS), developed by JPL
- The Deployable Antenna Assembly (DAA):
The antenna subsystem is being developed by JPL. Each antenna (right/left) consists of two feeds (one mono-polarized feed per swath on the body of the payload), a reflector and a deployable mast on which the reflector is mounted. The all-carbon antennas, each with a 5-metre boom, afford great stability (on the order of 1 micron at 10 metres)