Efficient Calibration of Active-Phased-Array SARs

Calibration of the first space-borne SAR instruments, with only one swath and based on passive antenna technologies, (e.g. ERS-1) required the in-flight characterisation of that single-swath instrument over well-known scatterers (reflectors, transponders and rainforest sensing data). Modern systems evolved to more access flexibility -therefore, multiple beams-, and multi-polarisation that led to the use of complex active phased-array antennas. In-orbit calibration of such complex SAR system can be a very time-consuming and expensive exercise that takes time off the spacecraft lifetime. The full characterisation on ground presents the additional risk of a limited validity if during the early phases of the in-flight operations a number of sub-arrays suffer degradation (i.e. failure of T/R module functions or irrecoverable phase and gain drifts). If this type of degradation occurs, an inflight re-characterisation of the instrument throughout the different beams (i.e. using repeating passes over the rain forest, etc.) is the only way to keep the required accuracy. After the lessons learnt during the ENVISAT ASAR commissioning, this paper present a new concept for SAR calibration built around a mathematical antenna model based on accurate on-ground measurement of the instrument, a set of post-launch external measurements to be performed during the initial commissioning period, periodic in-flight internal characterisation, and the internal calibration data to be taken during and together with the sensing data. Such an accurate antenna model is a very powerful tool both for pre-flight characterisation of all antenna beams, and for in-flight estimation of the actual patterns. 1 Radiometric Calibration The objective of the SAR radiometric calibration is the achievement of high radiometric performance (typically an overall radiometric accuracy better than 0.5 dB, 1σ). The radiometric uncertainties that calibration should compensate will in general come from: absolute signal and noise levels, relative variations across the field of view, and temporal variations along the operation. Therefore, calibration requires acquiring the knowledge -to the requested accuracyof the following parameters at the time of radar sensing: • Antenna Radiation Patterns, • Antenna Pointing (i.e. the reference frame of the radiation pattern), • Absolute Gain for absolute radiometry • Variations of these parameters during the data taking (i.e. the short-term Instrument Stability), and • Noise For an active antenna the radiation patterns and the antenna pointing are assumed to be stable in short term, i.e. they are dependent on the characteristics of the sub-arrays, their status, and the overall conditions of the antenna after deployment. The absolute gain is assumed to vary slightly throughout the data take because of temperature effects and limitations of temperature compensation schemes. 2 Calibration Concept Traditionally, the calibration of space-borne SAR systems (ERS-1, ERS-2, RADARSAT-1, etc.) has been considered as an in-orbit activity that relied on systematic radar acquisition over uniform-scattering scenes (e.g. rainforest) and required the deployment of well-known point targets (e.g. transponders, corner reflectors, etc). This concept worked very well for such single-mode instruments based on and highly stable passive antenna technologies. The calibration plan was defined independently from the system design and development, was executed during the system commissioning and supported the maintenance of the performance for the lifetime. Evolution to more flexible systems (i.e. multi-beam, multi-polarisation, azimuth scanning, etc.) led to the use of complex active phased-array antennas. Systems like ENVISAT ASAR applied the same calibration concept used for the ERS satellites resulting in a time-consuming and expensive exercise that takes time off the spacecraft lifetime and requires a periodic re-calibration because of the expected sub-arrays degradation (i.e. failure of T/R module functions or irrecoverable phase and gain drifts) in order to maintain the required accuracy. Figure 1 shows the calibration process related to the system design and development, including the characterisation tests that were defined for ENVISAT ASAR to support the system stability. Figure 1: Traditional Calibration Process New generation of space-borne SARs, also based on multibeam, multipolarimetry, azimuth scanning for spot-light and azimuth-sweep ScanSAR, are required to provide the sensing data in a much more operational basis and precise antenna beam control, for which time-consuming calibration and re-calibration activities are barely acceptable. The proposed system calibration concept incorporates the calibration process up to the system design and development in order to minimise the in-orbit calibration process and avoid the non-operational periods. It is based mainly on the use of on-ground pre-launch characterisation data and in-flight internal (i.e. not required external measurements) characterisation and calibration data. These will be complemented with a short set of post-launch external verification measurements to be performed during the initial commissioning period. This calibration concept requires that the system design is made suitable to the approach and that the characterisation of the critical elements is performed at their development. In particular: • An internal calibration network to be built in the antenna to loop back the internal calibration signals • Stable and well-calibrated transmitter and receiver functions to cope with the levels of the calibration signals • Stability requirements for the elements that will fall outside the internal calibration network (e.g. radiators, etc.) • Characterisation tests with the accuracy required by the calibration plan in order to keep the overall system performance • The development of Antenna Model that reproduces accurately the antenna radiation characteristics from the on-ground characterisation and the internal calibration data Design Build Assembly Integration Tests