Minimize RA2 Cal/Val
Minimize Calibration Activities

IF Calibration strategy

One of the components of the Envisat RA-2 sensor is 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 RA2 waveforms due to this component, the measurement of the filter impulse response is needed.
 

To retrieve the shape of the IF Filter impulse response spectrum, the RA2 is commanded to operate in the so-called IF Calibration mode.
This consists of the measurement of 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.
 

The different IF Calibration strategies applied during the whole ENVISAT mission are summarized in the table below.

Cycle Date Himalaya Rocky Mountains
From beginning of mission to cycle 30 Up to 04 Oct 2004 A -
From cycle 31 to 52 From 4 Oct 2004 to 13 Nov 2006 A D -
53 13 Nov 2006 A D

 

The CTI_IFC table with Rx_Distance = 90 microsecond; start orbit #24604 at ANX=0.
-
54 18 Dec 2006 A D CTI_IFC table with Rx_Distance = 100 microsecond; start orbit #25105 at ANX=0 -
55 22 Jan 2007 A D

 

CTI_IFC table with Rx_Distance = 81 microsecond; start orbit #25606 at ANX=0.
-
From cycle 56 to 61 From

 

26 Feb 2007 to

 

24 Sep 2007
A A D
From cycle 62 to 64 24 Sep 2007 A A D

New procedure for IF calibration (through Digital BITE Mode command)

65 07 Jan 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 Feb 2008 A

New procedure for IF calibration (through Digital BITE Mode command)

-

 

 

 

 

In-flight Internal Calibration
 

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

 

To be noted that the monitoring of correction factors on S-Band is stopped from cycle 65 onwards due to the loss of the S-band transmission power, which occurred on 17th January 2008.

 
?0 Absolute Calibration
 

The transponder calibration is based on the comparison between the theoretical power at the output of the altimeter instrument when it captures 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 ?0 absolute calibration of the RA-2 is performed using a reference target given by a transponder that has been developed at ESA-ESTEC. This was exploited during the 6 month Commissioning phase to generate early calibration results. In order to consolidate the calibration results and to monitor the RA-2 calibration of ?° during the Envisat lifetime, continual monitoring was accomplished by operating the transponder for as many Envisat overpasses as possible.

 

Minimize Cross Calibration
The Envisat RA2 cross-calibration reports summarize cross-calibration between Envisat, ERS-2 and Jason missions (Jasson-1 from 2005/06/05 to 2009/08/24, Jason-2 from 2009/12/29); from AVISO+ web portal.