RA-2 CAL/VAL

Cal/Val Plan

Calibration Activities

One of the components of the Envisat RA-2 sensor was an Intermediate Frequency (IF) Pass Band Filter. In theory the spectrum of the impulse response of such a filter should be a rectangle, but in reality this is not the case.

In order to compensate for the distortion on the RA-2 waveforms due to this component, the measurement of the filter impulse response was needed.

To retrieve the shape of the IF Filter impulse response spectrum, the RA-2 was commanded to operate in IF Calibration mode.

This involved measuring the noise power spectra by setting the instrument in listen-only mode (no power transmitted) and putting the receiving window delay to a value such that no echo from the surface could be received.

Flight test passes for IF calibration were made over both the Himalaya's and Rocky Mountains. The different IF Calibration strategies applied during the whole Envisat mission are summarised in the table below.

Cycle Date Himalayas Rocky Mountains
From beginning of mission to cycle 30 30 until to 04/10/2004 A -
From cycle 31 to 52 From 04/10/2004 to 13/11/2006 A D -
53 13/11/2006

A D 

The CTI_IFC table with Rx_Distance = 90 microsecond; start orbit #24604 at ANX=0

 

-
54 18/12/2006 A D CTI_IFC table with Rx_Distance = 100 microsecond; start orbit #25105 at ANX=0 -
55 22/01/2007

A D

CTI_IFC table with Rx_Distance = 81 microsecond; start orbit #25606 at ANX=0.

 

-
From cycle 56 to 61 From 
 
26/02/2007 to  
24/09/2007
A A D
From cycle 62 to 64 24/09/2007 A A D 
New procedure for IF calibration (through Digital BITE Mode command)
65 07/01/2008 A D 
New procedure for IF calibration (through Digital BITE Mode command) only over descending passes
-
From cycle 66 to end of mission 11/02/2008
New procedure for IF calibration (through Digital BITE Mode command)
-


In-flight Internal Calibration

The RA-2 Range and Sigma0 measurements were corrected to take into account the internal path delay and attenuation respectively. This was done by measuring those two variables in relation to the internal Point Target Response. The two correction factors were calculated during the L1b processing and directly applied. The Time Delay and the Sigma0 calibration factors were continuously monitored

Monitoring of correction factors on the S-Band stopped from cycle 65 onwards due to the loss of the S-band transmission power, which occurred on 17 January 2008.

 

Sigma0 Absolute Calibration

The transponder calibration was based on the comparison between the theoretical power at the output of the altimeter instrument when it captured the impulse emitted by the transponder, and the actual power detected by the instrument as produced by the RA-2 level 1b (L1B) processing software.

The Sigma0 absolute calibration of the RA-2 was performed using a reference target given by a transponder that was developed at ESA-ESTEC. This was exploited during the six month Commissioning phase to generate early calibration results. In order to consolidate the calibration results and to monitor the RA-2 calibration of Sigma0 during the Envisat lifetime, continual monitoring was accomplished by operating the transponder for as many Envisat overpasses as possible.


Cross Calibration

Download the RA-2 cross-calibration reports, which summarise cross-calibration between the Envisat, ERS-2 and Jason missions (Jason-1 from 5 June 2005 to 24 August 2009, and Jason-2 from 29 December 2009). The reports from were provided from the AVISO+ web portal.

Validation Activities

Download a synthesis of the error budget estimated for the whole ESA Envisat Altimetry Full Mission Reprocessing level 2 products. This takes into account the homogeneous reprocessed v3 data set, achieved in March 2018.

Altimeter v3 dataset verification documents

  • Envisat RA-2/MWR V3.0 reprocessing impact on ocean data - (to be released)

Altimeter v2.1 dataset verification documents

Envisat RA2 GDR quality assessment