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The SBS-1 Multilateration Trials Webpages | ||||||||
| Introduction Data acquisition A first glance onto trial data Receiver clock synchronization
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A first glance onto trial data The data acquisition trial was successfully completed on Monday Oct 27, 2008 between 20.00 and 20.02 UTC. We have acquired six sets of data files with around 18 MByte of data packets. SBS-1 receiver locations Thanks to your great support a suite of good locations could be used for our trial purposes. Ideally the four SBS-1 locations are grouped like a circle or rectangle around the area that is interesting. From the SBS-1 locations that have recorded data the below selection of stations was made. Also in the picture you can see a few tracks that were recorded during the trial, each of them about 2 minutes long.
The stations depicted R1, MC, P5 and G9 had the highest number of hits and in combination they are ideally located to track flights in the inner of the rectangle they span. Useable flights for clock synchronization A few tools are available here that can scan the log files for our purposes. The first step is to identify those flights that have been seen by all four SBS-1 stations at the same time. We do this with the following logic. For a moving target an ADS-B position squitter has a unique byte pattern. This is derived from the position encoding. The following general assumptions apply according to ICAO Annex 10:
This means that for high precision squitters we can well assume that a different position data packet is transmitted with every squitter. Now we have to search for those aircraft for who identical positions squitters have been seen by all four stations. Here is the result of this analysis:
The left window shows those aircraft that position squitters were received from by all four stations during the trial.The right hand list show those aircraft from who identical position squitter packets were received by all four stations. In the end data from 15 airframes are useable for synchronization purposes For demonstration of the trial results we will continue to use data from airframe 4006DB. This is a First Choice A321 G-OOAE. At this time we do not have more information about this particular flight (and we do not need any). In the above picture of the trial area it is the red track that approaches Coventry from the South.
Here is a view onto some data packets after they were sorted to time order:
8D 40 06 DB 58 71 B6 43 D7 BE EB 00 00
00 0 B868BA 20:00:24.716 You can see the raw position squitter, then the station number 0,1,2, or 3, the associated 24-bit counter value that came with the data packet (expressed as a hex number) and the PC time stamp from the log file. We can see that PC time stamps are only a rough estimation of time as much as they differ. Also it is obvious from the data received that there are some difficulties in receiving every position squitter by all four stations. This should happen every ca. 1 second, but in reality this is the exception and there are some gaps. Given that the 24-bit counter overflows after 0.8 seconds we have to take additional measures to assign the data packets to the correct counter period. Here is a sketch about the problem:
We will continue to work on this issue in the next chapter.
Interesting report on a similar subject: http://www.ruena.de/files/ACAS-Monitoring_IEV_TU-BS.pdf
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