Conceptual view of SRAL altimeter on Sentinel-3 (Credits ESA)
Operating over oceans, SRAL measurements are used to determine the ocean topography, thus supporting research into ocean circulation, bathymetry and marine geoid characteristics. Furthermore, SRAL also facilitate operation in coastal and inland waters and the gauging of the flow of large rivers. Measurement of the radar echo power and shape enables wind speed and significant wave height at sea to be determined, thus supporting weather and sea state forecasting. SRAL has a strong heritage of the instrument techniques implemented for the Poseidon-3 altimeter on Jason-2 and for SIRAL (SAR Interferometer Radar Altimeter) on CryoSat.
The altimeter emits a radar beam that is reflected back to the antenna from the Earth’s surface (see how altimetry works for details). SRAL operates at two frequencies (13.575 GHz in the Ku-band and 5.41 GHz in the C-band) to determine atmospheric electron content, which affects the radar signal path delay. These two frequencies can also serve to measure the amount of rain in the atmosphere.
The SRAL instrument includes measurement modes, calibration modes and support modes. The measurement modes are composed of two radar modes associated to two tracking modes:
- LRM (Low Resolution Mode). It refers to the conventional altimeter pulse-limited resolution mode (so far, the LRM mode is being used on all altimetry missions). It consists of regular emission/reception sequences at a fixed PRF (Pulse Repetition Frequency) of around 1920 Hz leading to an ambiguity rank of 10.
- SAR mode: This is a high along-track resolution mode composed of bursts of Ku-band pulses.
These modes are associated to two tracking modes which consist of the following:
- Closed-loop mode: refers autonomous positioning of the range window (ensures autonomous tracking of the range and gain by means of tracking loop devices implemented in the instrument).
- Open-loop mode: refers to the positioning of the range window based on a-priori knowledge of the terrain height from existing high-resolution global digital elevation models.
The open-loop is intended to be used (on coastal ocean, sea ice ice sheet margins or rivers/lakes) instead of the more conventional closed-loop tracking over some surfaces, to improve the acquisitions over inhomogeneous or rough topography. While in open-loop, the setting of the tracking window of the altimeter is driven by predetermined commands, stored on board, combined with real-time navigation information available from the GNSS receiver. The main advantage is that the measurements are continuous, avoiding the data gaps typical of closed-loop tracking, which has problems in tracking the rapid topographic changes at coastal margins and in mountainous regions.
|Emitted Frequency (GHz)||Dual-frequency (Ku, C) – 13.575 and 5.41|
|Pulse Repetition Frequency (Hz)||1920 (Ku|
|Pulse duration (microseconds)||20|
|Bandwidth (MHz)||350 (Ku) – 320 (C)|
|Specific features||Dual-frequency for ionospheric correction|