Strange values in SiRF-examples

[Marc]

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?
_________________
Best regards



Marc

[simontdc]

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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G1NTX AFRIN LRPS

[Marc]

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?

            
_________________
Best regards



Marc

[Marc]

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.
_________________
Best regards



Marc

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