[IGSMAIL-6987] North reference point (NRP) definition added to antenna.gra

[Ralf Schmid]

Dear IGS colleagues!

In order to account for horizontal antenna phase center offsets (PCOs) 
and azimuthal phase center variations (PCVs), it is essential to know 
how receiver antennas have to be oriented with respect to the north 
direction. So far, this information was only provided by the antenna 
calibration facilities, whereas it was not contained in any of the IGS 
antenna files.

At the IGS 2010 Workshop in Newcastle, the Antenna Working Group decided 
to follow the approach of a so-called "north reference point" (NRP), in 
accordance with the well-established "antenna reference point" (ARP) 
which provides the reference for measuring the antenna height above a 
certain marker. In contrast, the NRP designates the element of the 
antenna that has to be oriented toward the north direction.

Following a recommendation of the IGS 2014 Workshop in Pasadena, the NRP 
definition could now be added to the IGS file antenna.gra that is 
available at
ftp://ftp.igs.org/pub/station/general/antenna.gra

There are four major features:
   MMI: man-machine interface
   NOM: north orientation mark (placed on antenna by manufacturer)
   RXC: receiver connector (connect antenna to external receiver)
   UNK: unknown
Besides, there are twelve secondary features and connector designations 
that are only needed for specific antenna types. Generally, the north 
orientation mark (NOM) is chosen to be the NRP, in case it is present 
and clearly visible. If not, an alternative is needed.

The NRP definition can be found in two places:

1) below the ARP definition within each antenna sketch ("???=NRP"), e.g.:
> AOAD/M_T
>
>                          -----
>                      /           \
>                     |             |
>   +-------------------------------------------------+     <--  0.1020  TCR
>   |                                                 |
>   |                                                 |
>   |                                                 |
>   |                                                 |
> +-+-------------------------------------------------+-+   <--  0.0380
> +-------------------+-------------+-------------------+   <--  0.0350  BCR
>                     |             |
>                    =|             |
>                     +------x------+                       <--  0.0000  BPA=ARP
>                                                                        RXC=NRP
> <--                      0.3810                     -->

2) besides the ARP definition within the machine-readable section at the 
end of the file, e.g.:
> Machine-readable quick reference section begins here.
...
> AOAD/M_T BPA RXC

As regards the "JAV_GRANT-G3T   NONE" antenna, the NRP provided in 
antenna.gra (RXC=NRP) is not consistent with the robot calibration 
contained in igs08.atx so far. The latter referred to a north mark that 
was hardly visible. In consultation with the manufacturer, it was 
decided to harmonize the NRP for all JAV_GRANT/JAVGRANT antenna types. 
An updated robot calibration will be published with the next release of 
igs08.atx.

IGS station operators are asked to consider the NRP definition provided 
in antenna.gra for future antenna installations, but NOT to touch 
current installations. Instead, a possible misalignment should be 
reported in the site log:
> 4.x  Antenna Type             : (A20, from rcvr_ant.tab; see instructions)
...
>      Alignment from True N    : (deg; + is clockwise/east)
http://igscb.jpl.nasa.gov/igscb/station/general/blank.log

Manufacturers are asked to provide NRP information together with the ARP 
definition when registering new antenna types for the IGS files. 
Ideally, each antenna type should be equipped with an easily visible 
north orientation mark.

Help on identifying the NRP of outdated as well as uncalibrated 
equipment (all antenna types with UNK=NRP) is greatly appreciated. 
Please send a short note, ideally including a photo, to 
schmid at dgfi.badw.de! If you have further questions, please use the same 
e-mail address.

Many thanks to everybody who helped to compile the long list of NRP 
definitions, especially to Geo++ and NGS!

With best regards,
Ralf Schmid
on behalf of the IGS Antenna Working Group