The SRAL instrument comprises one nadir-looking antenna, externally mounted on the satellite's +Zs Earth panel, and a central electronic chain composed of a Digital Processing Unit (DPU) and a Radio Frequency Unit (RFU). The central electronic chain is mounted inside the satellite on the -Ys panel in a cold redundant configuration.
The main frequency used for range measurements is the Ku-band (13.575 GHz, bandwidth 350 MHz), while the C-band frequency (5.41 GHz, bandwidth 320 MHz) is used for ionospheric correction.
The SRAL instrument includes measurement, calibration and support modes. The measurement modes are composed of two radar modes combined with two tracking modes.
The two radar modes are:
- Low-Resolution Mode (LRM): conventional altimeter pulse-limited mode based on a 3 Ku / 1 C / 3 Ku pulse pattern
- SAR mode: high along track resolution mode composed of bursts of 64 Ku-band pulses surrounded by two C-band pulses.
The two tracking modes provided in the SRAL design are:
- closed-loop mode: autonomous positioning of the range window using the median algorithm
- open-loop mode: range window position based on a priori knowledge of terrain altitude derived from a Digital Elevation Model (DEM).
There are also two calibration modes (CAL1 and CAL2) that allow measurement of the internal instrument impulse response (by looping back a fraction of the transmit signal) and determination of the transfer function of the receive chain, derived from the acquisition and averaging of thousands of noise samples. The SAR mode provides an enhanced along track (azimuth) resolution of the order of 300 m.
Figure 1: SRAL Instrument (Credit: TAS-F)
Figure 2: SRAL Instrument (Credit: TAS-F)