| Date: | 2004-11-18 23:51:00 | |
| Sender: | Graham Appleby ,Philip Gibbs, Matthew Wilkinson <Philip Gibbs <pgib@nerc.ac.uk>> | |
| Subject: | [SLR-Mail] No. 1280: Herstmonceux Time bias QC | |
| Author: | Graham Appleby ,Philip Gibbs, Matthew Wilkinson | |
| Content: | ******************************************************************************** SLR Electronic Mail 2004-11-18 23:51:00 UTC Message No. 1280 ******************************************************************************** Author: Graham Appleby ,Philip Gibbs, Matthew Wilkinson Subject: Herstmonceux Time bias QC Colleagues The rapid laser range QC service developed at NSGF Herstmonceux has been in operation for some months now. The process was discussed at LW14 in San Fernando in the Summer, and is based upon our very rapid time bias function update system, which automatically uses the latest laser data to improve those functions for all available IRV sets. The functions are updated at least once an hour as new data becomes available from CDDIS and often more frequently as some stations send their NPs directly to us. The resulting plots of time bias values for each pass and the smooth time bias functions fitted to them are updated at 2-hourly intervals; see http://nercslr.nmt.ac.uk/timebias/timebias_plots.html Those observations which, at some level, produce apparent time biases that do not fit the smooth time bias function are flagged as possible erroneous data. We currently trigger this flag if a time bias value stands off by more than 15ms. In order to maximise the value of this early warning system, we use observations of most of the current ILRS targets, some of which undergo without-warning manoeuvres and some of which are susceptible to rapid changes in drag forces, for instance following a Solar storm. Depending upon the timing of such events, it is possible that post-event observations will produce time bias values that appear not to fit the smooth function. In such cases, the automatic email warning to the station and to the CB are seen in hindsight to be erroneous. A recent example involved GFO-1, ENVISAT and ERS-2. Solar storms on 8th and 10th November 2004 caused rapid atmospheric density changes unforeseen in the predictions and thus apparent jumps in the smooth time bias functions for these satellites. We apologise for the small flood of warning emails to stations on this and previous occasions. As time allows, we are reviewing our processes and hope to reduce to a minimum these false alarms. We would again ask for feedback on any erroneous message sent out to help us improve the service. STOP PRESS - Topex has today (18th November) undergone a large change in the timebias function. We will try to ensure no error messages go out. Regards, The NSGF team ------------- References. Werner Gurtner. Realtime Time Bias Distribution, in collaboration with University of Berne, Switzerland. SLRMail #0921, March 2002. Ingrid Bayer, Philip Gibbs and Matthew Wilkinson. The Herstmonceux Time Bias system as a possible real-time QC Tool, 14th Laser Ranging Workshop, San Fernando, Spain, June 2004. Matthew Wilkinson. New available Timebias plots, SLRMail number 1256, September 2004. Graham Appleby. A note on the recent Solar storms and their effects on LEO orbits. Via the NSGF website at http://nercslr.nmt.ac.uk From: ”Philip Gibbs” ******************************************************************************** |