Questions You Should Know about Geodetic Antenna

For Trimble ALLOY receiver: here are some suggestions as you navigate the Left Menu items. Under Network Configuration, I make some IT security suggestions, but you are the master of this receiver, so suit to your needs :

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I am using WYGI CORS as an example. You will replace WYGI with your 4-char ID

Receiver Status | Identity: System Name = 4-char ID (WYGI)

Data Logging | Summary: Auto Delete is ENABLED

Data Logging | RINEX Metadata: Station Name = 4-char ID (WYGI)

Receiver Configuration | Tracking: elevation mask is either at 0 or 5 degrees. Clock Steering is ENABLED. We request Everest Multipath to be DISABLED - but choose to your needs.

Receiver Configuration | Antenna: select your antenna model under "RINEX Name". Add your "Antenna Serial Number". (we see current antenna is set to True North, i.e. 0 deg alignment from True N)

Receiver Configuration | Reference Station: Station Name and Station Code are the 4-char code (WYGI)

Network Configuration | HTTP: HTTPS is enabled to default 443 port. Enable High Security.

Network Configuration | NTP: if you do NOT need this option, DISABLE "Enable NTP client" (lockdown for security)

Network Configuration | Zeroconf/UPnP: DISABLE "Zeroconf service discovery" and "UPnP service discovery" (this is akin to Player-to-Player, as I understand it. I choose to lockdown for security).

Security | Configuration: ACTION: Please create an account with "File Download" only permissions. You tell us what the credentials are. Can be something generic like user=data or user=download and then tell me what the password for that is. That way we can download via HTTPS, should you desire to shut down FTP in the future.

Firmware: please update to most current, or second to most current (if you are conservative). A lot of the ALLOY receivers in CORS are running 6.21 or 6.23, but go to 6.26 if you feel confident in it.

Once you are happy with the configuration, create a backup file and download it to your local computer. Go to:

Receiver Configuration | Application Files:
a) Under Operation pull-down menu, choose "Generate Clone File". Give it some Filename that is meaningful to you. I tend to do {station}_{yyyy}{ddd}_{firmware} like "SACR__620" - but suit yourself. Hit the "Enable All" button + OK.

b) Under Operation pull-down menu, choose "Download Clone File".

A "coordinate" file for a CORS contains the officially adopted position and velocity for the station's antenna reference point (ARP) as well as for its L1 phase center. Hence, this file is also called a "position-velocity" file. This file may also contain officially adopted positions and velocities for selected geodetic monuments located near the station. Note that the ARP usually corresponds to the center point on the bottom of the pre-amp on the CORS antenna.

A coordinate file presents positions and velocities in both the International Terrestrial Reference Frame (ITRF) and the North American Datum of (NAD 83). Also, it presents positions in both geocentric cartesian coordinates (X, Y, Z) and in geodetic coordinates (latitude, longitude, and ellipsoidal height) for the GRS80 ellipsoid. NAD 83 positions given in these files are identical to those contained in the National Geodetic Survey's Integrated Database

See NCN Data and Products page, section Published Coordinates and Velocities, for more information.

We supply 2 positions for our CORS users: the L1 Phase Center (L1PC), and the center of the bottom of the antenna which we call the antenna reference point (ARP). We supply these positions in 2 reference frames, NAD83 (,MA11,PA11)epoch .00 and ITRF (epoch .0) . We do not use the WGS84 to position points. You can read more about WGS here: Mordern Terrestrial Reference Systems: WGS84 and ITRS

The published coordinates and velocities of each CORS are stored in the coordinate file. See NCN Data and Products page, section Published Coordinates and Velocities, for more information of how to access the coordinate file.

We recommend that you use the published coordinates and velocities in the coordinate file, and consider the coordinate in the RINEX file header and station logs as APPROXIMATE coordinates. It is the policy of NGS to overwrite the appoximate position in the header with the published NAD83 position, but some older files may not have these corrections.

The L1 phase center is the theoretical point in space where the L1 carrier phase is received "on average." The actual location where this signal is received, however, varies as a function of the direction of the incoming GPS signal, and hence an averaging process is required.

The antenna reference point (ARP) is a specified physical point on the antenna.

The relative spatial relationship between these two points is determined via a calibration process in a laboratory-type environment. This process involves collecting and processing several hours of GPS data, and it involves several assumptions about antenna characteristics.

Are you interested in learning more about Geodetic Antenna? Contact us today to secure an expert consultation!

The National Geodetic Survey (NGS) has calibrated many antennas and has determined the average spatial relationship between these two points for each of several classes of antennas. See NGS's Antenna Calibrations (ANTCAL) page for more information.

For stations in the NOAA CORS Network, NGS provides spatial coordinates for both the L1 phase center and the ARP, as different GPS-processing software packages may use one or the other of these points or possibly even some other reference point.

To find out whether your GPS-processing software uses the L1 phase center or the ARP, you need to contact the company that produced this software.

NGS encourages GPS companies to have their software use the ARP, but our agency can not require this practice.

Most of the handheld units designed for recreation purposes are good for general location and navigation. They are not designed for precise data collection. The location coordinates are determined from using one GPS receiver without the benefit of corrections from a secondary source, it is known as single-point positioning. Consumer grade handheld GPS units generally provides coordinates with around 3 meters horizontal accuracy and usually with poor vertical accuracy. Users can get more realiable solutions by tracking more satellites from a secondary satellite constellation such as Galileo, GLONASS, etc.. Users can also improve postion accuracy by enabling connection (if capable) with satellite-based augmentation systems (SBAS) such as Wide Area Augmentation System (WAAS, avaialble in North America) or European Geostationary Navigation Overlay System (EGNOS, avaialble in Europe).

More information about the GPS augmentation systems can be found at GPS.gov.

This problem may result from several causes but the chance the NCN position is seriously in error is very small. First, you must ensure that you are comparing NAD 83 positions with NAD 83 positions or the equivalent. Different datums can introduce over 100 meters in position differences, e.g. NAD 27 to NAD 83. If the datums are the same, are you trying to compare an uncorrected point position (yours) with an NGS published position? An uncorrected point position can be in error by as much as 100 meters (horizontally when SA was on) 95% of the time due to Department of Defense signal dithering. If that is not the problem, have you checked to ensure that the reference position you think you are using in your computations is actually being used by the reduction software? The positions in the RINEX files are ONLY APPROXIMATE and may be tens of meters or more in error. You must enter into your reduction software an accurate position from the appropriate coordinate file. There are other possible problems, but these are the most obvious and most common.

Probably not the one you have now if you are asking this question.

We presume in this FAQ that you have obtained a low cost L1 device,
and that you wish to do RTK navigation to obtain ~2cm accuracy over moderate to short baselines.

If you received a small magnetic puck antenna as part of your GNSS device, replace it with something suitable.  While it is possible to use this antenna in some static conditions, it is rather unlikely you will be able to do any mobile work with it. And if BOTH your rover and and your reference station are using this sort of antenna, you are more or less doubling problems you can easily avoid.

The golden rule regarding how to invest your money in GNSS technology is not unlike that famous rule for real estate:  Location, Location, Location!   The first and very best thing you can do to ensure success is to select a suitable open sky location for your reference antenna site.  The next is to select a suitable antenna.  This is in fact more important than the GNSS system you select, because any corrupting influences introduced in the antenna propagate into the GNSS observation and become very hard to then remove.  [Finally of course a good L1/L2 GNSS device is wanted, although for baselines under 10~15km there are many applications that can achieve solid centimeter performance with inexpensive L1 only products such as the popular uBlox.]

Why does an inexpensive puck antenna not suit?  Because the phase delay and phase center of the antenna will vary as much as ~60 cm depending on the angle and elevation at which the GNSS signal comes into it.  And this will also vary significantly depending on the effective ground plane where you place it.   Do not even think about placing such an antenna “on the dash” for the rover side in a vehicle, or “on the ground” when standing nearby or over it, or “at your desk” indoors – even if you are in a wooden home.  All of these examples will be so corrupted by multipath as to be unusable.  A clear view of the sky is essential for success in the base station. In this discussion, integrated antenna solutions and hybrid antenna with other signals in the same physical design (such as GNSS and cellular) are typically also a poor choice for achieving high accuracy.

By contrast, the phase bias found in a “good” antenna has a variation of less than two centimeters (geodetic grade devices are in the range of fractions of a mm, see also Footnote A).   Recall that the carrier wave of the L1 signal is 19 centimeters, and that to achieve a “fixed” state (ambiguity resolution, or AR) requires that the precise number of these wavelengths must be determined and tracked for each GNSS signal.  Once locked on, the carrier is being tracked to a resolution of tens of picoseconds, pretty impressive for a signal that is 12,000 miles away!  In other words, if your choice of antenna is adding an unknown changing value of three wavelengths of measurement bias based on the angle of arrival for each signal, that is making it rather hard for your navigation filter to achieve a fixed AR solution.  And then, when the vehicle rotates and changes heading, it all shifts yet again.  The key message here is:  If you want to use an L1 system under such conditions, please invest in a suitable antenna.

So, back to which one?

In the absence of weight restrictions for your mission (such as a drone application), we recommend selecting your antenna from one of the many proven avionics antenna designs available for L1/L2 GPS/GLONASS use.  They are designed to be all-weather devices, fairly small, and of fairly moderate price.  Chose one with L2 support if at all possible.  From a strict theory point of view, the optimal antenna to meet a given requirement set does not admit other signals which are not required (matched filter theory), but selecting an antenna that supports L2 or GLONASS, even when they are not required for your day-one mission needs, is often a wise investment.

The antennas you will want to consider are all “active” in that they require a DC power voltage (supplied by your GNSS device over the coax) and they have a gain value you need to match to the expectation of the GNSS device to be used, minus the signal loss due to the the connectors and coax.  Typical gain ranges are 10~40 db, and this may also vary depending on the coax length and connectors you select as well.  Keep all coax lengths as short as practical to avoid signal loss.

Aside: If your reference GNSS maker has a suitable antenna they recommend, we suggest you use that.  In fact many GNSS manufacturers will not guarantee that their device will meet stated performance levels unless their antenna is also used.  Expect to pay $1k~$4k more (US dollars) for such antennas.

We hesitate to recommend one design over another, but here is a unit we have been using for urban automotive testing as well as for reference stations for over half a decade with great success.  Using this antenna model we routinely hold <2cm levels in moving vehicle equipment.  The maker, Antcom is a well established provider of GNSS antennas; their catalogs have many solutions serving a variety of RTK and other needs.  And unlike some others, the design of this unit has been calibrated. The unit below, in L1/L2 form, is about $450 US in single quantities.

The model that we often use is:   Antcom 53GOA4-XT-1 antenna.

This is an L1/L2 GPS/GLONASS design suitable for all but the highest accuracy needs.  In the above part#: The 4 indicates 40dB gain. The T indicates a TNC connector (use “S” if you prefer the smaller SMA). X means no cable pigtail is present.

Our own local representative / dealer contact is:

Rex Williams,
GeoNAV
387 Maryville Avenue
Ventura, California - U.S.A.
: 805-650-
Fax: 805-650-

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You will note that this unit, as do many others, comes with a 5/8 inch (and 11 TPI) threaded connector (you will see this size called “19mm” in some catalogs).  The 5/8 size and threads are an international standard for survey equipment use.  Most tripods accept this type of mounting, as do the magnetic mounts shown in the above picture.  If you will be using this on an automobile roof, the 3-inch extension post is essential in order to allow the SMA coax connector access to the bottom. We typically leave the small metal ground plane attached when testing.

Footnote A:
For high grade units and applications, a table of offsets for the antenna bias variations in phase delay over azimuth and elevation has been developed by independent testing processes (supported by various sources of government funding).  These tables can be obtained for many devices at this point (including the Antcom unit recommended above).  If you cannot find the model you want to use in these tables, it may be either new or just not suitable.  The reference station software then uses these bias values to subtract/add to the raw carrier phase observations before sending them out in the RTCM3 messages to achieve the highest possible accuracy.  As part of this process, the reference station antenna is installed with a known orientation (there is typically a mark that is pointed to the North). All that the operator needs to do is load in the antenna model that is being used. This process is typically not performed in the rover as the orientation angle changes with movement, but it is common to orientate the rover antenna forward.

Want more information on Quadrifilar Helix Antenna? Feel free to contact us.