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1.1.5 Peculiarities of RA2

The Radar Altimeter of second generation (RA-2) is an instrument for determining the two-way delay of the radar echo from the Earth's surface to a very high precision: less than a nanosecond. It also measures the power and the shape of the reflected radar pulses.

 

The RA-2 is derived from the ERS-1 and 2 Radar Altimeters, providing improved measurement performance and new capabilities.

 

The main objective of the RA-2 instrument is to collect, on global scale, radar echoes from ocean, land, ice without interruption. In the case of land regions this is a rather demanding task, since it implies design of a robust tracking algorithm for on board real time processing to avoid losing track.

 

This has been successfully met by the RA-2 design thanks to an innovative tracking algorithm developed in ALS known as Model Free Tracker (MFT). This algorithm does not perform precise estimate of radar to surface range, echo power or of other echo features, but concentrates its effort in maintaining the earliest part of radar echoes within the tracking window, independently of its shape. For this purpose, the tracking algorithm can adjust the radar resolution for the operations in Ku band among three values available, in order to optimally tune the width of the tracking window to the topographic features observed.

 

The following features are modified / improved in the RA-2 of ENVISAT-1:

 

  • Dual frequency instrument

RA-2 is a nadir-looking pulse limited radar functioning at the main nominal frequency of 13.575 GHz (Ku Band).

A secondary channel at a nominal frequency of 3.2 GHz (S band) is also operated to compensate the range error on altitude measurements caused by the propagation of the radar signals through the ionosphere.

 

  • Three different resolutions

The RA-2 is provided with three different bandwidths (320 MHz, 80 MHz, and 20 MHz, corresponding to resolution of about 0.5, 2 and 8 m, respectively) so that the resolution can be adapted to different scenarios (Ocean, ice, ice sheets, sea ice, and land).

 

  • Autonomous adaptative resolution

The change of the resolution is done autonomously by the instrument, through the so-called resolution selection logic, to adapt tracker performances to surface characteristics. Based on the Signal to Noise Ratio (RSN) of the on-board waveform and the waveform position (P) compared to reference values stored in the on-board memory, the MFT will decide whether the range window is using the adequate resolution, whether the resolution could be increased, or need to be decreased.

Further details on the complete functionality of the RA-2 on-board tracker and how its Resolution Selection Logic (RSL) works can be found in the Technical Note "The RA-2 On-board Tracker and its Autonomous Adaptable Resolution" PO-TN-ESA-RA-1316.

 

  • Robust tracking

The implementation of MFT in RA-2 provides very robust tracking over difficult surfaces as well as the capability to autonomously control the resolution

 

  • The new alpha-beta tracker

The only function of the RA-2 alpha-beta tracker is keeping the return waveform within the range window whereas the ERS tracker also had to estimate the engineering parameters. This estimation will now be done on ground instead of being done on board. Therefore the only function of the new tracker is the tracking itself and not also the estimation as it was performed in ERS. This fact extremely increases the robustness of the tracker. Another improvement on the alpha-beta tracker is the capability of compensating for the computation time lag.

 

  • Double dynamic range and two additional samples

RA-2 provides 128 samples of the waveform compared to the 64 samples of ERS. For extremely low SWH conditions, range and SWH estimates can be further improved by including in the precision processing the usage of two extra echo samples, computed by the on board digital processor through a standard DFT algorithm and made available for each Ku band radar echo down linked to ground. The location of the two additional waveform samples is programmable from ground so that it can be optimally tuned with respect to the position of the tracking point in the FFT bank. This is a unique feature of the RA-2 together with the possibility of transmitting to ground the In-phase and Quadrature components (i.e. raw data without any processing applied on board) of the echoes from 2000 consecutive Ku radar pulses. Experimental processing of these so called "individual echoes" on ground can provide more insights on surface topography at the boundaries.

 

  • Individual echoes mode

The RA-2 has the capability to provide limited bursts of individual, unaveraged echo sample data at the full rate for research purposes. In this concept the full-rate data are stored, for a short burst of about 1 sec, into an internal buffer memory, in parallel to the normal averaging and other functions of the instrument. The buffered data are subsequently read out at a much lower rate and appended to the normal science data. They will be available as a dedicated-data product. Users will be able to extract these data along with adjacent 18 Hz waveforms and geophysical corrections. Having easy access to the surrounding data is necessary for the research work on individual echoes.

 

  • MWR correction

RA-2 data will contain MWR corrections in all Level 2 products and MWR brightness temperature can be found in the Level 1b and level 2 MWR measurements data sets (MDS).

 

A summary of the main characteristics is reported in the following table.

 

Table 1.1

Instrument parameter

Range

Accuracy

Altitude

764 km to 825 km

< 4.5 cm (highest res.)

Backscatter Coefficient

-10 dB to +50 dB

< 0.4 dB (bias)

 

 

0.2 dB (residual)

Measurement Datation

 

100 μs (UTC)

Wave height

0.5 m to 20 m

< 5% or 0.25 m

Operating Frequency

13.575 GHz (Ku-Band)

 

 

3.2 GHz (S-Band)

 

Bandwidth

320 , 80 , 20 MHz and CW (Ku-Band)

 

 

160 MHz (S-Band)

 

Pulse Repetition Frequency

1795.33 Hz (for Ku-Band)

interleaved operation

 

448.83 Hz (for S-Band)

 

Pulse Width

20 μs

 

IF Bandwidth

6.4 MHz

 

Operation

continuously over a complete orbit

 

Data Rate

100 kb/s

 

Mass

110 kg

 

Power

161 W

 


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry