How GPS works

How GPS Works

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Fig 4.50

Everyone has heard about Global Positioning System GPS. Its found in cars, people are using it on some mobile phones, hillwalkers are using it. Every year its use is becoming more ubiquitous. It has been used by boat users for many years although some traditionalists scorn its use. However, it is an excellent piece of equipment that can give a very precise and reliable position.

It was developed for the sole use of the military, invented by USA but was made freely available in 1983. Although originally used by the military it is used by millions of people daily. The military have a fantastically accurate version, and although making it freely available have reduced its accuracy for civilian use through a process of selective availability. The service available for civilian use was made even better in 2000 and although not quite as good as the military service is still very accurate.

The GPS satellites

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Fig 4.51

At its backbone the GPS system is made up of at least 24 satellites orbiting the earth 11,000 nautical miles above its surface (Fig 4.51). Twelve of these GPS satellites are kept operating and are positioned so that everywhere on the earths surface is covered nearly 100% of the time. Although this may be true, there are still times when it take quite a while to receive a fix, perhaps because your receiver isn’t picking up enough of the satellites.

Each satellite has a pair of atomic clocks that keep extraordinary time within 3 nanoseconds and only loses perhaps 1 second every 100 years. Each of these satellites is able to transmit information indicating its location and the current time on the atomic clocks. They also emit an ‘ephemeris’ or list of all the other satellites and there location. All the satellites are synchronised so that signals are transmitted at exactly the same time.

The control stations

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Fig 4.52

Strategically located around the world are unmanned control stations, Hawaii and Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the Atlantic Ocean and Colorado Springs. There are also four manned antennae stations which broadcast signals, track and monitor the satellites (Fig 4.52).

GPS on the boat

Screen Shot 2015-11-19 at 20.23.19The GPS equipment on your boat receive, detect and process the information emitted by the satellites (Fig 4.50). This GPS receivers can be handheld mobile units if fixed installations on aircraft, ships, tanks, submarines, cars, trucks, even our sailing yachts.

The signals from the satellites, although moving at the speed of light arrive at slightly different times to the GPS receiver units on your vessel (Fig 4.53). The signals leave all the the satellites at precisely the same time and are synchronised, so your GPS in the boat can process these signals and calculate the amount of time it takes for the signals to reach the receiver unit. These times are synchronised and calibrated to the satellites atomic clock, so extremely precise calculations can be made to determine the exact distance of the satellites.

Spherical Position Lines

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Fig 4.54

The microwave transmissions from the satellites are received on the GPS antennae of your vessel. The time it takes to reach your GPS is compared with an internal clock and the distance of the satellite from the vessel can be calculated.

Its handy for us to think about these transmission from the satellites as spherical position line. In effect the satellite is at the centre of a hug sphere and our vessel would be on the surface (fog 4.54). The transmitted ephemeris, the signal from the satellite that tells us where other satellites are, gives our GPS receiver the information where to look for other satellite signals. As our GPS picks up other satellites it forms spherical position lines around the other satellites and able to perform something like a 3 point fix.

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Fig 4.55

The point where 3 spheres intersect can give a 2 dimensional fix. Because we are floating on a flat sea it is ideal for marine navigation. Fig 4.55 shows us how 3 spheres are able to give a fix. For aviation purposes we need to obtain a 3 dimensional fix to account for the plane flying up and down in the air and need 4 satellite spheres to get a fix.

That cocked hat again

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Fig 4.56

Just because we are working with space age technology does not mean that there are no errors made in satellite navigation. GPS can also get a fix with a cocked hat where there is uncertainty in the fix (Fig 4.56). This is primarily due to errors in the timing between the satellites, your inbuilt clock in your GPS isn’t working properly or satellites are not spread properly to get a good 3 point fix. The software in your GPS unit ameliorates any error by adjusting the internal GPS clock which in turn reduces the size of the spheres making the position fix more accurate.

You will find that the longer the GPS is left on the more accurate the fix becomes because the software has time to tune the spherical position lines until the cocked hat effect is reduced to lowest size of uncertainty. You will see on your GPS until that there is Horizontal Dilution of Precision (HDoP) which is a number indicating how good the fix is, which is an assessment of how big the cocked hat is and how the size changes by adjusting the internal clock in the GPS unit. An HDoP of 1.3 indicates a good fix while a larger number indicates less accuracy and that more internal clock adjustment was needed to get the spherical position arcs to meet and a position fix point.


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Fig 4.57

Differential GPS works quite well but has limited use for marine navigation. The receiver is placed on a known point on the land which is programmed into the set. The differential GPS receives satellite GPS signals and calculated the difference between the signals to get a better fix.

The land based unit can then send a radio signal to a beacon on the vessel and the dGPS receiver on our vessel can decode this message and determine its position (Fig 4.57). Because there is a land based unit involved it has limited applicability for vessels operating offshore or on ocean passages. It is likely that this system will be phased out in favour of sdGPS.


Satellite Differential GPS does not use radio beacons, instead it uses other geostationary satellites that redirect signals to vessels. The geostationary satellites are fixed in position above the earth as they orbit the earth at the same speed in is rotating. They provide accurate position fixes primarily because the control centre knows where it is all the time. There are different systems in North America, Europe and Asia and others which are expected to offer global coverage.

The main physical causes of inaccurate information

Power failure
Damage to antenna
Transmission from mobile phones
Changes and interruption of the signals from satellites

GPS accuracy and data

GPS is fantastic technology but it should never be relied upon. It sods law that when you need it the most, it will fail on you. There are other pitfalls, for instance it will quite happily navigate you over land and when used with chart plotter can be difficult to asses dangers when adjusting the scale of vector and raster charts on the small screens.

GPS accuracy

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Fig 4.58

You can expect that the GPS position fixed will be accurate to within 15 metres, 95% of the time. So we can assume that 5% of the time the fix will be inaccurate. Not only will it be inaccurate but also difficult to say how inaccurate or what size of the error is. sdGPS is much more accurate and for 95% of the time it will pinpoint your position to 3m.

To visually represent the accuracy of GPS, IN Fig 4.58 the blue circle with the vessel in the centre represents the 95% accuracy for a standard GPS unit, and the individual numbers represent the 20 fixes made. The point ‘K’ outside the circle represents one of the fixes that for 5% of time were over 15 metres.

GPS is not a compass

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Fig 4.59

The GPS on your yacht can only give direction when it moving and this is ‘after the fact’ in hindsight, basically it has taken two position fixes and worked out the true bearing between them. It will lag behind what is actually happening at the helm. This can be highlighted when at anchor as the GPS is continually fixing its position 95% of the time within 15m range (Fig 4.59). The direction between these individual fixes will continually change to read GPS direction is lost. The take home message here is that you have to be very careful if navigating under chart-plotter at slow speeds as directional information can be erroneous.

We know that GPS does not have a magnetic compass in it, and calculates its Course Over Ground (COG) by calculating difference between two positions, ie latitude and longitude coordinates. This is the course the vessel is taking over the seabed below and not where the vessel is heading.

GPS also shows Speed Over Ground which is calculated at the time taken to travel between two coordinated, latitude and longitude. Obviously this information is meaningless when at anchor, well unless the anchor is dragging along the seabed. Again like the COG the slower the vessel is moving the more the errors come into play and can produce erroneous results. However as long as you are aware of these and able to read and understand information from GPS unit, its still a remarkable piece of technology.

Watch out for Datum shift

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Fig 4.60

Remember back to the the module 1 and the TItle information block on the RYA charts (Fig 4.60) mentioning that the chart refers to a particular datum? The chart datum for GPS is World Geodectic Survey (WGS84) so we know that we can directly fix the positions form the GPS unit onto RYA Chart 3 and RYA Chart 4.

This information block on the charts tells us the datum that that particular chart is in. A lot of the time it may be in a different datum and therefore position data form the GPS will not be correct for the chart.

If we try to make a fix in datum on older charts with a different datum we get a phenomena called datum shift, which will give incorrect position and needs to be corrected. Luckily, most GPS units have the function to change between different datums and can synchronised so that chart and GPS position information is in the same datum. So its always important to make sure your GPS unit is in the same datum as the chart.

User Beware

Please remember that the data from your GPS may be more accurate than the chart you are using. This can be exaggerated in places that have not had good hydrographic surveys. This is true of many places in the greek islands where information on the chart was gathered using basic trigonometry and astronomical techniques. So in theory the information on the charts does not marry with GPS position data. Skippers must therefore exercise great caution when using GPS to navigate vessels close to land. The GPS will be correct, but the land and rocks may be in the wrong place on the chart!

Basic GPS Functions

We have had a brief look at the limitations of GPS navigation systems, now we can turn to have a look at how this system can assist our navigation. The displays on the GPS unit on our vessel tells us the latitude and longitude of our antenna (Fig 4.61), as we have noted 95% of the time this is within 15m on standard GPS and up-to 3 metres on sdGPS. As long as the datums on the GPS unit is the same as that of the chart we can plot these positions obtained from the GPS directly onto the chart


The readout gives us the COG, course over ground and the SOG, speed over ground which are continually calculated and updated as the GPS unit receives information from the satellites. The COG is always expressed as a True bearing so never any need to convert it to Magnetic bearing and is the actual direction the vessel is traveling in relation to the seabed (Fig 4.62). The SOG is calculated by measuring the time taken to travel between to sets of location coordinates. It is important to remember that COG is not the heading of the boat or Course To Steer CTS as the GPS unit does not allow for the effect of leeway, currents or tide.

Active Waypoint Information

One of the most powerful functions of GPS unit is the ability to place and calculate range and bearings to waypoint. A waypoint is a position anywhere on the earths surface and GPS can determine the range and bearing to any waypoint (4.63). You can enter many waypoints into the GPS, however the unit does not know if the route between two individual waypoints is across the water or land. We can activate a way point, so that the GPS is locked onto it and becomes an active waypoint.

The range and bearing to a waypoint are continually calculated as the GPS software continually adjust the position coordinates of the vessel as of moves through the water and the coordinates that have been entered for the waypoint. Waypoint accuracy depends on the information that the user has entered into the GPS on the vessel. If it is entered manually there is always potential error and should always be checked.

An error of 1 digit in 1/1000 if a minute (0.001) means an error of 2 yards
An error of 1 digit in 1/100 of a minute (0.010) means an error of 20 yards
An error of 1 digit in 1/10 of a minute (0.100) means an error of 200 yards (1 cable)
An error of 1 digit of 1 minute (1.00) means an error of 2000 yards (1mile)
An error of 1 digit of 1 degree (60 minutes) means an error of 60 miles!

Words of Warning

Any errors that have been entered into the GPS unit can put the position of the vessel out considerably. There are also inherent errors in the actual GPS unit as these systems rely upon a continual supply of electricity. Also the USA government and military has and can limit the accuracy of the information sent by the satellites. It is very important that navigators have the traditional skill set available to navigate without GPS and electronic navigation systems.

Positions should always be logged and plotted into charts. Systems do go down and when satellite coverage becomes a little poor the GPS do lose their position (Fig 4.64). The take home message is that GPS and other electronic navigation system should never be used as the primary form of navigation. The GPS is excellent at telling you where you are but cannot tell you what lies ahead, what is under the vessel and what action you should take to avoid hazards. When linked to a autohelm a GPS will follow waypoints faithfully even when they are haphazardly placed on top of a mountain.

Although a straight line ground track on a GPS might seem the best route to follow. It is sometimes not the best course to follow especially on slower moving sailing yachts. The tides will have more of an effect on them and when making a passage in tidal waters, for example a passage to Ireland from Scotland that expected to take 12 hours will experience one flood tide and one ebbing tidal stream. For 6 hours there will be one south flowing tide and the other 6 hours one north moving tide. If you were to follow the ground track from the GPS you would be stemming or fighting with the tide, slowing the vessel down. On the other hand if you maintain a course to steer that does not stem the tide, the vessel will sail faster, the tides almost canceling each other out, one carries it southwards the other carrying it northwards. This technique is often used by offshore racers and can leave the boats that slavishly follow ground tracks that are set by GPS without taking into account the effect of tidal stream, and leeway (Fig 4.65).

GPS Plotting Technique

To complete this section we will look at how to use GPS in position fixing, in addition to the more conventional plotting of latitude and longitude we will show you three techniques for a quick plotting of GPS position on a chart.

Using the centre of the compass rose as a reference waypoint
Construction a waypoint web and
Constructing a cross track error ladder

Well also introduce the concept of a head bearing to help avoid an area.

Bearing and Rage to Waypoint

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Fig 4.66 – The bearing and range to waypoint is an excellent way to quickly fix your position on a chart using information from the GPS unit. However, you should always corroborate this information.

One of the most useful plotting techniques you can use on your GPS, particularly suitable for slower moving sailing vessels is to use a bearing to waypoint technique. You enter a waypoint into your GPS which is normally the centre of a compass rose on the chart. When you activate this waypoint with the GOTO function on the GPS the bearing BTW and range RTW to this waypoint is continually displayed on the GPS display.

In Fig 4.66 we can see that the information from the GPS display can be plotted onto the chart easily by using the Portland Plotter and dividers. The readout displays BTW 056°T and the range RTW is 3.4 miles. Basically we set the Portland Plotter to read 056° and point it in the direction of the waypoint which has been set at the centre of the compass rose. We can then use the divider to measure a distance of 3.4 miles from the latitude scale and transfer this distance to the bearing line we have drawn with the portland plotter.

We have transferred the information from the GPS unit to the chart and by using the Bearing to Waypoint technique have accurately plotted our position on the chart. We could then check this position by reading the latitude and longitude and checking against the GPS or by using the handheld compass and taking a bearing on charted object. You should be aware that this bearing information is from the vessel towards the waypoint.

Waypoint Web

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Fig 4.67

The second technique involves drawing a web on the chart. It is particularly useful for sailors that are sailing in the same general area. You draw a concentric ring of bearing lines and range rings. Bearing lines are conveniently radiating from the centre at 10°True intervals. The range of the bearing lines could by 1 miles but really depends on what distance or area you are sailing and what scale of chart you are using. A waypoint is set at the centre of the waypoint web and GOTO function on GPS selected to activate the waypoint.

It really quite simple to use the waypoint web and positions can be plotted quickly by eye. If the GPS display tells us that that the waypoint is 245°T and 1.5 Miles we can quickly plot the position on the chart. You must remember that the range bearings can be written on the outside of the Waypoint Web ring anticlockwise and makes plotting very quick.

The waypoint web is useful for people that race or sail in the same area as it take some time to construct the web. You can draw the web in with pen, so that pencil marking can be erased leaving the ink web to be used again. Otherwise, laminating the chart may be a good idea then using felt ink pens to chart positions.

Cross track error ladder (XTE)

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Fig 4.68

The cross track error ladder is probably best for vessels that can move at speed through he water (Fig 4.48). The backbone of the ladder displays our straight line ground track COG. We add perpendicular lines to the ground track every mile so we can use cross track error technique. Like the Waypoint Web a little pre chart-work is necessary to use this technique but once prepared it is a fast and easy way to plot your position which may be necessary in a fast moving vessel. All you need to do is read the XTE error page on GPS display and plot your position onto the chart by by eye.

The limitation of GPS

There are limitations to the use of GPS and include

Owned and controlled by a foreign government
Coverage can be poor at times you might have to use your traditional navigation skills
Electronics can fail, especially if overdo the batteries
Datum shift can generate large errors
Chart survey age impacts on accuracy
Waypoints entry error, user entering waypoints can make its precision useless
GPS takes time to warm up and get accurate fix

However, it is an excellent tool but information should be confirmed by another source. One huge benefit of GPS is how it can be linked into the safety systems, particularly VHF of the yacht.

Errors with Reflected Transmissions

An echosounder works by transmitting a pulse of high frequency sound directly down to the sea bed, the pulse is then reflected back to the transducer (the transmitter). This is one reason for seemingly odd depth readings when passing over rocky shelves, thickly encrusted with seaweed or where the pulse is partly absorbed and not reflected. Another reason when passing though the wake of another vessel where all the bubbles in water change the density of the water and therefore the speed that the pulse of sound passes though it. The speed that the pulse (and the reflection is known as an echo) travel though sea water is a constant so half the time elapsed between transmission and reception of echo is used to calculate the depth.
Radar works in the same way but uses extremely high frequency microwaves that travel at the speed of light ie 300000 km per sec. In the illustration note how the 2 buoys reflect an echo back towards the radar transmitter while the pulse that travels between the 2 buoys carries onto infinity or until it is also reflected back as an echo by another object further away and thus providing differential distances of contacts (the object that reflects the pulse)