TerraSAR System Calibration

The ESA Earth Observation TerraSAR-L system features a L-band Synthetic Aperture Radar and strives for very high radiometric performance for spatial resolutions of 5 meters and swaths up to 200 km, in strip-map and ScanSAR operations, full-polarimetric capabilities, repeat-pass ScanSAR interferometry and a Wave Mode. Such levels of radiometric performance require the use of instrument calibration mechanisms. The rich capabilities of the L-SAR instrument in terms of antenna beams, polarisations and modes would make the traditional calibration approach, used for ERS and ENVISAT, a too expensive and time-consuming process. In addition, the L-SAR instrument is based on an 11 x 2.9 m active phase array antenna, with 160 transmit/receive modules arranged in 16 rows and 10 columns. With traditional calibration schemes, any deviation of the antenna characteristics, for instance by losing sub-arrays throughout the spacecraft lifetime, would cause significant degradation of the radiometric performance as these schemes would not be capable to correct the sensing data as efficiently as for the nominal fully operating system. Moreover, in order to maintain a comparable radiometric performance, a full recharacterisation of the on-board instrument should be carried out every time that a given number of sub-arrays change their characteristics. The experience gained thought the development and calibration of the ENVISAT ASAR and the lessons learnt during the commissioning period have been used to establish a novel calibration approach based on very accurate on-ground pre-launch characterisation data, a set of post-launch external measurements to be performed during the initial commissioning period, periodic in-flight internal characterisation data, and the internal calibration data to be taken during and together with the sensing data. This concept is conceived to achieve a very high radiometric quality but reducing as much as possible the in-flight characterisation, that requires deployment, maintenance and data collection of transponders, and the need of longlasting antenna beam characterisation using repetitive passes over the rainforest, for each antenna beam. THE TERRASAR-L MISSION ESA’s TerraSAR-L mission is designed for a lifetime of 5 years featuring a 14-day repeat cycle in a Sun-synchronous dawn-dusk orbit, with global imaging coverage, tight orbit control, high precision orbit determination and a 20-minute of high resolution and radiometric performance data per orbit [1]. These high radiometric performance requirements together with the operational concept and the systematic data-take approach of the mission, requires the implementation of a very effective calibration concept. TerraSAR-L calibration is intended to provide data with the information needed to achieve the required performance of radiometric accuracy better than 1dB (3σ), with a short in-flight commissioning time reduced to three months, and capable to cope with active antenna graceful degradation throughout the mission lifetime with the same radiometric performance. Moreover, the concept reduces the effort and cost of the in-flight characterisation that requires deployment, maintenance and data collection of transponders; and the need of long-lasting antenna beam characterisation using repetitive passes over the rainforest that, in the case of ASAR, required more than 5 good samples for each antenna beam, for 8 antenna beams and more than six months. The TerraSAR spacecraft is based on the novel Snapdragon configuration that simplifies the payload design and AIT. The main payload is an L-band SAR instrument based on an 11 x 2.9 m active phase array antenna with 160 transmit/receive sub-arrays laid down in 16 rows and 10 columns. The instrument operates Strip-map and ScanSAR modes, with full-polarimetric Strip-map capabilities, repeat-pass ScanSAR interferometry and a Wave Mode, with a bandwidth up to 85 MHz. Figure 1: Snapdragon Configuration CALIBRATION STRATEGY The end-to-end system calibration strategy is based on calibrating as much as possible pre-launch, which particularly applies to Instrument and Processor calibration, and: ! Validate rather than verify on ground and verify rather that validate in orbit ! Build a reliable and accurate characterisation data and implement internal monitoring of instrument variables in orbit Instrument Calibration Polarimetry Calibration Atmospheric Effects Spacecraft (AOCS, etc.) E2E System Calibration Processor Figure 2: TerraSAR-L Overall Calibration Obviously, the use external targets is necessary for atmospheric and polarimetric calibration. However, the effect from the instrument can be decoupled from the atmospheric and polarimetric uncertainties if it is characterised previously. The system calibration implementation is based on: ! on-ground pre-launch characterisation, ! post-launch external measurements during the initial commissioning period, ! periodic in-flight internal characterisation, ! and the internal calibration during and together with the sensing data. This calibration concept requires extra hardware to be built in the system, specific to routing the calibration signals through the instrument. This additional hardware consist of: ! Calibration network to be built on the antenna to loop back the internal calibration signals ! Auxiliary transmitter and receiver to cope with the non-operational levels of the calibration signals ! Switching hardware (switches, circulators, etc.) The calibration hardware introduces additional uncertainty to the calibration scheme, as it is also subjected to variations. This uncertainty is reduced by pre-launch characterisation of this hardware and monitoring the stability of the auxiliary transmitters and receivers by a specific calibration pulses. WHAT IS CALIBRATION? In-orbit calibration requires having the knowledge to the requested accuracy of the following parameters at the time of radar sensing for a data take: ! Antenna Radiation Patterns, ! Antenna Pointing (i.e. the reference frame of the radiation pattern), ! Absolute Gain (i.e. instrument and atmospheric effect) for absolute radiometry, ! Polarimetric characteristics (i.e. crosspolarisation and imbalance effects) ! Variations of these parameters during the data taking: The short-term Instrument Stability Tracking With the exception of the atmospheric effect on propagation loss and polarimetric imbalances, the rest of the parameters fall in the category of instrument calibration. The antenna radiation patterns and pointing are assumed to be stable in short term and are solely dependent on the subarray characteristics, T/R module status and a successful antenna deployment. The absolute gain is assumed to vary slightly throughout the data take because of temperature effects and limitations of temperature compensation schemes INSTRUMENT CALIBRATION Antenna Patterns The calibration of the antenna beams will be based on an Antenna Model that will be developed and validated during the on-ground pre-launch test activities. This Antenna Model will be built from accurate Embedded Sub-array Patterns and T/R module characterisation. This characterisation will be based on the Module Stepping tests to be taken under the same operating conditions and well correlated to the embedded sub-array tests. Once the latest updated T/R module data is provided, the antenna pattern of every beam is generated automatically by the Antenna Model when the corresponding beam coefficients are applied. Antenna Pointing The antenna pointing, i.e. the antenna boresight alignment with respect to the AOCS system of coordinates, is a consequence of the alignment of the various systems of coordinates during on-ground spacecraft integration, the in-orbit deployment of the instrument, and the radiating conditions of the phased array antenna. The Antenna Model will be built with respect to the system of coordinates of the on-ground deployed antenna and will be capable to determine the antenna pointing for every beam with respect to this system of coordinates. The spacecraft AIT alignment will provide the alignment of the antenna reference system to the AOCS system and, therefore, to the inflight attitude at any nominal mode during operations. The AOCS performance is defined: ! Knowledge of the orientation of the spacecraft up to an accuracy of 0.002o per axis, and ! AOCS orientation capability of 0.010o per axis For the TerraSAR-L snapdragon configuration, the in-orbit deployment may cause some deviations with respect to the assumed nominal antenna system of coordinates (e.g. planarity, etc.) that would affect the pointing of the antenna beams. In this TerraSAR-L specific case, this would, nevertheless, affect the azimuth but not the elevation pointing of the beams. Therefore, a different approach can be taken for elevation and azimuth pointing: Elevation The elevation pointing will be derived from the Antenna Model referred to the AOCS reference system. During commissioning the process of derivation of the antenna pointing will be verified by using the antenna pattern verification with repeat passes over the rainforest. Azimuth Similarly to the elevation pointing, the azimuth pointing will also be derived from the Antenna Model referred to the AOCS reference system, and also verified during the commissioning period. However, in order to consider the deviations from the nominal deployment and the planarity stability, a special in-orbit pointing calibration test might be performed periodically. This test would use Pointing Calibration Beams that would verify the assumptions from the Antenna Model and would account for the deployment deviations. This pointing calibration test would be based on the monopulse approach (i.e. by switching between a uniform-phase beam, sum, and a beam with the antenna longitudinal half turned 180 in phase, difference) over the rainforest. Antenna Gain The absolute instrument gain will be obtained from different sources from the pre-launch on-ground instrument tests to the in-orbit calibration tests. These various procedures will have different accuracies that will be correlated: a) Pre-launch antenna beam tests in the NF antenna range. The absolute gain will be measured together with the antenna beam patterns for the uniform-illuminated beam and some of the other beams. It is not expected to achieve an accuracy to the level of 0.1 dB, depending on the range gain calibration it would be expected of ± 0.2 or ± 0.3 dB b) Antenna Model. Module stepping results, T/R calibration data and central electronics delivered RF power can be combined in the Antenna Model to derive the expected radiated power. This will be correlated with the measured gain in the range. The overall accuracy will be assessed c) Transponder tests. During the commissioning period gain measurements will be performed by using calibrated transponders. This is expected to achieve a high accuracy and to be correlated to the pre-launch estimations d) Gain estimation with rainforest data to be done during the satellite lifetime Instrument Short-term Stability The variations of the instrument characteristics within every data take will be tracked with the Calibration Pulses. These calibration pulses will be routed through the signal and calibration paths for transmit and receive and the two polarisations. The calibration pulses will switch along the antenna sub-arrays at row level and will follow a Pulse Coded Calibration (PCC) scheme [2] that allows a continuous operation of all antenna sub-arrays simultaneously and a nominal load of the power supply units. ANTENNA MODEL The antenna model is the essential tool where the calibration concept is based. It will be capable of modelling the antenna radiating characteristics to a high order of accuracy, typically 0.1 dB of absolute gain and 0.02 dB of relative gain within the imaging swath. This high accuracy is given by a precise characterisation of each sub-array and a close monitoring of the transmit and receive characteristics of the T/R modules connected to them. Measurements of the antenna passive front end (Return loss, S11, and Insertion loss, S21) may be necessary to complete the Antenna Model. These measurements will be performed at tile-embedded radiator level: The validation of the Antenna Model will be done by correlating the results with Near-Field antenna tests of a uniformillumination test beam (all T/R modules at maximum gain and equal phase) and a limited selection of the operational beams (see below). A post-launch verification of the model will be performed during the commissioning period by using repeat passes over the rainforest. This verification can only be done for the part of the main beam that corresponds to the swath and can also be used to verify elevation pointing. Antenna Characterisation The main contribution to the Antenna Model and its accuracy are the Embedded Sub-array Test. It consists of the measurement of the radiation characteristics of each individual sub-array embedded in its location within the antenna final configuration. The interactions caused by the rest of the sub-arrays are present in the measurement in this set-up. Particular care must, therefore, be taken in order to terminate properly the inactive sub-arrays. Likewise, the radiation tests must be defined with the consideration of the whole antenna as the object to measure and not only the active sub-array. ( ) φ θ , h mn E Embedded Sub-array Pattern of sub-array (m,n) in horizontal polarisation ( ) φ θ , v mn E Embedded Sub-array Pattern of sub-array (m,n) in vertical polarisation In order not to repeat the NF range radiating test for the 160 sub-arrays and considering that the sub-arrays have a highly imbalanced radiation pattern (narrow-azimuth and broad-elevation beam) an alternative approach can be taken: The measurement can be performed at column level (i.e. all sub-arrays of the same column are activated and the rest are off) for azimuth, and row level (i.e. all sub-arrays of the same row are activated and the rest are off) for elevation, and the results combined for each sub-array. For sub-array (m,n): ( ) ( ) ( ) elevation h n row azimuth h m column h mn h mn E E K E φ θ φ θ φ θ , , , _ _ ⋅ ⋅ = for horizontal polarisation ( ) ( ) ( ) elevation v n row azimuth v m column v mn v mn E E K E φ θ φ θ φ θ , , , _ _ ⋅ ⋅ = for vertical polarisation