AUSPOS FAQ-5. GPS antennas

Online GPS Processing

• Question 5.1 - Why do I have to specify the antenna used?
• Question 5.2 - Exactly what is antenna height?
• Question 5.3 - What type of antenna do I have?
• Question 5.4 - What happens if I choose the DEFAULT(NONE) antenna?

[back to top]

Question 5.1 - Why do I have to specify the GPS antenna?

To achieve the highest quality processing results, attention needs to be paid to the type of antenna used. This is particularly important, as the nearby IGS stations may not have the same antenna type as you, and as a result, introduce bias and noise into the computed coordinates.

The actual observation point on a GPS antenna is called the electrical phase centre. The location of this phase centre is represented by a mean constant offset, from the physical point on a GPS antenna known as the Antenna Reference Point (ARP), and an additional variable offset that is dependant on the transmitting satellite's elevation.

The properties of the electrical phase centre is different for every type of antenna. Importantly, the electrical phase centre can be modelled provided you know the antenna type.

[back to top]

Question 5.2 - Exactly what is antenna height?

The service allows input of GPS antenna heights. This is the vertical distance from the ground mark to the Antenna Reference Point (ARP). The (ARP) is the physical point on a GPS antenna that measurements are typically reduced to.

The ARP varies between antenna types. As a WARNING, the ARP is rarely the top of the Antenna Ground Plane. The ARP height should NOT be confused with slant and other instrument height measurements commonly used in GPS processing.

[back to top]

Question 5.3 - What type of GPS antenna do I have?

The service uses the IGS antenna naming convention. This can make it confusing for users to relate their antenna to what the IGS calls it.

The IGS - Naming Convention Document provides more information.

If you are unsure what type of antenna you have, then the National Geodetic Survey (NGS) antenna calibration web site might help.

[back to top]

Question 5.4 - What happens if I choose the DEFAULT(NONE) antenna?

If you choose the DEFAULT(NONE) antenna type, then no antenna phase centre model is applied. If in addition to this you set the antenna height to 0.0000 m, then the position computed is effectively the antenna L1 phase centre. If you set the antenna height to 0.0000 m, and choose an antenna type, then the position of the ARP is computed. This functionality can be useful for those DGPS service providers using the service - because they are often more interested in the antenna phase centre rather than the position of a ground mark or ARP.

[back to top]