Activities performed based on the ERS-1 calibration plan are illustrated in the most recent paper on the performance of the ERS-1 SAR. This was presentated at the CEOS SAR Calibration Workshop held in Tokyo, Japan on 2-5 April 2001. The paper "The ERS-1 SAR Performance: A Final Update" can be found here and its contents are:
Doppler Centroid information
The piloting of the ERS-1 satellite throughout its lifetime was performed using three gyroscopes. SAR products wereused to monitor the Doppler centroid frequency of imagery acquired by ERS-1 as shown in the figure below. It can be seen that there were several occasions, when there was a larger than expected change in Doppler centroid frequency. This was due, primarily, to imagery acquired during orbit manoeuvres (imagery acquired during a manoeuvre can still be processed but it may be unsuitable for interferometry). The negative Doppler Centroid frequencies are from ascending pass imagery while positive values are from descending pass imagery. Each product header contains the Doppler information for the product.
Subset of ERS-1 Doppler Centroid Frequency over the mission lifetime.