Global measurement of terrestrial snow cover is critical to two of the NASA Earth Science focus areas: (1) climate variability and change and (2) water and energy cycle. For radar backscatter measurements, Ku-band frequencies, scattered mainly within the volume of the snowpack, are most suitable for the SWE (snow-water equivalent) measurements. To isolate the complex effects of different snowpack (density and snowgrain size), and underlying soil properties and to distinctly determine SWE, the space-based synthetic aperture radar (SAR) system will require a dual-frequency (13.4 and 17.2 GHz) and dual-polarization approach.
A transmit/receive (T/R) module was developed operating at Ku-band frequencies to enable the use of active electronic scanning phased-array antenna for wide-swath, high-resolution SAR imaging of terrestrial snow cover. The T/R module has an integrated calibrator, which compensates for all environmental- and time-related changes, and results in very stable power and amplitude characteristics. The module was designed to operate over the full frequency range of 13 to 18 GHz, although only the two frequencies, 13.4 GHz and 17.2 GHz, will be used in this SAR radar application. Each channel of the transmit module produces >4 W (35 dbm) over the operating bandwidth of 20 MHz. The stability requirements of <0.1 dB receive gain accuracy and <0.1 dB transmit power accuracy over a wide temperature range are achieved using a self-correction scheme, which does real-time amplitude calibration so that the module characteristics are continually corrected. All the calibration circuits are within the T/R module.
The timing and calibration sequence is stored in a control FPGA (field-programmable gate array) while an internal 128K×8bit high-speed RAM (random access memory) stores all the calibration values. The module was designed using advanced components and packaging techniques to achieve integration of the electronics in a 2×6.5×1-in. (5×17×2.5-cm) package. The module size allows 4 T/R modules to feed the 16×16-element subarray on an antenna panel. The T/R module contains four transmit channels and eight receive channels (horizontal and vertical polarizations). Each channel contains GaAs MMIC (monolithic microwave integrated circuit) amplifiers, a 5-bit phase shifter, and a programmable attenuator. To meet the compact size and maintain isolation between the channels, a two-sided module approach was adapted. The transmit side of the module houses the four transmit channels, the control FPGA, and the power regulators and logic circuitry. It also contains the 4-way stripline power splitter. The 4-dual channel receivers are packaged on the reverse side of the module along with the horizontal and vertical power combiners, and some logic and power conditioning circuits. The transmit and receive sides of the T/R module with the component identifications are shown in Figures 1 and 2.
The broadband, 4-channel, highly stable self calibrating T/R module is a useful building block for other radar and communication phased-array systems operating in this band.