This document was found on Peter Bennett's Archive site and edited slightly by Tim Hogard. This was sent to Peter as a recomendation of changes. Peter keeps his version of this at: ftp://sundae.triumf.ca/pub/peter/gpsfaq.txt Most of this is from: Saturday, June 15, 1996 3:07:54 PM Local time So find something newer! ------------------------------------------------------------------------- This article is a collection of information, and pointers to other information sources, on the Navstar Global Positioning System, and related topics. The following topics are addressed: 1) The Navstar Global Positioning System 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"? 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/? 3) Survey Systems 4) Aviation Systems 5) Other Satellite Navigation Systems 5.1) NAVSAT or Transit 5.2) GLONASS 6) Specific GPS Units 6.1) Garmin 6.2) Eagle Acunav 6.3) Magellan Meridian XL 7) Radio Navigation Systems 7.1) Loran-C 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? 9) References 9.1) Books 9.2) Internet sites 0 Disclaimer: I 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 it's limitations, as would be of interest to a casual user of a GPS navigation receiver. 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) 21 The Navstar Global Positioning System 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 clear/acquisition (C/A) code (I have also seen the C/A code called "coarse/acquisition"). The L2 signal only carries the P code. Each satellite transmits a unique code, allowing the receiver to identify the signals. 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 it's 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 centered 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 it's clock until they do, providing very accurate time, as well as position. 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). 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.) 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. Military receivers can use the encrypted P code to get 20 metre accuracy, or better, regardless of the state of SA. 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. 1.6 Does the time reported by GPS include "leap seconds" The GPS system time 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 10 second difference. This will change to 11 seconds on Jan 1, 1997. Most receivers will count 23:59:59, 23:59:60,00:00:00,00:00:01 when this happens. 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 physical measurements on land. This resulted in many slightly different 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 charts and maps are based on the North American Datum of 1927 (NAD27). Newer nautical charts (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 it's 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. 2.3 Why does the reported altitude vary so much? Primarily due to satellite geometry. To get the most accurate altitude, you should use satellites that are nearly overhead. However, the satellites are more likely to be near 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 at sea level, the real altitude is probably known better than the lat/long, making the error more obvious. 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. For marine navigation, the differential reference station computes the errors in the psuedorange measurements for each satellite in view, and broadcasts the error information, and other system status information, over a marine radiobeacon. 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 measurements it makes, and displays a corrected position. DGPS will eliminate the error introduced by Selective Availability, and errors caused by variations in the ionosphere, resulting in an accuracy of about 5 metres (16 ft.) for typical marine DGPS systems. The July 95 issue of Practical Sailor has a review of the Magellan and Garmin Differential Beacon Receivers (DBR) which quotes prices of about $450 for the units. These units require an antenna similar to that used for Loran C. In the US and Canada, the differential broadcasts from marine radiobeacons are available free of charge (if you can afford the receiver). However, Canada (and perhaps the US) claims that the service is still "experimental". The USCG system covers the coastal areas as well as the great lakes and major river routes. There are also commercial DGPS services broadcast by other means (VHF or UHF radio or communication satellite) that can be used by paying a subscription fee to the service provider. Some of these can apparently provide accuracy down to 1 metre, with a suitable GPS receiver. 2.5 What are waypoints and routes? A waypoint is just a position stored in the GPS receiver. 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. 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. 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 3. Survey Systems GPS survey systems were one of the first uses of commercial GPS. These units are more actuate than the typical navigation units but rely on post-processing of the data and fixed base stations. These systems can have an accuracy of less than 10cm for the very expensive models. Low end units cost about US$20,000. 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. 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. 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 better than GPS with SA on and worse than GPS with SA off. 6. Specific GPS Units One of the common questions is "What unit should I get?" 6.1 Garmin The most popular Garmin units are the handheld GPS40 (US$250) and the GPS45 (US$300). The only difference is the GPS45 has an external antenna and the GPS40 does not. The GPS45 gets better reception in poor coverage situations but the GPS40 might be a better choice if there is a risk to an external antenna. The GPS40 should be slightly more waterproof than the GPS45. The GPS90 is a GPS45 with an aviation database. These units are about 16x5x1.25mm. Garmin also makes the GPS75 (replaced the GPS55) as well as an aviation model GPS95. The GPS75/95 are in a case that is about twice as wide as the GPS40 and have more buttons. The Garmin units can use a closed protocol to send out more information than the NMEA protocol. The unit can upload and download routes and waypoints and can download a track. The non aviation units will not display useful data if they are moving at a speed over 100kts (115mph) for marketing reasons. There is a Garmin FAQ posted occasionally. 6.2 Eagle Acunav 6.3 Magellan Meridian XL The XL has a feature where it can wake up, take a fix and go back to sleep. It does not support uploading waypoints. 7. Radio Navigation Systems 7.1 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.) 8 ABOUT THIS FAQ 8.1 Who put this FAQ together? 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 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. 8.2 How can I contribute to this list? Any comments, suggestions, corrections, or contributions (including new questions) should be sent to: Peter Bennett bennett@triumf.ca (flames may be sent to prodigy@aol.com :-) ) 8.3 What newsgroups will this FAQ be posted to? Initially, it will only be posted to sci.geo.satellite-nav (any other suggestions, once it's been reviewed a bit...?) It will also be available by ftp from: ftp://sundae.triumf.ca/pub/peter/gpsfaq.txt or: (if you want to see the NMEA-0183 and other stuff I have) ftp://sundae.triumf.ca/pub/peter/index.html 8.4 May I post this FAQ to another newsgroup or my local BBS? This being version 0.01, I would still prefer that it not be distributed outside this newsgroup. 9. References 9.1 Books Trimble Navigation publishes two booklets on GPS: 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. 9.2 Internet sites ftp://sundae.triumf.ca/pub/peter/index.html NMEA-0183 interfacing info files and programs (and this FAQ) http://w.navcen.uscg.mil The US Coast Guard's Navigation Information Web Site http://w.utexas.edu/depts/grg/gcraft/notes/gps/gps.html University of Texas http://degaulle.hil.unb.ca/Geodesy/index.html University of New Brunswick Dept. of Geodesy and Geomatics http://www.abnormal.com/~thogard/gps/ Misc GPS data and Garmin FAQ ftp://ftp.tapr.org. http://www.BSRG.org/ Look for the links: 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.realtime.net/~dfowler Information on the free DGPS. 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.