GPS signals arrive at the antenna phase centre. Therefore, the position that is actually computed is the position of the antenna phase centre. This must be reduced to the point on the ground that is actually being measured by the surveyor. As a result very accurate antenna information must be recorded by the surveyor. Furthermore, the surveyor must be certain about the antenna type used.

The image left shows the 'Compact L1/L2 antenna with a ground plane' used in association with the Trimble 4000SSI GPS receiver. The Compact L1/L2 antenna without a ground plane is shown upper right. Note that these antennas are identical internally. The ground plane is not associated with the antenna itself. Rather it is a feature designed to block signals that have been reflected from the ground from reaching the antenna. It is a multipath reduction feature.

Both type of antenna contain information on their dimensions stamped on the side. In fact, the internal dimensions of the antennas (ie the locations of their phase centres) are identical.

Compact L1/L2 antenna without ground plane