Fieldwork

Reach site, hunt around,

Setup, precise centering,

Yellow box takes data.

- A concise guide to GPS fieldwork

Perhaps the most important thing to remember about GPS fieldwork is that with a 24-hour GPS survey you can measure the position of the GPS antenna to within about 2 mm horizontal precision and 6 mm vertical precision, and you can measure the vector between two GPS antennas even more precisely than that. What this means is that the tolerances for setup error are very tight and you have to be very careful. If not, YOU are likely to be the limiting error source in the measurements. It also means that you really can't hide it if you screw up. So do your best if you make a mistake or something odd happens, carefully record it on the log sheet. It is far better to know we have a bad observation than to find out years later.

Where can I find site information?

We maintain a GPS Site Database that in theory stores site information from all the GPS sites we use. In practice, some are missing, but the database is mostly complete for Alaska and environs. You can search the database by 4 characeter ID, or radial or rectangular searches, and other ways as well. To access the database, you need a login and password, and for that you need to ask Jeff.

Orienting the antenna to TRUE NORTH

Every antenna has a reference line or arrow that must be oriented toward TRUE NORTH. The reason for this is that the position actually measured by GPS is something called the antenna phase center, which in general is not located on the axis of the antenna. So if you orient the antenna in some other direction, you introduce an error into the resulting postion that for many antennas is much larger than the measurement uncertainty.

The reference arrow for the Trimble 4000's antenna is marked on the antenna.

For other antennas, the antenna cable connector should point to NORTH. There will normally be something marked on the top of the antenna along that same line.

To orient the antenna, you need to know the magnetic declination. It should be recorded on the site description. If not, it can be found on any USGS topo map. If all else fails, remember 25 degrees east, which is not too far off for most of central and southern Alaska.

If the declination is EAST, set your compass to 360 - declination and orient by the north arrow on the compass.

If the declination is WEST, set your compass to 360 + declination and orient by the north arrow on the compass.

If you find that you oriented the antenna the wrong way, and it is already recording data, don't do anything rash! The best thing to do in this case is to carefully record the actual orientation of the antenna.

Using the Trimble 5700 and R7 receivers

These two receivers are basically the same thing. Neither one is convenient to program in the field. Turn them on and they should work.

What the lights mean.

Where do data log sheets go

In the filing cabinet, in a hanging file stored by GPS campaign.

Every year, we send a copy of all log sheets to UNAVCO for permanent archiving, along with the data files. Sometimes after a while it is easier to find the information from the UNAVCO Data Archive than it is to find our own log sheets in the filing cabinet.

Converting files to RINEX

The final step in the fieldwork is to convert the data files to the RINEX format. RINEX stands for Receiver INdependent EXchange format. It is an ASCII format that is publicly documented and agreed upon worldwide. In contrast, the Trimble .dat and .t00/.t01 files are proprietary formats that are only used by Trimble. Naturally, every receiver manufacturer has its own proprietary format. There is a more compact binary format called BINEX that is meant to store the same information. BINEX has been adopted as the standard format for PBO, and may one day become the standard instead of RINEX.

There are enough steps in the process that a complete guide is on its own page.