As part of an experiment I was conducting using TGO, I needed to add a CORS antenna type not defined in the TGO antenna

Creating a New Antenna Definition in Trimble Geomatics Office

As part of an experiment using TGO, I needed to add a CORS antenna type not defined in the TGO antenna.ini file. Thanks to some help from Trimble Support, I added the antenna and was able to process the baselines needed.

The antenna in question is a 3S Navigation Temperature Stabilized Antenna (http://adriang.com/AACE-Industries/products.htm). This antenna is used at CORS USN1 (http://sopac.ucsd.edu/cgi-bin/dbShowArraySitesMap.cgi?site=usn1). This antenna will be used as an example later to demonstrate the addition of an antenna to TGO.

Before going through the trouble of creating a new antenna definition, check that you have the latest antenna definitions from Trimble. Visit the Trimble web site at: www.trimble.com and navigate to the support page. From there download the latest Trimble Office Configuration Utility (http://www.trimble.com/trimbleconfiguration_ts.asp ). Often another antenna will have characteristics very similar to a defined antenna. For example, the addition of a radome will minimally affect the antenna modeling. If the antenna needed is still not available from Trimble, the procedure detailed below will rectify the situation.

Adding a New Antenna Definition

Adding an antenna for use by TGO involves creating a Phase Center Variation file and modifying the antenna.ini file. The antenna.ini file references the phase center variation file. In a default installation, these files are located at \Program Files\Common Files\ Trimble\ Config.

The easiest way to create a phase center variation file is to modify an existing file. The file consists of two sections. The first section lists the L1 and L2 phase center offsets. The second section lists the variations of these components with satellite elevation. Antenna phase center offsets and variations for antennas not defined by Trimble can found at the NGS and IGS web sites. NGS does relative and absolute antenna calibrations and publishes the results at http://www.ngs.noaa.gov/ANTCAL/. TGO uses relative antenna calibrations for phase center modeling. IGS antenna calibrations are at http://igscb.jpl.nasa.gov/igscb/station/general/igs_01.pcv . Both the NGS and IGS phase center variation files use the same format, which differs from the format used by TGO. For more information on antenna calibrations in general, see http://www.ngs.noaa.gov/ANTCAL/images/summary.html.

The major differences between the NGS / IGS format and the Trimble format are the reference point for the electrical antenna phase center and the order of the elevation based variations. The NGS / IGS format refers the phase center offset from the antenna reference point while the Trimble format references the phase center offset from the nominal antenna phase center.

The following phase center variation table illustrates this difference. The Trimble version of the phase center variation table for the Leica AT502 antenna shows the L1 electrical phase center as 16.2 millimeters above the nominal antenna phase center and the L2 electrical antenna phase center as 19.8 millimeters above the nominal antenna phase center. This file is named lat502.ngs.

;PCT converted from ant_info.003 <MLM-00/10/26-86>

;LEI AT502 Aero element L1/L2, External NGS ( 2) 99/06/14

;

;Mean phase center (mm) North East Up

L1NominalOffset = 0.3 2.0 16.2

L2NominalOffset = -1.4 1.8 19.8

;Elevation range (deg) Start Stop Step

ElevationRange = 5 90 5

;Azimuth step size (deg)

AzimuthStep = 0

;Asimuth/elevation corrections (mm)

AZ=0

;L1

0.0 0.3 3.3 5.5 7.1 8.3 8.9 9.1 9.1 8.8

8.2 7.4 6.4 5.3 4.2 2.9 1.5 0.0

;L2

0.0 -4.6 -2.7 -1.1 0.5 1.6 2.4 2.8 2.9 2.6

2.0 1.1 0.3 -0.6 -1.3 -1.5 -1.2 0.0

In the NGS version of the antenna phase center variation table for the same antenna below, the L1 electrical antenna phase center is 61.8 millimeters above the antenna reference point and the L2 electrical antenna phase center is 65.4 millimeters above the antenna reference point.

LEIAT502 Aero element L1/L2, External NGS ( 2) 99/06/14

.3 2.0 61.8

.0 1.5 2.9 4.2 5.3 6.4 7.4 8.2 8.8 9.1

9.1 8.9 8.3 7.1 5.5 3.3 .3 .0 .0

-1.4 1.8 65.4

.0 -1.2 -1.5 -1.3 -.6 .3 1.1 2.0 2.6 2.9

2.8 2.4 1.6 .5 -1.1 -2.7 -4.6 .0 .0

The north and east offsets are the same in both versions of the phase center variation table.

Looking at the variations of the electrical antenna phase center by elevation, in the NGS version of the table the differences are listed from ninety degrees to zero degrees in five-degree increments while the Trimble version of the file lists the elevation differences from zero degrees to ninety degrees in five-degree increments. The values are the same but in reverse order.

In the antenna.ini file tells TGO which phase center variation file to use with each antenna and relates the antenna reference point to the nominal phase center. To add an antenna to the antenna.ini file, add a line to the [RINEX] section of the antenna.ini file. The line for the Leica AT502 antenna reads:

Ant55=LEI AT502,0,default=0

The first field is an arbitrary one up number that identifies the antenna and a name. This number is unique for the section of the antenna.ini file where the antenna appears however, an antenna may appear in more than one section and have different numbers in each section. To add an antenna for use with CORS add the antenna to the [RINEX] section. The name references the antenna to a later section in the antenna.ini file. The next field is the RINEX measurement method and the third field is the default measurement method.

The final section of the antenna.ini file relates the phase center variation table and the nominal antenna reference point. This section also provides a link to the antenna names given in a RINEX observation file. An example of the entry for the Leica AT502 antenna follows:

[LEI AT502]

Name=Leica AT502

Manufacturer=Leica

Class=Survey

PartNumber=

CharCode=L6

Type=65

MeasMethod0=0.00000,0.04560,0.00000,"Bottom of antenna mount"

RINEXMethod=0

RINEXName=LEIAT502

RINEXName=LEI AT502

RINEXName=LEICA AT502

PhaseCorrTable=

NGSCorrTable=lat502.ngs

Freq=2

GraphicsFile=Leica_AT502.jpg

The first line must match the entry in first field of the line in the [RINEX] section of the file. The second line provides the antenna name that will appear in a TGO pull down menu. The Class field is optional. Classes include Survey, Mapping, Positioning, Machine Control and Ag. Any antenna added from a CORS would fall in the Survey class. The CharCode is an optional two-character code that can be used to identify the antenna where space is limited. The characters can be any combination of letters and numbers. If entered this code must be unique. These codes are used in Trimble's binary DAT-files and in CMR-transmission data for RTK surveys. The Type field must also be unique. This number does not have to match the number in the Ant= field detailed above.

The MeasMethod field relates the nominal phase center to where the antenna height is measured. There can be more than one MeasMethod for an antenna. The first method is method 0 followed by 1 and 2. Following the measurement method, the first value is the radius of the antenna or ground plane if a slant measurement method is used, the second value is a vertical measurement and the third value is a correction if an indirect measuring device such as an internal tape is used. The last field is a description of the measurement method that will appear in TGO when selecting the method. When a slant measurement method is used, the first value is used to reduce the slant measurement to vertical; the second value corrects the derived vertical measurement to the nominal phase center. In the case measuring to the bottom of antenna mount, as with a fixed height tripod or a CORS, the first and third values are zero and the second number is the distance from the ARP to the nominal phase center.

The nominal phase center of the Leica AT502 is 0.04560 meters above the ARP. The phase center offset from the Trimble format phase center variation file is 16.2 millimeters.

ARP to nominal phase center per antenna.ini file:	0.04560
+ Nominal phase center to electrical phase center per Trimble lat502.ngs:	0.01620
ARP to electrical phase center per NGS PCV file:	0.06180

The RINEXMethod field is the default method for measuring the antenna height for RINEX files. The RINEXName field(s) are the names that will appear for this antenna in the header of the RINEX observation field. If this name does not match the name in the header of the RINEX observation file, TGO will default to Unknown External when importing the RINEX file. Never use an undefined antenna for any survey grade baseline processing. If Unknown External appears as the antenna type, investigate and edit the antenna type before processing.

The PhaseCorrTable field, the NGSCorrTable and possibly the IFECorrTable fields point to a phase center variation table file. These phase center variation files, regardless of their origin, all use the Trimble format. The PhaseCorrTable field, if present, points to a phase center variation file derived from Trimble antenna calibration. These files have an extension of PCT. The NGSCorrTable field, if present, points to a phase center variation file derived from NGS antenna calibration. These files have an extension of NGS. The IFECorrTable field, if present, points to a phase center variation file derived from Institute for Geodesy absolute calibration. These files have an extension of IFE. When defining a new antenna use the NGSCorrTable field and point it to the phase center variation file you create.

The Freq field is 1 for single frequency antennas and 2 for dual frequency antennas. The last field, GraphicsFile, points to a photograph of the antenna, if available.

Example

Using the procedure outlined above add the 3S Navigation Temperature Stabilized Antenna to the antennas TGO can use. The NGS calibration table does not include the 3S antenna. The IGS has phase center variation table has the following entry.

THREE-S        3S-02-TSADM         TSA-100 w/ Dorne Margolin  NGS (  1) 00/05/08

       1.7       3.6     272.5

    .0    .0    .2    .5   1.0   1.5   2.0   2.5   2.9   3.3

   3.6   3.9   4.2   4.5   4.9   5.4   6.0   0.0   0.0

        .8       4.2     291.6

    .0    .1    .3    .4    .6    .9   1.1   1.5   1.8   2.2

   2.5   2.9   3.3   3.6   3.9   4.2   4.3   0.0   0.0

The up phase center offset from the ARP for this antenna is 272.5 millimeters. We do not know distance from the ARP to the nominal phase center however, that can be reconciled in the antenna.ini file. Create the NGSCorrTable using the Trimble format:

;PCT converted from ant_info.002 <MLM-99/04/07-60

;3S-02-TSADM 3S Navigation NGS ( 1) 00/0/08

;

;Mean phase center (mm) North East Up

L1NominalOffset = 1.7 3.6 0.0

L2NominalOffset = .8 4.2 19.1

;Elevation range (deg) Start Stop Step

ElevationRange = 0 90 5

;Azimuth step size (deg)

AzimuthStep = 0

;Azimuth/elevation corrections (mm)

AZ=0

;L1

0.0 0.0 6.0 5.4 4.9 4.5 4.2 3.9 3.6 3.3

2.9 2.5 2.0 1.5 1.0 .5 .2 0.0 0.0

;L2

0.0 0.0 4.3 4.2 3.9 3.6 3.3 2.9 2.5 2.2

1.8 1.5 1.1 .9 .6 .4 .3 .1 0.0

The north and east offsets remain the same. Set the L1 up offset to 0.0 and the L2 up offset to the difference between the IGS values for the L1 phase offset and the L2 phase offset. For the elevation dependent phase center variations copy the IGS values but in the opposite order they appear in the IGS table. Save the file with the name 3S02TSADM.ngs

Add the following entry at the end of the [RINEX] section of the antenna.ini file:

Ant161=3S-02-TSADM,0,default=0

The last entry in the current version of the antenna.ini file is Ant160=Unknown Ext. One up the Ant= number to 161 and name the antenna.

At the end of the antenna.ini file, add the following:

[3S-02-TSADM]

Name=3S Nav Temp Stabilized Ant

DCName=

Manufacturer=3S Navigation

Class=Survey

PartNumber=

CharCode=TS

Type=187

MeasMethod0=0.00000,0.27250,0.00000,"Bottom of antenna mount"

RINEXMethod=0

RINEXName=3S-02-TSADM

PhaseCorrTable=

NGSCorrTable=3S02TSADM.ngs

Freq=2

GraphicsFile=

The name in brackets matches the entry in the [RINEX] section of the antenna.ini file. This name will appear in TGO pull down menus.

In the MeasMethod0 field set the phase center offset to the value in the IGS table. This has the effect of making the distance from the ARP to the nominal phase center equal to the distance from the ARP to the electrical phase center. Following the pattern above regarding the relationship between the nominal phase center, the antenna reference point and the electrical antenna phase center:

ARP to nominal phase center per antenna.ini file:	0.27250
+ Nominal phase center to electrical phase center per custom 3S02TSADM.ngs :	0.00000
ARP to electrical phase center per IGS PCV file:	0.27250

For the antenna type, search the antenna types to find an unused number. The highest number in the current antenna.ini file is 255. In this case, a gap in the numbers allowed the use of a lower number.

Save the modified antenna.ini file and launch TGO. Test the configuration by processing baselines to the new antenna. Phase center offsets include north and east components however, the majority of the effect of antenna modeling appears in the ellipsoid height. Compute L1 Fixed solutions, L2 Fixed solutions and L3 Fixed (IonoFree Fixed) for the baseline. These three solution types test the L1 and L2 phase center offsets. The L3 solution is a combination of the L1 and L2 observables and provides further confirmation.

The accepted value for the ellipsoid height of the ARP of CORS USN1 is 57.400 meters. This value is based on a least squares adjustment of seven baselines processed using PAGES with 24-hour observation sets. The ellipsoid heights resulting from processing one 24-hour observation set with TGO are as follows:

Baseline Solution Type	Ellipsoid Height	Difference from Accepted Value
L1 Fixed	57.401 meters	0.001 meters
L2 Fixed	57.402 meters	0.002 meters
L3 Fixed	57.399 meters	-0.001 meters

The close conformance of the ellipsoid heights confirms the efficacy the new antenna definition.

After confirming the changes, backup the antenna.ini file and the phase center variation file. When updating TGO or other Trimble utility programs the antenna.ini file, with the added antenna, is backed up and overwritten. Having the changes in a safe place is a surer way to ensure that your work is not lost at the next update.

Appendix: Defining Baseline Processing Styles to Force L2 and L3 Solutions

L2 Fixed Solution

To specify an L2 fixed solution, in TGO use the Survey pull down menu to access the GPS Processing Styles dialog. Create a new style, naming it appropriately. Specify a Solution Type of Fixed and click on the Advanced button to edit the style. Click on the Global tab and select a Frequency Type of L2 as shown below.

L3 Fixed Solution

By default, the minimum baseline length for receiving final pass ionospheric corrections is five kilometers. By specifying a minimum length of zero all baselines, regardless of length, receive ionospheric corrections during the final pass. This compels the solution type to Ionospheric Free Fixed, assuming it is possible to fix a minimum of three integers, also known as an L3 fixed solution.

To specify a minimum baseline length of zero for final pass ionospheric corrections use the Survey pull down menu to access the GPS Processing Styles dialog. Create a new style, naming it appropriately. Specify a Solution Type of Fixed and click on the Advanced button to edit the style. Click on the Iono tab and specify a length of zero in the Apply to all baselines longer than: text box as shown below.