VITA 49 Integer Timestamp with GPS Option installed

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  • Updated 2 years ago
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I made some good progress working with the VITA49 packets directly to implement my HTML5/Websockets webclient (E.g. small steps here:  DAX Audio reconstruction from pcap file e.g. makes me pretty happy: https://twitter.com/HB9FXQ/status/913490432570642432 :-)  ). Now I came across the integer timestamp value of the VITA packets. 

If I'd install the GPS option: Would that give me some accurate abs time value? I'd guess something like TDOA with a small network of radios that could be built. Is my current interpretation correct: UnixTimestamp derived based on the system uptime of the radio? Maybe there is a hidden command to set/sync the system time?

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Frank, HB9FXQ

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Posted 2 years ago

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Andrew Russell

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Frank,
I don't have the programming skills but would love to contribute to such a network in a multi client environment.
Andrew VK5CV
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Steve - N5AC, VP Engineering / CTO

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Official Response
First, the current timestamps are derived from the processor clock and are only accurate to around 100ms so I wouldn't count on them for TDOA.  There is a plan to put timestamps from the GPS into the software and in this case, you would be accurate to 50ns.  

More importantly, however, is that TDOA requires an accurate ionospheric map to get good results at HF since your path is not ground wave, but is skywave instead.  If you're communicating between two points that are 1,000km apart and the ionosphere is at 100,000km, your path length is 200,006km.  The distance between the points hardly matters at all!  Time-stamping is really the easy part.
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kr4k

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The ionospheric specification is getting much better. Technologies are coming together.

Check out a first level assimilative model, IRTAM = IRI Real-Time Assimilative Mapping. It uses the 45+ digisonde sites,  http://ulcar.uml.edu/DIDBase/  - with updates about every 15 minutes. IRTAM produces a “smooth” background model so that TIDs in the regions of digisondes do not corrupt the background model accuracy elsewhere. But it gets the major parameters much more accurate. IRTAM results are now available via:  http://giro.uml.edu/GAMBIT/

The latest Lowell digisondes produce a skymap where you can see the TIDs in detail. You can see this in: http://www.digisonde.com/pdfs/LDI-Presentation-ProposersDay.pdf

73/gus
(Edited)