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TBC Sky Plot utility

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(@big-al)
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Two or three months ago, I ran a session using a Trimble 4000 series receiver and Compact L1/L2 antenna, and the results of that session were less than expected, and I discovered that only a few satellites had been tracked and high PDOP values were perhaps a factor in the poor results. I haven't used the pair since.

Things have slowed down a bit, and yesterday, I ran a test session with the suspect receiver and antenna, to see if I could figure out anything. I don't suspect it has a large effect, but I have since acquired and mounted a groundplane on the suspect antenna, and whereas for the old session I had not oriented the antenna to north, yesterday's session included proper orientation to north.

In the past, I have usually downloaded the DAT file using a utility called Trimble GPLoad which is part of the old Trimble Survey Office (predecessor of TGO). Then, I have used the Trimble DAT to RINEX converter utility to produce a RINEX file. I did that today as well. The header of the resulting file is at the end of this post. However, now that I own a license of TBC, I wanted to see if I could download the DAT file directly from the 4000 receiver into a TBC project, and I was successful at that, using the "View>Device Pane", selecting the 4000 series receiver, and watching the download proceed. I was pleased that my investment in TBC apparently supports the old Trimble hardware. Oddly, the logged data file was identified as 3828-024-0 in the receiver, but was renamed/converted to 38280240#0#3.R in the TBC software.

Here's what I'm getting at - the SV's tracked do not seem to match what the Trimble Sky Plot utility predicts, and I don't understand why. I have compared the RINEX file (the header of which is below) to the session editor in TBC which lists which SV's were tracked, and when, and the results are consistent with one another. I tracked 10 GPS satellites during the course of this session 5,9,14,15,18,21,22,26,27,and 29.

However, when I run a "Sky Plot" for the occupied location using TBC's "Tools>Planning" I get a plot which indicates an almost completely different set of satellites available, and I don't understand why. I've downloaded and imported the latest almanac information (almanac.alm) from the Trimble website, but it seems to have had no effect.

Below is the sky plot I get in TBC and the RINEX header. Any suggestions would be most welcome. Thanks.

2.11 OBSERVATION DATA GPS(GPS) RINEX VERSION / TYPE
cnvtToRINEX 2.11.0 A. R. Thorp 25-Jan-12 21:52 UTC PGM / RUN BY / DATE
----------------------------------------------------------- COMMENT
MARKER NAME
3828 MARKER NUMBER
A. R. Thorp ACCORD Engineering & Surveying, LLC OBSERVER / AGENCY
3550A13828 4000 SSI SITE SURVEYOR7.29 REC # / TYPE / VERS
00044631 COMPACT L1/L2 WITH GROUNDPLANE ANT # / TYPE
1355179.2849 -4535326.2007 4260861.2636 APPROX POSITION XYZ
1.5240 0.0000 0.0000 ANTENNA: DELTA H/E/N
1 1 0 WAVELENGTH FACT L1/2
6 C1 L1 L2 P2 S1 S2 # / TYPES OF OBSERV
2012 1 24 18 57 15.0000000 GPS TIME OF FIRST OBS
2012 1 24 21 31 0.0000000 GPS TIME OF LAST OBS
0 RCV CLOCK OFFS APPL
15 LEAP SECONDS
10 # OF SATELLITES
G05 99 99 99 99 99 99 PRN / # OF OBS
G09 616 616 616 616 616 616 PRN / # OF OBS
G14 230 230 225 225 230 225 PRN / # OF OBS
G15 616 616 612 612 616 612 PRN / # OF OBS
G18 616 616 616 616 616 616 PRN / # OF OBS
G21 616 616 616 616 616 616 PRN / # OF OBS
G22 446 446 436 436 446 436 PRN / # OF OBS
G26 82 80 22 22 82 22 PRN / # OF OBS
G27 563 563 555 555 563 555 PRN / # OF OBS
G29 24 24 20 20 24 20 PRN / # OF OBS
CARRIER PHASE MEASUREMENTS: PHASE SHIFTS REMOVED COMMENT
END OF HEADER

 
Posted : January 25, 2012 2:22 pm
(@sicilian-cowboy)
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A guess would be something like having the wrong time zone setting, or some other easily overlooked setting.

 
Posted : January 25, 2012 2:26 pm
(@big-al)
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That's what I thought too, and I looked and looked but didn't find anything. I'll look again. Maybe it's a 12 hr vs 24 hr thing. Do you use the Sky Plot utility, Cowboy?

 
Posted : January 25, 2012 3:47 pm
(@dane-ince)
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some difference could be

Eleavtion mask the same for sky plot & RCVR?
Same Almamac for both?
Same location for both?

 
Posted : January 25, 2012 5:06 pm
(@big-al)
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Figured it out. My bad.

It is so easy to overlook a setting, and often so difficult to find the offending mistake.

It comes down to this - when setting up a station for a Sky Plot in the Planning Utility, time must be entered in 24 hour format. You can make it local time if you want, but still must be in 24 hour format. I had entered it in 12 hour format, somehow overlooking the associated need there would be to specify "PM", for which there was no option. Thus the sky plot previously generated showed the path of satellites that were up 12 hours previously....Duh!

 
Posted : January 25, 2012 5:24 pm
(@big-al)
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some difference could be

Dane,

Does the receiver have its own almanac, or is the almanac information received from the transmitting satellites as part of its package of data?

Al

 
Posted : January 26, 2012 7:16 am
(@dane-ince)
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GLAD YOU GOT IT WORKED OUT

The rcvr collects the current broadcast ephemeris(almanac) along with the other GPS data/signals. You select what ephermis you will in post processing or by the fact of when you send in the files for Opus.

 
Posted : January 26, 2012 10:54 am
(@mightymoe)
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some difference could be

Big Al, there are a couple of things from the old days with Trimble receivers.

Hopefully, I've got this correct: The north pointing indicator on the antenna only means something when you have two antennas of that same type up collecting data at the same time. The idea was that they will both be shifted off the points the same direction and distance so they cancel each other out. If you are collecting data as a standalone and processing against a newer antenna then it doesn’t really matter where you point the L1/L2 antenna. In fact, if you do more than one session it would be best to point it in a number of different directions.

Also, the older receivers would only track a limited number of satellites. I don’t know how few the 4000 would track, but with one of my old Trimble units if 12 were up it would not track all of them and it would struggle. I would “turn off” some of them and things would quickly improve.

 
Posted : January 26, 2012 1:36 pm
(@big-al)
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some difference could be

Moe,

As I understand it, antennas have a differential in three dimensions XYZ each of which change slightly with time and with changes in the satellite constellation, and the differential is related to the physical orientation of the antenna.

So, theoretically, if that is the case, whether you are collecting data from one antenna or two, or twenty, setting up the antenna so that it points to north, will enable the highest precision in the solution results. I'm still learning, but my impression is that it is incorrect to assume that the orientation of the 4000 series antenna is irrelevant when processing against a newer antenna. That said, perhaps the differentials involved are insignificant with the 4000 series antennas and as a practical matter, the theory is put aside for that reason?

But the gist of what you post in any case seems to reinforce what I am thinking about the orientation of the antenna - that whether or not the antenna is oriented to north will not (as a general rule) affect which satellites are tracked, but may affect the quality of the final solution. I think it was Loyal that had posted earlier about some older used Trimble antenna that he acquired which had a bad sector or quadrant and so as a result didn't track satellites properly within that quadrant. I don't know enough about the electronic behavior of these antennas to really understand how that could happen. The receiver has 18 channels. If one or more of those channels were not OK, I suppose that might affect the receiver's ability to track satellites?

Your suggestion that certain satellites might be "turned off" in the 4000 series receiver is very interesting. If there are in the vicinity of 32 GPS receivers in the constellation today, but my receiver only has 18 channels, I suppose I wouldn't be able to track all of them, and specifying which ones to track and which ones to give up on makes sense. Off the top of my head, I don't think I've read how to do that in the 4000 series literature I've been reading. I intend to look that up to see if its an option.

Thanks for your post.

Al

 
Posted : January 26, 2012 2:58 pm
(@mightymoe)
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some difference could be

So, theoretically, if that is the case, whether you are collecting data from one antenna or two, or twenty, setting up the antenna so that it points to north, will enable the highest precision in the solution results

Actually, I don't think that's true. I'm pretty sure that the old L1/L2's were set up to work as a pair. If say antenna #1 was off the point .02' to the northwest when you point the antenna to the north, then antenna #2 was also off the point .02' to the northwest when pointed to the north. So therefore, they cancel out the error if they both are pointed correctly.

Then the micro centered antennas came out and it became a bit irrelevant how the L1/L2 was pointed when they get matched up with one of those. At least that's what I was told. So if you have a L1/L2 antenna paired with micro centered antenna, pointing north just doesn't really matter.

However, the error is small and hard to trace in any event.

I'm not sure how too many satellites affect the 4000 when it's in static mode. I can say that it got weird when there were too many up with my old RTK units. It probably means less when the receiver is collecting a static session.

 
Posted : January 26, 2012 3:52 pm
(@big-al)
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some difference could be

Forgive me, I was thinking of static work when I posted, and I believe you refer to a convention that has more to do with RTK work involving a pair of Trimble 4000 receivers.

As in, if you used a single 4000 setup to log a static session, and submitted the logged data to OPUS for processing, NGS would use its modeling of antenna behavior as part of its solution process.

 
Posted : January 26, 2012 4:45 pm