NASA missions utilize active, passive, or both, microwave sounders with a large reflector antenna as an important component. In most of these applications, design engineers have realized that desirable science requirements (spatial and temporal resolutions) can be met only by compromising between conflicting engineering design parameters. A microwave remote sensor designed to achieve high spatial resolution would result in longer revisit time, yielding low temporal resolution and vice-versa. To overcome these conflicting requirements, the present technology advocates use of a cluster of feed horns arranged in the focal plane of the primary reflector antenna. Each feed horn produces a different footprint with appropriate overlaps covering a wide swath, allowing a high temporal resolution. Each feed horn, since they act independently, is designed to produce high spatial resolution. However, this approach has many disadvantages compared to an antenna system in which the cluster of horn feeds is made to act as a Focal Plane Array (FPA). Furthermore, the current approach does not enable maximization of the antenna gain, or immunity for radio frequency interference (RFI).
A Focal Plane Array (FPA) feed concept was developed for NASA’s Snow and Cold Land Processes (SCLP) Mission. The main objectives of SCLP are to provide global observations of Snow Water Equivalent (SWE) and snow wetness, with sufficient temporal and spatial resolutions (6-day repeat cycle and 100-meter or less spatial resolution) to reduce measurement uncertainty to levels enabling assessment of variability in fresh-water storage, and help improve predictive weather and climate models. High temporal and spatial resolutions are essential for estimating accurate temporal and spatial gradients of science parameters to study interconnectivity of Earth’s physical processes. This technology has the potential for achieving high spatial (<100 meters) as well as temporal resolution (3- to 5-day revisit time) simultaneously without increasing weight and volume of the microwave remote sensing instrument. The technology, when fully matured, will enable scientists to obtain quality science data at significantly reduced mission cost.
The proposed FPA feed also offers other advantages including a 15% increase in the antenna gain for steered beams with the same reflector antenna size, reduced side lobe levels, and RFI mitigation. The FPA system leverages successfully tested methodology used to design phased array feeds (PAFs) for astronomical applications.
Basic components of the FPA feed system are an array of wideband, primary, dual-polarized feeds placed in the focal plane of a large reflector antenna; dualband duplexer/multiplexer to transmit/ receive V-pol signals at 10- and 15-GHz frequencies; multiband duplexer to receive H-pol signals at 10-, 15-, 19-, and 37-GHz frequencies; and other standard components of microwave radars. The FPA excitation can be appropriately adjusted such that optimum illumination of the main reflector antenna is achieved. The optimum illumination allows the designer to achieve enhancement in the antenna gain and reduce side lobe levels. On receive, the FPA feed system allows users to process the received signal to suppress the RFI that may be present in the unknown direction.