Calculating GTS to UTC offset from sirf binary protocol

[ckuethe]

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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http://www.ualberta.ca/~ckuethe/gps/

[ckuethe]

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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http://www.ualberta.ca/~ckuethe/gps/

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