Author: Sven Bauer

Dear colleagues,

following our presentation at the ILRS workshop in Riga we (Jens Steinborn and me) developed a first beta version of our time bias prediction service. The website of the current interface is:

http://slr.gfz-potsdam.de:5000/tb/v1

Feel free to use and share it internally. However, this is currently not a public website so please do NOT post this publically yet - as in the NESC forum for example.

The interface is pretty much straight forward. On the website you see satellites listed with the available predictions from different providers and days as well as the predicted time bias value for the current point in time. There are four groups where you can find the individual satellites listed which can be found at the top of the page and which are:

- Watch List

- LEO Targets

- Geodetic Targets

- Debris Targets

So if you are about to track a satellite, find the group, search for the satellite in the list, check the currently predicted time bias value and use that as an a priori start value durning acquisition. In brackets first you see the RMS of all time bias values/passes with respect to the applied fit and second the number of time bias values/passes used for that fit. From that you can get a first idea about the reliability of a predicted value. If you click on a linked provider name you will see a graphical representation of the time bias values of all available passes and the predicted one. This allows you to evaluate the quality of the predicted time bias value but also of the prediction itself by checking the trend and the scatter. It is interesting to compare predictions from the different providers even with satellites like Lageos 1 where you do not really expect a time bias. However, when you take a look at the HTS prediction over multiple days you can see modeling errors periodically adding up which is not the case with the JAX predictions. In some cases you might also see that a predicted time bias value does not look too good with respect to the trend. In such cases just do an ”extrapolation on the screen” for a better estimate since we are currently experimenting with data weights which do not always work well with sparsely distributed data.

Let us know if you encounter issues, if you are missing information, satellites, providers, anything else or if you have suggestions. The contacts are given on the site and are Jens and me. Please remember that this is a beta test, so the service might encounter outages or other issues.

As we have shown in our Riga presentation, the time bias value prediction becomes better with passes being processed and submitted to EDC quick (e.g. after a calibration and not after a shift) as well as with overlaps of concurrent predictions. Overlaps of e.g. 6 hours help to predict time bias values from data already collected with a past prediction instead of starting with a new prediction without any data.

Thanks and best

Sven

--
Dr.-Ing. Sven Bauer
Section 1.2 Global Geomonitoring and Gravity Field,
Earth System Parameters and Orbit Dynamics,
POT3 Satellite Laser Ranging Station

Phone:          +49 331 288 1738
Fax:            +49 331 288 1169
E-mail:         sven.bauer@gfz-potsdam.de

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Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Foundation under public law of the federal state
of Brandenburg
Telegrafenberg, D-14473 Potsdam