[teqc] Signal/Noise values from Z-Extreme receiver data

Lou Estey lou at unavco.org
Tue Jul 20 08:00:29 MDT 2004


Dario,

> I received a large set of TEQC Rinex translated files from receivers 
> ASHTECH-ZXII y Ashtech Z-Extreme.
>  
> I found in all files that Rinex files from ASHTECH Z-XII files have 
> "normal" S/N values, but Rinex files from Z-Extreme receivers have 
> inusual low values.
>  
> Here is an example:
>  
> *** fragment of 0101323a.obs (ASHTECH-Z12) ***
>  
>      2.10           OBSERVATION DATA    G (GPS)             RINEX 
> VERSION / TYPE
> teqc  2000Feb29                         20031126 18:30:14UTCPGM / RUN BY / DATE
[...snip...]
> 03 11 19 16 53 15.0000000  0  6G31G25G 3G20G11G14
>     163881.32049    121992.80947  20614307.799    20614316.657    20614307.412  
>    -135169.76049    -99226.04645  23476649.000    23476663.221    23476649.744  
>     401236.84149    291232.02346  22088848.354    22088861.183    22088848.818  
>     -47342.94949    -34561.55546  22833207.043    22833219.599    22833206.840  
>    -150271.80449   -109618.26847  21851897.708    21851905.008    21851896.847  
>     352130.13149    241932.75946  22364268.146    22364278.171    22364268.989

> *** fragment of 0701322b.obs (Z-Extreme) ***
>      2.10           OBSERVATION DATA    G (GPS)             RINEX 
> VERSION / TYPE
> teqc  2000Feb29                         20031126 18:31:57UTCPGM / RUN BY / DATE
[...snip...]
> 03 11 19 16 53 15.0000000  0  8G 1G31G14G11G15G 3G20G25
>   -2727568.31443  -2054893.07342  24332321.138    24332341.154    24332321.645
>  -17569012.83643 -13641080.70642  20508584.291    20508594.406    20508585.718
>  -14182208.39943 -11007367.76742  22228090.048    22228101.811    22228092.867
>  -16590976.58643 -12893430.70842  21736040.151    21736048.901    21736041.451
>    -169784.17243    -83278.68442  24298154.772    24298172.032    24298154.260
>  -12586189.95443  -9802697.97242  21980369.384    21980381.242    21980370.239
>  -10854547.65943  -8415804.11442  22732908.967    22732921.839    22732909.781
>   -8885066.75443  -6887797.97942  23349973.796    23349991.605    23349976.198

> I pick for example the same epoch on both files but the same happen in 
> all files (all sites)along one month of measurements.
>  
> Seems to be S/N from Z-extreme receivers maps to very low values in 
> (TEQC) Rinex files?.
> Is there a way to correct this during translation?
> Is that a problem? in TEQC or ASHTECH data?

For everybody else on the list, this question is about the RINEX SNR 0-9 flag
on the L1 and L2 data: high values (e.g. 9 for L1, 5-7 for L2) for the Z-12
and low values (e.g. 3 for L1, 2 for L2) for the ZXtreme in the RINEX fragments
above.

The situation is that Ashtech is using the SNR slot in their binary formats
for two different measurements of SNR depending on the receiver.  The Z-12
outputs SNR in Ashtech's old "ireg" value, and the micro-Z and ZXtreme output SNR
in dBHz.  The mapping in any official release of teqc (14 Mar 2002 or earlier)
was based on the older "ireg" values only, which are numerically larger than
the equivalent in dBHz.  This was resolved in the teqc development code in
May 2003 (see http://www.unavco.org/facility/software/teqc/log.html for
20 May and 29 May 2003) -- but it relies on the code correctly identifying
the receiver type first, because _there is no other reliable way_ to tell
whether the SNRs are stored as the older "ireg" values or in dBHz.

There is also an additional piece of new code in teqc that will convert the
"ireg" SNR to dBHz SNR if the receiver is an Ashtech Z-12 (the only case) if
you are outputting S1 or S2, assuming that the code correctly identifies the
receiver as a Z-12. I sent out an announcement on this topic on 22 May 2003 to
the old teqc email list (see http://www.unavco.org/facility/html_mail/teqc/msg00326.html
-- and, BTW, no one ever responded about the HP-UX host.  hint, hint).

There is the same trouble with Trimble receivers: the older ones store SNR
in Trimble's "AMU", and the newer ones store SNR in dBHz -- and the only
way to tell which is which is to know what the receiver is.

This topic will continue to plague us for years.  My suggestion if you
are using a mix of older and newer Ashtech and/or Trimble receivers is
to minimize your dependence on the RINEX SNR 0-9 flag and instead use
the S1 and S2 observables -- though even there you will have to know what
the specific receiver is outputting.

BR,
--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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