Tag Archives: NGS

What to do for Absolute Antenna Calibrations, when there are not any?

I could write a book on the difference between relative and absolute antenna calibrations, but I don’t need to and you should not spend much time wondering about why. Or if. You ‘Just need to make the change to Absolute Antenna Calibrations’ for everything.

But, depending upon which antennas you own, you may find there is not an absolute calibration for your antenna! What to do?

First a little background. Let’s look at a couple of sample antenna:

Relative ASH111660       NONE L1 GPS/GLONASS, base of RF connector->N  NGS (  3) 10/06/02
       0.3      -0.7      75.4
Absolute ASH111660       NONE L1 GPS/GLONASS, base of RF connector->N  NGS (  3) 10/06/02
       0.9      -1.1      57.3
The Absolute calibration is -18.1 mm from the relative calibration.
Relative MAG105645       NONE L1/L2 GPS                                NGS (  2) 00/12/21
       0.6       3.6      65.6                              
Absolute MAG105645       NONE L1/L2 GPS                                NGS (  2) 00/12/21
       1.2       3.2      47.5
The Absolute calibration is -18.1 mm from the relative calibration.                   

In general, this trend continues for most antenna because they were NOT actually re-calibrated for absolute calibrations, the absolute values are 18.1 mm less than the relative values. This is because this is the difference between the reference AOAD/M_B Dorne Margolin B Chokering:

Relative AOAD/M_B        NONE Dorne Margolin B, chokerings (Rogue)     NGS (  0) 97/10/27
       0.0       0.0      78.0                              
   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0
   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0
       0.0       0.0      96.0                              
   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0
   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0   0.0
Absolute AOAD/M_B        NONE Dorne Margolin with chokerings (Rogue, A IGS (  0) 11/03/25
       0.6      -0.4      59.8                              
   0.0  -0.2  -0.9  -1.9  -3.2  -4.6  -6.0  -7.1  -7.9  -8.2
  -8.1  -7.4  -6.2  -4.6  -2.3   0.7   4.5   9.1  14.2
      -0.1      -0.6      88.3                              
   0.0  -0.1  -0.5  -1.1  -1.8  -2.6  -3.4  -4.2  -4.8  -5.2
  -5.3  -4.9  -4.0  -2.8  -1.3   0.5   2.8   5.7   9.4

The L1 difference (59.8 – 78.0) = -18.2 mm (sorry, I don’t know why it is 18.2 instead of 18.1)
The L2 difference (88.3 – 96.0) = -7.7 mm

So if you have an antenna like the NAP 100 with a relative calibration north : 1.0mm, east : 3.8mm, up : 73.2mm above ARP; which would look like this:

Relative MAGNAP100         NONE L1    GPS
       1.0     3.8      73.2

I believe it is safe to say the absolute calibration would be:

Absolute MAGNAP100         NONE L1    GPS
       1.0     3.8      55.1

By the way, if you are still using L1 only receivers to do static work (I will forgive you if you are running Stop and Go) you need to sell them (PM2, PM3) on ebay and trade-up to X90-OPUS receivers. The days of L1 only should be numbered!


Filed under GNSS Solutions, PM3

OPUS Projects: Training

I am two days into the four day NGS ‘OPUS Projects’ training being held in at the Corbin VA observatory. Wow, this is cool.

Located next to Fort AP Hill the Corbin Training center which also houses the Fredericksburg Magnetic Observatory.

The NGS Antenna calibration piers are out back:


ImageThats the robotic absolute calibration fixture on the right in the foreground.

Anyway, back to ‘NGS OPUS Projects.’ OP is going to change the way we all do control going forward. It will take me awhile to digest everything but here are some high points:

o OP has been around for a 1/2 decade

o OP is a piece of cake to submit observation files to using the existing OPUS web submission page. (You can have your crews submit files directly from the field.)

o OP allows you to delegate functions: crews can submit and attribute shots; sessions can be administered by a project leader and the final adjustment can be performed by the manager.

o OP will magically fix observations. I have test files that only use 80% of the observations and have 60% fixed observations when processed in OPUS. Running the same files in OP, with other nearby observations results in 98% used and 90% fixed. Magic.

o OP takes care of all the framing issues. Everything is processed internally in IGS08 (or whatever the latest frame might be.) Then OP converts to NAD83-2011.

o OP is easy to use. Really easy to use.

o OP is free.

o I trust OP. Today I ran a fairly large job that encompassed 3-days, 24-hour observations for 5 south western utah TURN stations. I processed against 8 CORS sites. Results look excellent. Then I unconstrained all but one of the CORS sites and everything still fell into place (within +/- 1 cm.)

o OP does all of the dirty work for Bluebooking (that’s tomorrow!)

o OP generates some killer reports.

o OP is going to come out of beta in September (maybe, probably.)

In any case, you’re going to hear a lot about OPUS Projects over the next year. It is the real deal and it (if you doing work with accurate positions) it is going to change the way you do things. OP is going to save you time. OP is going to produce a better work product. In short, I think that OP is going to significantly change geodetics in the United States. And since it works anywhere, I suppose the world.




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