Question of the Month

Archive

May 17, 2003

Q: I have noticed a discrepancy between the antenna phase center offset in OPUS relative to the published phase center offset for our antenna.  I submitted a RINEX file specifying the above antenna and got a NAD83(CORS96)2002.0000 Ellipsoid height of 247.592 m.  I submitted the same RINEX file with no antenna specified and got an ellipsoid height (same datum) of 247.673 m.

To my understanding, the difference (0.081 meters) should be equal to the antenna phase center offset from the Antenna Reference Point.  However, the L1 phase center offset for our antenna is 0.110 meters. How can this be??

A: When we submit the file, bkly2710.02o to OPUS with correct identification of the antenna, the NAD83 ellipsoid height of the ARP is 692.593 meters with peak-to-peak uncertainty of .025 meters.  In contrast, the published NAD83 ellipsoid height of the ARP is 692.580 meters.  The discrepancy between the one-day OPUS height and the multi-day published height is :

```
692.593 m  OPUS-derived ARP
- 692.580 m  Published ARP
-------
.013 m  discrepancy
+/-  .025 m  OPUS peak-to-peak uncertainty

```

The answer looks pretty good to me when we calculate the height the correct way--in other words, by correctly identifying the antenna.

The antenna calibration for antenna model NOV600 states that the ARP-to-L1PC offset is 0.0905 meters (see http://www.ngs.noaa.gov/ANTCAL/Models/NOV600.shtml )

The NAD83 ellipsoid height of the L1PC for the OPUS-derived solution of bkly2710.02o is therefore

```
692.593 m  OPUS-derived ARP
+    .091 m  ARP-to-L1PC offset
-------
692.684 m  OPUS-derived L1PC

```

Now you want to solve the problem the wrong way by failing to identify the antenna.

When we submit the same file to OPUS without identifying the antenna (NONE), the NAD83 ellipsoid height of the default nominal L1PC is 692.719 meters.  The discrepancy between the right answer and the incorrectly determined answer is:

```
692.719 m  Incorrectly Derived OPUS-derived L1PC
- 692.684 m  Correctly determined OPUS-derived L1PC
-------
.035 m  discrepancy
+/-  .021 m  Incorrectly Derived OPUS-derived peak-to-peak uncertainty

```

The discrepancy is outside the peak-to-peak uncertainty by 14mm. Moreover, the difference between the incorrectly determined one-day OPUS solution and the published solution is:

```
692.719 m  Incorrectly Derived OPUS-derived L1PC
- 692.671 m  Published L1PC
-------
.048 m  difference

```

Note that this nearly 5cm difference contains two sources of error: first, the comparison is between one day of data versus the published mean of 10 days of data; second; one value is calculated incorrectly and the other is calculated correctly.

Why does the incorrect method give such a large 5cm discrepancy?  First, the ARP-to-L1PC offset of 0.0905 m is only half the calculation.  The remaining half of the calculations have neglected the variations in the phase centers due to the elevation angles of the arriving GPS signals.

Note from the antenna calibration table that when the signal arrives from elevation angle 85 degrees (i.e., almost overhead) the L1PC is another 0.7mm higher than the nominal value.  When satellite signals arrive from 45 and 40 degrees above the horizon, the L1PC is another 8.1mm higher than the nominal value.  These elevation-angle dependent terms are totally neglected from the incorrect method of L1PC calculation.

Where do the neglected terms go?  They spill into the tropospheric corrections for atmospheric water vapor that directly affect the height calculations, thereby giving unpredictable and incorrect heights.

The correct answer depends on identifying the antenna, so that one form of calculation (phase center variations) doesn't corrupt another form of calculations (tropospheric corrections).  If the antenna model were unimportant, we would not bother with antenna calibrations and ARP coordinates.  Everyone could always choose "NONE" for the antenna, regardless of make and model, and life would be easy--except the heights would be wrong!

So, if you know your antenna and we have the phase pattern then select it from the drop-down list in OPUS and enjoy.       :)

Phase patterns from NGS - http://www.ngs.noaa.gov/ANTCAL

National Geodetic Survey - CORS Team