With a total station the height of the instrument is usually measured to the height of the “eye” or the center of the optic which makes it relatively simple. With GPS there is no optic, only an imaginary plane that is used to determine elevation by figuring the distance from the ground to this plane. Ok, maybe it is not imaginary, but the phase center measurements are neither physical positions nor stable points.
The phase center is variable on any GPS receiver based on directional signal information from satellites. These variations affect the antenna offsets used to connect the measurement to the position being recorded. To handle this, the National Geodetic Survey calibrates each receiver model creating a measurement between the receiver phase center and it’s antenna reference point (ARP). When we plug in the height of the receiver to our data collector we key in the measurement from our point to the ARP.
Some receivers have “measure points” on the receiver other than the ARP that are easier to get to for measuring. You can then either do the simple math to reduce the height from the measure point to the ARP, or in some cases the data collection software already has that equation built in so you only enter the distance from the point to your measure mark. Some units such as the Leica receivers have a tripod hook that has been calibrated to their ARP. This allows the user to affix the “hook” to the base of the receiver, pull the tape to the point and know the number read includes the distance from the end of the tape to the ARP.
The simple answer here is to do the same thing you do for your rover. Use a fixed height base. There are a few options available such as the Fixed-Height GPS Antenna Tripod with Collapsible Center Staff from SECO Manufacturing (SECO PN: 5119-00). This particular tripod features a collapsible center staff with twist locking mechanism that rotates 360° to check calibration of plumbing with a 10-minute vial, and quick release & twist locking mechanisms on the legs. It is dual graduated and adjusts to three heights: 2 m, 1.8 m, and 1.5 m. It collapses to 50 inches (127 cm).
I highly recommend these for use with the Altus receiver and have a few clients using them with their Sokkia and TopCon GPS units as well. For more details on how to use this tripod visit SECO’s website http://www.surveying.com/tech_tips/details.asp?techTipNo=13.