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The Radar Altimeter on board Envisat has shown, since February 2006, an anomalous behaviour concerning the Ultra Stable Oscillator (USO) clock period, needed to obtain an exact datation and range for the measurements. The anomaly, consisting of a constant offset of about 5.6 metres and orbital oscillations in the Envisat RA-2 range values, has hindered any geophysical application by the scientific community since then. The investigation of the problem has led to a predictive model and new corrective algorithms that have been implemented on-ground in the form of external files containing the correction to be applied to the ranges by users, at a 1 s rate.


Another correction, applied directly on the L1b products, and completely transparent to users, has been implemented at ESRIN and used for some pilot projects (i.e. River&Lakes and RAIES).



Starting in February 2006, the RA2 ranges have been biased significantly for several weeks (part of cycle 44 and 45 and most of cycle 46) due to an offset in the Ultra Stable Oscillator clock frequency. In all cases these USO frequency anomalies appeared after an instrument reset. When the anomaly occurred the USO period increased rapidly and then started to oscillate (see fig. below).


USO Tx/Rx clock values, wrt nominal, during the latest anomaly period.


In fact two different problems could be individuated:


- a long term variation (USO drift or mean bias) that could be accounted for by external auxiliary USO files produced by the IECF

- a short tem variation (additional 1-cpr non harmonic variation around the orbit) that could be accounted for inside the Ground Segment only with an algorithmic upgrade.


The USO anomaly was first detected on the data of 1st February 2006, after an instrument anomaly recovery, and has been present for the following periods:

1st feb '06 - 11 feb '06

13th mar '06 - 2nd apr '06

8th apr '06 - 15th may '06 (when RA2 was switched to side B)

21st jun '06 (back to side A)- 1st march '07 (when the anomaly disappeared)



Whilst the Envisat Ground Segment is still trying to incorporate the USO correction inside the official operative processor (i.e. Level 1b Instrument Processor Facility or IPF), a pilot processor was put in place at ESRIN to determine the combined long-term and short-term USO variation of Envisat RA-2 RF side A. The goal was to correct the original Level 1b 18 Hz window delay values, with a very slight approximation, such that new corrected Level 1b products were available to be given as input to the RA2 Individual Echoes and River&Lakes processors for a correct use. The scheme of the pilot USO correction processor developed at ESRIN, with the input/output data flows is illustrated in the next figure.


Input/Output data flow of the pilot USO correction processor


Fortunately, since the 1st of March 2007, the USO frequency went back to its nominal value with no round orbit oscillations, after an on-board RA2 anomaly recovery. Neither range jumps of several metres, nor orbital oscillations, have been detected since, whereas the RA-2 actually suffered from other anomalies that triggered a reset, such as single event upsets. Only nominal bias and drift are present since the 2nd of March.


More info on the pilot USO processor and its validation can be found in the following Technical Note (link to uso_correction_note-v.1.4.pdf)