1. What is GPS?

2. Why buy and use a GPS receiver?

3. Who use GPS receivers?

4. How accurate is my GPS?

5. What is WAAS?

6. GPS antennae - How do they differ?

7. Latitudes and longitudes?

8. What is a map datum ?

9. How do I convert a location's waypoint to a different map datum?

10. What is Selective Availability?

11. What reference point is used for the waypoints of towns, cities and other large surface areas?

12. How do I convert degrees from decimal degrees, degree-minutes and degree-minute-second formats?

13. How do I calculate the distance between two waypoints?

14. Map projections ? What is it all about ?

15. Pseudo Differential GPS - What is it?

16. The GPSWaypoint register?

17. Why does the GPSWaypoint register expresses co-ordinates in the decimal degree format?

18. Taking waypoints on the move - Convenience vs Accuracy

1. What is GPS?

GPS is an acronym for Global Positioning System. 

The Global Positioning System (GPS) is a space-based radio navigation system, consisting of 24 satellites and ground support, that provides accurate, three-dimensional position, velocity, and time, 24 hours a day, everywhere in the world, and in all weather conditions.  It was initiated in 1973 and the first GPS satellite was launched on 21 February 1978.

Currently there are two main systems in the world used for Global Positioning and navigation, that of the United State's NAVSTAR GPS and that of the Russians, called the  GLONASS (GLobal Navigation Satellite System). Due to the inherent limitations and relative inaccurate positioning obtained from the GLONASS , all commercial and most military use is based on the United State's GPS system.  

The GPS Global Positioning system consists of three main components. 

    1.   GPS Ground control stations. (operated by the US Defence force)

The ground control component includes the master control station at Falcon Air Force Base, Colorado Springs, Colorado and monitor stations at Falcon AFB, Hawaii, Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, and Kwajalein Island in the South Pacific. The control segment uses measurements collected by the monitor stations to predict the behaviour of each satellite's orbit and atomic clocks. The prediction data is linked up to the satellites for transmission to users. The control segment also ensures that GPS satellite orbits remain within limits and that the satellites do not drift too far from nominal orbits.

    2.   GPS satellites (The space component)

The space segment includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral in Florida, United States.  GPS satellites orbit in circular orbits at 17,440 km altitude, each orbit lasting 12 hours. The orbits are tilted to the equator by 55° to ensure coverage in polar regions. The satellites are powered by solar cells to continually orientate themselves to point the solar panels towards the Sun and the antennas towards the Earth.  Each satellite contains four atomic clocks. 

Delta Rocket launching NAVSTAR GPS Satellite

Look at the Delta Rocket Boosters that fell in the Western Cape !

    3.   GPS receivers  (what you buy and use to determine your position and support your personal navigation)

When you buy a GPS, you are actually buying only the GPS receiver and get free use of the other two main components, worth billions of dollars - compliments of the Government of the United States. (If you don't have a GPS, BUY ONE NOW, before losing out on the free use of billions of dollars of technology !) 

The ground stations send control signals to the GPS satellites, The GPS satellites transmit radio signals and the GPS receivers, receive these signals and use it to calculate its position. 

      

The calculations used to determine your GPS receiver's position is based on very small time differences, from when the satellite transmitted the signal, to, when the GPS receiver received the signal. These small differences are then used to calculate the distance from the receiver to the satellite. However, when receiving only one signal, we can only calculate how far away from the satellite we are. When receiving two signals, we can determine two likely positions where we are. We need three satellite signals to determine our exact position on the earth's surface. (2D/2 Dimensional positioning). When more than three satellites are "visible" to the GPS receiver, it will also calculate the altitude of the receiver (3D/3 dimensional positioning).

Your GPS receiver requires signals from at least three satellites to determine your unique position on the earth's surface. With a fourth signal your altitude can also be determined. Receiving signals from more than four different satellites, the position of the GPS receiver can more accurately be determined. (See accuracy hereafter)  

The GPS satellite constellation is designed in such a manner as to guarantee that at least 4  satellites are visible from any place on earth at any moment in time. Most of the time (+95%) however, you should have at least 6 satellites visible. Many commercial GPS receivers can receive and process signals from 12 satellites for increased reliability and accuracy. 

GPS satellites carry atomic clocks that measure time to a high degree of accuracy. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the range to the satellite. The receiver must account for propagation delays caused by the ionosphere and the troposphere. With three ranges to three satellites and knowing the location of the satellite when the signal was sent, the receiver can compute its three-dimensional position.

To compute ranges directly, however, the user must have an atomic clock synchronized to the global positioning system. By taking a measurement from an additional satellite, the receiver avoids the need for an atomic clock. The result is that the receiver uses four satellites to compute latitude, longitude, altitude, and time.

        Back to FAQ's

2. Why buy and use a GPS receiver ?   

"Many people look and ask why?, I look and ask why not?" 

Why should you use a GPS receiver ?

        All these benefits and much, much more!

          Back to FAQ's      

3. Who use GPS receivers ?

Although the GPS system was initially created as a means of supporting the navigational and positioning requirements of the military, it has entered our personal lives and will become a more and more indispensable aid to personal navigation.

GPS receivers are currently used by:

        1.    Company personnel to get to appointments, customers, suppliers:

                        Representatives, 

                        Managers, 

                       Technical staff , 

                       Customer support, 

                       Buyers,

                      etc.

        2.    Companies to track assets, vehicles and personnel.

        3.    Off-road travelers in:

                        desert areas (e.g.. Namib) , 

                         in the bushveld, 

                         swamp areas (e.g.. the Okavango), 

                         mountainous areas (e.g.. Drakensberg), 

                        ice and snow fields (e.g.. Antarctica)

                        unknown or newly visited areas.

        4.    All those in aviation and shipping, from pleasure trips to all commercial and military cruising, sailing and flying

        5.    Tourists to:

                        locate their own positions in unknown areas,

                        find the position of their destinations,

                        Get route from where they are to where they want to go

                        avoid identified ",dangerous or hazardous", areas

                        let them enjoy the scenery instead of worrying where they are.

        6.    To mark and find all those special:

                        Fishing spots

                        Diving spots

                        Look-out points

                        Game-viewing spots

                        Adventure sport venues

                        etc.

        7.    Rescuers in search-and-rescue operations

        8.    New vehicle owners with the built-in GPS receivers

        Back to FAQ's

4. How accurate is my GPS?

See the press release from the Clinton Administration regarding GPS Accuracy ! 

The physical laws governing the theory on the GPS and navigational issues are exact and should, in the ideal world, leave us with exact navigational calculations and positioning. The GPS relies on the integration of many aspects in order to eventually calculate the exact or true position at any point in time. However - we live in an imperfect world and the GPS, as smart as it is, is also not perfect. Hence the inherent inaccuracy of GPS positioning. To understand accuracy factors when using GPS receivers for positioning, we should understand the different errors and their combined (non-linear) impact on positioning accuracy. Error analysis can be performed by a so-called ",truth",-model, but again, is only as good as the ",truth",-model itself. However, for general Traveling purposes, we should not be to worried about GPS errors and inaccuracies. How close to an oasis do you need to get in the desert before you actually can see it ?

For all practical purposes we can combine all error effects and rely on the following accuracies:

        1.    Military purposes                   - Accurate to 1 meter (not available for commercial or other use)

        2.    Civilian GPS receivers           - Horizontally :  between 7 and 20 meters (Have a look at our Error histogram)

                                                            - Vertically: between 60 and 100 meters 

        3.    Civilian DGPS** receivers      - Horizontally between 2 and 7 meters 

                                                            - Vertically  between 60 and 100 meters 

            ** DGPS = Differential GPS (with differential beacon receiver) A differential receiver can be fitted to most

                      GARMIN GPS receivers, including some handheld's.

        4.       Civilian survey class differential instruments 

                  (with acknowledgement to Anton Reynecke (Land Surveyor - Thanks Anton !)

 

Civilian survey class differential instruments gives accuracies in the order of one to two centimetre for horizontal, and three to four for vertical, depending on the distance from the base (fixed) station. The centimetre level accuracy is for anything up to the order of 20 km from the base, and degrades slightly for longer distances.

These accurate receivers were available and operative even with Selective availability still active since some 12 (I think) years ago.

These type of instruments are rather pricey, in the order of R 300 000.00 for a rover and a base station.

GPS errors can be classified according to the sources of the errors:

1.    Environmental induced errors

These errors arise from inaccuracies in the modeling of the environment, ionospheric conditions and changes in ionospheric conditions, inability to determine or predict the direction and magnitude of the gravitational vectors at any given time at a given location, inaccuracies in modeling the earth's shape  etc. Ionospheric interference on GPS signals are counteracted by using dual frequencies to completely remove the impact of the ionospheric interferences in military receivers. Unfortunately, only one frequency is available for civilian GPS receivers and they have to rely on mathematical models to estimate the distortion impact of free electrons on the satellite signals.  Of the inherent inaccuracy of 15 meters for civilian GPS receivers without SA, about 5-10 meters are due to the influence of ionospheric interferences, and unless that improves, we can do nothing else to increase the accuracy of one-frequency, civilian GPS receivers.

Satellite geometry or constellation, is another source of errors or inaccuracy. DOP (dilution of precision) is the term used to describe the impact of the satellite geometry on the accuracy of GPS receiver readings. DOP has a number of components: horizontal, vertical, position, time and geometric. When visible satellites are in a straight line and relatively close to each other, DOP will be at its greatest. GARMIN GPS receivers calculates the different DOP figures from all visible satellites and uses the ",best", positioned satellites based on the LPDOP, (lowest position DOP)  A calculated DOP value of more than 6 might lead to a GPS receiver not locking onto the satellites due to the large error that might result. The best thing to do in such a case is to wait a few minutes and the try again. 

Reflected satellite signals (i.e.. from mountains) might lead to a multi-path signal (that is more than one path of one satellite's signal) which cannot be corrected by normal civilian GPS receivers and readings based on multi-path signals will therefore not be accurate.

2.    Computational errors

All our GPS components rely on digital computer systems to perform all calculations. Rounding errors, Overflow errors, limited stack space etc. all contributes little by little to the inherent accuracy capability of our GPS receivers. When performing velocity calculations, we can use State-estimation techniques, relating to the motion of the sensor (i.e.. acceleration), to perform measurements with the resultant impact on accuracy. 

3.    Alignment errors

GPS sensors should all, theoretically, be perfect aligned with their ",assumed", directions. Again, differences in actual alignment and ",assumed", alignments lead to small errors in the ",chain of calculations", and influence the accuracy of our GPS receiver coordinate read-outs.

4.    Instrumentation induced errors

From the GPS ground control stations to the GPS Satellite constellation to our GPS receivers, uses sensors to sense various variables. These sensors may sense variables that do not equal the actual physical quantity it is measuring. Impurities in sensor material, random noise sensed, sensor noise (especially at high frequencies), inaccurate zero-settings or bias, sampling rate, scale factors etc. may all influence the accurate sensing of physical sensed quantities and lead to instrument induced errors.

The above errors lead to the so-called inherent accuracy limitation on commercially available GPS receivers which is currently in the order of 15m. Again nothing to get really worried about when navigating to that oasis in the desert.

5.    Human induced errors.

Very obviously but sometimes forgotten - are your map-datum on your GPS corresponding to the Map datum that was used for determining the original Waypoint ?? Large errors might result from using two different map-datums to reference the same location.

For a view on the impact on accuracy while marking waypoints on the move - look here

 Back to FAQ's

 

 

6. GPS Antennae - How do they differ ?

For recreational and traveling purposes, standard GPS receiver antennas can be classified into two groups:

1.   Upright antennas (or rather Quadrifilar helix antennas):

        Rectangular in shape, mostly visible and external to the main housing of the receiver

        Can detect satellites right on the horizon

        Cannot normally detect satellites directly overhead

        Should be held upright for best reception

2.   Flat antennas (or rather Patch microstrip antennas)

        Flat patch, usually not visible and internal to the receiver's housing

        Can detect satellites directly overhead

        Cannot detect satellites on the horizon

        Should be held flat for best reception

External antennas linked via cable to the GPS receiver, are normally much more sensitive than internal antennas and allows for comfortable positioning within a vehicle, boat or other enclosures. Some ",active", external antennas are available to actively amplify the antennas signal before sending it to the GPS receiver in order to compensate for the signal loss through the cable.

        Back to FAQ's

 

7. Latitudes and Longitudes 

        Back to FAQ's

8. What is a map datum ?

A map datum is a reference surface which is defined mathematically and approximates the shape of the earth in particular areas. At different areas across the world, different map datums were (and some still are) used due to the differences in the earth's general surface shape at different places. Specific map datums are more applicable to  particular areas or regions than others.

A particular map datum is physically represented by a group of surface based trigonometric stations, whose positions have been accurately measured and is used to determine the reference surface (or map datum)

A map datum enables us to calculate the position of a specific location accurately and consistently. However, there are significant deviations, when a waypoint (a longitude and latitude set) located with a particular map datum, is used with a different map datum as reference. So it might appear that specific points on the earth's surface, have different latitudes and longitudes, depending on the map datum that was used, or alternatively, one waypoint (a longitude and latitude set) may seems to point to different physical positions ! 

Currently there are many, many map datums in use. The Garmin products support more than 100 different map datums (see our list and applicable countries) to reduce the inaccuracy caused by the deviations of different map datums. Theoretically this implies that one waypoint can locate more than 100 different places, all depending on the map datum that was used as reference.

All GPS and map users should be aware of this peculiarity and take note of the most applicable map datum to be used. If you use waypoints from any source, or use a map with longitudes and latitudes, make sure that your GPS is using the same map datum as used originally to determine the waypoint.

In South Africa, most older maps use the CAPE datum (based on the modified Clarke 1880 ellipsoid), (not to be confused with the CAPE CANAVERAL map datum). So if you are working with older South African maps and would like to use waypoints indicated on them, SET YOUR GPS to the ",CAPE", map datum.

The World Geodetic System 1984 ellipsoid, or WGS84 in short, is the basis for the Hartebeeshoek94 datum, suitable for all GPS users. All waypoints references are moving towards this internationally accepted datum. So if you use data with this map datum, SET YOUR GPS to the ",WGS84", map datum.

THE GPS WAYPOINTS in the GPS WAYPOINT REGISTER, use the WGS84 map datum, unless indicated otherwise. 

For another good view on Map datums, click here.

For a comprehensive (VERY GOOD) description of the WGS 84 map datum and formulas on map conversions, in South Africa look at this.

For related info try these links:

        Map Projections

        Co-ordinate systems

        GPS system

Back to FAQ's

 

 

9. How do I convert a location's waypoint to a different map datum?

The mathematics involved in map datum conversions are quite involved and will not be discussed here. Have a look here for software capable of transforming between different map datums. However, there is quite and easy way to convert from one map datum to another - available directly on your GPS !

Say for example that you would like to convert a waypoint from the CAPE map datum to the WGS84 map datum.

1.     Set your GPS map datum to ",CAPE",.

Enter a waypoint such as
    S 25d50.653m E028d09.282m.

2.     Now set your GPS map datum to ",WGS84", and note the converted waypoint.
    S 25d50.687m E028d09.264m

The differences in the longitude and latitudes in the above example, translates to an error of 69 meters if map datums are used incorrectly. 

Back to FAQ's

10. Selective availability (SA)

The GPS satellites are owned and controlled by the U.S. Department of Defence and they used to degrade the accuracy of the GPS signal available to commercial users. This is known as ",Selective Availability", (commonly known as ",SA",).

Since 01 May 2000, SA was however dropped by the Clinton Administration and we no longer have to worry about it. See the press release. (however, it is not to say that SA may not be switched on again for whatever reason !!!)

        Back to FAQ's

11. What reference is used for the waypoints of towns, cities and other large surface areas ?

Traditionally, post-offices was used by some institutions as a reference point for cities and towns. Due to the mapping of other large surface areas like dams, lakes and pans, the currently used method is to use the calculated centroid of the area as the reference point.

        Back to FAQ's

12. How do I convert degrees from decimal degrees, degree-minutes and degree-minute-second formats?

By using the following formulas in a MS Excel spreadsheet:  (which you can Download)

• To convert decimal degree (DD) to degrees-minute (DM) format:

           DM =  TRUNC(DD)&",d ",&INT((B6-TRUNC(DD))*60*1000)/1000&",m",

           which will yield DM=12d 7.407m when DD=12.12345

• To convert decimal degree (DD) to degree-minute-second (DMS) format:

           DMS =  TRUNC(DD)&",d ", &TRUNC((DD-TRUNC(DD))*60)&",m",  

                       &INT(((B6-TRUNC(DD))*60-(TRUNC((DD-TRUNC(DD))*60)))*60*10)/10&",s",,",",)

           which will yield DMS = 12d 7m 24.4s when DD=12.12345

• To convert degree-minute-seconds (DMS) to decimal degrees (DD) format:

           DD = INT((LEFT(DMS,2)+MID(DMS,4,2)/60+MID(DMS,7,4)/3600)*100000)/100000&",d",

           which will yield DD = 12.58241d when DMS = 12.34.56.7

To do these calculations, you can download the degree format conversion calculator as an MS Excel spreadsheet. (Note that it has macros which you need to enable - if your browser asks you)

        Back to FAQ's

13. How do I calculate the distance between two waypoints?

To calculate distances between two waypoints requires the use of the ",great circle formula", because the earth is round (approx) and we have to take that into consideration when working with relatively large distances.

The formula is:

• Distance = acos(sin(lat1)*sin(lat2)+cos(lat1)*cos(lat2)*cos(lon1-lon2))

                  where  lat1,lon1 = latitude and longitude of waypoint1 (in radians)

                  and     lat2,lon2 = latitude and longitude of waypoint2 (in radians)

         When using decimal degrees (instead of radians as above) to perform the calculations it becomes:

• Distance=ACOS(SIN(lat1/180*PI)*SIN(lat2/180*PI)+ COS(lat1/180*PI)*COS(lat2/180*PI)*COS(lon1/180*PI-lon2/180*PI))*180*60/PI)

                  where PI = 3.141592654.......

or you can download the Distance calculator in MS Excel format.

      Back to FAQ

14. Map Projections ? What is it all about ?

All map projections have one aim: To get a suitable representation of our 3-D earth onto 2-D paper.

Unfortunately there is not a single "best" projection to standardise on. For different areas in the world and for different purposes, different projections would provide a "best" projection.

Also look at map datums.

The University of Colorado provides excellent explanations on the different Map Projections.

Back to FAQ

15. What is Pseudo-differential GPS?

PDGPS is sometimes referred to as "Inverted DGPS". PDGPS works on the principle that if the system base station is told it's exact location, then a high quality GPS receiver can be used to monitor the difference vector from that known to other GPS's

The compromise is that the system assumes that the other GPS's and the base see the same satellites and resolve position with the same degree of error. The corrected position is accurate to better than 8m for > 90% of the time. Careful monitoring algorithms can reject false or large vectors and time averaging can minimize errors.

Back to FAQ

16. Why use the GPSWaypoint register? 

Currently there are a few sources of GPS waypoints available on Internet that is available to non-commercial Internet users. These web-sites are mostly the efforts of individuals in a ",hobby-like", fashion. Large GPS waypoint databases, on the other hand,  are the property of organizations and are not made available on a free-of-charge basis, as is the large database of The GPS WAYPOINT REGISTER at www.gpswaypoints.co.za. 

        The GPS WAYPOINT REGISTER - Your only real choice for GPS waypoints and info on Southern African destinations.

        Back to FAQ's

17. Why does the GPSWaypoint register expresses co-ordinates in the "decimal degree" format ?

Degrees are the POPULAR method of measuring angles. Normally we do that by expressing an angle in the ",degree-minute-second", format. However, degrees are not the NATURAL measure of angles. If we look at scientific calculations we realise that RADIANS are the natural means of calculating angles. RADIANS are always expressed as decimal figures and hence are easy to use in calculations. However, indicating an angle as 0.785398163 radians is not as meaningful as expressing it as 45 degrees.  So, we end up between the need to calculate and the need to quickly understand. Radians calculates easily in formulas (i.e. to determine the distance between two waypoints) but degrees, minutes, seconds are more easily understood.

In the GPS WAYPOINT REGISTER, we express all co-ordinates in the decimal degree format. This is a compromise that we do to still allow easy calculations but also to be easily understood. Using decimal degrees therefore allows us to easily do calculations (12.3 degrees + 12.6 degrees = 24.9 degrees) instead of some more complicated calculations involved when working with degree-minutes or degree-minute-seconds (i.e. 12d36.55m + 12d29m49s = ??) while still being able to produce an understandable figure. (How large is an angle of 0.66519 radians ??) 

In the 4x4 environment, a number of people have standardised on the dd mm.mmm format (degree and decimal minute format) to allow easier location on maps (indicating degree and minute lat/long lines)

The GARMIN series of GPS's allow you to shift between different formats of waypoints very easily.

The conversion between decimal degrees and radians are straight forward when using the following formulas:

      degrees = radians * 180 / PI

      Radians = degrees * PI / 180

         Back to FAQ's