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Strange values in SiRF-examples

 
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Marc
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Joined: 18/03/2003 14:19:29
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Location: Germany

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Strange values in SiRF-examples Reply with quote

Hello,



seems like I'm the first one to use this forum. I don't know how I deserved this honour. Anyway:



In the documentations for the SiRF messages, to be found, for instance, at Falcom s website, there are a few examples whose values I don't understand. Take chapter "2.2.21 Navigation Library DGPS Data - Message I.D. 29" of that paper: There is a "Pseudo-range rate correction" in the unit m/s with a value of 1017817771 - that is three times the speed of light. And there's a correction age of 1069547026 seconds given - more than 33 years, where you'd expect just a few seconds or minutes. In fact I received a similar correction age from my EMTAC BTGPS.



Something is strange here. Either there's really a mistake on the manufacturer's side, or I simply don't understand what pseudo-range rate correction and correction age really mean. Can anyone shed a little on this, please?
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Marc
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simontdc
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Joined: 11/03/2003 08:29:43
Posts: 64
Location: United Kingdom

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Strange values in SiRF-examples Reply with quote

Looking at the documentation, it may just be that the default data is old, or more likely it is meant to be scaled i.e. multilply by 10^-6 or similar. Looking at message 28, the figures there do seem to make sense (i.e. a Pseudorange of 24,921,113,696 metres).



If it's not scaled, I'd imagine this data is not 'real' - a correction age of a thousand million seconds would be useless. It would be interesting to see some real data from this message - I may well try it.
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Marc
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Joined: 18/03/2003 14:19:29
Posts: 8
Location: Germany

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Strange values in SiRF-examples Reply with quote

Hi Simon,



this is an interpretation of some real data from a sample taken with a PC from an EMTAC BTGPS:



A0 A2 00 1A 1D 00 19 00 E2 01 C0 BE 1A D6 3D 4C CC CD 3F E6 64 73 40 A0 00 00 40 A0 00 00 09 65 B0 B3



A0 A2:           Start sequence

00 1A:           Message length (26 Byte)

1D:                 Message ID (29)

00 19:           Satellite ID (25)

00 E2:           IOD (226)

01:                Source (1)

C0 BE 1A D6:     Pseudo-range correction (3233684182 m) (*)

3D 4C CC CD:     Pseudo-range rate correction (1028443341 m/s) (**)

3F E6 64 73:     Correction age (1072063603) (***)

40 A0 00 00:     Reserved, they won't tell us what this means

40 A0 00 00:     Reserved, they won't tell us what this means

09 65 B0 B3:      Checksum and end sequence



(*) They must be kidding: That's 3233684.182 km or more than eight lunar distances.

(**) They must be kidding again: That's 1028443.341 km/s or more than three times the speed of light.

(***)They're kidding all day: The correction age is almost 34 years. If the unit was microseconds we'd still have more than 17 minutes. Possibly this is not an age but a GPS time stamp, given to the correction at the moment of its publication?

            
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Marc
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Marc
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Joined: 18/03/2003 14:19:29
Posts: 8
Location: Germany

PostPosted: Sun Sep 14, 2003 8:04 am    Post subject: Strange values in SiRF-examples Reply with quote

Hi there,



I found the reason: There is indeed a little error in the documentation. The bytes in question must not be interpreted as integer values but as floating point numbers. SiRF has confirmed this suspicion.
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Marc
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