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To get the best use out of a GPS, you need to understand both its capabilities and its limitations.

This first part of a three part article explains how a GPS does its most fundamental thing - working out where you currently are.

 
 
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A Beginner's Guide to Using GPS Part 1

How the GPS Knows Where You Are
 

If the GPS appears to be telling you to turn left at the rail line, don't automatically accept its advice!

As amazingly accurate as GPS receivers can be, they still can make mistakes.

Part 1 of a 3 part introduction to GPS, as part of our broader series on GPS - see links to additional articles in the series on the right.

GPS technology is truly amazing and close to magic, but there are still limitations on what it does and how it does it.

It is only when you appreciate the limitations as well as the capabilities of GPS that you'll be able to get reliable best use from your unit.

There are too many stories of drivers who have blindly trusted the information on their GPS screen, ignoring the conflicting real world information on the road.  Never do this.  Use common sense and understand that if there is any doubt, what you see outside the car is of course more correct than what the GPS is telling you!


Your First GPS?

For many people, their first introduction to GPS is finding a unit installed in a rental car.  Or maybe they receive one as a gift.

The anticipation and expectations of this marvelous magical new technology are probably very high.  But, as amazing as it is, it is not perfectly flawless in all ways, at all times.

The information in this three part article will help you better appreciate what, why, and how your GPS can and can't do certain things.

How a GPS Unit Knows Where You Are

It is amazing, really.  You turn on a unit that is about the size of a couple of decks of cards, and it tells you exactly where you are.  How does it know this?

Those of you who remember your high school geometry and trigonometry can 'fill in the gaps' of the following explanation.  In simple terms, the GPS unit measures the distance between itself and a number of satellites in the sky, and then, based on this information, can calculate where on the planet it is.

That's the simple version.  But it helps you to understand a bit more than this, so you can anticipate and understand when the GPS unit can not work out where it is, or when it can't make this calculation as accurately as in theory it could.  These errors are not the 'fault' of the unit, but are often inevitable results of the underlying math in its calculations.

How many satellites must a GPS see?

A GPS needs to be able to measure its distance to at least three different satellites in order to calculate a two dimensional fix of its position (ie latitude and longitude).

Not only does it need to be locked on to at least three satellites, but they need to be in different parts of the sky.  The more widely spread out the satellites are, the more accurate the calculation can be.

If the GPS can receive data from a fourth satellite, it can then start to make a three dimensional fix of its position (ie, latitude, longitude, and altitude).  It helps if one of the satellites is more or less directly overhead, and, of course, if all four of the satellites are reasonably equally spread around the sky.

Altitude calculations are much less accurate than the latitude/longitude calculations.  Adding extra satellites - 5, 6, 7, even 8 or more - to the number the GPS receiver is locked on to and using for reference points will improve the accuracy of its calculation.

But as important as the number of satellites is the location of them.  Ten satellites all bunched up in the same far corner of the sky might result in the unit being unable to make any location calculation at all, whereas just three or four, evenly spread, might give a very good calculation.

It can sometimes be interesting and helpful to look at your unit's satellite receiving map (if it has such a feature).  This feature (on most Garmin units) shows a map of the sky and then the location of the satellites in the sky, and highlights the ones which the unit is locked onto and receiving good signals from.  You can quickly see at a glance if the unit is likely to be receiving good reliable data or not from where the satellites are.

Here is an example of the satellite screen display on a Garmin GPS.  On the left is a representation of the sky - the outer ring is the horizon, the inner ring is a 45 angle from the horizon, and the center is directly overhead.  The right hand side shows the strength and quality of signal for the satellites being received.  You can see the unit is locked on to seven satellites, with two more that it is getting partial data but not yet locked on to, and various other satellites that it 'knows' are out there but which it can't get any signal from (due to obstructions).

The computed 28' accuracy is, as always, an overly optimistic number, but it gives you a relative feeling for the quality of the fix - obviously 28' accuracy is better than 100' accuracy, but not as good as 10' accuracy.

Looking at where the satellites are in the sky, you'll see that there is a reasonable amount of triangulation - there are satellites to the left, right, and behind, with good angles between them, making it easy to get a fairly good fix, as is suggested by the 28' accuracy.

Satellite number 48 is a WAAS satellite.

How many satellites can a GPS see?

The GPS satellites move in what can be considered semi-random patterns; sometimes there is a bunch of them in one part of the sky and not many in other parts of the sky, and sometimes they are evenly spread out.

Although there are typically 8 - 12 satellites in the complete 180hemisphere view of the sky above you, your GPS seldom has an unobstructed view of all of these.  A GPS basically needs a line of sight view to the satellites - the very weak radio signals from the satellites can't punch through much in the way of obstructions, and so as a reasonable rule of thumb, if there is any blockage between the receiver and the sky, satellites behind that blockage will not be able to be seen and their signals received.

If your receiver is mounted in your car, you've immediately got the car roof obscuring much of its view of the sky (mount the unit as far forward on the dash as possible so as to reduce this obstruction) and even the presence of yourself and other people in the car will block some of the sky, too.

If there are hills or buildings, these will limit the amount of the sky visible, too.  Go through a tunnel or drive into a covered parking lot and you'll lose just about every satellite.  Even things like a dense tree cover can block the signals, which sometimes can't even make their way through the tree leaves.

So while you start off with, in theory, the ability to get signals from up to 12 or so satellites, the inevitable nature of obstructions massively reduces the amount of visible sky and satellites.

Your GPS receiver always works best when it is receiving multiple signals, and from well spread out satellites.

The Delay When You First Turn a GPS On

When you first turn your GPS on, it needs to 'tune in' to the satellites in the sky, and to do this, it needs to know 'where to look' for the satellites.  (The phrases in quotes represent major over-simplifications of what is actually happening.)

If the unit was turned off just a short while ago, and then turned on again in the same place it was turned off, the unit 'remembers' where the satellites where, and assumes that it is still in the same place it was a short while ago too, so it knows where to 'look' for the satellites and can quickly 'find' them again and lock onto their signals.

This is variously referred to as a warm or hot start, depending on exactly how up to date the unit's information is.

But if the unit has been turned off for a day or two, and/or if it has been moved while it was switched off (for example, you take your unit out of your car, fly with it somewhere, and then turn it on in the rental car at the destination), not only has it forgotten where the satellites are, but it also doesn't know where it is, and so it has to start afresh from the very beginning of looking everywhere for any satellites and computing its position potentially anywhere on earth.

This is referred to as a cold start.

More detailed explanation

In order to lock onto satellites and use their location signals, the receiver needs to do two things.  Firstly, it needs to know about where in the sky each satellite is likely to be.  This information, which is called the 'almanac' is transmitted by all satellites once every 12.5 minutes, and gives a list of all currently working satellites and their paths/positions over the next up to six months.

Once the GPS has found any satellite, it downloads the almanac information, and then knows approximately where to look for satellites (assuming the GPS also knows more or less where itself is located).

After it finds each satellite, it then needs to download exact detailed information from each satellite about exactly where the satellite is.  This information, called the 'ephemeris' information, is transmitted by each satellite every 30 seconds, and is good for up to four hours, so needs to be continually updated.

If the GPS unit already accurately knows where it is, and has current ephemeris data, when it is switched on it only needs to do a hot start, which can be very quick.  If it knows where it is (to within about 65 miles) and has up to date almanac information but needs ephemeris information, it does a 'warm start' which takes a bit longer.  And if it has out of date almanac information, or has moved more than 65 miles, it will need to do a cold start, which is the longest/slowest process.

Normally, with a GPS in our car, and with us turning the GPS on (probably automatically when the ignition goes on) every time we drive the car, and with driving the car every day or two, the GPS unit will be doing warm or hot starts.

How long this all takes

The time it takes from turning a unit on until when it has completed calculating its position is referred to as the unit's 'TTFF' (Time To First Fix) or acquisition time.

In the 'bad old days' (ie about ten years ago) units with single channel receivers could take half an hour or more to calculate their position from a cold start, and could take five minutes to calculate their position from a hot start.  Nowadays, units with multi-channel receivers (ie, they can listen to many satellites simultaneously rather than just one at a time) can do a hot start in 5 -  30 seconds, a warm start in about a minute, and a cold start in no more than 15 minutes (assuming the unit can clearly see a sufficient number of satellites).

If you're getting into a rental car, the GPS unit in the rental car may not have been used for some time, and will need either a warm or cold start.

There's one additional complicating factor in the time it takes for a GPS to acquire its first satellite fix.  If you start driving around, you can lengthen the time this takes considerably, because as you drive, various satellites will come into view and go out of view again, so as some satellites disappear, you 'lose' the time the unit has take to tune those in, and the unit has to restart the process again to get more satellites, and so on.

If you're collecting your rental car from an open lot with a clear view of the sky, the very first thing you should do is turn on the GPS, even before you load your bags in the car.  And if you've flown somewhere with your own GPS, as soon as you're at the rental car lot or somewhere else, stopped, with a clear view of the sky, turn the unit on and have it update itself on where it is and where the satellites are.

If your rental car is in a parking garage, then the GPS receiver will probably not be able to find any satellites until you drive out of the garage and onto the street.

Losing and regaining satellite signals

Say you're driving along and go underneath an overpass on the freeway, and briefly lose satellite signals.  Or perhaps you turn a corner and a building that had been blocking your view in one direction now shifts its position as you turn and blocks other parts of the sky briefly, obscuring some satellites that you'd formerly been receiving.

In such cases, you'll of course lose some or all of the satellite signals you'd been receiving.  If you only lose a few satellite signals, the GPS might still be receiving signals from enough satellites as to still know where you are.  And, of course, with moving obstructions, while they are blocking some satellites they might be unblocking other satellites at the same time.

In effect, as satellites shift in and out of view, the receiver is continually doing 'hot starts' to regain the signals from the temporarily obscured satellites.  This means you might not even notice the occasional loss of signal, or, if you do, it should quickly be restored as the temporary obstruction disappears.

Dead Reckoning

This sounds like the title of a mystery or western novel.  But it refers to the ability of a GPS to use other methods of calculating where you are and where you are going.

Dead reckoning can be useful if the GPS briefly loses satellite signals, or gets 'confused' (for example, with bad signals and lots of streets to choose from when driving in a high rise concrete jungle downtown).

Dead reckoning, in its most sophisticated form, adds a compass sensor to the GPS to tell the GPS, on the rare occasions when it doesn't already know this, what direction you are heading in, and then adds extra sensors from the vehicle to tell the GPS about the vehicle's speed and perhaps also supplements the compass sensor with turning information from the vehicle too.

In a simpler, self-contained form, dead reckoning might comprise a compass sensor and an accelerometer within the GPS.  This enables it to know what direction you're heading, and to sense any increase or decrease in speed (and turns) through the accelerometer.

In its simplest form, if/when the GPS loses satellite signal, it simply assumes that you are continuing in the direction and at the speed you'd been traveling at immediately prior to losing the signal, and it continues to plot you along that path for a while before it decides it can no longer continue with this assumption and gives up.

Dead reckoning, if used intelligently, can make a GPS enormously more helpful.  I've watched regular GPS units get very confused, particularly in downtown areas, with the location pointer seemingly randomly jumping from street to street and back again.  But my Landrover GPS, with dead reckoning, never does that, because the GPS supplements its location calculation from the satellites with vehicle information and so it 'knows' that there's no way the vehicle is magically jumping from street to street and back again.  Instead, if it has a temporary conflict between its calculated position and what the dead reckoning tells it is happening, it makes a (usually sensible and correct) decision with this extra information.

It can also be helpful when driving along a freeway that has underpasses and overpasses in a spaghetti type interchange, and you are being instructed to take a series of one of many different sets of exit combinations, and just when you most need to be walked through your turns, the GPS suddenly freezes and flashes a 'bad satellite coverage' message at you.

If a unit does offer dead reckoning, it is important to appreciate that the accuracy of its dead reckoning is vastly less than that of its GPS receiver, and, importantly, the dead reckoning accuracy gets worse and worse over time, and if it isn't updated by GPS information fairly quickly, it will become unhelpful and may become misleading.

A clever enhancement to dead reckoning is an extension of the 'snap to road' concept.  If the dead reckoning calculates you to be 100 yds from an intersection, and then suddenly senses you turning left, it may be clever enough to tell itself 'hmmm, I guess I was wrong, we are already at the corner' and update its location calculation based on the assumption you turned on to the road rather than into a driveway it knows nothing about.  This type of self-correcting can extend the period of time for which dead reckoning remains accurate and useful.

If you have a chance to choose a unit with dead reckoning capabilities, you should do so, even if it costs more than a unit without this capability.  It significantly improves the value, accuracy and reliability of the information your GPS will provide you.

Read more in the GPS articles series

See the links at the top right of the page to visit other articles in our extensive GPS series.

This particular article is part 1 of a three part article introducing you to GPS receivers, and what they can and can't do.  Please also visit

1.  Beginner's Guide Part 1 - How the GPS Knows Where You Are
2.  Beginner's Guide Part 2 - Maps, Routing and ETAs
3.  Beginner's Guide Part 3 - Errors, Inaccuracies, POIs, Speed

 

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Originally published 6 Jun 2008, last update 19 Dec 2013

You may freely reproduce or distribute this article for noncommercial purposes as long as you give credit to me as original writer.

 
 

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