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understanding GPS

by Andrew Gliganic

  Several years ago, a commercial airplane crashed in the Atlantic, just off Long Island.  A multi-agency task force was created and had responsibility for recovering victims, and later, the wreckage.  The technology used in this effort was breath taking.  Several square miles of ocean floor, 120 feet deep, were mapped with SONAR and Laser Line Scanning devices.  The resulting images revealed a huge field of debris along  with the exact latitude and longitude for individual pieces.  Next, dive teams were deployed to  recover these targets.  The challenge that remained was how to find these exact points on Earth, without any visual landmarks, and place divers precisely enough to avoid lengthy searches?

     The answer came from NAVSTAR GPS.  Short for Navigation System with Time And Ranging Global Positioning System, this technology is operated by the U.S. Air Force.  It consists of a constellation of 24 artificial satellites in high Earth orbit, each transmitting data to land based GPS instruments.  The receiver/computer (R/C) GPS instruments used by the recovery dive teams were able to determine position to within a meter of accuracy.  These devices guided the teams to their target, where they dropped an anchor connected with a descent line to a surface float and launched their divers.  In this fashion many of the victims, and much of the wreckage was ultimately recovered.

     Originally created for national defence, GPS is now available to all of us.  Unlike any other navigation system or technique, it works 24 hours a day, in any weather, in any visibility, and to high levels of accuracy.  Inexpensive and dependable instruments are available and they have revolutionized navigation.  With minimal training, users and find their exact positions, determine course and speed, local variation, time to destination, and cross-track error (how far off course you are).  They can also keep a library of pre determined positions (way points), and can determine the bearing and range between any two way points.

How Does it Work?

     Starting in 1978, the U.S. Department of Defence began launching GPS satellites into orbit, approx. 11,000 nm above the Earth.  This network now has 24 operational satellites plus several back ups.  Each is assigned to one of six different orbital paths which keep them to within 60 N and 60 S latitudes.  These flight paths, together with the high altitude of the satellites, allow GPS signals to reach any point on Earth.  Each satellite is about 2000 pounds, is partially solar powered, and transmits at a maximum of 50 watts.

     So what are these satellites telling us?  Well each is assigned a unique identification code called a pseudo-random number.  This tells the land based GPS instrument which satellite is transmitting, and this number (1-32) is displayed on the R/C display.  Next the satellite transmits date and time data as well as orbital data for ALL of the NAVSTAR satellites.

     To find position, the GPS instrument takes the satellite data, determines the exact time and calculates the exact predicted position of the satellite.  Next it measures the amount of time that the satellite's signal takes to reach the instrument and calculates the distance to the satellite.  By doing this with at least three different satellites, a 2 dimensional fix (latitude and longitude) can be triangulated by the GPS R/C.  With data from 4 or more satellites, a 3 dimensional fix can be obtained showing lat/long as well as altitude.  


Fig. G1

Selective Availability

  Author's note:  As of midnight, 5/1/00, the U.S. government has set Selective Availability to Zero.  While reserving the right to degrade the signal if strategically necessary, the Department of Defence will be transmitting accurate signals for all GPS users.  This section on SA will remain in the module for (hopefully) historic reasons.

     Oh boy, this is great!  But what's the catch?  It's called Selective Availability (SA).  Uncle Sam didn't spend $10 billion bucks just so we could get to our favourite fishing site.  GPS was developed and is maintained primarily for national defence, so the system is degraded slightly when used for civilian purposes.  This is so that hostile countries or organizations don't have the precise accuracy available to the U.S. military.

     Each satellite transmits over three frequencies, one being assigned to civilian use.  Under Selective Availability, an intentional error is introduced to the civilian signal, creating positional variation of from 0 to 300 feet.  For most navigational purposes this is good enough, but where greater  accuracy is needed, Differential GPS (DGPS) technology can correct the SA error for an accuracy of within 3 meters or better.  This system, administered in selected areas by the U.S.C.G., uses a GPS receiver/computer located at a precisely known position.  This receiver then determines the exact amount of SA error for that moment and transmits correction data to DGPS equipped R/Cs within the coverage area.  Some of the limitations of DGPS are the added cost of the equipment and the limited coverage area.

Satellite Geometry     

       Another factor which affects the accuracy of  GPS is satellite geometry.  Remember in the plotting module, when we talked about bearings?  The most accurate fixes were obtained when bearings were about 90 from each other.  The same is true for GPS "bearings."  A fix from three satellites located in line with each other will not be as accurate as one obtained from four satellites spread out around our position.  Because of the orbital paths that the satellites take, this is usually not a problem unless the ship is sailing in a river or fjord, where steep banks block a satellite's signal. 

How to Buy a GPS Receiver/Computer

     There are dozens of different instruments available.  How do you pick one?  Well first ask yourself several questions:

  • What do you need a GPS R/C for?  How much accuracy do you need?  Obviously an inner harbour tug boat would require a much greater degree of accuracy than a family cruiser.  Maybe you don't really need one, but would like to have one aboard for fog, or emergencies.

  • What features do you need?  How many waypoints will you be keeping? What type of display do you like?  Is it easily visible at night?  Does it have a map display?  Do you want the portability of a handheld or do you need the large display size of a mounted model?  Do you need to interface the GPS with your RADAR or electronic course plotter?

  • How much do you want to spend?  Quality instruments start at under $200 and go up into the thousands.  I have used an inexpensive hand held Garmin 45 for several years now, and it's never disappointed me.