[time-nuts] nuts about position (cheap receiver)

[Mark Sims]

You could add doppler to the RINEX file.   All the receivers with raw messages 
seem to output that.

I am playing with the Furuno GT87 output.  It does not output carrier phase 
data (only pseudorange / doppler / SNR).   CSRS-PPP can still process that.   
On the first run  (3 hours of data, after a 20 minute self-survey ) it did 
reject 81% of the epochs.  Sigmas were 3/2/5 meters.   The GT87 outputs 
altitude as MSL, not WGS84.   

I just submitted a new 14 hour run (with the receiver doing a self-survey... 
not in position hold mode).

-

> I submitted 4 days (118-121) of RINEX files from an IGS station to
CSRS-PPP, but with only the GPS C1, L1, and S1 observations.
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Re: [time-nuts] LEA-6T TCXO measurements

[Bob kb8tq]

Hi

Two different meanings of the term sawtooth.

The first plot shows a very normal phase rollover process. That (when 
incorrectly 
unwound) can create errors. In some cases those sorts of errors are inevitable 
and 
it is always wise to see if data “noise” occurs right at a wrap around point. 

The other meaning of sawtooth in the case of GPS is rooted in the same process. 
The GPS time solution and local time have a pattern very much like the first 
plot. 
As long as it is running as shown in the first plot, it averages out (provided 
there is 
averaging).  The problem comes in when the plot does *not* look like the first 
graph. 
If the TCXO comes to a flat spot on it’s compensation curve, it can “hang out” 
to one
side of the error band. Instead of a sawtooth you get a curve. That process of 
staying
to one side of the error band does not average out and you get a bump in the 
time
record. 

If you dig into the archives, there are lots of plots showing typical data.

Bob

> On May 4, 2018, at 5:18 AM, Gabs Ricalde  wrote:
> 
> On Thu, Apr 26, 2018 at 8:36 PM, Bob kb8tq  wrote:
>> Hi
>> 
>> What you are looking at appear to be sawtooth jumps. Simply put, the module 
>> is
>> looking at the closest edge on the TCXO to do it’s timing. When the device 
>> drifts,
>> it can “slip” to another cycle. If you watch the PPS out, there are also 
>> artifacts that
>> result from this process operating in a non-ideal fashion. Lots of details 
>> on that in
>> the archives.
>> 
> 
> Hi Bob,
> 
> The raw data has a sawtooth pattern due to millisecond clock jumps
> [1], but TimeLab unwraps it and they happen more frequently than the
> frequency jumps.
> I redid the the experiment using the same receiver (pink trace) and
> another LEA-6T (green trace), there were no frequency jumps. SBAS was
> enabled in the old measurements (blue trace). I'm not sure if that
> explains it, but the manual does not recommend SBAS for timing.
> 
> 
> [1] "GNSS Receiver Clocks" http://www.insidegnss.com/node/2512
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Re: [time-nuts] nuts about position (cheap receiver)

[Gabs Ricalde]

I submitted 4 days (118-121) of RINEX files from an IGS station to
CSRS-PPP, but with only the GPS C1, L1, and S1 observations. This is
similar to what cheap receivers would be generating, but with probably
lower noise. emr clocks/orbits were used. The east/north/up
differences between the PPP solutions and the latest IGS solution
(week 1996) in meters are:

   E  N  U
 -0.29  -0.05   0.14
 -0.01   0.12   0.03
  0.10   0.11  -0.04
 -0.04   0.02   0.65

The reported 95% sigma is 0.26m (longitude), 0.24m (latitude), 0.63m (elevation)
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