Tag Archives: L1 Offset

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!

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