What GPS? Telling you about what GPS and SatNav is, and which GPS is best

What GPS? Telling you about what GPS and SatNav is, and which GPS is best. This websites purpose is to give information about SatNav systems based on GPS, to allow consumers to make an informed choice about which is the best system for them to buy, whether it is the cheapest or the fully featured.

It describes how GPS works, how SatNav works, key features, common issues, and which GPS is cheapest and which GPS is best.

What is GPS and how does GPS work?

GPS, which is short for Global Positioning System, is about figuring out where in the world your location is.

GPS is a technology that is based on the concept of triangulation. If you know where some known locations are, and you know the distance to each known location, then you can figure out where you are.

In GPS, the known locations are satellites, and the GPS constellation (the name given to a group of satellites) consists of 24 satellites. The more satellites that you use for your location fix, then the more accurate your location fix will be. The GPS satellites orbit the Earth every 12 hours and hence the satellites that your receiver is tracking can change. They all have difference trajectories and typically you can only see a maximum of 10 or 11 at one time. More likely you will see 7 or 8 satellites depending on how much of the open sky is visible from your location.

The following examples illustrates this.

Figure 1 - GPS with 1 Satellite

Figure 1 shows a person receiving a signal from one satellite. The signal transmits a time based message so the user can figure out how far it is away. The dark blue circle shows the signal radiating from the satellite at a certain time snapshot when the user receives it. As the signal is omni-directional, the user knows that the satellite is somewhere in the path of the light blue circle, based on the time and the speed of light (the circle that shows all directions from the user the same time difference the satellite is away). Within the signal of the satellite, its position is described, hence the user has one known location.

Figure 2 - GPS with 4 Satellites

Figure 2 shows a person receiving a signal from four satellites. With just two satellites the signals will overlap in two positions and hence it will not be clear which position the user is at. This is illustrated by the user receiving the signals from the yellow and green satellites and intersection points I1 and I2.

With three satellites the users position can be triangulated in 2 dimensions (I2) and with four satellites in 3 dimensions.

The GPS chipset contains an "almanac" containing information on where it approximately believes all the satellites in the constellation to be . This almanac remains valid for a long period of time, and newer ones will be automatically downloaded from the satellites over a 15 minutes period.

This almanac assists the GPS chipset to know where to start looking. It is not easy for a GPS to find and recognise signals as they are transmitted from the satellite. They transmit a long sequence of pseudo random code (basically a sequence that is uniquely recognisable as being from a particular satellite) that contains the "ephemeris", basically a set of information that allows the GPS to know more accurate information on the orbit of the satellite, its position, its time (which can be used to estimate the distance from your position), and some additional information. The ephemeris is transmitted in 30 second cycles.

Once the GPS chipset has recognised one satellite and downloaded its ephemeris, then this assists it in figuring out where the other satellites should be and it can begin to calculate where your location is.

Once you have one satellite in view you then know the current time (well actually the UTC time that the satellite was at when it broadcast that particular pseudo random code sequence). Three satellites will give you a 2D fix - that is, a fix that ignores altitude variation. Four satellites give a 3D fix (longitude, latitude and altitude), and more give you a more accurate 3D fix.

How quick can the GPS get a fix? / TTFF / Time To First Fix

When you first start up your SatNav device, it most likely has no idea where you are and what the time is. This is known as a "Cold Start" condition, and it can take a long amount of time to download the ephemeris and give you your position. This can be in the order of minutes. It does get better. Once you have used your GPS, and if it has a battery in it, then it should be able to remember the current time. The GPS can use this time, and the last known location, apply it to the almanac and more quickly figure out where the satellites are. This is known as a "Warm Start" and is in the order of 30 - 50 seconds. If you have been using your GPS in the past 4 hours then the downloaded ephemeris may still be valid, and you may be able to get a "Hot Start". This can be as quick as 5 - 10 seconds.

For warm starts and cold starts, delays occur when the necessary downloading of ephemeris data is interrupted (such as interference from an Urban Canyon). If this happens then the receiver then has to wait another 30 seconds to fill in the missing data.

Some GPS chipsets from different manufacturers vary from these approximate times, and it is seen as the holy grail to be able to get a GPS that is instant fix when turned on. Presently technologies that utilise the cellular networks are bring trialled, but they are not common yet.

Other Satellite Systems

GPS is the US funded global position system. GLONASS is the Russian equivalent system, but is still struggling to reach full operation, and has yet to be used in consumer SatNav systems. Galileo is the European equivalent system and is still in the phases of research.

Once all three systems are deployed there will be over 70 satellites in orbit providing satellite navigation services. In the future it will be possible to get multi-mode SatNav receivers that can use signals from all three to get a very accurate fix, but these are several years off yet.

What is SatNav? / What is Satellite Navigation? and How does SatNav work?

SatNav, which is short for Satellite Navigation, is about figuring out how to get to your desired destination, using knowledge of your current position.

SatNav uses GPS and combines it with a computer to give a product that gives guidance to a user to help them find their way to a destination.

Most SatNav systems use an ARM based CPU (like a lot of mobile phones and PDAs), a bunch of dynamic memory (RAM) and some sort of persisted storage - either internal FLASH memory, some sort of memory card (eg: SD), or a hard drive. They allow user input via a touchscreen or buttons, and they provide guidance via a screen and loud speaker.

The most important part of a SatNav system is the software. This is where the key intelligence lies. It usually consists of some sort of navigation engine which allows a user to search for a destination (via a Point of Interest, Street, or some other form), calculate the best route (whether it is the fastest, shortest, most scenic, etc), and then give good guidance via the display and loud speaker.

Tuning the navigation engine to give good accurate guidance and optimal routes takes years of testing and experience in the field, and hence the best navigation experience comes from the longer established SatNav companies.

Tied to the navigation engine is the map database. This is a database of all the points of interest, streets and roads, places, cities, parks, topographical and geographical features. There are two main map data providers in the world - NavTeq and TeleAtlas. Both have there pros and cons in each geographical region, but they both provide all the basic information required. Typically they release their raw data to SatNav companies every 6 months on 40+ CDs per continent. The SatNav companies then extract and compress the data they need to make their navigation engine work, typically down to 1 - 2 GB per continent.

GPS produces errors. Most GPS chipset manufactures state that the accuracy can be in the order of 5 - 10 metres. Map data is gathered by many different means, from driving roads and recording their location using GPS, to digitising aerial photos, all of which also create data that has inherent errors. Hence the navigation engine needs to be smart enough to compensate for both these errors and make assumptions about where it thinks you are on the map, other it would end up drawing your location somewhere off a road! This process is known as map matching.

All SatNav systems are smart enough to automatically reroute if you choose to not follow the given guidance. They will automatically calculate a new route to your desired destination and then start to give you guidance.

Once you have your hardware, a navigation engine, and maps, then you have all the components for a basic SatNav system.

Which SatNav system is best?

The first decision in choosing a SatNav system is also the easiest, as the decision is made for you. This is whether to choose a Transferable Navigation system or a factory installed navigation system. Generally speaking, Transferable systems are off the shelf systems that the user can purchase from any consumer electronics store and install easily without any specialty wiring. Factory installed systems, as suggested either come standard with the car or require a professional to install and integrate in the car. They are usually part of the main car console display or installed in a DIN slot (eg: Navman's iDN 3000).

Transferable systems have several advantages over factory fitted systems: they tend to be much cheaper, they can be transferred easily between vehicles (useful if you have more than one vehicle or if you sell it), and as the consumer electronics development cycles are much quicker than car auto electronics cycles, they usually have a richer, more advanced, feature set.

There are now a lot of transferable SatNav systems on the market. However the best SatNav systems come from the established players, as they have had experience in the market place and have tuned there map engines. TomTom is the current market leader and has a good mature product range, as does Garmin and Navman. All these companies release products or updates to their core products every six months, so it is worth checking out their websites for the latest information.

Hints and Tips for buying a GPS / SatNav system

Before you research SatNav systems write a list of what you want out of a navigation product, and then compare to the product range of each. Many products have many features, many of which you may not "need" outside the core navigation features. Look for products with touch screens (for easy text entry), a good loud speaker (for clear instructions), a non reflective screen (there is noting worse that not being able to see the map!), and a screen that is polarised out of phase to your sunglasses (some SatNav devices use portrait polarised screens in landscape style devices - take your own polarised sunglasses along to check, you may find that you cannot see the screen when wearing them!).

Other useful features include: lots of POIs (Point of Interest - things like restaurants, hotels, stores, airports, car parks etc), speed camera / safety camera warnings (these are proximity alarms that can warn you if there is a speed camera site nearby - applies to some countries only), and traffic updates, either via FM RDS (usually free over radio) or GPRS (over mobile network, often subscription based) - this is a great feature that allows you to see traffic jams or other events that can affect your route, and allow you to avoid them. A recent feature that Navman has introduced is NavPix - effectively pictures with a latitude and longitude. This allows you to navigate to a place just by selecting a picture from an album (eg: the Eiffel Tower in Paris - who really knows the address of this tourist attraction? :-)) or you can take a picture of a place using your Navman device and send it to your friends.

One of the biggest features for SatNav devices is accurate navigation that starts up quickly. The recognised market leader is the SiRFStar III GPS chipset, which has the best acquisition times and highest sensitivity. The next best option is the SiRFStar II GPS chipset with the SiRF XTRac high sensitivity firmware.

Another consideration is the availability of extra maps if you plan to travel outside your initial region. Most systems from TomTom, Garmin and Navman have maps available for Western Europe (full coverage), Eastern Europe (limited coverage), USA and Canada (full coverage), South Africa (limited coverage), Australia (full coverage) and New Zealand (full coverage).

A hot item at the moment is support for WAAS (in North America) and EGNOS (in Europe). These are SBAS - Satellite Based Augmentation Systems, essentially Geostationary satellites that provide another signal to the GPS constellation and help provide more accurate GPS fixes. The reality is that SatNav (especially with Map Matching) is good enough without these, and they are more of a marketing gimmick.

What is the cheapest SatNav system?

Presently the cheapest and best entry level SatNav systems around are the Navman F Series which is all based on the Navman F20 (this replaced the iCN 320 and iCN 330) which has an easy to use interface, SiRFStar III, 3.5" touchscreen and is traffic compatible.

The Navman F Series GPS range consists of the F20 (the satnav with country level maps), the F20 Europe (the satnav with continental maps), the F30 (the F20 with an FM RDS TMC traffic cradle), the F40 (the F20 with a Bluetooth Handsfree kit for your mobile phone - shows Caller ID on the screen), and the F50 (the F20 with a Bluetooth Handsfree kit for your mobile phone and FM RDS TMC traffic receiver).

The Navman F Series is extremely easy to use and features an industry leading intuitive interface - even your grandma could use it.

In the mid range there is the Navman N40i (which replaces the popular iCN530) which has 3.5" touchscreen, NavPix, Camera and SiRFStar III GPS chipset, the Garmin c320, c330 and c340 (not all are available in all regions), and the TomTom 300 Series (300 has 3.5" touchscreen and SiRFStar II GPS chipset, 310 has a 4" touchscreen and SiRFStar III GPS chipset) and TomTom One (3.5" touchscreen and SiRFStar III GPS chipset).

At the high end there is the Navman N60i (which replaces the iCN 720 and 750) with a 4" widescreen, SiRFStar III GPS chipset, NavPix and Camera, and the TomTom 500, 700 and 900 (500 and 700 have 3.5" touchscreen and SiRFStar II GPS chipset, 510, 710 and 910 have a 4" touchscreen and SiRFStar III GPS chipset).

Common Issues with SatNav Systems

The GPS antenna in a SatNav system needs to be able to see the sky. Newer GPS systems are more sensitive and are able to acquire in more difficult conditions, however they work best when they can see the sky overhead. Hence trying to acquire in a garage or parking building is unlikely to be very successful.

In fact, GPS is often treated similarly to mobile phone coverage, when in fact it is almost the opposite. Mobile phones work well in cities near cellular towers, and tend to work poorly in the country side where there are fewer cellular towers. Whereas GPS tends to behave more poorly in cities due to urban canyons and other obstacles, and works very well in the countryside where it can see more of the open sky.

Athermic Windscreens

One of the biggest issues with transferable SatNav devices is that many new cars have athermic windscreens. These are windscreens with metallic coils embedded in them that are used to defrost the screen. However they also are great at preventing the GPS signals from coming through the windscreen and being picked up by the SatNav device. Hence, the best way to mitigate this issue is to purchase an external antenna kit which allow an antenna to be mounted in a more optimal position.

Urban Canyons / Tunnels

Another area where people have trouble getting a GPS fix is when they are starting off in a garage or Urban Canyon. An Urban Canyon is a fancy name for a road with tall buildings on each side that can obscure the GPS signal or cause significant multi-path problems.

If you live in a city with significant Urban Canyons (eg: New York, Chicago, Tokyo), then you will need a system with a SiRFStar III chipset to maximise your chances of picking up a signal, and holding onto it once you start moving.

If you live in a city with a large number of tunnels, where tunnels have branches or in significant Urban Canyons then you may want to consider a GPS system with Dead Reckoning. At the moment all good Dead Reckoning systems need to be professionally installed and rely on additional sensors to the GPS such as gyros, wheel ticks and accelerometers.

Long TTFF / Long Acquisition

Some systems suffer from long acquisitions. Typically, some older generation chipsets, such as SiRF Star II can take up to several minutes to get a Cold Start. This can be exasperated by being in an Urban Canyon or if you have an athermic windscreen. If you suffer poor performance consistently you may want to try an external antenna, it can make a big difference.

Also, some GPS chipsets have satellite search algorithms that are not necessarily optimised for city use. eg: if you start in your garage the GPS may start looking for the last know satellites, if it cannot find them, it will widen the search to a larger number of satellites and so forth. By doing so, it can take much longer to get an acquisition, even if you do exit the garage and are in open sky (as it is now looking for other satellites). Hence you may want to experiment with just turning on the GPS once you can get a view of the sky.

Incorrect or old map data

Another issue that users can experience is when roads change. This may be due to the construction of new roads, changes to intersection rules (eg: no right turn), or any other reason.

Even though a road may of changed in the last 12 months, and a new set of map data has been released, there is no guarantee that the new road will be in it. While NavTeq or TeleAtlas may release there updated raw data every 6 months to the SatNav companies, they may have updated other areas, not necessarily the one you are in. Its best to go to a store and try out new data first before choosing to upgrade.