Calibration Data for Retracking TOPEX Data

Philip Callahan (Jet Propulsion Laboratory, United States)

CoAuthors

Jean-Damien Desjonqueres (Jet Propulsion Laboratory, United States); Shailen Desai (Jet Propulsion Laboratory, United States); Matthieu Talpe (Jet Propulsion Laboratory, United States); Joshua Willis (Jet Propulsion Laboratory, United States)

Event: 2018 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Measurement and Retracking

Presentation type: Type Poster

We will show the collection of calibration data assembled to support the retracking of TOPEX data for the climate data record. The data include the Cal-1 and Cal-2 data acquired twice each day throughout the mission and “Cal Sweeps” acquired pre-launch, toward the end of Alt-A and during Alt-B operations. Unfortunately, review of the data does not provide significant new insight into the waveform (WF) leakages.

The Cal-1 and Cal Sweeps provide measurements of the altimeter point target response (PTR). As is now well known, the Alt-A PTR changed with time becoming significant after about cycle 140 (July 1996) until turnoff at cycle 235 (February 1999). Alt-B operated for cycles 236 until the end of mission at cycle 481 in October 2005, including the colinear period with Jason-1 for cycles 344-364 and the interleaved orbit from cycle 366. The twice-daily Cal-1 data are averaged by cycle. Unfortunately, the Cal-1 data provide bare Nyquist sampling of the PTR and are centered at a different gate than regular tracking. Nonetheless, the data provide a clear picture of the changing Alt-A PTR and the stability of the Alt-B PTR. The Cal Sweeps provide very detailed information on the PTR by sweeping the Cal-1 data through a range of delays. Cal Sweeps were performed for both Alt-A and Alt-B prelaunch. Unfortunately, Cal Sweeps of Alt-A were then not done until the changes in the PTR had become quite obvious beginning in cycle 220. For Alt-B, Cal Seeps were done approximately monthly. From these data it was determined that separate estimates of the PTR for Ku and C bands should be used.

Cal-2 data provide information on changes in the low pass filter response. This response is the WF “weights” used both in standard processing and retracking. The Cal-2 data show essentially no change in weights for either Alt-A or Alt-B. The proper WF weight files have been recovered and used for retracking.

We have also examined the time correlation data, which were used to track the oscillator drift correction. While the data show a 59-day variation that is well-correlated with instrument temperature, the magnitude of the effect is less than approximately 1 mm and so is not the source of the previously discussed 59-day variations in TOPEX data.



Part of the work reported here was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Copyright 2018.

 

Poster show times:

Room Start Date End Date
Foyer, Salao Nobre & tent Thu, Sep 27 2018,18:00 Thu, Sep 27 2018,20:00
Foyer, Salao Nobre & tent Fri, Sep 28 2018,14:00 Fri, Sep 28 2018,15:00
Philip Callahan
Jet Propulsion Laboratory
United States
philip.s.callahan@jpl.nasa.gov