Global Positioning System GPS and its application in Forestry How a GPS Receiver works?  Fig.4: GPS Receiver This is a handheld device weighing about 8 ounces and measuring about 6 inches by 2 inches (ignoring the antenna). The antenna is the vertical stub on the right, roughly three inches long. This unit has a small LCD screen and a set of buttons to activate its different features. When you turn on a GPS receiver, its first task is to try to find the radio signals for the satellites it can "see". GPS satellites live in very precise orbits about 11,000 miles up (for comparison, the space shuttle orbits at about 200 miles and geosynchronous satellites orbit at about 24,000 miles). Because the satellites are so far away, their radio signals are fairly weak. Therefore, for the GPS receiver to "see" a satellite, the satellite must be above the horizon and unobstructed by buildings, mountains, etc. At any given moment at any point on the planet there are between 6 and 9 satellites above the horizon. During the process of acquiring the satellites, the GPS display will look something like this: 
On this screen, the larger circle represents the horizon and smaller circle represents 45 degrees. The dot in the centre is straight overhead. The numbers within the circles represent satellites that are visible, and the bar chart on the right represents the relative strength of the signals from the different satellites. Once the GPS receiver has locked on to 3 satellites, it can display your longitude and latitude to about 100 foot accuracy. If the receiver can see 4 satellites it can also tell you your altitude. With this information you know exactly where you are. Most modern GPS receivers are able to store your track. As you move, the GPS periodically stores your position in its internal memory. It can then show you the path you have followed on the display so that you can see exactly where you have been. Tacks also make backtracking easy. Most modern GPS receivers also support the concept of waypoints and routes. A waypoint is a specific point (longitude and latitude) that you have stored in memory. A route is a series of waypoints connected together to form a path from one point to another. The user can imagine that if you are a boater, he might mark certain buoys as waypoints, or store the coordinates of your favourite fishing spots as waypoints. A hiker might store different landmarks or resting points along a trail as waypoints. The user can then string a collection of waypoints together into a route. At the start of a trip you tell the GPS receiver which route he wants to follow. The receiver will then tell you the heading you need to take to get to the next waypoint in the route. As the user passes each waypoint, the GPS receiver gives the heading to the next waypoint on the route. Some of the newer receivers, like the GPS III shown above, have road maps stored in memory. The unit is therefore able to show the user exactly where he is on a map of the area. Internal maps usually show major highways. By hooking a GPS up to a laptop computer the user can locate you on very detailed road or topographic maps loaded from CD-ROMs. So over all the GPS receivers helps the user to:
Types of Receivers Aviation Airborne GPS receivers are generally used for navigation and attitude determination. There is a wide range of receivers available to fit every budgetary requirement. There are handheld GPS receivers available, which can accept data cartridges containing Jeppson charts. High-end GPS units are being built into jumbo jets and are being tested for automated landing. Computer Boards These receivers are designed to fit inside a computer of some sort. A general purpose IBM-PC based receiver is made by Novatel while another GPS designer kit is made by GEC-Plessey. Handheld Quite a variety of these are now available. Many are intended to fit a specific purpose such as land navigation, boating, aviation and even industrial mapping. The range of features is therefore, fairly diverse. Some are available at prices as low as $300 with some high end-mapping units running up to $4000. Mapping These receivers are intended for mapping items for later inclusion in databases, maps or drawings. All will have DGPS capability and most will have the ability to store lots of points and add text or menu information to the points. These units will start around $1000 and go up from there. OEM Modules OEM modules are bare GPS receiver boards that are intended to be incorporated into other equipment. Many will be outfitted with one or two RS-232 ports from which the user is responsible for programming the unit and interpreting the output. These can run from a few hundred dollars to several thousand dollars. PC Card (PCMCIA) There are only three of these: The NavCard is made by Rockwell while Trimble Navigation makes the GPScard and the Gold GPScard. The Gold card accepts differential correction while the regular card does not. All of these cards are priced between $300 and $1600. Marine These are used almost exclusively for navigation. Many will have NMEA-183 interfaces for connecting to other ship electronics. Space borne Space born GPS receivers are used for satellite navigation and attitude determination. Most space borne receivers are radiation-hardened versions of ground-based receivers. Surveying This grade of receiver is used by surveyors to derive "measurements" rather than "position". For surveyors it is the relative relationship between two receivers that is important; from this relationship an absolute "position" may be derived if and as necessary, on whichever datum is appropriate for the survey. Surveying receivers are generally capable of the highest accuracies and cost the most (up to $30K per set). |