Brunton compasses and measuring tapes are inexpensive, highly portable, and good for mapping small areas. Total Stations are exceptionally accurate and excellent for mapping larger areas, but they are also quite expensive. But what about mapping very large, even huge, areas? Or, what if it is impossible to walk from one point to another, or develop a line of sight between points? Here is where the global positioning system (GPS) is important.
The history, purpose(s), operation, and economics of GPS are really quite simple. The concept of GPS was born in the 1960s, but serious work on the program did not begin until the 1970s. It was initially designed as a military navigation system with no serious thought about possible private or non-military applications. Now it is available to anyone with two "Ben Franklins." The Department of Defense, however, still controls the satellite system and could turn the whole thing off anytime. It also controls a small amount of built-in bias (or "error") known as Selective Availability or simply as S/A. We will deal with this, other sources of error, and correcting for them later.
The DoD has four ground-based monitoring stations, and a master control station. The monitoring stations are on Hawaii (eastern Pacific), Kwajelin (western Pacific), Diego Garcia (Indian Ocean), and the Ascension Islands (Atlantic). The master control station is at Falcon AFB, Colorado. The monitoring stations track the satellites as they pass overhead, collecting data about satellite paths and times. These data are then forwarded to the master control station where models compute precise orbital data (Ephemeris) and satellite clock corrections for each satellite. New data are uploaded to the satellites at least once a day. The satellites then retransmit subsets of the orbital data as radio signals which can be received by GPS receivers.
In other words, the exact location of each satellite is known, and, accordingly, every point on earth can be determined in reference to the global positioning system. In effect, GPS is a high-tech version of triangulation; not unlike taking a couple of compass bearings on benchmarks or points with known locations. With GPS, the satellites are the benchmarks.
In the way of an analog, think of two people with synchonized watches standing some unknown, but to be determined, distance apart. At a predetermined moment, one person starts yelling each second..."1, 2, 3, ...." The second person is listening while monitoring his or her watch. This person records the second at which he or she hears "1." This second is then multiplied by the speed of sound (1,088 feet/second at 32 degrees F) to determine the distance between the two people. Carrying-out this procedure with two people yelling will allow the third person to calculate his or her relative location by triangulation.
Precision Positioning Service (PPS) is used only by the military and select civilian users. It has horizontal accurracy of 17.8 m, vertical accuracy of 27.7 m, and time accuracy of 100 nanoseconds. This system uses the Precise Code (P-code), which is encrypted at the satellite and needs to be decrypted by users with a security module. P-code repeats PRN every 7 days, rather than the C/A code.
Standard Position Service (SPS) is available to everyone. It has horizontal accuracy of 100 m, vertical accuracy of 156 m, and time accuracy to 167 nanoseconds.
1. Clocks, as good as they are, are not perfect. Also, there can be differences in satellite and receiver clocks
2. Ephemeris errors or variations in orbit, including satellite "wobbling."
3. Troposheric and ionospheric errors result from radio waves traveling through air, clouds, dust, etc., and ionized electrons. Signals are slowed and distorted.
4. Pseudo-range noises are from galactic atmospheric sources.
5. Receiver errors result from electronic noise generated with the circuitry.
6. Multipath errors involve radio signals bouncing off of buildings, cliffs, etc., thereby "ghosting," and causing delays.
7. Selective Availability (S/A), as mentioned earlier, is bias built-in and controlled by the military. It can be as much as 32 meters.
8. Root-Mean-Squared error is the square root of the sum of the squares of the differences between the amount of each type of error and the mean of all the errors. It is exactly the same as one standard deviation.
A great deal of error can be corrected or compensated for by using Differential Positioning (DGPS). The principle of DGPS is quite simple, relying on two receivers rather than only one. One receiver is placed in a location known and not moved. It is known as the "reference" or "base" receiver, and is used to figure out exactly what errors the satellite data contain. This receiver transmits error correction messages to other GPS receivers, "roving" or "remote" receivers in the area that use the message to correct their position solutions. It is essential that the reference receiver track all or more of the satellites tracked by the roving receivers
In addition to these sources of error, and, therefore, affecting the accuracy of GPS is something called the Geometric Dilution of Precision or GDOP. This is the effect of the unique geometric relationship between the receiver and each of the satellites on the final solution. In general, the fewer the satellites available, and the closer they are together, the less accurate the readings. Since GPS is a four dimensional system (X, Y, Z, and time) there are four types of GDOP. Horizontal Dilution of Precision or HDOP involves latitude and longitude (2-D); Vertical Dilution of Precision or VDOP involves elevation or altitude; Time Dilution of Precision or TDOP involves clock offset; Position Dilution of Precision or PDOP also known as Spherical Dilution of Precision or SDOP involves three dimensions (3-D). PDOP is the overall best measure of dilution of precision. It is also the one to which most geographical projects need to pay special attention.
GPS receivers attempt to select the set of satellites that will provide the best PDOP. These will be those that are high enough above the horizon to minimize the thickness of the atmosphere and interference from buildings, yet not so high that they are too close together. When a receiver finds four usable satellites, it solves for three dimensions (3-D), or position, and time. When a receiver can find only three usable satellites it solves for two dimensions (2-D), or horizontal or locational information. If the receiver is set in the "AUTO" mode, it will switch back and forth between 2-D and 3-D. This is not acceptable if you are recording positions and need measurements from four satellites.
Finally, DOP can compound range errors by a factor ranging upward from 4. Low DOPs are better than high ones. Errors of 6 and under are generally considered acceptable. For example, at a PDOP of 4, an RMS error of 1.6 m is actually 6 m. A typical range error of 15 m is actually 60 m. At a PDOP of 6, typical range error approaches 100.
This "100 meter horizontal accuracy" is, however, even more misleading. If one was to stand on a point with a precisely known location and take 100 GPS readings over a period of time, 68 percent of these readings would fall within a circle delimited by a 50 meter radius of the known point (within 1 standard deviation). Ninety-five percent (95%) would fall within a 100 meter radius (within two standard deviations). Finally, 98 % of the readings would be within 150 meters of the known point.
S/A Turned Off!
On Monday, 1 May 2000 Selective Availability was turned off.
There was much public fanfare, including items in all major newspapers
and spots on the national television news. Almost immediately, numerous
people began thinking their problems with GPS inaccuracy were a thing of
the past. It was widely thought that horizontal accuracy had now
improved from 100 m to 25 m. On 4 October 2001 the U.S. Coast Guard
finally published the new standards ( U.S.C.G.
Navigation Center ). On page 14 of this report, it says that the U.S.
Government is committed to providing SPS service reliability to >99.94%
accuracy within 30 m. Thinking the government would be conservative, many
non-governmental GPS experts conducted independent tests and think the
accuracy is well within 10 m. As accurate as this may seem, 10 m is still
not all that accurate for some tasks. Furthermore, the government
can still turn-on S/A anytime, without notice.
Data Collection. Out in the field, mapping is a simple as going to a point and "clicking" the GPS receiver switch. You then read the data directly from the screen and record it in your field notebook, or you can store it in memory. If you are mapping lines or areas, simply walk, drive, or move along the line or perimeter while the GPS receiver logs positions continuously. While you are collecting data, the reference receiver should be continuously collecting base station data.
Data Processing. After you have collected and stored the data, you can download them into your personal computer for processing. Or, if you recorded the data in a notebook, you can draft your map manually.
The particular unit to be used in this course has been preprogrammed and customized to facilitate the assigned exercise. Simply follow the steps enumerated below.
1. Press PWR. A cryptic warning will appear on the screen.
2. Press EXIT. The Status Screen will appear on the screen. In essence, it is a map of the sky above you, and shows the locations of the satellites. The bar graph on the lower part of the screen shows the strength of each satellite's signal. Wait a few moments while the satellites lock onto your location. When they do, the message "POSITION ACQUIRED" will appear on the screen.
3. Press PAGES. A menu with four choices will appear on the screen, Status, Nav #, Plot #, GRP #.
4. If GRP # is not highlighted, press the Up Arrow or the Down Arrow until it is. You should notice that the image on the screen beneath or behind the menu will change each time you press the arrow key.
5. If GRP G or GRP H does not appear, press the Left Arrow or the Right Arrow until either one does.
6. Press EXIT. The menu will disappear and four boxes will appear. The large top box shows your horizontal position in terms of latitude and longitude. This is all the information you need for the exercise.
7. Record this information.
8. Press PAGES. The menu will once again appear.
9. Press the Right Arrow until GRP I appears.
10. Press EXIT. The menu will disappear and five boxes will appear. The top box shows the time.
11. Record this information.
12. Press and hold the PWR key for three seconds. A countdown message appears on the screen. When the image disappears, the unit is off.
Created by William E. Doolittle. Last revised 12 December 2001, wed
Thanks to Peter H. Dana
