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3.2.2 Inflight Performance Verification

In-orbit beam calibration of an active phased-array antenna is a major task and it will include measurements in special modes, like module stepping and external characterisation, as well as acquisitions over rain forest. A sophisticated antenna model will combine the various data together with the pre-flight characterisation and provide the elevation and azimuth patterns for the ground processor.

Any drift in the gain and phase characteristics of the TRMs can distort the antenna beams. Deviations in antenna pattern and antenna gain will potentially contribute to radiometric errors in the SAR image. figure3.25 shows an overview of the various processes defined to maintain the calibration of the ASAR antenna beams.

ASAR Instrument Calibration Tasks
Figure 3.25 ASAR instrument calibration tasks

Monitoring any instrument gain drifts requires a separate calibration network, to couple out part of the transmit signal or to inject chirp signals into the receiver chain. This data will be included into the high-rate data stream and will be analysed by the ground processor in order to estimate the necessary gain drift corrections.

The absolute overall system gain can be most accurately determined from the image response of point targets with high and well-known RCS. ASAR high precision transponders will be deployed in the Netherlands and will serve as the main external calibration targets. A special transponder operation mode and well-characterised distributed targets will be used for the low-resolution Global Monitoring mode.

In the following sections the pre-flight measurements and the above methods for internal calibration, module stepping, external characterisation and rainforest measurements and absolute gain calibration will be explained. Furthermore the strategy for maintaining the ASAR antenna beams will be presented.

Preflight Characterisation Measurements


In order to provide the required image quality, the two-way antenna beam pattern should be known to a high degree of accuracy (0.1dB). The transmit and receive antenna patterns have been accurately measured on ground Ref. [3.5 ] for all eight beams (IS1-IS7 and SS1) and both horizontal and vertical polarisations. The radiating performances of the antenna were measured in the Large Planar Near Field Range at Astrium Ltd (Portsmouth). Full characterisation was performed: 8 beams, two modes (transmit and receive), two polarisations (V and H), in copolar and crosspolar components. Example patterns are shown in figure3.26 .

image
Figure 3.26 FM Elevation Beam Pattern Measurement

Internal Calibration


The ASAR instrument incorporates a very comprehensive system for internal calibration. There is an individual calibration path for each of the 320 transmit/receive modules. Internal calibration will be carried out on a row by row basis for each of the 32 rows. The calibration pulses are included in the instruments timeline during imaging and consist of the following (see also figure3.27 ):

- Transmit Calibration Pulses P1 (representative of T/R module load)
The T/R modules of the four adjacent rows in a tile share the same power supply. In order for the calibration sequence to be representative of the nominal operation, the ten modules of the selected row are set to their nominal phase and amplitude settings whilst the phase of the modules of the three rows sharing the same power supplies, are set so that their combined contribution out of the calibration network is nominally zero. Thereby minimising their interference to the measurement of the selected row.
- Transmit Calibration Pulse P1a
A second type of transmit pulse is added in order to characterise the residual parasitic contribution of the three unwanted rows during P1. During P1a, the three unwanted rows are set as for P1 and the previously wanted row is now switched off. Even though the load conditions on the power supplies are not exactly representative, the small error introduced into the estimation of P1a is negligible.
- Receive Calibration Pulse P2
The receive path of the instrument is also characterised but since no variation is expected from power supply load variations it is possible to characterise on a row by row basis.
- Central Electronics Calibration Pulse P3
The central electronics transmit and receive paths are included in the P1/P1a and P2 characterisations. The central electronics are therefore characterised independently by means of P3.

image
Figure 3.27 ASAR Internal Calibration Diagram

Using the amplitude and phase of the calibration pulses (P1/P1a, P2 and P3) for each row it is first necessary to calculate the amplitude and phase of P2 relative to P3 and to subtract P1a vectorially from P1. From these values for each of the 32 rows and together with the external characterisation factor, it is possible to calculate the elevation beam pattern. This is then used to detect any deviation to the reference instrument gain pattern as characterised on ground. The typical update rate for this calculation is 5 to 35 seconds (mode dependent).

A replica of the chirped pulse is calculated from a complete calibration row cycle using the P1/P1a, P2 and P3 measurements, the ground characterised row patterns and the external characterisation data. This is also typically updated every 5 to 35 seconds.

Despite the comprehensive nature of the internal calibration system, it is not possible to use it to calibrate the passive part of the antenna, which falls outside of the calibration loop. This is achieved through external characterisation by using the ground transponders.

Module Stepping


ASAR has a dedicated Module Stepping Mode, which is used to gather data from all 320 transmit/receive modules automatically. The entire procedure takes less than one second. The data are downloaded to the ground for processing. After processing, the results are compared with the reference data from on-ground tests in order to determine any TRM module gain or phase drifts, temperature behaviour and any eventual module failures. Using this information it is possible to implement any necessary correction to the TRM coefficients and eventually re-synthesise the antenna beam patterns if required.

External Characterisation


ASAR can be put into External Characterisation Mode while flying over a calibration transponder. This involves sending a series of pulses from each of the 32 rows in turn followed by each of the 10 columns in turn. These pulses are detected both by the internal calibration loop and the receiver embedded in the transponder (see figure3.28 ). Comparison of these data allows characterising the passive part of the antenna and the calibration network. The baseline is to repeat measurement every six months.

image
Figure 3.28 External Characterisation

Rain Forest


The reasons images of the Amazonian rain forest are used for the characterisation of the antenna beam pattern are that it is a stable, large-scale, isotropic distributed target with a relatively high backscatter and a well-understood relationship between backscatter and incidence angle.

In order to determine the two-way beam pattern, an uncorrected rain forest image is averaged in the azimuth direction. In the final processed image, the inverted beam pattern is applied and hence the effect of the pattern on the backscatter is removed.

Alternative distributed targets at different latitudes are being investigated. Promising results have been found from ERS data over Lake Vostok in Antarctica. Antenna pattern estimates at different latitudes could be used to verify the round-orbit performance of the ASAR.

Gain Calibration


The purpose of the ASAR gain calibration is to provide the users of ASAR data with the possibility to determine the absolute level of backscatter from any target, point (s) or distributed (s0). For ASAR this is achieved in fundamentally the same way as for ERS, namely by providing an Absolute gain Calibration Factor (ACF) in the header of the (processed) product. Since ASAR, however, has a total of eight beams and five different modes and up to four polarisations more ACFs will need to be determined for ASAR than for the ERS single beam, single polarisation with two modes.

The method to be used to determine the ACFs is to image a target of known radar cross-section, integrate the power in its Impulse Response Function (IRF) corrected for the associated background (clutter) power and hence calculate the correction (the ACF) which must be applied to the image values in order to arrive at the same cross-section for that target. For this purpose, precision calibration transponders are deployed in the Netherlands. The radar cross-section of these transponders is 65dBm 2 and is known to within ±0.13dB and they are stable to 0.08dB Ref. [3.6 ] . It is necessary to use active radar calibrators (transponders) as opposed to passive ones (e.g. corner reflectors) since the ratio of signal to clutter determines the accuracy to which the calibration can be made.

Once the ACF for a particular configuration has been calculated it will be possible to make a direct comparison with the on-ground measurements of the end-to-end system gain carried out during FM testing.


Global Monitoring Mode


This is a special case since the spatial resolution of 10001000m makes the normal use of the transponders unfeasible. For a reasonable calibration to be made (3s value of ± 0.5dB), a signal to clutter ratio of better than 30dB is required. If the clutter at the calibration sites typically has a sigma nought of -6dB then this would require a transponder RCS greater than 84dBm2. This would inevitably saturate the receiver invalidating the calibration. As a result, it is necessary to come up with an alternative scenario for calibrating this mode.

The first option (baseline) is using the other modes (namely Wide Swath and Image) to calibrate GM mode by means of the Amazonian rain forest. Since the sigma nought of the rain forest is stable to within 0.3dB it will be possible to use the sigma-nought value obtained from a previous (or subsequent) pass in WS or IM to calibrate GM mode. In addition, other relatively stable distributed targets may be used such as the ice caps and specific desert regions (Gibson, Gobi etc).

The second option will allow direct calibration using a special global monitoring mode setting and a modified calibration transponder. The intention is to use the ASAR's digital chirp generator to offset the centre frequency by 5MHz. This is possible since the chirp bandwidth in GM mode is only around 1MHz. In the calibration transponder, the received signal is shifted back by 5MHz allowing it to be received by the ASAR. As the clutter return will all be outside the range of the reduced bandwidth filter in GM mode, only the transponder response will be seen in the processed image against a background of noise. The result of this operation is to provide a transponder signal to clutter ratio of between 25 and 30dB allowing for reliable calibration of Global Monitoring Mode.


Calibration Transponder


Since the commissioning phase is planned to last only six months following switch-on of the instrument it is necessary to make use of every possibility of imaging the transponders during that period. Furthermore the placement of the transponders must be optimised to ensure that each of the seven image mode swaths can be acquired over the transponders sufficiently to allow for reliable calibration of each beam. Based on a detailed coverage analysis four locations in the Netherlands have been selected. Three transponders will be fixed. One mobile unit will be used for calibrating the Wave Mode and to support interferometric investigations during later phases of the ENVISAT mission.

Three precision calibration transponders have been developed by MPB (Canada) based on an ESA prototype and were delivered to ESTEC in April 2000 (figure3.29 ). Validation of the calibration performance will be supported by the four RADARSAT transponders deployed over a latitude range from 45-74o in Canada [3]. As RADARSAT operates at 5.3GHz, the actual RCS of these transponders at 5.331GHz will decrease by about 0.4dB.

image
Figure 3.29 Prototype ASAR Calibration Transponder Deployed at ESTEC

Beam Maintenance Strategy


The strategy for beam maintenance throughout the instrument lifetime is schematically shown in figure3.30 . Initially, the on-ground characterisation data will be used. TRM drifts occurring during the first years of operation will be detected during module stepping and compensated for, individually, by applying corresponding offsets, in order to bring the TRMs back to the initial conditions. Should compensation not be sufficient after TRM eventual failures, the antenna beam characterisation data will be updated in the ground processor with a new calculated antenna pattern. At the end of the instrument lifetime, beam optimisation by re-calculation of new sets of beam coefficients will also be possible.

The use of rain forest images, antenna synthesis software based on pre-flight embedded row tests, and eventually the actual near-field raw data acquired during the pre-flight beam characterisation will be used, in parallel, to verify any change.

image
Figure 3.30 ASAR Beam Maintenance Strategy

Conclusions


ASAR antenna beam calibration is based on combining several inputs: pre-flight characterisation measurements, internal calibration, module stepping and external characterisation, rain forest and transponder measurements. Absolute calibration will rely on ASAR precision transponders. RADARSAT transponders will support the verification of round-orbit calibration performance.

It is the intention to have the Image, Wave and Wide Swath modes calibrated within six months of launch and the all modes within nine months of launch.

REFERENCES
Ref 3.5
J-L. Suchail, C. Buck, A. Schnenberg, R. Torres, M. Zink, ESA/ESTEC. "The ASAR Instrument Verification: Results from the Pre-Flight Test Campaign". Proc. CEOS SAR Workshop April 2001, Tokyo, Japan.
Ref 3.6
H Jackson ESA/ESTEC, I. Sinclair, S. Tam. MPB Technologies Inc. "ENVISAT ASAR Precision Transponders". CEOS SAR Workshop 26-29 October 1999, ESA-SP450
Ref 3.7
R.K. Hawkins, et al. "RADARSAT Precision Transponders". Adv. Space Res. Vol. 19, No. 9, pp. 1455-1465, 1997.