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Calculating GTS to UTC offset from sirf binary protocol

 
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ckuethe
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Joined: Aug 08, 2004
Posts: 18
Location: ECEF X/Y/Z: -1514741/-3483783/5106784 m

PostPosted: Thu Aug 12, 2004 6:42 pm    Post subject: Calculating GTS to UTC offset from sirf binary protocol Reply with quote

I'm working on adding support into OpenNTPD (and possibly ntp.org ntpd too) for using a SiRF gps as a reference clock. I've got a prototype working pretty well; with a very naive filter and no kernel time discipline I've got my laptop's clock stabilized to +/- 0.01sec. The big problem is "how do I calculate the number of leap seconds, easily?"

I know that the data I need is in message type 8, I'm plodding my way through the ICD-GPS-200C which tells me the magic is mostly in page 4, subframe 18. By their InfiniteWisdom(tm) SiRF have completely bollixed the 50bps data stream, and made it a royal pain to parse. Even moreso than the format already is. So to anyone with a SiRF dev kit, or anyone brave/foolhardy enough to dive into the aforementioned spec: care to share any observations?

I admit it... I'm lazy. I hate re{invent,design,implement,test}ing the wheel unless it's absolutely unavoidable. Thus my plea for code or enlightenment. Interested perl hackers can grab a snapshot of the code from my homepage..

ps. yes, I know I could flip back to nmea mode to grab the UTC time and use that to calculate the leap second offset, but that's a really ugly hack.
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ckuethe
Occasional Visitor


Joined: Aug 08, 2004
Posts: 18
Location: ECEF X/Y/Z: -1514741/-3483783/5106784 m

PostPosted: Mon Aug 16, 2004 6:37 am    Post subject: Reply with quote

My, aren't we all chatty this week!

Other leads turned up no results, so it looks like I'm stuck with the unenviable task of doing the calculations myself. Between the hassle of re-re-re-re-reading the ICD-GPS-200, and the hunting down of compiler bugs as I reimplement this all in C, I'm starting to feel much more sympathetic to geomatics companies who charge thousands per copy of their software.
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