The Global Positioning System FAQ Version 9.001 - July 30, 1997 Additions, comments, and corrections should be sent to: Karen Nakamura, karen@gpsy.com This article is a collection of information, and pointers to other information sources, on the Navstar Global Positioning System (GPS), and related topics. Originally authored and edited by Peter Bennett for many years, it is currently being maintained by Karen Nakamura. This FAQ may be found on the Web at: http://www.gpsy.com/gpsinfo/ The following topics are addressed: 1) The Navstar Global Positioning System (GPS) 1.1) The Space Segment (AKA the satellites) 1.2) How does it work? 1.3) What accuracy can I expect? 1.4) What (and why) is Selective Availability? 1.5) How do some users get centimetre accuracy? 1.6) Does the time reported by GPS include "leap seconds"? 1.7) What is the August 1999/Year 2000 problem? 1.8) What are the speed and altitude limitations? 2) Navigation Receivers 2.1) Why are my GPS positions consistently wrong? 2.2) What is a horizontal datum, and which should I use? 2.3) Why does the reported altitude vary so much? 2.4) What is DGPS? 2.5) What are waypoints and routes? 2.6) Can I connect the GPS to a computer/autopilot/? 2.7) Will my GPS work in a car/plane/forest/cave...? 3) Survey Systems 3.1) DGPS Survey Systems. 3.2) Static Survey Systems. 3.3) Kinematic Post-processed Systems. 3.4) Real Time Kinematic GPS 4) Aviation Systems 4.1) Wide Area Augmentation System (WAAS) 5) Other Satellite Navigation Systems 5.1) NAVSAT or Transit 5.2) GLONASS 6) Radio Navigation Systems 6.1) Loran-C 6.2) Decca 7) Sources of GPS equipment 8) ABOUT THIS FAQ 8.1) Who put this FAQ together? 8.2) How can I contribute to this FAQ? 8.3) What newsgroups will this FAQ be posted to? 8.4) May I distribute this FAQ or post it somewhere else? 8.5) Acknowledgements 9) References 9.1) Books 9.2) Internet sites 0 Disclaimer: We believe the information provided is reasonably accurate. This FAQ is not intended to provide rigorous technical information on the system, but just to provide a general discussion of the GPS system and its limitations, as would be of interest to a casual user of a GPS navigation receiver. More detailed information can be found via the WWW in our "GPS Web Resource Guide" (http://www.gpsy.com/gpsinfo/gps-resource.txt) GPS is an _aid_ to navigation, and does not free the navigator from the need to know and use more traditional piloting and navigation techniques. "The prudent navigator will not rely solely on any single aid to navigation" (USCG Notices to Mariners) 1 The Navstar Global Positioning System (GPS) The Global Positioning System consists of three interacting components: 1) The Space Segment -- satellites orbiting the earth 2) The Control Segment -- the control and monitoring stations run by the DOD 3) The User Segment -- the GPS signal receivers owned by civilians and military This FAQ covers the Space Segment and User Segment, but does not discuss the Control Segment. 1.1 The Space Segment The space segment consists of a constellation of 24 active satellites (and one or more in-orbit spares) orbiting the earth every 12 hours. Four satellites are located in each of six orbits. The orbits are distributed evenly around the earth, and are inclined 55 degrees from the equator. The satellites orbit at an altitude of about 11,000 nautical miles. (Earlier plans for the system called for 18 or 21 active satellites.) 1.2 How does it work? Each satellite transmits two signals: L1 (1575.42 MHz) and L2 (1227.60 MHz). The L1 signal is modulated with two pseudo-random noise signals - the protected (P) code, and the coarse/acquisition (C/A) code. The L2 signal only carries the P code. Each satellite transmits a unique code, allowing the receiver to identify the signals. When a feature called "Anti-Spoofing" is active, the P code is encrypted, and known as P(Y) or Y code. Civilian navigation receivers only use the C/A code on the L1 frequency (although some high-end civilian surveying GPS receivers can utilize the carrier frequency of the L2 band for more precise measurements). The receiver measures the time required for the signal to travel from the satellite to the receiver, by knowing the time that the signal left the satellite, and observing the time it receives the signal, based on its internal clock. If the receiver had a perfect clock, exactly in sync with those on the satellites, three measurements, from three satellites, would be sufficient to determine position in 3 dimensions. Unfortunately, you can't get a perfect clock that will fit (financially or physically) in a $300 (or even $3000) receiver, so a fourth satellite is needed to resolve the receiver clock error. Each measurement ("pseudorange") gives a position on the surface of a sphere centred on the corresponding satellite. Due to the receiver clock error, the four spheres will not intersect at a single point, but the receiver will adjust its clock until they do, providing very accurate time, as well as position. Since the receiver must adjust its clock to be precisely in sync with GPS time, a GPS receiver can be used as a precise time reference. Some receivers provide a 1 pulse per second output for this purpose. For further information on this topic, see Tom Clark's information on his "totally accurate clock" at ftp://aleph.gsfc.nasa.gov/GPS/totally.accurate.clock/ 1.3 What accuracy can I expect? The Standard Positioning Service (SPS) available to civilian users should give 20 metre horizontal accuracy, however it is normally degraded to 100 metres (95% of the time) due to Selective Availability (SA). (That is, the reported position will be within 100 metres of the true position 95% of the time.) The vertical accuracy is about 1.5 times worse than horizontal, due to satellite geometry. (Satellites are more likely to be near the horizon, than directly overhead.) Trimble Navigation, in their booklet "GPS - A guide to the next utility" give the following error budget for commercial navigation receivers: Satellite clock error 2 ft. Ephemeris error 2 ft. Receiver errors 4 ft. Atmospheric/ionospheric 12 ft. Selective Availability 25 ft. Total (root-sum-square) 15 - 30 ft depending on SA The predicted acuracy is calculated by multiplying the above figure by the PDOP (Position Dilution of Precision) which typically will range from 4 to 6. This gives accuracies of 60 - 100 ft (30 m) without SA, up to 350 ft (100 m) with SA. The accuracy can be improved by averaging readings over some time. When taking readings for this purpose, there is apparently no point in taking the readings more often than every 15 min, or so. One user reports the following results: Averaging for 15 - 20 hours: 10 metre accuracy 24 hours 5 metres 48 hours 3 metres The error values are given as "2*sigma" values - for those (like myself) who don't do statistics, this means that the readings should be distributed as follows (for a 2*sigma of 100 metres): 32% of values are worse than sigma (50 metres) 5% of values are worse than 2*sigma (100 metres) 1% of values are worse than 2.6*sigma (130 metres) 0.3% of values are worse than 3*sigma (150 metres) 0.006% of values are worse than 4*sigma (200 metres) 0.00006% of values are worse than 5*sigma (250 metres) When Hans Pfeifer posted the above, David Salonimer suggested that, since SA is an artificially generated error, the error distribution may be clipped somewhere between 2 and 3 sigma, to virtually eliminate the extreme deviations. 1.4 What (and why) is Selective Availability? Selective Availability (should really be called Selective Unavailability :-) ) is an intentional degradation of accuracy intended to prevent "the enemy" from making tactical use of the full accuracy of GPS. SA is normally on, but was turned off during the Gulf War, and during the invasion of Haiti, presumably because the military didn't have enough military receivers to go around. (At least, it is widely rumoured to have been off at these times, but there is apparently no official confirmation...) They apparently now have sufficient military grade receivers, so we shouldn't expect SA to be turned off for this reason again (although some have noted SA being turned off for civilian emergency use). Military receivers can use the encrypted P code to get 20 metre accuracy, or better, regardless of the state of SA. In early Feb. 96, the US government passed a law that appears to require the military to turn SA off by May 1, 96. Apparently the first bill that covered this was vetoed, but the language was added to another bill that did pass, and was signed by the president. However, that bill is worded such that the military could (and did) find a way to legally leave SA on. On March 29, 1996, the White House announced that SA would be removed in four to ten years (i.e. somewhere between 2000 and 2006), so we appear to be stuck with it for some time, unless there is a change in policy. 1.5 How do some users get centimetre accuracy? The 20 to 100 metre accuracy mentioned above applies to single frequency navigation receivers, which are capable of updating the position every second or so. The high accuracy measurements are achieved with much different equipment covered below under "Survey Systems". These systems use both frequencies, and differential measurements, comparing the data from a roving receiver with that from a fixed receiver at a known location. They may also average the measurements over some period of time. These measurements actually determine the _difference_ in postition between the fixed and roving receivers to great precision, rather than determining the absolute position of either one. 1.6 Does the time reported by GPS include "leap seconds" The GPS system time (as used by the system itself) does not include leap seconds, but the difference between GPS time and UTC is included in the data sent by the satellites, so receivers can (and most navigation receivers do) display current UTC or zone time, rather than the GPS system time. Currently there is a 11 second difference. Tom Clark posted the following to sci.geo.satellite-nav on Dec 31, 1995: The GPS satellites have been sending the bit that says a leap-second is pending for some time now. At 00:00:00 UTC (=00:00:11 GPS) on Jan.1 your receiver is *SUPPOSED* to apply the correction. Not all receivers will do this properly because of shortcuts that their makers put into the receiver's firmware: a - Some receivers have the GPS-UTC time offset *HARD-CODED* in their internal ROMs. Some older receivers are already several seconds off because of the manufacturer ignoring the GPS message specification! [This *BAD* approach has caused GPS to get a "bad name" in some circles for having inaccurate timing. If your receiver persistently is several seconds in error, it is not the fault of the GPS system! It is because your receiver's manufacture didn't know what he was doing!] b - Instead of using the LEAP-SECOND PENDING bit and applying it at 23:59:60 UTC time, they extract the GPS-UTC time explicitly from the downlink message. In this case, your receiver may not make the change until the first full GPS (30 second duration) message is received after the New Year. c - Some manufacturers only grab the GPS-UTC offset from the first satellite they lock onto after they are powered up, using the explicit offset like case (b). If this is the case, then you will have to turn your receiver off and then back on to have the leap-second be applied. In most receivers, the output of the current UTC to the display and the RS232 lines is the lowest priority task in the receiver. Receivers often have the display lag the UTC second by a fraction of a second. If you are using an NMEA message the UTC time is in the $GPGGA, $GPZDA or $GPRMC message. These messages take a while to transmit, so you may be well into the second before the output is visible to you. 1.7 What is the August 1999/Year 2000 problem? Rather than counting days, months, and years, the GPS system keeps a count of weeks since Jan 6, 1980. It uses a 10 bit counter for this, which means that it can only count up to 1023 weeks. so, at midnight Aug21/22 1999, this counter will "roll over" to zero. Someone noticed this, and forecast the end of the GPS system at that time. However, this roll over is an expected event, and will be handled correctly by the GPS system itself, and by most GPS receivers. I have seen a report that one very early navigation receiver would not handle this, but all currently produced receivers are apparently OK. Garmin, Lowrance, Eagle and Rockwell have all stated that their receivers will work correctly (Rockwell receiver boards are used in receivers sold under other brand names). 1.8 What are the speed and altitude limitations? The system has no inherent speed or altitude limitations (GPS has been used on satellites for position determination), but the US requires that commercial receivers be limited to operate below about 900 knots and 60,000 ft. It is apparently possible to get permission to bypass these limits for specific applications (research rockets, etc.). Garmin used to limit their non-aviation models (40 and 45 at least) to operation below 90 knots. Above this speed, the receiver displays an error message and stops updating the position. This is apparently a marketing decision to force aviators to purchase the more expensive aviation models which incorporate an aviation waypoint database. They discontinued this practice with the advent of their 12 channel parallel units (GPS 12/12XL/II+/III). 2. Navigation Receivers 2.1 Why are my GPS positions consistently wrong? The chart probably uses a different horizontal datum than the GPS. 2.2 What is a horizontal datum, and which should I use? (Geodeticists will have a more complex explanation, involving differing non-goecentric ellipsoids, etc., but the following should provide a sufficient explanation for navigators. See some of the WWW sites for the more scientific explanations) A horizontal datum in effect defines where on the earth the lines of latitude and longitude are drawn. In earlier times, surveys were based on points determined by astronomical observations, and by physical measurements on land. This resulted in many slightly different regional Lat/Long grids. The GPS system forces us to use a consistent, world-wide grid. Positions reported by GPS are based on a horizontal datum called "World Geodetic System of 1984" (WGS84). In the US and Canada, most older (but still current) charts and maps are based on the North American Datum of 1927 (NAD27). Newer nautical charts (including Canadian charts prepared since mid-87) are on NAD83 which is, for all practical purposes, identical to WGS84. The difference between NAD27 and NAD83/WGS84 varies across the continent. In the Pacific Northwest, an NAD83 position plotted on a NAD27 chart will be about 0.65 seconds (65 ft) south and 5 seconds (330 ft) west of its true position. In some areas of the world, the local datum may differ from WGS84 by a mile or more. Many GPS receivers can be set to display positions in a local datum rather than WGS84. Most Garmin receivers can display positions in more that 100 different datums. It appears that Garmin receivers store waypoint positions as WGS84 co-ordinates, and convert between the currently selected datum and WGS84 as needed, so that the physical location of a waypoint should not change as you change the current datum on the receiver. 2.3 Why does the reported altitude vary so much? Primarily due to satellite geometry. To get the most accurate altitude and location, you should use satellites that located at close to right angles to each other and one directly overhead. However, the satellites are more likely to be nearer the horizon, and a receiver will likely choose satellites nearer the horizon in the interests of getting a more accurate horizontal position, since that is what most navigators are interested in. The error in altitude is typically about 1.5 times the horizontal error. The altitude may also _appear_ to vary more than the horizontal position, since it is given in "normal units" (feet or metres). Also, particularly for those of us at sea level, the real altitude is probably known better than the lat/long, making the error more obvious. (I know the altitude of my boat is 0 +/- tides. When the GPS shows an altitude of 200 ft (or -150 ft) I _know_ it is wrong!) 2.4 What is DGPS? Differential GPS (DGPS) is a means of correcting for some system errors by using the errors observed at a known location to correct the readings of a roving receiver. The basic concept is that the reference station "knows" its position, and determines the difference between that known position and the position as determined by a GPS receiver. This error measurement is then passed to the roving receiver which can adjust its indicated position to compensate. Unfortunately, the error depends on the particular satellites used to compute the position, so the reference station can't just say "move all positions 100 metres south". The differential reference station computes the errors in the pseudorange measurements for each satellite in view separately, and broadcasts the error information, and other system status information, by some means. A differential beacon receiver receives and decodes this information, and sends it to the "differential ready" GPS receiver. The GPS receiver combines this information with the individual pseudorange measurements it makes, before calculating the position. For marine use, the US and Canadian Coast Guards (and corresponding agencies in other countries) have established DGPS reference stations that broadcast the correction data over the existing 250 - 350 KHz marine radio beacons. This marine service is available free of charge in the US and Canada, but may only be available by subscription in some countries. Commercial DGPS providers use subcarriers on FM Broadcast stations, and other means, to distribute the correction data. DGPS will eliminate the error introduced by Selective Availability, and errors caused by variations in the ionosphere, resulting in reported postions within about 10 metres (33 ft.) of the true position 95% of the time, for typical marine DGPS systems using inexpensive navigation receivers. Better receivers can get within 3 meters, or so. The DGPS correction data can be used as far as 1500 KM from the reference station depending on the DGPS setup -- if the DGPS is part of a larger monitoring network. (Note that the advertised range of the marine radiobeacons is only 50 - 200 miles, so other means of data transmission must be used at greater distances.) The July 95 issue of Practical Sailor has a review of the Magellan and Garmin Differential Beacon Receivers (DBR) which quotes prices of about US$450 for the units. These units require an antenna similar to that used for Loran C. Differential correction data is commonly transmitted using the RTCM-104 standard. This standard defines a number of different data messages in binary format. The first set of messages (1 through 17) were intended for use with C/A code tracking navigation receivers, and result in about 10 metre accuracy. They correct for ionosphere, SA, and other errors, but with the inherent limitation of C/A code quantization error. Messages 18 through 21 deal with carrier phase GPS corrections, as used in surveying applications. The RTCM-104 Recommended Standards for Differential Navstar GPS Service (ver. 2.1) is available for purchase from: Radio Technical Commission for Maritime Services 655 Fifteenth Street, NW, Suite 300, Wachington, D.C., 20005 U.S.A. The document is copyrighted and is not available on the WWW. 2.5 What are waypoints and routes? A waypoint is just a position stored in the GPS receiver's memory. The receiver can calculate the distance and direction (and time-to-go) to the waypoint, and, if interfaced to an autopilot, will direct the autopilot to steer the boat to the waypoint. A route is a series of waypoints. When navigating a route, the GPS will automatically change the destination waypoint to the next waypoint on the list as it reaches each waypoint. The GPS receiver or autopilot normally sounds an alarm, and requires an acknowledgment, before making any course change. 2.6 Can I connect the GPS to a computer/autopilot/? ? Most navigation receivers have NMEA-0183 data outputs to send data to autopilots and other instruments. Unfortunately, NMEA-0183 provides for several different "sentence formats" to transfer the same data, so it is possible to have two pieces of equipment which both legitamately claim NMEA-0183 compliance, but can't communicate because they disagree on the specific sentences used. NMEA-0183 is a standard developed by the National Marine Electronics Association for data communications between marine instruments. NMEA-0183 data is plain ASCII text sent at 4800 baud. The signal levels are not really RS-232 (as used on most computer serial ports) but will usually work when connected directly to an RS-232 port. Further information on NMEA-0183, and PC programs for monitoring the data are available from: ftp://sundae.triumf.ca/pub/peter/index.html Many receivers also have proprietary data formats which are used (in the case of navigation receivers) to transfer waypoint lists, track logs and other data between the GPS and a computer, and also to pass data which is not covered by the NMEA standard. 2.7 Will my GPS work in a car/plane/forest/cave...? The GPS signals are absorbed by most materials, so a GPS receiver needs a fairly clear view of the sky to operate. There are some reports that a true multi-channel receiver, such as the Eagle Accunav, will perform better in marginal conditions than a single-channel receiver, such as the Garmin 45 or Magellan 3000. Other reports seem to indicate that the difference between these receiver types is very small, or non-existent. Some of the varying reports may be due to different definitions of "forest canopy" or "dense tree cover". Many users have good results placing the GPS receiver on the dashboard of a car, right against the windshield. However, some cars (Pontiac was mentioned specifically) have a transparent metallic film embedded in the windshield as a heater for defogging. This film apparently also acts as an excellent shield against the GPS signals. In those cars, you will need an external antenna. Some users have had good results in commercial aircraft, with the antenna held against a window, preferably on the south side of the aircraft (in the northern hemisphere), to be able to see the maximum number of satellites. Note that Garmin's non-aviation models will not report navigation information if the speed exceeds 90 knots, so they will not be any use in flight. *** WARNING *** Government or airline regulations may prohibit the operation of radio receivers (including GPS) and other electronic equipment during some or all of a flight. Ask permission from the flight crew before using GPS on a commercial flight. As indicated above, forest cover will block the signals to some extent. If you can receive some satellites, some reports indicate you may have a better chance of getting a fix if you keep moving, than if you stand still. Land masses such as a cliff (or a concrete building) will block the view of a major portion of the sky, and make getting a fix more difficult. 3. Survey Systems GPS survey systems were one of the first uses of commercial GPS. These units are more accurate than the typical navigation units but rely on post-processing of the data collected by roving receivers and a fixed reference receiver, and on averaging the data collected over a period of time, by using carrier phase tracking, (and other techniques) to get the increased accuracy. These systems can have an accuracy of better than 1 cm for the very expensive models. Survey systems range in price from US$7,000 to $30,000, or more. 3.1 DGPS Survey Systems. For some survey work short range DGPS systems are used. They operate over short distances and achieve accuracies of 0.5 to 1m. In this case the accuracy is mostly down to the quality of receivers and the short distance between the reciever and base station. Clearly the base station is usually owned by the user as well as the mobile which does increase the cost by a factor of 2! 3.2 Static Survey Systems. Two GPS recievers can be placed in separate locations for a period of time (for short distances 2mins upto 1hr) and the raw pseudo range data can be collected. This data can then be post-processed and a baseline established i.e.range and bearing. The position can be as good as 1mm but may be less good under poor satelite geometry or larger distances. This method can then be used to transfer the knowledge of one accurate point (e.g. a trig pillar (that's what we call them in UK) or in our case the GPS antenna on the roof of our office) to a new point. We use this regularly to survey in new DGPS stations. 3.3 Kinematic Post-processed Systems. In the same way as static surveys, raw pseudo range data is recorded at a fixed site and a mobile. You can then post process the data to see accurately where you have been. Again the distances between the static and mobile systems determine the accuracy but generally 0.1m is possible. 3.4 Real Time Kinematic GPS The newest thing on the block! RTK systems work in similar ways to DGPS short range systems but mathematically like Kinematic post processed and can achieve accuracies of arround 7cm in real time. (3.1 - 3.4 by Dave White ) 4. Aviation Systems The FAA is currently looking at overcoming some of the problems with accuracy with the GPS system. The intent is to be able to use GPS for approaches or even landings. The current systems being experimented with consist of a wide area DGPS systems as well as pseudo satellites. A pseudo satellite is a ground based transmitter that sends out the same signals as the GPS satellites. This may cause some early receivers problems since they assume the satellites are moving and in orbit. Most modern GPS receivers work fine in small planes even though most manufactures have special version for aviation that contain navaid and airport databases. These receivers typically cost from US$700 up. (Note that Garmin's non-aviation models have a 90 knot speed limit, so they will not be useful in aircraft.) 4.1 Wide Area Augmentation System (WAAS) The US FAA is developing a differential GPS system known as WAAS. This system uses a number of reference stations (WRS) scattered around the US. These correspond (somewhat) to the differential correction stations of the Coast Guard marine DGPS system, but do not transmit the correction signals themselves. They monitor the GPS signals, ionospheric conditions, and the WAAS correction signal, and transmit the data to the WAAS master stations (WMS). The WAAS master stations take the data from the WRS, validates past correction signals, and generates a new WAAS correction signal. This correction signal is then transmitted to the InMarSat geosynchronous communications satellites which retransmit the correction signal to the entire US. The InMarSat satellites transmit the correction signal on the GPS L1 frequency, but use a different pseudorandom (PN) code than any of the GPS satellites. The WAAS beacon receiver could apparently be incorporated directly into a GPS receiver. The experimental WAAS system is expected to be in operation in the summer of 1996. 5. Other Satellite Navigation Systems 5.1 NAVSAT or Transit The Navy Navigation Satellite System (NAVSAT, also known as TRANSIT or Sat-Nav) is an older system using four or five satellites in polar orbits. It provides fixes every hour or so, rather than continuously as with GPS. It also requires the receiver to be in a fixed position while taking a fix, or to be moving on a known course at a known speed. 5.2 GLONASS GLONASS is a Russian system similar to GPS. There are apparently no inexpensive GLONASS receivers at present. This system provides accuracy that is nominally better than GPS with SA on and not as good as GPS with SA off. However, due to funding problems within the Russian military, the GLOSNASS system currently has a number of failing satellites and empty orbital slots that can cause high DOP/positional errors, so the actual reliability of a GLOSSNASS fix is lower than that of GPS. Ashtec produces a combined GPS+GLOSNASS system. 6. Radio Navigation Systems 6.1 Loran-C Loran-C is a land based system consisting of groups of transmitters (called "chains") operating on a frequency of 100 KHz. A receiver measures the difference in time between receiving a signal from the master station, and from two secondary stations. The navigator determines his position by plotting these time differences (TDs) against a TD grid overprinted on a chart. Actually, all but the earliest (and cheapest) Loran-C receivers include provision to calculate Latitude and Longitude from the TDs. This feature should be used with caution since the speed of propagation of a 100 KHz signal varies depending on the terrain it travels over, thus the TD lines may not be where the theoretical calculations would place them. (The plotted grid on most charts should already be corrected for these effects.) There are two programs in my ftp directory to convert between Loran-C TDs and Lat/Long. These programs have provision to enter correction factors to allow for the varying speed of propagation. The results of these programs must be checked carefully to ensure that the correct correction factors are used. (Personally, I would plot the TDs on a suitable chart, and read off the Lat/Long, rather than using a program to do the conversion.) 6.2 DECCA The DECCA system is much the same as the Loran-C system (i.e. it is a ground based hyperbolic system). It is shorter range than Loran-C but also more accurate (less coarse). It was set up by The DECCA Navigator Co. (Now Racal DECCA) all around European waters and is used almost to the exclusion of Loran. There are also a few chains in other parts of the world but these are mostly for special purposes. (from Dave White ) 7. Sources of GPS equipment Marine navigation and other "consumer grade" receivers are sold by many marine electronics dealers, outdoors/hunting suppliers, including the major US mail-order companies such as West Marine, Boat/US, Defender, Cabella's, etc. John Beadles has a comprehensive listing of GPS manufacturers on his WWW site: http://galaxy.einet.net/editors/john-beadles/introgps.htm 8 About this FAQ 8.1 Who put this FAQ together? Peter Bennett (bennett@triumf.ca) Having recently been attacked by a swarm of questions from a new GPS user, and having seen many repeated questions re GPS in the sci.geo.satellite-nav newsgroup (not the least of which was "Where's the FAQ??") I thought it was about time a FAQ was created. I am a sailor, and a member of Canadian Power and Sail Squadrons. I have taught the CPS Advanced Piloting and Marine Electronics courses which give (very) limited coverage of GPS and other electronic navigational aids. To support the course material (and my interest in "neat toys"), I have gathered information on GPS and related things from various sources. Karen Nakamura took over stewardship of the FAQ in December of 1997. She is the President of a small GPS/GIS company named Global Mapping Systems that provides consulting services and a Macintosh-based GPS/GIS software package named GPSy -- http://www.gpsy.com In her other "civilian" life, she's a cultural anthropologist focusing on Deaf identities in Japan. 8.2 How can I contribute to this list? Any comments, suggestions, corrections, or contributions (including new questions, and their answers, if possible) should be sent to: Karen Nakamura karen@gpsy.com 8.3 What newsgroups will this FAQ be posted to? It will be posted to sci.geo.satellite-nav about twice a month. It will also be available by HTTP from: http://www.gpsy.com/gpsinfo/gps-faq.txt or FTP from: (also if you want to see the NMEA-0183 and other stuff Peter Bennett has on his site) ftp://sundae.triumf.ca/pub/peter/index.html 8.4 May I post this FAQ to another newsgroup or my local BBS? Yes, providing it is posted unchanged. (I would appreciate email to karen@gpsy.com advising me of any further distribution.) 8.5 Acknowledgements I would like to thank the following for their contributions, suggestions and corrections: Peter Bennett, bennett@triumf.ca Dave White, dave@ormtec.demon.co.uk Tim Thogard, thogard@soulcage.inmind.com Guido Lenz, 100575.3342@compuserve.com Tom Clark, clark@tomcat.gsfc.nasa.gov Brooke Clarke, brooke@pacific.net and other readers of sci.geo.satellite-nav 9. References 9.1 Books Trimble Navigation publishes two booklets on GPS which I found to be useful references in preparing this document: GPS - A guide to the Next Utility Differential GPS Explained Trimble Navigation 645 North Mary Avenue Post Office Box 3642 Sunnyvale, CA 94088-3642 Phone 1-800-827-8000 or 408-481-8000 Fax 408-481-2000 Basic Geodesy Smith, JR, 1988, Landmark Enterprises, Rancho Cordova ISBN 0-910845-33-6. Check your Library. Guide to GPS Positioning prepared under the direction of David Wells ISBN: 0-920-114-73-3 May be ordered from: Canadian Institute of Surveying and Mapping Box 5378, Station F Ottawa, Ont. Canada K2C 3J1 9.2 Internet sites ftp://sundae.triumf.ca/pub/peter/index.html mirrored at: http://vancouver-webpages/peter/index.html ftp://ftp-i2.informatik.rwth-aachen.de/pub/arnd/GPS/peter/index.htm NMEA-0183 interfacing info files and programs (and this FAQ, an NMEA-0183 FAQ, and a Garmin 40/45 FAQ) http://galaxy.einet.net/editors/john-beadles/introgps.htm http://www.navcen.uscg.mil The US Coast Guard's Navigation Information Web Site - the official source for civilian GPS information. http://www.utexas.edu/depts/grg/gcraft/notes/gps/gps.html University of Texas http://www.lib.unb.ca/GGE University of New Brunswick Dept. of Geodesy and Geomatics Engineering http://www.abnormal.com/~thogard/gps.html Misc GPS data and Garmin FAQ ftp://ftp.tapr.org. http://bsrg.org Amateur Packet Reporting System (APRS) (current ver 7.1) ftp://ftp.hawaii.edu/mirrors/info-mac/sci/larrys-mac-gps.hqx The latest release of MacGPS, version 0.3d1. http://www.rssi.ru/SFCSIC/SFCSIC_main.html Coordinational Scientific Information Center(CSIC) Russian Space Forces (GLONASS) http://satnav.atc.ll.mit.edu/ MIT Lincoln Lab GLONASS homepage. http://www.starlinkdgps.com Marine DGPS beacon information http://www.dgps.com DCI (Differential Corrections Inc) home page - DCI is a commercial DGPS provider http://www.ngs.noaa.gov/PC_PROD/pc_prod.html Info on UTMS. http://www.fys.uio.no/~kjetikj/fjellet/GPS1.html Details about UTM and Grid Zone Designation points. http://www.inmet.com/~pwt/gps_nav.html Introduction to GPS navigation http://igscb.jpl.nasa.gov International GPS Service for Geodynamics (IGS) High-accuracy scientific uses of GPS