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Re: gpsrinex and Trimble


From: Gerry Creager - NOAA Affiliate
Subject: Re: gpsrinex and Trimble
Date: Tue, 5 May 2020 22:37:44 +0000

Since you've got a reasonable antenna, the other elements in this, from a history of post-processing geodetic measurements, are proximity to the tower and height above ground. The zephyr appears to be an enclosed calibrated ground plane reflector, with a similarly calibrated preamplified patch antenna. Reasonable design, been in use for decades. A choke ring might mitigate multipath a little better (I used to question some of that contention with Gerry Mader, antenna geek extraordinaire at the National Geodetic Survey).

Trimble has always claimed to have advanced multipath mitigation but urban canyon geodesy taught me they had better marketing than factual content until you got to the choke ring antenna (Mader was right) but it was better for incident multipath from above and sides than from below. Your position above the roof suggests it's likely to see reflections from whatever is nearby and below. Ground planes also are prone to some knife-edge diffraction. Nearby trees will almost certainly be in the far field and likely not contributing to multipath so much as absorbing signal when they can. Of course, the tower is likely a multipath contributor. Still, getting sufficient data to derive cross-arm azimuth shouldn't be a big deal for dual-frequency resolved data.

The coax is likely not contributing to much beyond lower signal levels. 

One thing I recall is that Trimble liked high power preamps, They were working, back in the bad old days, with 45 dB preamps, and testing with 2 Trimble receivers and preamplified antennas side by side tended to cause ringing between the antennas! Based on published specs, I suspect they're still doing that. I doubt that's an issue here with a completely different manufacturer's antenna; Ashtech antennas didn't cause ringing when used in tests close to Trimble hardware.

For what it's worth, we did have a pair of antennas on a secure mount on the roof of the building where the geodesy lab was and collected L1 data for differential corrections, and L1/L2 RINEX to support CORS, and for local campaigns where we'd process against its long-term derived position (which was pretty bad vertically, and changed with each heavy rain). 

The work I did that led to the rewrite of NGS-58 was done with both Trimble and Ashtech receivers, and Trimble ground planes and Ashtech choke rings. 

gc


On Tue, May 5, 2020 at 9:54 PM John Ackermann N8UR <address@hidden> wrote:
Hi Gerry --

Good question.

The antenna is engineered, I hope, to make the best of a mediocre
situation.  Unfortunately for this purpose (and UHF contesting), my
house is on a quite wooded lo with nearby tall trees in just about every
direction.

The antenna is a Trimble Zephyr Geodetic.  To get a better sky view,
it's mounted on a very stiff (1/4 inch thick aluminum channel) side arm
that extends about 30 inches out from both sides of a Rohn 45G tower at
about 10 feet above the roof of the house.  There's only another 20 feet
of tower above that, and the antennas are pretty small, so while there
may be some twist in strong winds, I don't think it's very substantial.


The feedline is about 60 feet of LMR-400 going to a couple of HP
8-channel distribution amp/splitters, one with filters removed so it's
not limited to L1.

The worst tree cover is to the north, where thankfully it does the least
harm.  The trees in other directions are a bit more distant.

There's a Chinese L1/L2/L5 antenna on the other end of the side-arm so I
can do base and rover tests without leaving home. :-)  And actually
doing that gave me a *very* accurate bearing for the sidearm which means
sooner or later I'll get the Zephyr fiducial mark aligned more
accurately to north (it could well be 15 or 20 degrees off now).

I need to do some experiments now that the leaves are coming out, but I
suspect I should be running with an elevation mask quite a bit higher
than the default 7.5 degrees.

John
----

On 5/5/20 5:29 PM, Gerry Creager - NOAA Affiliate wrote:
> Yo, I believe is the appropriate salutation; it's been a long COVID...
> First, a question for John: What was the antenna and where was it
> (indoors, outdoors, sitting on a pvc pole waving in the wind?
>
> On Tue, May 5, 2020 at 8:23 PM Gary E. Miller <address@hidden
> <mailto:address@hidden>> wrote:
>
>     Yo John!
>
>     On Tue, 5 May 2020 16:15:06 -0400
>     John Ackermann N8UR <address@hidden <mailto:address@hidden>> wrote:
>
>     > Well, I was actually able to get TrimbleRTX to accept my F9P RINEX
>     > file created by gpsrinex.
>
>
>     Good.  Someone reported otherwise this week.
>
>     > It would *not* accept the one created by
>     > rtklib's convbin tool -- no useful explanation what was wrong, just
>     > unrecognized file format.
>
>     Those are called "Lassie Errors".  Content free.
>
>     Does anyone here have Timb;e contacts?
>
>  
> Not anymore, thankfully but let me reach out to someone who might. 
>
>
>     > The bad news is that the results were pretty awful.  Since the results
>     > are just a single-page PDF, I've attached it here.  But in summary:
>     >
>     > Out of 2873 total observations, 100% were usable, but only 207 (7%)
>     > were used.  That raises a lot more questions than it answers...
>
>
> A couple thoughts
>
>   * Antenna wasn't stable
>   * Multipath due to an inadequate antenna
>   * Misrepresented as static which tightens the error criteria
>   * Trimble is sometimes obnoxious in its interpretations 
>
>     No thanks.  We already have enough unanswetred questions here.  :-)
>
>     > The sigmas (not clear if 1 or 2 stdevs) are 0.185m, 0.131m, and 0.066m
>     > for XYZ, and 0.062m, 0.195m, and 0.130m for lat/lon/height.  The
>     > report came with this warning:
>
>  
> 1 sigma; Top of the column...
>  
>
>     Which are about 10x worse than NRCan.
>
>     > Of course, those numbers don't actually appear in the report itself.
>
>     Typical Trimble.
>
>
> What he said
>
> gerry
>  
>
>     RGDS
>     GARY
>     ---------------------------------------------------------------------------
>     Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
>             address@hidden <mailto:address@hidden>  Tel:+1 541 382 8588
>
>                 Veritas liberabit vos. -- Quid est veritas?
>         "If you can't measure it, you can't improve it." - Lord Kelvin
>
>
>
> --
> Gerry Creager
> NSSL/CIMMS
> 405.325.6371
> ++++++++++++++++++++++
> /The way to get started is to quit talking and begin doing./
> /   Walt Disney/




--
Gerry Creager
NSSL/CIMMS
405.325.6371
++++++++++++++++++++++
The way to get started is to quit talking and begin doing.
   Walt Disney

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