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This paper describes the receiver that has been designed for the PAU instrument [1]. The design's main challenge has been the integration in a single 7 cm x 11 cm x 3 cm box of a receiving unit consisting of 4 channels (two per polarization) and merging two different sub-systems: an L-band radiometer (PAU-RAD) and a GNSS-Reflectometer (PAU-GNSS-R)...
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... at the device input by mean of a variable resistor. With this method it can only be measured a temperature range from 307 K to 400 K because of the thermal noise introduced by the variable resistor, instead of the expected normal range from 6 K to 400 K. At last, the measured sensibility is 109.2 µV/K. Fig. 9 shows the scheme assembly used, and Fig. 10 shows different responses for each input stimulus. Figure 9. Schematic to test receiver's sensibility. Figure 10. Output responses for each input ...
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... Once the direct and reflected signals have been collected, they are amplified, filtered, down-converted, and sampled. The receiver used in griPAU is the one that has been developed in the frame of the PAU project (Ramos-Perez et al., 2006). This receiver has two chains with 120 dB gain, 2.2 MHz bandwidth, and the output signal is centered at an intermediate frequency of 4.309 MHz, and the sampling frequency is 5.745 MHz. ...
In recent years Global Navigation Satellite System’s signals Reflectometry (GNSS-R) has stood as a potential powerful remote sensing technique to derive scientifically relevant geophysical parameters such as ocean altimetry, sea state or soil moisture. This has brought out the need of designing and implementing appropriate receivers in order to track and process this kind of signals in real-time to avoid the storage of huge volumes of raw data. This paper presents the architecture and performance of the Global Positioning System (GPS) Reflectometer Instrument for PAU (griPAU), a real-time high resolution Delay-Doppler Map reflectometer, operating at the GPS L1 frequency with the C/A codes. The griPAU instrument computes 24 × 32 complex points DDMs with configurable resolution (ΔfDmin = 20 Hz, Δτmin = 0.05 chips) and selectable coherent (minimum = 1 ms, maximum = 100 ms for correlation loss Δρ < 90%) and incoherent integration times (minimum of one coherent integration period and maximum not limited but typically <1 s). A high sensitivity (DDM peak relative error = 0.9% and DDM volume relative error = 0.03% @ Ti = 1 s) and stability (Δρ/Δt = −1 s−1) have been achieved by means of advanced digital design techniques.
... And, 4) the number quantization of bits. Without lost of generality the described algorithms are tested using a PAU receiver [17] with the following parameters: gain G = 112 dB, noise figure The power level is adjusted with the frequency synthesizer. To minimize receiver's noise, 200 consecutive PRN sequences are averaged, i.e. the integration time is T i = 200 PRN . ...
The calibration of correlation radiometers, and particularly aperture synthesis interferometric radiometers, is a critical issue to ensure their performance. Current calibration techniques are based on the measurement of the cross-correlation of receivers' outputs when injecting noise from a common noise source requiring a very stable distribution network. For large interferometric radiometers this centralized noise injection approach is very complex from the point of view of mass, volume and phase/amplitude equalization. Distributed noise injection techniques have been proposed as a feasible alternative, but are unable to correct for the so-called "baseline errors" associated with the particular pair of receivers forming the baseline. In this work it is proposed the use of centralized Pseudo-Random Noise (PRN) signals to calibrate correlation radiometers. PRNs are sequences of symbols with a long repetition period that have a flat spectrum over a bandwidth which is determined by the symbol rate. Since their spectrum resembles that of thermal noise, they can be used to calibrate correlation radiometers. At the same time, since these sequences are deterministic, new calibration schemes can be envisaged, such as the correlation of each receiver's output with a baseband local replica of the PRN sequence, as well as new distribution schemes of calibration signals. This work analyzes the general requirements and performance of using PRN sequences for the calibration of microwave correlation radiometers, and particularizes the study to a potential implementation in a large aperture synthesis radiometer using an optical distribution network.
... Each element has a dual-polarization antenna (horizontal and vertical) and each polarization is thereafter divided in two using a Wilkinson power splitter. The receiver topology is described in detail in [3]: The input signals are demodulated at an intermediate frequency (IF) of 4.309 MHz and have a 2.2 ...
In ESA's SMOS mission it is expected to improve the sea surface salinity retrieval using multi-angular information to correct for sea state effects. A multi-beam, digitally steered radiometer with polarization synthesis has been proposed for the first time as a part of the PAU instrument (Passive Advanced Unit for the ocean monitoring project). This paper provides theoretical and experimental results of extending the calibration algorithms of a digital radiometer obtained in to any number of receivers in order to achieve the digital beamforming.
... PAU [12] will retrieve the SSS from radiometer outputs (Stokes parameters) and correcting the sea state effect using information from the reflectometer ( ) and from the IR-radiometer (SST). PAU-RAD consists of an array of 4x4 dual polarization receivers integrated behind a patch antenna [13], whose outputs are digitized, and then properly calibrated and combined to produce several beams using a digital beamformer [14]. In this implementation the PAU-RAD operates at the GPS L1 frequency, sharing the RF-IF front-end with the PAU-GNSS/R. ...
... A detailed noise wave analysis of this novel topology is described in [12]. The receiver topology is described in detail in [13]: The input signals are demodulated at an intermediate frequency (IF) of 4.309 MHz and have a 2.2 MHz bandwidth. The analog signals are then digitalized at 8 bits at a sampling frequency of 5.745 MHz. ...
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 4x4 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.
... II. PAU INSTRUMENT CONCEPT PAU is a new type of instrument, which is a hybrid of: • an L-band radiometer formed by an array of 4 × 4 dual polarization (vertical and horizontal) receivers [10] integrated behind a patch antenna, whose outputs are digitized and then properly calibrated and combined to produce several beams using a digital beamformer [11], • a GPS-reflectometer that uses the same hardware as the Lband radiometer, but the digital beamformer synthesizes beams pointing towards the specular reflection points of the GPS signals [12] while synthesizing a LHCP or a RHCP, and • a pair of commercial 8-14 µm thermal IR radiometers to measure the atmospheric downwelling radiance and the upwelling radiance from the sea surface + atmosphere. ...
It is generally accepted that the best way to retrieve sea surface salinity (SSS) is by means of L-band radiometry (1400-1427 MHz). However, in addition to the polarization and the incidence angle, the SSS and the sea surface temperature (SST), the sea surface brightness temperature depends on the sea state. This work describes a hybrid L-band radiometer & GPS L1 reflectometer proposed to infer sea state information at similar surface roughness scales and therefore improve the sea state correction in surface salinity estimates.
... The L-band radiometer is an array of 4 × 4 dual-polarization receivers [24] integrated behind a patch antenna, whose outputs 0196-2892/$25.00 © 2007 IEEE are digitized and then properly calibrated and combined to produce several beams using a digital beamformer (DBF) [25]. The GPS-reflectometer uses the same hardware as the L-band radiometer, but the DBF synthesizes beams pointing toward the specular reflection points of the GPS signals [26]. ...
Sea surface salinity can be remotely measured by means of L-band microwave radiometry. However, the brightness temperature also depends on the sea surface temperature and on the sea state, which is probably today one of the driving factors in the salinity retrieval error budgets of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission and the NASA-Comision Nacional de Actividades Espaciales Aquarius/SAC-D mission. This paper describes the Passive Advanced Unit (PAU) for ocean monitoring. PAU combines in a single instrument three different sensors: an L-band radiometer with digital beamforming (DBF) (PAU-RAD) to measure the brightness temperature of the sea at different incidence angles simultaneously, a global positioning system (GPS) reflectometer [PAU-reflectometer of Global Navigation Satellite Signals (GNSS-R)] also with DBF to measure the sea state from the delay-Doppler maps, and two infrared radiometers to provide sea surface temperature estimates. The key characteristic of this instrument is that both PAU-RAD and the PAU-GNSS/R share completely the RF/IF front-end, and analog-to-digital converters. Since in order to track the GPS-reflected signal, it is not possible to chop the antenna signal as in a Dicke radiometer, a new radiometer topology has been devised which makes uses of two receiving chains and a correlator, which has the additional advantage that both PAU-RAD and PAU-GNSS/R can be operated continuously and simultaneously to perform the sea-state corrections of the brightness temperature. This paper presents the main characteristics of the different PAU subsystems, and analyzes in detail the PAU-radiometer concept.
... The L-band radiometer is an array of 4 x 4 dual polarization receivers [22] integrated behind a patch antenna, whose outputs are digitized and then properly calibrated and combined to produce several beams using a digital beamformer [23]. The GPS-reflectometer uses the same hardware as the L-band radiometer, but the digital beamformer synthesizes beams pointing towards the specular reflection points of the GPS signals [24]. ...
... shows the circuitry which has been split in two: on the left there are the power supplies and thermal controller, while on the right there are the ADC, the two FPGAs and the CNS. The analog receiver [22] is electrically and thermally isolated in a separated box (Fig. 5). On the back of this box there are several connectors (USB, Ethernet) to send the processed data to a host computer, where it is stored and/or further processed. ...
... Each element has a dual-polarization antenna (horizontal and vertical) and each polarization is thereafter divided in two using a Wilkinson power splitter in a new radiometer topology [1], thus having at 64 (16x2x2=64) input channels. In [2] the receiver topology is described in detail: the input signals are demodulated at an intermediate frequency (IF) of 4.309 MHz and have a 2.6 MHz bandwidth. The sampling frequency is 5.745 MHz, higher than the IF, and the " band-pass sampling " [3] technique produces the PAU-RAD input signals to be centered at 0.25 times the digital frequency, which simplifies the I/Q demodulation. ...
This paper presents the calibration of the PAU-RAD instrument: a novel pseudo-correlation radiometer with digital beamforming, it consist on a Wilkinson power splitter and two receiving chains whose outputs are cross-correlated. To types of calibration are required: an internal hardware, relative calibration to perform the phase and an amplitude correction, and internal radiometric calibration. The simplified and unified calibration procedure is presented using internal well-known temperatures.
... At the moment this reflectometer is in its implementation stage. It has been tested off-line with real data gathered from one of the PAU receivers [9] , and the first realtime field measurements are expected to be available in the near future. ...
The use of global navigation satellite systems (GNSS) reflected signals has been pinpointed as a promising technique for the retrieval of geophysical parameters. This article describes the implementation of a GPS reflectometer to obtain real-time delay-Doppler maps (DDM) to infer sea-state data, needed to correct the L-band brightness temperature and measure sea salinity. This instrument will be based on a FPGA-embedded system, and is part of the PAU concept.
... This article presents and describes the PAU instrument concept at system level and the predicted performance in terms of angular resolution and radiometric sensitivity. Three companion articles [22] [23] [24] describe in detail the receiver topology, the implementation of the digital beam-former and the calibration strategy, and the implementation of a real-time Doppler-Delay Map (DDM) generator. ...
... It consists of a suite of three instruments operating synergetically: 1) PAU-RAD: an Lband radiometer to measure the brightness temperature of the sea surface, 2) PAU-GNSS-R: a GPS-reflectometer to measure the sea state, and 3) and PAU-IR: and IR radiometer to measure the sea surface temperature. The L-band radiometer is an array of 4 x 4 dual polarization receivers [22] integrated behind a patch antenna, whose outputs are digitized and then properly calibrated and combined to produce several beams using a digital beamformer [23]. The GPS-reflectometer uses the same hardware as the L-band radiometer, but the digital beamformer synthesizes beams pointing towards the specular reflection points of the GPS signals [24]. ...
... This article presents and describes the PAU instrument concept at system level and the predicted performance in terms of angular resolution and radiometric sensitivity. Three companion articles222324 describe in detail the receiver topology, the implementation of the digital beam-former and the calibration strategy, and the implementation of a real-time Doppler-Delay Map (DDM) generator. ...
Sea surface salinity can be remotely measured by means of L-band microwave radiometry. However, the brightness temperature also depends on the sea surface temperature and on the sea state, which is probably today one of the driving factors in the salinity retrieval error budgets of SMOS and Aquarius/SAC-D mission. This work describes the architecture design of the Passive Advanced Unit (PAU) for Ocean Monitoring, its subsystems and its main characteristics. PAU combines in a single instrument three different sensors: an L-band radiometer with digital beamforming to measure the brightness temperature of the sea, a GPS-reflectometer also with digital beamforming to measure sea state, and an infrared radiometer to provide sea surface temperature estimates. The key characteristic of this instrument is the fact that both the L-band radiometer and the GPS-reflectometer share completely the RF front-end, and to be able to track the GPS-reflected signal is not possible to chop the antenna signal as in a Dicke radiometer, therefore a new radiometer topology has been devised.
