|Lecture GPS #2 Lecture Notes
Text Coverage: Wolf and Ghilani: Study: Ch 13: 6 – 11. Problems: 13:11-20
A. GPS Hardware
1. GPS is revolutionizing many aspects of society. Automobile navigation systems are becoming commonplace. For example Rand McNally, the mapping company, markets a GPS auto navigation system for less than $100. Visit their website to review their street finder GPS solution for a laptop: Rand McNally - Welcome!
2. Low cost hand-held receivers are available from Magellan, DeLorme, Garmin, and Lowrance are available. For example, visit: GPS Receivers, Garmin GPS, Lowrance GPS, Magellan GPS and DeLorme GPS. These receivers are “CODE PHASE”, whereby the ranges are measured as a integer number of whole 300m code blocks plus a measured fractional one. Since the wavelength is large (300 m), the uncertainty in the measured fractional phase shift is also large; therefore 3 – 10 meters accuracy is the best to be expected. Code-phase receivers can be used alone for navigational solutions. If two are used in differential GPS, the accuracy can be improved by a factor of 10.
3. The basic L1 “carrier wave” of the GPS signal has frequency 1575.42 MHz. Dividing this into the speed of light gives the CARRIER WAVELENGTH of about 0.19 meters, 19 centimeters. Geodetic-grade receivers have more accurate clocks so that the integer count of the carrier wave’s 19 cm wavelength can be determined. Such a “CARRIER PHASE” receiver measures the fractional wavelength of the 19 cm carrier wavelength and therefore is much more accurate than a code-phase receiver. Typical accuracies approach 1 to 2 cm in X and Y coordinates plus 2 to 5 cm in Z.
A second “carrier wave” is broadcast by the GPS satellite, the L2 signal, having a frequency of 1227.60 MHz. This second signal is not modulated into message packets of 300 m like L1, and therefore it can only be used for “carrier phase” GPS solutions, not “code-phase”. A carrier-phase receiver that picks up both carrier signals is called a “dual frequency” receiver. Also, the number of channels a receiver has is important. A 12 channel receiver can track and measure from up to 12 satellites at one time.
Trimble, Ashtech, and Leica are big names in carrier phase GPS. Trimble Navigation has the largest market share in geodetic receivers. They also have a strong line of code-phase receivers. These can be seen at: Trimble Land Survey Home. Another good brand is Leica, headquartered in Switzerland: Leica Geosystems solutions and systems for positioning, surveying, mapping, construction, navigation, industrial measurement, defense and security, machine guidance, cadastral, photogrammetry, mining and engineering and other surveying and measurement app
4. There are several Internet sites dedicated to GPS. Here’s one: GPS Resource.com (www.gpsresource.com) -- The Consumer's GPS Resource. A trade magazine is dedicated to GPS, GPS World. Here’s the magazine’s web site: GPS World Home Page.
B. Summary of GPS Methods and Terminology
1. The NAVIGATIONAL solution
a. also known as NAVIGATIONAL-GRADE GPS because positions generally shown graphically on a small scale map so that errors of 3 to 10 meters are not visible.
b. uses one CODE PHASE receiver
c. 3-10 m accuracy
d. low cost receivers: Magellan, Garmin, etc.
e. applications: hiking, boating, road navigation
2. The MAPPING solution:
a. DIFFERENTIAL GPS solution, DGPS
b. Two CODE PHASE receivers, one on a BASE STATION, one ROVER
c. Base stations measure compute X, Y, and Z corrections.
d. Corrections are applied to the rover’s raw measured position. Four approaches can be used for applying the DGPS corrections
e. Four alternate methods of applying corrections:
(1) Post-processed DGPS CORRECTION – USER BASE UNIT. Two code-phase GPS receivers are used, one on the base station of known coordinates, one a rover. Each receiver is hooked to a data collector that records the measured positions for each second of time. At the end of the work day both data sets are read into a computer, and corrections are applied.
(2) REAL-TIME DGPS – USER BASE METHOD -- the base station has a radio transmitter. The owner applies to the FCC for a license to transmit data on a certain frequency. The broadcast differential corrections are received by a radio antenna attached to the rover GPS receiver, and these are mathematically applied to the raw measurement. This gives the corrected DGPS values real time.
(3) REAL TIME DGPS CORRECTION – CORS BASE STATION – The U.S. Coast Guard is responsible for the civilian use of GPS. The USCG maintains CORS stations (continuously operated reference stations) around the coastline and on major rivers of the U.S. – Egmont Key north of Tampa for example. The Coast Guard broadcasts X, Y, and Z corrections over a certain frequency to points up to 250 miles away. Many GPS receivers can access this frequency and thereby apply the Coast Guard corrections. Using the Coast Guard CORS station as a base, only one rover receiver is needed.
(4) POST-PROCESSED DGPS CORRECTION – CORS BASE STATION -- The next day, the office PC can “dial up” a CORS (continuously operated base station) base station by modem or internet and download the corrections for each second of time for the previous day. The PC software then applies the corrections to the rover’s measurements to improve the accuracy.
f. Accuracy of code-phase DGPS: 0.3 to 1.0 m.
g. Applications: GIS mapping, natural resource mapping.
3. GEODETIC or SURVEYING solutions because the accuracy approaches that of traditional theodolite methods.
a. Also known as CARRIER PHASE solutions or SURVEYING solutions since an integer count of whole 19 cm carrier waves are measured from satellite to receiver plus a partial one – the “carrier phase”.
b. Always differential – minimum two receivers needed.
c. Dual frequency – measures L1 and L2 signals
d. Cost -- $10,000 - $20,000 per receiver, higher costs because of more accurate clocks, stable electronics, dual frequencies, and other factors.
e. Accuracies attainable: 1 –2 cm in horizontal, 3- 5 cm vertical.
f. Applications: geodetic surveying of new control points, construction layout, topographic mapping, general survey use.
g. Two alternate approaches:
(1) the STATIC NETWORK solution for GEODETIC SURVEYING
(a) a NETWORK of survey points are selected for the survey – county-wide, region-wide, state-wide, nation-wide.
(b) a SESSION usually consists of three receivers erected over three points in the network reading the satellites simultaneously for a certain length of time 10 min, 30 min, 60 min. More time means more accuracy.
(c) data files for the GPS receivers are downloaded into the same desktop computer for post processing. The software subtracts the positions between observed stations to compute a GPS "vector" equaling the change in X, change in Y, and change in Z between receivers. Each combination of receivers will produce one vector for each session.
(d) GPS vectors form a redundant survey system which is adjusted by least squares for final positions.
(2) RTK, Real Time Kinematic. The base station reads the point position each cycle (1second usually), subtracts this from the known point, to get X, Y, and Z corrections differential corrections. The base unit broadcasts these on an FM radio band to the rover unit, which then applies the corrections to its point position for a real time accurate position.