Internal calibration uses calibration signals which are routed as closely as possible along the nominal signal path. The calibration signals experience the same gain and phase variations as the nominal measurement signals. The ground processing then evaluates the calibration signals to identify gain and phase changes and correct the acquired images accordingly.
Transmit power, receiver gain and antenna gain are subject to instrument noise due to temperature changes or other effects over time. Internal calibration provides corrections for changes in the transmit power and the electronics gain as well as validating the antenna model. The resulting calibration data are used in ground processing to correct image data.
Internal calibration also covers the signal phase. The overall phase of the echo signal depends on two major elements: measurement geometry and instrument internal phase stability. As the hardware cannot generally provide the required phase stability, it is a task of the internal calibration scheme to cover the internal phase variations by adequate measurements. All internal calibration measurements, either for gain or for phase, are used in ground processing to correct data products and achieve the required stability.
Internal calibration uses a Pulse-Coded Calibration (PCC) technique to embed a unique pulse code on a signal such that it can be identified and measured when embedded in other signals. This allows the amplitude and phase of individual signal paths to be measured while operating the complete antenna. The PCC technique is implemented by sending a series of coherent calibration pulses in parallel through the desired signal paths. The individual successive signals are multiplied by factors of +1 or -1. Factor -1 is implemented by adding a phase shift of 180°, while factor +1 means no additional phase. Each path is identified by a unique sequence.
The PCC technique can be applied if:
- the receiver detects the signals coherently
- the whole sequence is executed in a sufficiently short time such that the parameters to be measured are stationary
- the system is linear with respect to the individual signals.
The PCC technique can measure the signal paths via individual Transmit (TX) /Receive (RX) Modules (TRMs) or via groups of TRMs (either TX or RX paths, either polarisation).
The average properties of rows or columns of TRMs can be measured by a short PCC sequence. The length of a PCC sequence is always a power of two. There are 20 rows of waveguides, therefore the PCC sequence has a minimum of 32 pulses. Although the 14 columns (14 tiles) could be measured by a PCC sequence of 16 pulses, it is assumed that a sequence length of 32 pulses is also used. All 20 rows are operated together, meaning the antenna is in a full operational state. The overall signal from all rows is received, digitized and packed into calibration packets. These packets are evaluated (on the ground) to determine the properties of the individual rows. The approach for measuring the average azimuth excitation coefficient is similar to the elevation pattern, using columns of TRMs instead of rows.
The PCC-32 measurements described above need approximately 129 pulses. Additional warm-up pulses may also be needed. Such a large number of calibration pulses represent a significant interruption in image generation when operated within the image acquisition of the Stripmap mode. For intermediate calibration pulses in Stripmap mode, and also for calibration pulses related to each sub-swath measurement in the TopSAR modes, a shorter sequence is needed. The Sentinel-1 acquisition timelines foresee slots of 20 calibration pulses block wise interleaved within the imaging operation (before each burst for TopSAR modes). In order to derive the combined calibration signals 5 or more of these slots shall be processed together. Moreover, due to the high stability of measured internal calibration parameters, since May 2015 it has been decided to avoid interleaved calibration sequences in stripmap data, demanding them only to the pre and post-amble acquisition sequences.
For the antenna model, the reference patterns of all beams are derived for radiometric correction of the SAR data. The active antenna of the SAR instrument allows a multitude of different antenna beams with their associated gain patterns. All these patterns are described by the mathematical antenna model which provides the antenna patterns as functions of the commanded amplitudes and phases within the front end EFEs and within the tile amplifiers. The quality of the patterns is ensured by the on-board temperature compensation controlled by the tile control units. The internal calibration signals measure the actual phases and amplitudes and allow verifying the correct function and performance of all included elements. The antenna model is established on-ground, based on pattern tests at various integration levels up to the complete antenna.
RF Characterisation Mode
The RF characterisation mode is a self-standing mode and is not associated with the individual imaging data-takes. It is operated at least once per day during a convenient point within the long duration of wave mode.
The RF characterisation mode verifies in-flight the correct function and characteristics of the individual TRMs. Operating it two or more times at different temperatures during the cool-down phases between the high dissipating imaging modes can provide in-orbit characterisation versus temperature where necessary. The RF characterisation mode performs measurements with internal signals and is designed to achieve a number of goals. The RF calibration mode will:
- cover all those measurements needed in-orbit but which are not required for each individual data-take
- provide data sets to assess the instrument health and performance as far as possible
- verify the correct function of the individual TRMs, both within the front-end and the tile amplifiers
- verify the excitation coefficients for the TX and RX patterns to ensure the validity of the antenna model.
This mode is based on the same measurement types as the internal calibration. The mode has to address the individual TRMs while operating the full antenna in representative thermal conditions and with nominal power consumption. This can be achieved using the PCC technique. As a standalone mode, it is not forced to use the signal parameters of a dedicated imaging mode, but instead an optimised set of parameters can be used. The calibration mode is to be operated for both TX polarisations. The receiver will measure both polarisations in any case.