PAU/RAD: Design and Preliminary Calibration Results of a New L-Band Pseudo-Correlation Radiometer Concept

Abstract

The Passive Advanced Unit (PAU) for ocean monitoring is a new type of instrument that combines in a single receiver and without time multiplexing, a polarimetric pseudo-correlation microwave radiometer at L-band (PAU-RAD) and a GPS reflectometer (PAU-GNSS/R). These instruments in conjunction with an infra-red radiometer (PAU-IR) will respectively provide the sea surface temperature and the sea state information needed to accurately retrieve the sea surface salinity from the radiometric measurements. PAU will consist of an array of 4×4 receivers performing digital beamforming and polarization synthesis both for PAU-RAD and PAU-GNSS/R. A concept demonstrator of the PAU instrument with only one receiver has been implemented (PAU-One Receiver or PAU-OR). PAU-OR has been used to test and tune the calibration algorithms that will be applied to PAU. This work describes in detail PAU-OR's radiometer calibration algorithms and their performance.

Description and Performance of an L-Band Radiometer with Digital Beamforming

“…Actually, a residual standard deviation error of 2° for the phase and 0.1 dB for the amplitude were chosen as targets. These target values (2° and 0.1 dB) were already achieved in a preliminary work in which only one receiver was considered [11]. These are the specifications adopted for the whole hardware calibration performance.…”

“…A first analysis, using only one receiver demonstrator, of the PAU-RAD calibration algorithms and their performance was described in [11]. Then, the first results, analysis and digital beamforming antenna diagrams where presented in [17].…”

“…The second handicap concerns the interference that GPS signals can incur on the radiometric measurements. Although the spread-spectrum modulation GPS signal is lower than the noise level (−27 dB below the noise level), the impact of the GPS interference on the radiometer measurements has to be quantified [11]. For this purpose, two extreme scenarios are distinguished considering a sea surface reflection coefficient of Γ = 0.7, a height of h = 110 m (the same height as in [14]) and an antenna's effective area of A eff = 0.16 m 2 (D = 17.5 dB, where D stands for antenna directivity) for both cases.…”

“…The Passive Advanced Unit (PAU) for ocean monitoring [10] is an instrument that combines, in a single receiver and without time-multiplexing, an L-band microwave radiometer (PAU-RAD) [11] and a GPS-reflectometer (PAU-GNSS/R) [12][13][14] which, in conjunction with an infra-red radiometer (PAU-IR) [15], simultaneously provide the sea state information and surface temperature needed to accurately retrieve the SSS. Furthermore, PAU-RAD, as it has been designed, is a concept demonstrator ground-based instrument, which is expected to measure multi-angular incidence ocean brightness temperature from a terrain elevation, for instance a cliff.…”

“…This frequency plan simplifies signal processing operations since the digital frequency is 0.25. Digitized signals are properly low-pass filtered, I/Q demodulated and processed in an ALTERA Field Programmable Gate Array (FPGA), where the full digital radiometer is embedded [11] (described in VHDL-97 digital description language [18]). Finally, after data calibration and through data processing, values proportional to the Stokes parameters can be achieved as follows: The first and the second Stokes parameters are obtained adding and subtracting the results of the same polarization channels correlation (Figure 2).…”