Trimble 4000 SSi receivers, High PDOP values

I had a somewhat similar problem with a Trimble 4000se many years ago. It turned out that one of the two antennas that I had received with the unit (used), had a “sector” in it, and didn't track birds in that particular “quadrant” (a rough and probably inaccurate description of the problem).

Several years ago, I received some data from a Surveyor in Utah that had similar “weirdness,” (Trimble Compact L1/L2). A quick test indicated that the antenna in question was simply NOT tracking birds within a particular “zone” (sector/quadrant). When a given [tracked] bird entered that “zone,” it “went away,” only to reappear an hour or so later once it had passed through the “dead zone.”

I dunno enough about GPS antennas to say anymore than that, but it's something that I would check out (AFTER checking the “mask angle” that might be set in the receiver of course).

Your planning software might also give you a clue IF this is the problem.

Loyal

Wow that's interesting. I will check to see if this pattern exists when using either of my two antennas. Yes, they are Compact L1/L2 by Trimble, one with groundplane and one without. Thank you, Loyal!

I've looked at my RINEX files to see if I could find any pattern. The number of satellites tracked varies by job from 7 to 15, typically around 11 for a 2.5-3 hour log session.

I've only used that particular receiver (#3828) in conjunction with the particular antenna (the one without groundplane) ONCE, so I have nothing direct to compare to. I have used the 3828 receiver at another job with my other antenna (with groundplane), and it tracked 10 satellites on that job.

I have used other receivers with the suspect antenna (without the groundplane) on a few jobs, and the resulting log sessions show between 10 and 13 satellites tracked. This session, resulting in only 7 satellites tracked, appears to be an aberration.

I checked each of my receivers, and the mask angle varies between 10 and 13 degrees.

Howdy,

I'd test the hypothesis that it is a bad "sector" by using the "sky plot" feature in planning software. The Trimble software includes this feature. There are also web sites with this feature. As you have data observed at two sites during the same time interval, differences in SVs tracked should be examined to determine the cause. Obviously, if all the SVs missing are in the same quadrant/area of the sky, you have a clue.

For this test to work you should have oriented your antennas to north. While an antenna problem could be the problem, did you monitor the site during observations? If someone or something interfered with the signal coming to the antenna the problem could also occur. This may be a more likely answer if the problem is intermittent. Of course electronics are notoriously perverse when attempting to determine reasons for failures,

HTH,

DMM

Are these files from the same rec'r or different?
If different, what are the elevation masks set at? If different then that may be the culprit. Some of the data is being screened off by the mask, i.e. 15 degree vs. 10 degree.

yeah, get rid of the elevation mask altogether.

DMM,

Thanks. I'll see if I can find the sky plot software. I recently acquired TBC software, so maybe that has it.

As for orientation of the antennas, one of the antennas has the groundplane, and was properly oriented to north during the session. The other antenna is without the groundplane and was the one which only tracked 7 satellites. It was not oriented to north, as I have found no markings thereon which would indicate how it should be oriented. I'll look into that further.

Al

> Howdy,
>
> I'd test the hypothesis that it is a bad "sector" by using the "sky plot" feature in planning software. The Trimble software includes this feature. There are also web sites with this feature. As you have data observed at two sites during the same time interval, differences in SVs tracked should be examined to determine the cause. Obviously, if all the SVs missing are in the same quadrant/area of the sky, you have a clue.
>
> For this test to work you should have oriented your antennas to north. While an antenna problem could be the problem, did you monitor the site during observations? If someone or something interfered with the signal coming to the antenna the problem could also occur. This may be a more likely answer if the problem is intermittent. Of course electronics are notoriously perverse when attempting to determine reasons for failures,
>
> HTH,
>
> DMM

Two different receivers, run simultaneously. Each of the two receivers had identical mask. +13 degrees.

I've reset the receivers to default setting which is +10 degrees mask.

Howdy,

I found a web-based tool that generates skyplots on the Ashtech site. It is a Beta version: http://asp.ashtech.com/wmp/ My data input is shown in this link: http://asp.ashtech.com/wmp/#tab5

Using the extracts of the RINEX file in your original post, I used the ECEF coordinates to compute geodetic coordinates. They indicate a location near Becket, MA at N42-18-04, W073-03-59. The two sites are about 231 meters apart. Google Earth shows it to be a densely wooded area.

I used these geodetic coordinates and the date and times shown as GPS Time in your posting as input into the linked tool. Here I encountered some problems.

De-selecting all GLONASS, SBAS and Galileo SVs and without specifying an elevation mask, I do NOT get a plot with all the SVs you logged. The plot shows only the following SVs: 1,4,8,11,17,20,27,28 and 32.

Looking at the plot generated does not show that the differences in satellites tracked to be due to their position in the sky relative to the site. Of course, the plot does NOT include all the SVs you show.

Possibly my problem is a bad XYZ value in the extract posted and therefore a wrong value for the station location. The starting location does NOT need to be very accurate with tens of miles not making a difference. Also the tool is a beta and may not have a current ephemeris.

Your Trimble software should include a mission planning module. The package is easy to use. It does require that you specify a current ephemeris.

HTH,

DMM

Is the firmware the same?

Try switching antennas and see if the problem "follows" the suspect antenna.

I'll check the firmware. Thanks for that idea!

used to be on trimble's site

It is probably still there. it is a stand alone program search Mission PLanning

I used Ashtech's Web Mission Planning software, as suggested, but my results are different, not sure why. I get many more satellites than you obtained. Thanks for the effort.