Calibration

Internal calibration
   T/R modules temperature compensation
   Calibration loop
   Calibration pulses
   Internal calibration processing
External characterization
External calibration
Ground processing calibration

Internal calibration

This part consists of the following sections:

• T/R modules temperature compensation
• Calibration loop
• Calibration pulses
• Internal calibration processing

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.

T/R modules temperature compensation

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 T/R module

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.

Calibration Pulses

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 nominally coherent.
For each of the 32 rows, the antenna and the central electronics are characterized with 3 types of pulses. (Figure 2). Pulse P1 characterizes the transmit chain of the instrument.
However, since T/R modules of the 4 adjacent rows share the same power supply, the 10 modules of the 'wanted' row are set to their nominal phase and amplitude settings for pulse P1, while the phase of the modules of the 3 'unwanted' rows are set so that their combined contribution out of the calibration network is nominally zero. Thus, their interference to the measurement of the 'wanted' row is minimized.
A second type of transmit pulse, referred to as pulse P1A, is added, in order to characterize the residual parasitic contribution of the 3 unwanted rows during P1. During P1A, the 3 unwanted rows are set as for P1, and the previously wanted row is now switched off. Even though the load conditions on power supplies are not exactly representative, the small error introduced on the estimation of P1A can be considered as small enough to be neglected. The receive path of the instrument is also characterized with a so called pulse P2, but, on receive path, no variation is expected from power supply load variations, and a row by row characterization is possible.
The central electronics transmit and receive paths are included in both P1/P1A and P2 characterizations. It is therefore necessary to characterize the central electronics independently by the use of the internal pulse P3.

Internal calibration processing

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 cycle.
As well as providing internal calibration during the measurement modes, ASAR includes a special module stepping mode, in which individual T/R module characteristics can be measured. This mode can be used to investigate T/R modules failures and aging effects. In this mode, only one module is activated at a time, either on transmit or in receive.
The internal calibration scheme also includes measurements of the instrument noise level. The measurements are included in the initial calibration sequences, at the beginning of a mode. In the modes which have natural gaps in their imaging sequence (i.e., wide swath and global monitoring modes), noise measurements are also made during nominal operation throughout the mode.

External characterization

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.

External calibration

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.

For the wide swath mode using the scansar technique the external calibration approach is similar to the one used for the narrow swath mode.Fig: Transponder on ESTEC site for test purposes

Ground processing calibration

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.
As part of the Ground Processor Internal Calibration, the amplitude and phase of calibration pulses (P1, P1A, P2, P3) for each row are used. The amplitude and phase of P2 relative to P3 are calculated and P1A is vectorially substracted from P1 as discussed earlier.
External characterisation data and the derived amplitude and phase values for the 32 rows on transmit and receive are used to measure any deviation of the instrument reference gain pattern from its ground characterised value.
The replica of the transmitted pulse is calculated from the P1, P1A, P2 and P3 measurements, the ground characterised row patterns and the external characterisation data. The constructed replica tracks variations in all the transmit and receive circuits and is used to determine the range reference function for range compression processing.
The ground processor includes a Doppler Centroid Estimator with specified accuracy of 50 Hz for image and wave mode like ERS and 25 Hz in ScanSAR modes in order to control azimuth radiometric errors. The ASAR ground processor Functional Block Diagram is outlined in the following figure.

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