Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/692436
Satellite evolution There's a bit of digital magic in- volved in making this all work, which we will explain, but to avoid confusion, let's start by defining some terms. You'll often see this technology referred to as GPS or GNSS or as we put it, GPS/GNSS. What's the difference? In the early days, the only satellites available to beam this positioning information down from space were global positioning satellites, or GPS, put into space by the United States government. In the early 2000s, ac- cess to the Russian Satellite Constel- lation, Glonass, became available. Europeans have plans to complete their own series of satellites called Galileo. This combination of satel- lite constellations created the Global Navigation Satellite Systems or GNSS. Today, people use the terms GPS and GNSS somewhat interchangeably. The base station At the heart of any GPS/GNSS system is the base station. Via antennae, it receives the positioning signals from satellites orbiting the earth. But because these signals travel a great distance through the atmosphere, their accuracy tends to be in the +/- 30-feet range. Good enough for automobiles or airplanes, but not civil engineering. The key to a base station is that it is set up precisely over a known, surveyed point. As the satel- lite's signal travels though the atmo- sphere, there is an error in its position when received on earth. Since the base station knows precisely where it is located, it can deter- mine the error. The error experienced at the base station is also seen by the rovers. The base station broadcasts this "differential correction" across the jobsite to all available man and machine rovers. The corrected signal is referred to as real time kinematic or RTK. This improves the accuracy to within millimeter tolerances, says Hilbig. This base station broadcast is typically delivered through a radio signal. Where radio transmissions are problematic, however, the RTK trans- mission can also be accomplished via cellular modem. A good analogy for the base sta- tion is the construction laser, says Hilbig. "Like a laser on a tripod, it can broadcast to an infinite number of receivers or multiple end users on the site," he says. "But with GPS, you can cover huge elevation changes and great distances depending on terrain, and you don't have to move it like you do a laser. Depending on the radio type and terrain you can often cover a three-mile radius." As mentioned, for a base station to function properly it must be set up over a control point or a known sur- vey point. To do this, many contrac- tors will mount it to a jobsite trailer or put it on a 4x4 post anchored in June 2016 | EquipmentWorld.com 26 GPS/GNSS 101 | continued The antenna for machine rovers are typically mounted on the blade and use the corrected GPS signals to guide the dozers or motor graders to the exact coordinates of the finished surface. On excavators, the GPS/GNSS antenna is usually mounted on near the counterweight. Man rovers in the field conduct topos on the site, measure stockpiles, layout the project and stake out pads, roads and other project elements.