How GPS Works

What is GPS & How it works. span>

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 28 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the U.S. government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

How it works

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

How accurate is GPS ?

Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design.12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. GPS receivers are accurate to within 15 meters on average.

Newer GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. No additional equipment or fees are required to take advantage of WAAS. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. The U.S. Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

The GPS satellite system
The 28 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 7,000 miles an hour. GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.Here are some other interesting facts about the GPS satellites (also called NAVSTAR, the official U.S. Department of Defense name for GPS):
The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
Transmitter power is only 50 watts or less.

What's the signal?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

A GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on your GPS unit's satellite page, as it identifies which satellites it's receiving.
Ephemeris data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.
The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system

Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

• Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
• Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
• Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
  Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
• Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
• Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exits when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
• Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Guide to Choosing GPS

How useful is a GPS?
GPS navigation can make the difference between an off-road adventure and disaster.

Points of Interest4WD
vehicles can take you to some amazing places. When you find a great back-road, marsh, desert flat, etc., record the spot with your GPS receiver. The GPS can guide you back there again in the future so you can relive the adventure. Or you can share your trip into with others, even over the Internet.

Pre-Trip PlanningGPS
is great for pre-trip planning. Mark the location of good places to set up camp and mark off dangerous areas that you want to steer clear of. Off road racers can use GPS receivers to determine paths that will cut time off a course. Also use GPS to establish rendezvous coordinates so you can meet up with fellow off-roaders.

Safety
Getting lost is not always dangerous but it is a hassle that can be avoided. A GPS will let you know where you are at all times so it is very helpful especially when you are heading into unfamiliar territory.
If you get into serious trouble however, you can use a GPS unit and a two-way radio to communicate your exact position to rescue teams.

What do I need?
Nice features to have in a GPS: big screen with good back lighting, long battery life, ergonomics (easy-to-use buttons and menus, etc.), reliable brand with good service support, ability to have an external antennae, etc…

What accessories should I get?
Consider an external antenna, as it will improve your reception. The Vortech Reradiating GPS Antenna is a good choice. The Vortech Antenna System sends boosted signals to your GPS and since it doesn't require a direct connection to the GPS, it will work with any model and is ideal for vehicle use where roofs and overheads block satellite signals.

What else should I know?
Learn the basics of how to read topographical maps. Mount the GPS unit in a good, visible location where you and perhaps also your passengers will be able to see it.

GPS for Hunting

GPS For Hunters
GPS navigation can be the difference between successful hunting and wandering aimlessly around in the woods. Mark the location of your deer blind, or find your way back to that great duck-hunting spot.

GPS as a planning and analysis toolUse your GPS with topographical maps to make your scouting time more efficient. If you look on a map and notice possible deer funnel areas where ridges come together or a creek or river abuts a bluff you can mark the latitude and longitude as a waypoint and so you can find the spot quickly.

You may have a difficult time remembering all the places where you find buck scrapes, feeds, and beds. Build a database with a GPS receiver by making a waypoint for each, and naming them with a code you can understand (i.e. "S" for scrapes, "R" for rubs, "T" for tracks or "S" for sightings, etc). When you get home, record this information in a hunter's log or a computer program. As you study the data you'll begin to notice patterns that will tell you where and when you need to be for a successful hunt. For example, when you hunt for deer you can use your GPS receiver to mark the spot where you discover deer have fed. You can return to those same sites when you hunt next year and you'll probably find the deer feeding in about the same locations.

SafetyGPS is useful too because it allows you to safely and easily navigate your way back even if you get caught in bad weather and visibility is compromised.

Easily navigate back to base camp if you get separated from the rest of the group.
If you get in serious trouble, you can use a GPS unit to communicate your exact position to rescue teams.

Knowing where you areHunters often become so preoccupied with the pursuit of game that they forget to pay attention to where they are going and when the game is finally down, they realize they don’t have a clue as to where they are. This is when a GPS becomes useful.
Use GPS and your two way radio to communicate your location for an impromptu rendezvous with fellow hunters.

In the dark, it is easy to get turned around by mistake. You may not want to use a flashlight because you want to be as inconspicuous to game as possible. But traveling to and from a stand is no problem with a backlit GPS.

Sometimes hunters down a large animal and have to hurry back to camp to get some help and then have trouble finding his trophy again, especially in the dark. Marking the location of the downed animal on your GPS before heading back to camp makes it easy to relocate.

GPS Tracking and Application

Being able to pinpoint the location of a device on planet Earth raises some interesting ideas and applications. Primarily, GPS (Global Positioning System) was intended to be released to the consumer market as a way to aid navigation.

However, since the price of the technology surrounding GPS has fallen, many companies have found new ways to apply it. Indeed, the price of associated technologies has also fallen dramatically since the inception of GPS, which has lead to many innovations, amongst them GPS Tracking.

GPS and how it works

Before we look at GPS Tracking in detail, we first need to establish what it is about GPS that makes this such a unique and useful application of this technology.

The principle behind GPS systems is that they are able to use triangulation to calculate their coordinates on Earth by measuring the time taken for signals from various satellites to reach them.

The GPS software is also intelligent enough to compensate for any irregularities in the signal strength and clock differences between itself and the GPS satellite network by using signals from four separate satellites to improve accuracy.

Usually the coordinates are then used to locate the GPS system on a map, which is either displayed to the user or used as a basis for calculating routes, navigation, or as input into mapping programs. For example, specific coordinates can be stored as waypoints allowing the user to retrace their steps by calculating the direction and distance to each waypoint that they have stored.

GPS Tracking

In fact, it is this use which represents the simplest form of GPS tracking. The user is able, using a portable GPS system, to keep a track of where they have been, in order to be able to either retrace their steps, or follow the same path again in the future.

When combined with other technologies such as GPS phones, this also gives the possibility for other users of GPS to follow in the footsteps of the initial user; which can be a useful application of GPS tracking for field activities.

Where GPS tracking comes into its own, however, is when it is combined with other broadcast technologies such as radio. GPS watches, for example, can be fitted with a GPS receiver which is capable of calculating its position, whilst also broadcasting that using a miniature radio transmitter.

The signal is relayed to a central command center equipped with GPS software systems which can track the position of the wearer, and either store it as a path or relay that information to a third party.

That third party could be an anxious parent, or the police. In fact there are a variety of GPS phones and wristbands which are sold in conjunction with a service which enables third parties to find out where their charges are at any time of the day or night.

GPS Vehicle Tracking

This is particularly useful when using GPS units attached to vehicles which have distinctive identification such as chassis numbers. The same principle applies as to a GPS tracking device designed to be worn by a human, except that the GPS is integrated with the vehicular entertainment system.

This serves two purposes. On the one hand, it provides the driver with an integrated GPS system, without the necessity to purchase a car navigation system, or a PDA GPS system, whilst also offering the possibility to relay that information via a radio or cell phone transmitter.

In fact, these systems have already been tried in the field, primarily as a vehicle locator in the event that the vehicle to which the GPS vehicle tracking system is attached is stolen. The police, once informed, can find out from the control center when the vehicle is, and proceed to track it physically.

A useful consequence of being able to use GPS vehicle tracking to locate a vehicle is that the manufacturer can also use the information to alert the driver as to when they near a service center.

If, along with the GPS coordinates, the system relays telemetry information such as the status of the engine, time since the last service, or even information not relating to defects, the receiver of this information can make a decision as to what kind of alert to pass on to the driver.

Coordinated Tracking

This also opens up the possibility to allow for coordinated vehicle tracking, in which GPS tracking is used to share location information between several vehicles, all pursuing the same end goal.

It is an approach that has been used successfully in conjunction with GPS fishfinder units which help fisherman to locate, track and catch fish. These units are more sophisticated than the average GPS unit, comprising other features such as depth gauges, tide time information and so forth.
The basic GPS functionality is the same however, and units can either share that information with each other, or a central point. The central point can also be one of the fishing vessels, and it has on-board computer systems capable of reconciling all the locator information along with a map, thus allowing the different vessels to coordinate their actions.

This also has military applications, of course, where units can share, in real time, information about their location, even when line of sight is no longer possible. In the past, this was done by relaying often inaccurate map co-ordinate estimations; now the locations can be called in with almost absolute accuracy.

Consumer GPS Tracking

Despite its’ hitech military and commercial fishing applications, as well as use in aviation GPS, the principle application of GPS tracking will be in providing an enabling technology to augment existing systems.

These systems will include cell phones and vehicles, usually in conjunction with a central point of service designed to keep track of the location. The reason for this is to keep the cost of the actual GPS unit down as much as possible in order to provide useful technology to consumers at the right price.

How it Works

In order to be in a position to choose from the various GPS solutions that are present in the consumer marketplace, it is necessary to have a reasonable understanding of what they can offer, as well as what the user is expecting from the solution.

There are several components that need to be looked at. There is the GPS receiver (or antenna) itself, which plays an important part. Then there is the portion of the system which uses the received information to estimate where the device is located on an internal map.

Then there are the various capabilities for route planning and recording, which vary from unit to unit. Finally, the display portion of each solution type can vary considerably, from limited location information to full color three dimensional maps of the immediate surroundings and advised route.

How it Works

All GPS solutions are based on the same premise, and it is a good idea to at least be aware of the underlying technology so that a reasonably informed decision can be made as to which application of the GPS technology is appropriate for the user in question.

GPS revolves around the possibility to capture signals from satellites which orbit the Earth at known positions. Each satellite has a separate identification and sends a signal which the GPS software can use to calculate the distance from the device to the satellite.

Much like regular orienteering triangulation, this allows the software to estimate its own position from several (at least 3, preferably 4 or more) signals, by finding the point in space at which the circles representing the distance from each satellite intersect.

From this location information, the software can then pinpoint the devices location on a map and use the information for a variety of different operations, from simply displaying the location, to adjusting a proposed route based on the actual position of the device compared to the desired position.

Applications

There are three broad classes of application – road, hiking and maritime – and each is further subdivided into other types which offer different features depending on the exact use to which they will be put and the environment that they will be operating in.

One important thing to remember is that this technology does not allow the transmission of actual mapping data via the satellites. It is purely concerned with the location of the device in space.

The first application we shall look at is the in-car (or in-vehicle) GPS. These units need to be accurate, safe, and contain details of all the roads, and important addresses (gas stations, railroads etc.) and points of interest for the geographical area in which the device will be operating.

These devices start with those which are part of the in-vehicle entertainment system, which ties them to a single vehicle. Obviously this will usually make them much more integrated with the vehicle itself, but also suffers drawbacks such as not being able to remove the unit from the vehicle.

Portable devices suitable for in-vehicle use exist, and offer a great alternative to those which are integrated with a particular vehicle. However, they will not be able to override the in-vehicle entertainment device (CD player, etc.) and so the voice commands might be drowned out by the music.

Then there are portable devices which are designed for general use, which have a limited road database, and reasonable terrain information. They are small enough to be carried in the palm of your hand, but can suffer from inaccuracies stemming from the built-in antenna and relatively low power software.

However, a recent trend has seen PDA manufacturers attaching a GPS antenna, and providing sophisticated software and maps which use the power of the PDA to give an excellent solution. They might not be rugged, but they do provide a very good base for both in-vehicle and general navigation.

Rather than having a direct connection, many PDA GPS solutions use Bluetooth as a means to communicate with the GPS receiver. This makes them more easily installed in, and transported from, vehicles.

Finally, maritime units need very specific features, not least being waterproof, and containing additional information relating to tides and information that can be plotted on nautical charts. Their maps are also very specific, giving undersea elevations as well as navigable channel information.

This includes devices such as fishfinders, which build other fishing related features into the basic GPS unit. A fishfinder is very useful for positioning, tracking and helping the fishing enthusiast to locate fish at sea.

Associated technologies such as GPS tracking have also evolved, where the user wears a watch which relays their position via radio to a central control center. This tracking information is then used in conjunction with an alert system to track anything from vehicles to people.

Updates & Mapping

When choosing a GPS, it is important to bear in mind that each type will be updated in a different way. For example, dedicated in-vehicle units tend to be updated via CD, which has to be purchased, usually at a high price, from the manufacturer.

Those which are attached to a PDA, as well as most handheld dedicated GPS devices are usually updated via a PC. They also need maps, but said maps can usually be acquired at a much lower price than the dedicated branded ones needed for other systems.

Being able to update the device easily and at a reasonable cost is a very important part of the decision process; unlike other consumer devices that you purchase, the cost of ownership of a GPS unit is proportional to its usefulness.

The less you spend on maintaining it, the less useful it becomes since the road networks are always changing. This is perhaps less important for devices aimed at hiking and orienteering, but could be a factor in deciding whether a multi-function device is better than one dedicated to a specific use.

Specific software vendors have solutions for preparing maps which can be uploaded into the GPS unit. For example, many are based around the mapsource system which permits users to define their own maps, perhaps scanned from a real one, in order to get the most out of their GPS.

Accuracy & Portability

If accuracy is paramount, then it is also important to choose a unit that is equipped with WAAS (Wide Area Augmentation System). This is a terrestrial service which provides additional information to the GPS which enables it to increase its accuracy.

WAAS equipped devices might be especially useful for road users and those involved in either door to door selling, or providing a service which requires them to be able to pinpoint their position right down to a few feet.

On the other hand, it is useless to purchase a WAAS capable system if the service is not available in the area in which the device is to be used. Most big cities should provide WAAS coverage, and it can also help to compensate for lack of GPS signals due to blocking by tall buildings.

The other way to avoid this is to use a better antenna, but this will hurt the portability of the device. If portability is a key part of the decision process, then it will usually be a trade-off against accuracy, whether that accuracy stems from poor GPS signal acquisition, or less powerful triangulation software coupled with a less detailed map.

The Choice

In the end, the choice is reasonably straightforward – buy the most expensive GSP that fits your needs; if price is a deciding factor, then buy the most expensive one you can afford which fits your needs.

Try to think about the following:

▪ Portability
▪ Features
▪ Mapping and Updating
▪ Environmental Features

Portability is affected by weight and size, as well as the antenna. This could have an effect on the features that the device offers – clearly if the screen is tiny, and the whole unit is about the size of a cell phone, then some advanced features will not be fitted.

The mapping and updating capabilities are also important. Should it be able to accept any old map from a PC, or is it enough that the unit is replaced after 5 years? Technology moves almost as fast as the roads are updated, so this could be an option.

Finally, are there any specific environmental options that are needed, such as waterproofing, rugged shock-proof design, or other features that make one device more attractive than another?

Answering these questions will help determine what type of GPS is right for you, and at the right price.