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KaRIn: Ka-band Radar Interferometer

Credits Nasa/JPL
Credits Nasa/JPL

KaRIn (Ka-band radar Interferometer) is a new class of altimeter, taking into account all the experience of previous missions (altimetry, but also SAR sensors), proposing with brand-new capabilities of measuring surface heights. 

The primary SWOT instrument is an interferometric altimeter which has a rich heritage based on 
(1) the many highly successful ocean observing radar altimeters, 
(2) the Shuttle Radar Topography Mission, 
and
(3) the development effort of the Wide Swath Ocean Altimeter (WSOA).

It is a near-nadir viewing, 120-km wide, swath based instrument that will use two Ka-band synthetic aperture radar (SAR) antennae at opposite ends of a 10 m boom to measure the highly reflective water surface. Interferometry SAR processing of the returned pulses yields a 5 m azimuth and 10 m to 70 m range resolution, with elevation precision of 50 cm. Spatial averaging over areas of 1 km2 improves the height precision to less than 2 cm. KaRIn will provide continuous mapping operating in single transmit antenna mode. 

See Techniques>Altimetry page dedicated to altimeter-interferometer for more information how it is working.

To form the required baseline between the two antennae, KaRIn will deploy two 5-m-long and 0.25-m-wide reflectarray antennas on opposite ends of a boom; both antennas can technically transmit and receive radar pulses, but in the normal scheme, only one will transmit (emit), and both will receive. The interferometer is a dual-swath system, alternatively illuminating the left and right swaths on each side of the nadir track. This is accomplished by an offset dual-feed design operating with orthogonal linear polarizations (V and H polarizations), which enables the emitting reflectarray antenna to generate two separate beams, one for each polarization. KaRIn will make near-nadir measurements (roughly ranging from 0.5° to 4.5° look angle on each side of the nadir track) at Ka-band (0.84-cm wavelength, 35.75-GHz center frequency). The look angles entails swath coverage of about 120 km (from 10 to 70 km in cross-track distance on both sides of the nadir track).

The KDES mounted on the metering structure
The KDES mounted on the metering structure

The KaRin architecture consists of:

The KaRIn Digital Electronics Subsystem (KDES):

The Unit receives KaRIn commands from the spacecraft (S/C) and DORIS data (for altitude and navigation information) via 1553 and sends KaRIn telemetry to the spacecraft via the same interface, acquires and packetizes telemetry from the rest of the instrument. It also receives and digitizes the downconverted I & Q echo information from the RFU. The KDES controls the radar hardware (RFU, HPA, alignment). To limit the data rate to the ground, the KDES converts the digitized data to interferograms. The maximum output data rate is 360 Mbps. The data is output to the S/C Solid State Recorder (SSR).

 

KaRIn RFU
KaRIn RFU

The Radio-Frequency Unit (RFU)

It is made of all the analog components needed to modulate/demodulate, amplify and convert in frequency the chirp reference signal up to the High Power Amplifier and from Duplexers. The two main subsystems which form the RFU  are the Duplexer, which is the switching matrix of the RF signals from/to HyperBox, EIKs, antenna feeds and the HyperBox, which implements all other functions of the RFU : Pulses generation/emission & Echoes reception / Ka band up & down conversion / LO IF & RF supply.

 

The HPA (High Power Amplifier)

The HPA amplifies the chirped RF signal send by the RFU to 1500 W for transmission to the ground. The HPA consists of a CPI-built Extended Interaction Klystron (the EIK is a pulsed load on the supply which envelopes the RF chirp), a JPL-built High Voltage Power Supply and a NASA-provided High Power Isolator/Filter. Unit is block redundant due to lack of experience with amplifiers in this class.

Deployable Antenna Assembly

It is a high precision, high stiffness, and stable deployable structure which locates radar panels relative to the feeds mounted on the metering structure. The assembly consists of an integrated Mast Assembly (IMA) and an Integrated Array Assembly (IRA). It shall be as rigid as possible to guarantee a stabilized baseline. It is mostly built of carbon composite parts. The IRA includes an alignment mechanism which can correct the pitch misalignment between the two reflectarrays.

One side DAA deployed
One side DAA deployed
The KaRIn Integrated Mast Assembly
The KaRIn Integrated Mast Assembly
KaRIn Integrated ReflectArray testing
KaRIn Integrated ReflectArray testing
The KaRIn equipped metering structure
The KaRIn equipped metering structure

The KaRIn Inertial Reference Unit (Gyro)

KaRIn includes a high performance IRU to measure the orientation of the mechanical baseline . The unit is part of a flight proven product line. It is a fiber optic gyro, sensing the angular rate using an optical signal traveling through a 5 km coil of optical fiber. There are no moving parts. Optical fiber is available with very long life. The IRU has built in fault tolerance –four entirely independent channels, any three of which are sufficient. It is a straight-to-flight development.

KaRIn System parameters

Parameter Value
Center frequency 35.75 GHz 8.4 mm wavelength
TX Bandwidth 200 MHz 0.56 % bandwidth
TX Pulse length 5.7 us Capability to operate within 3 - 9 us
Pulse Repetition Frequency (average) 2 x 4420Hz Varies by < 5% along the orbit
Peak Transmit Power (EOL) 1500W
Physical Baseline Length 10 m
Antenna size 5 mx 0.25 m
Boresight Look Angle +/- 2.65 deg Swath look angles -0.65° to 3.9°
Polarization, Right Swath VV Alternating polarizations between
Polarization, Left Swath HH consecutive pulses to 'illuminate both swaths.
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