Sp-461 ers-envisat Symposium

PERFORMANCES AND BUDGETS

The inherent principle of the Synthetic Aperture Radar impedes the direct measurement of the ASAR instrument

performances on Ground. The selected alternative is the computation of the instrument performance characteristics from

measurable lower level parameters throughout the various stages of instrument tests. These parameters are called

Verification Input Parameters (VIPs). For most of the VIPs, tests at instrument level are not possible and, in order to

measure them with high accuracy, they are measured at subassembly level.

From the VIPs and a certain number of assumptions and physical constraints, the ASAR high level performance

parameters, called Engineering Parameters (EPs), are computed by using a Performance Evaluation and Analysis

Software (PEAS).

During the on-Ground test campaign, a certain number of Characterisation Parameters (CPs) are also measured or

computed from lower level test results. The CPs will be used by the Ground Segment to produce the final data products

within the required specifications.

Finally, a limited set of additional parameters will be acquired during the in-flight external calibration and

Characterisation and provided to de Ground Segment.

Based on the VIPs measured at FM subassembly level, the predicted end-of-life performances of the ASAR instrument

in terms of Spatial Resolution, Ambiguity Ratios, Noise equivalent 

σ

0

 and Radiometric Resolution, Accuracy and

Stability are summarised in Table I. They are to be within the agreed specification. In most of cases, they present a large

margin with respect to it. Nevertheless, an area of some concern remains in the Distributed Target Range Ambiguity

Ratio of IS3 and, to a lesser extent, the farther swath. It should, however, be born in mind that the 

σ

0

 model for land and

see/ice used for the computation of the ASAR performances is very severe, making the specification extremely difficult

to achieve when Near-in Range Ambiguities are present due to geometry.