USO Clock Period Measurements

One of the critical elements of the Radar Altimeter systems is the USO (Ultra Stable Oscillator) as it provides the clock for the Altimetric Range Measurements. The variation in the USO clock period generates errors in the Range Measurements, and it is recommended that the end user apply a correction.

Users should be aware that starting from 2nd February 2010 (with processor v6.02L04 onwards) the Ultra Stable Oscillator (USO) clock period correction will be included in the RA-2 product and already applied. For this reason, the external USO clock period correction should not be applied anymore to the RA-2 products starting from absolute orbit 41443 and the generation of the external USO clock period correction files will be stopped.

Two sets of corrections are available:

• USO clock period measurements and correction before the USO Anomaly (before 1st February 2006)
• USO clock period measurements and correction after the USO Anomaly (from 1st February 2006 to 2nd February 2010)

The first set should be used for data processed with the Envisat Level 1B Instrument Processing Facility (IPF) versions up to V4.58,24th October 2005, which should be corrected for the USO clock period long term drift and bias.

The second set should be used for data processed with PF versions 5.02, 5.03 and 5.06, starting from 1st February 2006 up to 2nd February 2010 (orbit 41443). Data should be corrected for the USO clock period long term drift, bias and orbital variations due to the abnormal behavior of the USO Clock Period.

Data acquired from 24th October 2005 until 1st February 2006 should not be corrected given that the proper value of the USO clock period has been used within the L1b processing.

Click on the links above for complete descriptions of the measurements, corrections, and file-naming conventions.

USO clock period measurements and correction are available via HTTP (links at right) or FTP, at ftp://ftp.esrin.esa.it/pub/RA2_MWR/USO/auxdata/