How GPS is Used in Land Surveying
Land surveying is a profession that greatly utilize the GPS satellite system.
Today, man-made constellations are in orbit. Their purpose is to provide instant, exact and global positioning information. This global positioning system (GPS) is revolutionizing the modern construction sites, automating and guiding land surveying equipment, providing instantaneous points and positions, and simplifying control surveys. Nearly everyone involved in the land surveying and construction industry has heard something about satellite positioning technology or maybe you or someone you know is already using it. Whether an operator or a casual observer, there are still many questions about how this exciting system really works.
These positioning satellites don’t stay in one place like the stars; they orbit the earth about twice a day. Constantly transmitting uniquely identified radio signals that can be perceived by specialized receiving units on the earth. These signals are used to precisely measure the distance to the receiving instrument from each satellite overhead.
Just as stars have exact locations in the sky, the locations of the GPS satellites are also defined, becoming orbiting reference points. Using a process based on triangulation, a GPS receiver processes the signals from multiple satellites to compute an very precise position on Earth, which is why it is widely use in land surveying.
For mobile applications that require high accuracy, such as three dimensional machine control, there are two requirements that must be met. First, the signals from a minimum of five satellites must be received at all times. And second, those five satellites must be distributed across the sky. This is called Dilution of Precision or DOP. The simple rule of thumb is this, the more satellites received the better your system will perform.
All land surveying receivers incorporate standard GPS tracking technology; unfortunately the GPS satellite constellation alone cannot continuously make the minimum requirement of accurate mobile positioning. For any given location, there are times in a day when there may be less than five available satellites or very poor DOP.
Now, consider a land surveying job site with trees, buildings, or other obstructions when satellite reception is further reduced. All land surveying GPS receivers suffer this same problem because they all use the same satellites to operate.
Even the planned signal modernization to the GPS satellites won’t solve this problem completely. An obstruction that blocks one signal blocks all signals from that satellite. The solution isn’t so much adding more signals from the same satellites; it’s adding more satellites.
Today, there is a second positioning satellite constellation in operation, the Glonass System. This positioning system is operated and maintained by the Russian government; much like GPS is operated and maintained by the United States government; combining these two satellite constellations give land surveyors access to 55 different satellites; 24 more than a GPS-only system. (UPDATE: As of 2017 there are also 18 of the planned 30 Galileo satellites by the European community and 5 BeiDou Constellation launched so far by the Chinese.)
The additional satellites mean stronger and more accurate positions and better performance in obstructed land surveying areas. These additional satellites mean maximum up time in all land surveying locations, in all conditions.