The objective of the ASAR instrument internal calibration scheme is to derive the instrument internal path transfer function, and to perform noise calibration. This objective is realised by dedicated calibration signal paths and special calibration pulses within the instrument for making the required calibration measurements and by using these measurements to perform corrections within the ground processor.
The T/R module amplitude and phase characteristics vary principally as a function of temperature. Therefore the instrument includes a scheme to compensate for drifts over temperature. This scheme provides the antenna with a high degree of stability; however, it does not compensate for aging effect or T/R module failures. Also, under conditions of rapid temperature variation, such as eclipse, the compensation performance may be degraded. Therefore, it is necessary to include the active antenna components within the calibration loop.
The instrument calibration loop is used to perform three distinct functions. Firstly, it is used to characterize the instrument transfer function during the measurement modes. Secondly, it is used to characterize individual T/R modules. Finally, it is used in the special external characterization mode.
The calibration loop in ASAR is in fact comprised of a distinct calibration path to each of the 320 T/R modules. This enables transmit pulses at each T/R module output to be sampled, and allows calibration pulses to be injected into the receiver front end of each T/R module. Effectively, the scheme provides a multi-pathed calibration loop that encompasses all the active electronics in the instrument transmit and receive paths. In particular, aging of T/R modules characteristics and T/R module failure can be sensed.
There is no active switching within this network in order to maximize its reliability and stability. The calibration distribution network acts as a combiner when the loop is being used to sense T/R module transmissions, and as a splitter when the loop is being used to inject pulses into the T/R module receivers. The antenna calibration port can be switched either to an auxiliary receiver or to an auxiliary transmitter, both of which are located within the instrument central electronics. These elements can be used to sense or inject calibration pulses at the antenna calibration port. The detailed use of the calibration loop is partly controlled by the operating states of the T/R modules themselves (i.e., ON/OFF, Tx/Rx, H/V), because there is no switching within the calibration network.
During normal operation in any of the ASAR
measurement modes, a sequence of calibration pulses is interleaved with the normal radar
pulses. These pulses characterize the active array, both on transmit and receive, on a row
by row basis (i.e. only 10 modules along one row are activated, while the 310 remaining
modules are off). For different pulses within the sequence, different rows are activated.
The rationale for row by row characterization is that ASAR is essentially an elevation
plane beam steering instrument. Thus, the amplitude and phase settings applied to the T/R
modules along a row are nominally uniform, and the calibration signals from them are
One consequence of row by row characterization is
that the instrument transfer function cannot be simply calculated from a few pulses, as
this was the case in the AMI SAR. Instead, the ground processor must utilize the
calibration pulses from a complete cycle through the 32 rows to estimate the transfer
function. Also, a replica pulse for the instrument must be calculated from a complete row
The internal calibration scheme monitors drifts in the transfer function of the majority of the instrument, excluding the passive part of the antenna, the calibration loop itself and the mechanical pointing of the antenna. As part of the overall calibration strategy to monitor these elements a dedicated mode of ASAR called External Characterisation Mode is used nominally every six months.
During this mode a sequence of pulses sent by each antenna row in turn is simultaneously sensed by the antenna calibration loop and recorded on ground by a special ground receiver built in the ASAR transponder.
From data recorded in the transponder and data down-linked from the instrument the relative phase and amplitude of the pulse from each row are compared in the ground processor. The relative amplitude and phase is used to characterise the row of radiating sub-arrays and the calibration path from the row.
The external calibration scheme with the objective to derive the overall calibration scaling factor uses the successful methodology developed for ERS-1/2 for the narrow swath mode .
Three specially built high precision transponders with a radar cross section high enough compared to background backscattering coefficient and noise are deployed across the ASAR swath and observed several times during every 35 days orbit cycle. Images acquired over suitable area of the amazon rain forest were be used to derive the in-flight elevation antenna pattern. Absolute calibration factors derived from transponder measurements and across swath correction derived from the radar equation were be used to calibrate the final image product.
As part of the processor Data Handling and
Reformatting I/Q science data are uncompressed and are subject to an I/Q correction (bias,
differential gains, non-orhogonality). Like ERS any non linearity correction may be
applied in the Ground Processor using pre-launch instrument ADC characterisation.
The ASAR ground processor Functional Block Diagram
Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry
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