First I direct your attention to the FGCS Instrument Work Group memorandum dated June 22, 2001, subject: Test of Ashtech Z-Xtreme and Ashtech Locus GPS Survey systems.

According to this memorandum, Locus units were used in tests with National CORS stations, using Ashtech Solutions ver. 2.0, AND USING PRECISE SP3 FORMATTED SATELLITE EPHEMERIDES IN DATA PROCESSING.

I would really like to add this capability to my working knowledge of Ashtech Solutions Software. Please advise.

Secondly, given the above testing done on Locus receivers, can you furnish the required Locus antenna parameters to one Kent McMillan for addition to his "yellow box" software.


Thanks Guys

J.D.

J.D.,

Solutions doesn't use precise orbits (yet). The FGCS data must have been post-processed with other s/w. I'm told that a future release of Solutions will be able to use precise ephemerides, but can't give you a date at this time.

NGS hasn't performed antenna testing on Locus, so we do not have antenna parameters for that product. Antenna testing is separate from FGCS.

Regards,
Brian
Modified By Brian Ewing, PLS on 1/17/2002 at 1:51 PM

Brian Ewing, PLS said:

"Solutions doesn't use precise orbits (yet). The FGCS data must have been post-processed with other s/w."

To which J.D. Billings replied:

"I must have totally misinterpreted the 06/22/01 memorandum, but have no doubts about your knowledge of this product testing."

When using the Locus unit data in trimble software, wouldn't the vertical offset (0.125m) be the only parameter needed to get from the L1 phase center to the ARP?????

J.D.

J.D.,

You didn't misinterpret the FGCS memorandum (I just reread it). I didn't participate in the Locus FGCS test, but I've been working on Solutions since 2.0 and have yet to see precise ephemeris capability, so I can only assume that there is a discontinuity between the FGCS memo and how the testing was performed.

Regarding antenna info, yes and no. The other parameters would be azimuth and elevation-dependent variations in the electrical phase center (not tested by NGS).

Hope this makes sense.

Regards,
Brian

!

there is a discontinuity between the FGCS memo and how the testing was performed.

They must've hired someone from our local newspaper to write/report the info. There is always a "discontinuity" when they are invloved !!!

;0)

Jimbo
Modified By James Webb on 1/17/2002 at 6:47 PM

I don't even read our "weekly wipe". They would define a word such as "discontinuity" as one that would require the use of adult diapers.

jd

JD,

For what it is worth, I mentioned the FGCS memo/note about Locus to my State NGS advisor this past summer. He more or less chuckled and let me know in so many words that it didn't mean much.
As for the electrical phase offset, it has been explained to me that this point is somewhere above the antenna, and not in the center of the antenna. The trick to "cancelling out" the "EPO" is to orient your antennas the same way, thus eliminating a systematic error. As for me, I always have a compass handy and have the dispaly side of the Locus on the South.

Dave,

If you are using all antennae of the same type, orienting theem in the same direction effectively cancels the azimuth and elevation-dependent phase center errors on relatively short lines (where all are using very nearly the same constellation in highly similar geometry). That's why all Ashtech antennae have a North mark.

When mixing antenna types, or measuring long enough lines that the elevation for a given SV is significantly different from the opposite end of the line, it is important to know the phase center characteristics. Of course, this only applies to high-precision work, but is applicable to the ORGI experiments.

J.D. and Jimbo,

I used PC wording like "discontinuity" because I wasn't there, and don't know what went on.

Regards,
Brian

Regards,
Brian

I didn't realize that the orientation of the locus receivers made any difference. The manual doesn't say anything about it. I just assumed that the antenna was over the center of the rod. If there is an offset, it looks like it could potentially make a lot of difference, especially with the kinematic init. bar and with short baselines. Perhaps I sould be paying more attention to reciever orientation?
regards,
terry strickland

Terry,

Others can back me up, or correct me, but my experience with Locus units has shown no measureable eccentricities with centering. That is, when using Locus only I've seen no measureable difference when not worrying about the North orientation of all units. It probably is a very good suggestion to follow, and I do normally. Just not all the time.

.

Gents:

Here are the NGS calibration results:

ASH 110454 PROANTENNA L1 NGS ( 4) 01/10/30

Phase Center offsets in mm:
[N] [E] [Up]
-3.2 1.6 69.4

Vertical phase center variation (in mm) with SV elevation angle in 5 degree steps:

.0 .0 .7 1.8 3.0 4.5 5.8 7.1 7.9 8.4
8.4 8.0 7.0 5.4 3.2 .5 -2.8 .0 .0

These figures indicate that the Ashtech antenna tested had an phase center eccentricity of -3.2mm N and +1.6mm E. If I've got the NGS format right, it also means that the L1 phase center was found to be 69.4mm above the ARP.

As others have pointed out, these phase center variation values are most important when mixing antenna types, not when using the same type of antennas over short baselines.

Best regards,
Kent McMillan, RPLS Austin TX

That's all well and good, but don't confuse this antenna with having anything to do with my ORGI sessions. I'm not sure how you got off on the ProMark 2 antenna, but don't mix it in with the Locus L1/CORS ORGI experiment.

J.D. Billings

J.D.:

Yes, I didn't read Jim Frame's post carefully enough. That Ashtech antenna that the NGS tested was the ProMark 2 antenna. Do I take it that no antenna phase center models exist for the Locus receivers beyond a sort of average offset from ARP to L1 Phase Center? I guess I just would have thought that the manufacturer would have derived them.

Best regards,
Kent McMillan, RPLS Austin TX
Modified By Kent McMillan on 1/19/2002 at 3:14 PM

the case. I have no idea why the Locus antenna hasn't been NGS tested, but that seems to be the way it is. It does seem a bit perplexing considering the Locus as being so dependable, durable and just a damn good unit. Must be some corporate political B.S. involved somewhere.

For now, I've just been using the L1 Phase center to ARP offset as 0.125 m. I hope the tech guys will correct me if I'm wrong.


J.D.

JD,

The 0.125 meter number you shared with Kent is the right one to use. It represents what is used in Ashtech Solutions for that L1 phase center offset.

A careful reading of the information on the antenna calibration web page will tell you that information is based on a test and that test conditions can change the results. It's the old "Your results may vary." The tests you guys are doing matter to me since it relates to what surveyors do. You may not end up with antenna calibration information but you may recall that my efforts on the ProMark2 antenna missed the numbers from the NGS testing by 0.001 meter. Not too bad for a one legged fat man. They are right, I am wrong. But I was not far wrong.

I know that a lot of what you guys are dealing with the last several weeks is about millisquinches, particularly with Kent's comments about the advantages of the ITRF.

I feel the same way about debates about millisquinches and the relative merits of the antenna testing that I do about the precise ephemeris. I never had a client content to wait long enough for data from me to make use of a precise ephemeris. Even though my software would use it I never used that feature on a survey I did for money. Only science projects saw the use of the precise ephemeris. Nobody ever paid me for a science project.

If we need to sweat a millimeter or two then it is time for a "Z" and a choke ring antenna. Don't let them get you down, J.D., your objective was to determine the performance for a Locus, not make it into something else. What it is, is so very much more than what it is not.

JD has described the Locus in words I can relate to. If the objective is good answers, use the numbers you know and go to work. I have mixed ProMark2, Locus, SCA-12, and Z-12 data all together without heartburn about the quality of the numbers. In business class they taught me Management by Results. Doesn't that work for GPS as well?

If Kent's final answer falls half way between your Locus results and the dual frequency test results will you feel bad about your answer?

Thanks. I really appreciate the comments. You understood my project completely as being a test of the Locus unit, stretched to the max. My assumption is that it will be the same for any high quality L1 unit.

As for "feeling bad"?????/ Hell no. I was able to reproduce results well within anyone's margin for L1, not once, but twice. Three comparable sessions to me proves my original mission. It can be done. And it's gonna be done again on a bit larger scale. Same procedure and all.

Thanks

J.D.