New Antenna for the SWOT Satellite Allows for High Data Rate Communication

Wednesday, May 6, 2015

NASA's Surface Water and Ocean Topography (SWOT) satellite is set to make a first-ever global survey of Earth's surface water by collecting detailed measurements of how bodies of water change over time. The satellite will survey at least 90 percent of the globe, studying Earth's lakes, rivers, reservoirs, and oceans at least twice every 21 days. The data collected by these surveys could improve ocean circulation models and weather and climate predictions, while aiding in freshwater management around the world.
 

NADIR deck
Top view of a simplified model of SWOT’s NADIR deck including a 1.0m-diameter radiometer and a 1.2m-diameter altimeter.

Taking such frequent and extensive surveys of Earth’s bodies of water requires SWOT to communicate a large amount of data to ground stations. SWOT will be capable of collecting and transferring more than 7.2 terabits (Tb) of science data per day. This requires a downlink data rate of 620 megabits per second (Mbps) to deliver this volume in the brief time intervals when SWOT is in view of a ground station.

The 620 Mbps downlink data rate could be supported if the satellite had a steerable high-gain antenna with a gimbal capable of continuously pointing toward the ground station. However, using a gimbal generates vibrations that would degrade the accuracy of one of SWOT’s instruments, the Ka-band Radar Interferometer (KARIN). Instead, a low-gain antenna (LGA) with a broad-beam radiation pattern is necessary to provide maximum contact time with ground stations. An isoflux radiation pattern allows the antenna to cover the full visible Earth and to compensate for range attenuation.

Using an LGA at the 8.025-8.400 gigahertz (GHz) band allocated for Earth-exploration satellite service, the data will need to be divided into two channels of 310 Mbps to satisfy the 620 Mbps downlink data rate and the allocated microwave band. The two channels are transmitted using different polarizations: right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP).

In order to use the LGA as a viable option, self-interference issues needed to be addressed. The first type of inference that needs to be accounted for is referred to as polarization interference. If SWOT uses two antennas with different polarization (RHCP and LHCP), undesired polarization from one antenna will generate interference to the other one. An antenna with a high cross-polarization discrimination (XPD) will receive an insignificant amount of undesired polarization. The SWOT telecom antenna needed to demonstrate high XPD (~25dB) to mitigate this interference, which is very difficult across the large frequency band (8.025-8.400 GHz) and especially at an angle of ± 60 degrees from antenna boresight. No commercially available antennas met such stringent requirements.
 

prototype
Antenna prototype model of the ECH antenna.
ECH antennta
Nacer Chahat inspects the ECH antenna before measuring its performance in the JPL’s anechoic chamber.

The nadir deck of the SWOT satellite is a rich environment with a 1-meter aperture radiometer reflector antenna, a 1.2-meter altimeter reflector antenna, a Doppler orbitography and radiopositionning integrated by satellite (DORIS) antenna, and a laser reflector array (LRA). These antennas generate the second type of interference -- multipath interference. This occurs when one antenna is radiating RHCP, for instance, power will be reflected by the large reflectors and becomes LHCP.

To mitigate these interference issues, Nacer Chahat and colleagues at the NASA’s Jet Propulsion Laboratory, Pasadena, Calif. developed a new antenna design that would minimize both the cross-polarization interference and the multipath interference. “We had to develop an antenna meeting very stringent requirements to minimize cross-polarization and multipath interferences,” says Chahat. “In addition to that, we developed an OMT polarizer that could generate RHCP and LHCP on the same antenna. This would allow us to use only one antenna instead of two antennas and therefore reduce the weight of the satellite. Of course, this polarizer had to present outstanding performance across the operating frequency band to maintain the antenna performance.”

Between October and January over 200 designs were iterated before a prototype was built and tested to meet requirements. This new antenna the team developed is called the Externally Corrugated Horn (ECH) antenna. The name makes reference to Chahat's Early Career Hire status at the Jet Propulsion Laboratory, as he received his doctorate two years ago.

The antenna prototype was optimized, fabricated, and measured at JPL in four months. The calculated performance was validated with measurements.

For more information about SWOT, visit http://swot.jpl.nasa.gov