[teqc] Leica Navigation Messages

Lou Estey lou at unavco.org
Wed Jul 27 08:13:12 MDT 2005


Henry,

> We processed raw data from one Trimble 4000SSE receiver (using TEQC) and one
> Leica MC500 receiver (using Leica Geo Office) and found there is difference in
> the Navigation file. Below is a sample section.
> 
> === Leica Data ===
> 19 05 07 26 22 00  0.0-2.455757930875D-05-4.547473508865D-13 0.000000000000D+00
>     8.600000000000D+01 5.431250000000D+01 4.455542601534D-09 9.839382338912D-01
>     2.756714820862D-06 3.425075556152D-03 9.367242455482D-06 5.153715379715D+03
>     2.520000000000D+05-2.235174179077D-08-1.453271533084D+00-7.078051567078D-08
>     9.585451900661D-01 2.017500000000D+02-1.585713739022D+00-8.062121636954D-09
>    -1.464346699454D-11 1.000000000000D+00 1.333000000000D+03 0.000000000000D+00
>     0.000000000000D+00 0.000000000000D+00-1.443549990654D-08 3.420000000000D+02
>     2.520000000000D+05 0.000000000000D+00
> 
> === Trimble Data ===
> 19 05  7 26 22  0  0.0-2.455757930875D-05-4.547473508865D-13 0.000000000000D+00
>     8.600000000000D+01 5.431250000000D+01 4.455542734096D-09 9.839382338912D-01
>     2.756714820862D-06 3.425075556152D-03 9.367242455482D-06 5.153715379715D+03
>     2.520000000000D+05-2.235174179077D-08-1.453271533084D+00-7.078051567078D-08
>     9.585451900661D-01 2.017500000000D+02-1.585713739022D+00-8.062121534008D-09
>    -1.464346710204D-11 1.000000000000D+00 1.333000000000D+03 0.000000000000D+00
>     2.000000000000D+00 0.000000000000D+00-1.443549990654D-08 3.420000000000D+02
>     2.448000000000D+05 4.000000000000D+00
> 
> You can see that the difference with IDOT and Delta N values. The two receivers
> are very close.  Do you know the reason for this problem? 

There's also a discrepancy in the SV accuracy, time of transmission, and fit
interval. Time of transmission is probably correct for both.  For the SV accuracy,
my guess is that the Leica software is (incorrectly) reporting the URA index
N (= 0 for this case), and teqc is (correctly) converting this into the
nominal URA value X in meters (as specified in RINEX 2.xx), which for
N <= 6, X = 2^(1 + N/2).  Not all receiver formats give you enough information
to determine the fit interval: the Trimble .dat format (record 21) does, and
I'm pretty sure the same for Trimble RT17 (record 55h), and I'm not going to
bother looking up the rest.

As for IDOT and Delta n, these are both converted into floating point by the
receiver firmware: for both Trimble RT17 and .dat formats, IDOT and
Delta n are recorded from the compressed broadcast bits and converted into
_8-byte floats_ in the two formats which are then read by teqc; for Leica MDB
record 15, IDOT and Delta n are converted into _4-byte floats_.  Does this
mean the Trimble firmware conversion is better?  No, because there isn't
enough precision in the broadcast bits(*) for these quantities to warrant
the added precision of 8-byte floats.  The IEEE-754 standard for floats
will only give you 6.9 significant digits from a 4-byte float, and we see
agreement in that many digits in the two ASCII representations of IDOT
and Delta n -- and beyond 7 digits the numbers are basically noise for both,
and actually it's worse than that given the number of the broadcast bits
for these two quantities.

(*) Broadcast Delta n is 16 bits, and IDOT is 14 bits: 2^16 = 65536, 2^14 = 16384

Now, wasn't that fun for everyone? ;)

cheers,
--lou

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Louis H. Estey, Ph.D.              office:  [+001] 303-381-7456
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"If the universe is the answer, what is the question?"
                                                -- Leon Lederman
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