Cal/Val Plan: Download Derivation of the Backscattering Coefficient Sigma0 in ESA ERS SAR PRI Products

Calibration Activities: The calibration plan is described in a paper on the performance of the ERS-1 SAR. This was presented at the CEOS SAR Calibration Workshop held in Tokyo, Japan from 2 to 5 April 2001.
Download the paper "The ERS-1 SAR Performance: A Final Update" its contents include:

• Image Quality Assessment
• Radiometric Calibration (Internal Calibration, ADC Power Loss Correction, Stability, Noise Equivalent Radar Cross-Section, Nominal Replica Pulse Products)
• Distributed Target Radar Cross-Section Derivation
• Point Target Radar Cross-Section Derivation
• ERS-2 Attitude and Doppler Variations

Cal/Val Plan: Download the technical note Derivation of the Backscattering Coefficient Sigma0 in ESA ERS SAR PRI Products

Calibration Activities: Activities performed based on the ERS-2 calibration plan are illustrated in a paper on the performance of the ERS-2 SAR. This was presented at the ESA Living Planet Symposium in 2010 held in Bergen, Norway.

Download the paper "The Performance of the ERS-2 SAR after 15 Years", its contents include:

• Elevation Antenna Pattern
• Quality Assessment
• Radiometric Calibration (Internal Calibration, Raw Data Quality, Stability, Noise Equivalent Radar Cross-Section, ADC Power Loss Correction, Nominal Replica Pulse Products)
• Distributed Target Radar Cross-Section Derivation
• Point Target Radar Cross-Section Derivation
• Attitude and Doppler Variations
• Image Localisation
• Processor Evolution
• Sampling Window Start Time Anomaly

The piloting of the ERS-1 and ERS-2 satellites was initially performed using three gyroscopes (out of six available), with the number of functioning gyroscopes reducing during the mission lifetime due to failures. SAR products were used to monitor the Doppler centroid frequency of imagery acquired by the ERS missions as shown in the figure below.

There were several occasions, during the early years of the ERS missions, 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.

Due to the continuing gyroscope failures, a mono-gyro (1GP) piloting mode began operation on 11 February 2000. A consequence of this change was a loss of attitude stability but the impact on SAR product quality was small. For the 1GP mode the Doppler Centroid frequency increased for localised areas and time intervals but values were mostly limited to the first ambiguity number (-1600Hz to 1600Hz). During this IGP period, new on-board software to pilot the satellite without using any gyroscopes by exploiting the capabilities of the Digital Earth Sensor and Digital Sun Sensor was developed and tested.

A first coarse version of this software, the Extra Backup mode (EBM), was uploaded to ERS-2 in December 2000 as a first step for gyro-less operations and was operationally used since mid-January 2001 due to further problems with the remaining gyroscopes. Significant degradation of the ERS-2 attitude (particularly for yaw) occurred during the EBM. Very large Doppler Centroid frequencies are difficult to estimate in SAR processor – particularly for values outside ±4500Hz with the ESA SAR processors.

Improvements in attitude control took place in June 2001 with the introduction of the Zero Gyro Mode (ZGM) and again in March 2002 with ZGM-Yaw Control Monitoring (ZGM-YCM) – 90% of the Doppler Centroid frequencies remained within ±4500Hz. A further refinement involved deriving yaw values in real-time from wave-mode data to automatically correct the instrument attitude during the next orbit pass if necessary. Some changes to the strategy were required after the failure of the on-board tape recorder in June 2003, since wave mode acquisitions all around the orbit could not be used any more. Using a network of receiving ground stations, it was possible to continue receiving wave-mode data although coverage was reduced to the northern hemisphere. This strategy was called Regional YCM (R-YCM).

Download 'Review of the Impact of ERS-2 Piloting Modes on the SAR Doppler Stability' (FRINGE 2003 Workshop) paper describing the gyro operational status and impact. This paper also provides more detail of the changes in the interferometric dead-band indicated below due to the impact of gyroscope failures.

A small number of products generated within the ESA ground segment (< 1%) are processed with a nominal replica pulse rather than a replica generated at the time of imaging (an extracted replica). This occurs when the extracted replica pulse is corrupt. Products generated with a nominal replica have significantly higher pixel values than products generated using an extracted replica. These products can be corrected. For ERS-2 SAR products, the correction depends on the extracted replica pulse power at the time of data acquisition. This can be estimated from the quarterly averaged values given in the table below.

For 2003 Q1, two corrections are given, one before 26 February and the other after 28 February due to the gain change between these dates (increase of the Image up-converter level by 2.5dB on 26 February 2003 and increase of the image receiver attenuation gain by 1dB on 28 February 2003). Replica pulse powers between 4 September 10:04:14 and 14 October 2004 14:37:11 UTC were increased by 4dB to correct for the erroneous calibration attenuation gain setting during this period. The correction factor is applied such that the image intensity values need to be reduced. Note that as the correction given in the table has been derived from replica pulses from the start of imaging sequences, no account is made for any variations in replica power between the start and end of imaging sequences.

Download the technical note ERS calibration derivation of the backscattering coefficient sigma nought in ESA ERS SAR PRI products for more information.