As indicated elsewhere in this thread, there is a big difference in the types of surveys users perform. For the most precise surveys the absolute antenna calibrations are used. I consider orienting the antenna to part of "best practices."
The impetus for the first antenna calibrations was to account for the mixing of different antenna models on projects. Large height errors result from the failure to account for the different ways antennas capture the signal. See: https://geodesy.noaa.gov/ANTCAL/FAQ.xhtml#faq11 for more information.
The first calibrations were done in a relative sense by the US NGS. They tabulated elevation-dependent effects. Subsequently technology advanced to allow absolute antenna calibrations. These calibrations tabulate the impact of both horizontal orientation and elevation angle.
I show an extract from the absolute calibration file for the AOA D/M TA NGS NONE (i.e. no radome) antenna. Note that the first column of the table is the azimuth, other column values are vertical angle in five-degree intervals and rows are the azimuth.
Note that the first row of the table is for NO AZI. In other words the antenna was not oriented to north. Scanning through the values shows differences as a function of azimuth and elevation.
I have no current experience with GNSS equipment. I am surprised at comments that there is no way to orient the antenna.
T
note severe or rapid changes during data logging/collection.
Yeah.?ÿ I once ended a 4+ hour session just as a thunderstorm broke.?ÿ Got soaked retrieving the gear.?ÿ That session was useless, with a huge pk-pk range.
micro strip antennae, and utility grade antennae. Geodetic grade is kept in the office for special projects.
Of course, integrated receivers do not have an antenna connection point. The rule of thumb I have used for NGS/FAA measurements was to orient the faceplate to north, if there was no NRP specified. That way, no matter what we knew there was a consistent setup for each type of receiver/antenna combo.
Then again, I have processed data from repeat trips to the same network, with similar observation times, when I knew the original data was for a blue-book project (i.e. collected with north orientations) and the subsequent verification dataset was not collected with NRP orientations. I could not detect any appreciable difference, systematic or otherwise, between the two datasets - "appreciable" in this case meaning a few hundredths between the original and re-measured values.
antenna orientation could have no possible effect on the vertical.
The vertical effect probably won't be significant, but there is at least theoretical reason to think orientation could change it. If there is a large horizontal offset and the satellites aren't symmetrically located on average, then having more slant ranges from one side (e.g. south) than the other (north) would produce a vertical offset.
The National Geodetic Survey (NGS) Antenna Calibration pages contain important information about antennas, here: https://geodesy.noaa.gov/ANTCAL/index.xhtml
At the top of the page, you'll find a drop-down menu - "Browse Antenna Information by Company Brand and Model."
Say, for example, you are interested in a Trimble antenna, choose "TRM - Trimble Naviagation Ltd." and you will then see a long list of all the Trimble antennas that NGS (or the International GNSS Service (IGS)) has calibrated.
For each antenna, you will find listed the following info: antenna code, radome code, images, calibration data files, brief description, date calibrated, ARP (Antenna Reference Point), and NRP (North Reference Point).
NRP tells you the location/feature on the specific antenna model that should be oriented to TRUE (not magnetic) NORTH so that the calibrations can be correctly applied.
The possible values of NRP are all listed here (you can also get to this page by clicking on the "NRP" link at the top of the last column): https://geodesy.noaa.gov/ANTCAL/FAQ.xhtml#faq5
Some common NRP values are:
- RXC = receiver connector (connect antenna to external receiver)
- MMI = man-machine interface (the keypad/control panel)
- NOM = north orientation mark (placed on antenna by manufacturer)
(there are many others in the list)
In antenna FAQ #7, it says: "... If neither (ARP or NRP) is available, use for ARP the lowest physical point on the antenna, and orient the antenna cable attachment point to True North."
There is a lot of information on the antenna pages, including 24 FAQs, so I suggest taking a look there. There are a number of antenna subtleties that might be of interest to some.
Antenna calibrations include both phase center offsets (PCO) and phase center variations (PCV), and they can impact both horizontal and vertical (primarily) positioning results, and correct orientation of an antenna allows the calibrations to be applied correctly. So, it is definitely a best practice to correctly orient your antenna whenever using processing software that applies antenna calibrations (most software should and OPUS definitely does). It's possible it will make little difference for some antennas or situations, but why tempt fate?
Happy (antenna) orienting ... and stay healthy! -- Bill
Or back in the day when there only 4-5 hrs of satellite availability: night in winter, day in summer. 5 channel, L1, and car batteries.
Thanks to everyone who got involved in this discussion!
Congratulations on your first post to the forum, and thanks for adding good info to this thread.
