GPS: Never Get Lost Again
Have you ever been lost in the woods? Have you ever wanted to go back to that great fishing spot but can’t remember where it was? Have you ever found yourself wandering aimlessly in an unfamiliar part of the city having to ask people for directions? It happens more often than we would like that we find ourselves taking a wrong turn, and taking twice as long to find our way back. Well luckily, there is the perfect tool to guide us safely: It’s called Global Positioning System. But do you know how it really works?
Short for Global Positioning System, GPS (Global Positioning System) is basically a modern day compass and more. It can store maps and locations, guide you to your destination, and even measure your speed as you travel down the interstate. It has even helped explorers find their way through the deep rain forests of the Amazon.
When was GPS developed? GPS is a satellite-based, radio-navigation system developed in the early ‘70s by the U.S. Department of Defense. Initially, ground-based pseudo-satellites were used for testing. Because it worked so well during development, the first GPS satellite was launched into orbit in 1978. GPS was intended to be a purely military tool, but after a civilian airliner was shot down over Soviet territory in 1983, President Ronald Reagan declassified the GPS project and opened it for civilian use. This decision has saved countless lives during natural disasters and expeditions that have taken a turn for the worst.
GPS is used for land, sea, and airborne navigation as well as surveying, geophysical exploration, mapping and geodesy, vehicle location systems, and any other applications that may need location information.
The most common use for GPS is navigation. In-car navigation systems use GPS to guide you around the city or through unfamiliar territory. It can suggest the best route from point A to point B, giving you turn-by-turn directions. Some can detect where you’ve deviated from the suggested course and recalculate another route to bring you to your destination. There are versions specifically designed for maritime as well as airborne use. GPS has helped ships navigate around icebergs and other vessels, ensuring passengers and cargo a safe trip to their destinations.
Handheld GPS receivers work similarly, but are usually designed for recreational activities such as hiking, fishing, hunting, mountain biking, just to name a few. Although hand-help navigation products like Garmin and TomTom are now very popular.
GPS has been an essential part of expeditions through Amazon rain forests to exploring the frigid Arctic. It has helped plot safe routes and mark danger zones for the safety of other explorers who venture into the same area. [Editor’s note: I, myself have used them on my own archaeological expeditions in Costa Rica and Peru!]
Another use for GPS is tracking a person, vehicle or object. This does require hardware and software specifically designed for that purpose. GPS tracking can be broken down further into three categories – Data Loggers, Data Pushers and Data Pullers.
A GPS Data Logger records the position of the device at intervals that can be downloaded to a computer for analysis. This can be useful for monitoring numerous things such as an athlete’s performance in a long-distance run where data from previous runs can be analyzed on a computer. In sports where marshalling competitors over vast areas can prove impractical, GPS data loggers can be installed in each vehicle or attached to belts in order to confirm that participants indeed passed through the specified route in that particular competition.
A GPS data pusher takes a more active role. It constantly sends out its location to a receiver via radio or cellular network connections. Delivery and taxi companies usually make use of this system to keep track of their vehicles. It can also help recover a stolen vehicle by activating the GPS tracker, transmitting its location and movements continuously until the authorities arrive.
Then there are GPS data pullers. Not as popular as the GPS data pusher, this one works passively. It is always on, but requires a query before it sends out its location.
Some GPS receivers and GPS equipped hardware such as Notebook PCs and Handheld devices may be used for both navigation and tracking.
GPS is basically a positioning system that utilizes signals from a constellation of about 24 Medium Earth Orbit satellites to define exact position with a small circle of error, and from this you can determine location, speed, direction, and time duration. These satellites orbit at 11,000 nautical miles above the earth and transmit signals that can be detected by anyone with a GPS receiver, anywhere around the world! In order for it to work, GPS is comprised of three segments. They’re known as the Space Segment, the Control Segment and the User Segment.
With all the three segments working together, we can now see how each segment plays a major role in locating and tracking GPS receiver’s accurate position. The whole principle behind it is pretty simple. The GPS receiver is the device that we hold on to or the device that is attached to a moving vehicle. This device serves as what its name implies – a receiver that receives information from GPS satellites. The GPS satellites on the other hand, work as transmitters. They constantly send out signals towards the earth for GPS receivers to receive. But what kind of information does the GPS receiver really receive?
This is where it becomes clearer as to how GPS really works. The GPS receiver needs to know two things to properly determine its location on Earth. It should the location of the satellite in space, and it should also know how far away the satellite is from the receiver.
To find out where the satellites are in space, the GPS receiver needs two types of information from the satellites. The first one is called the “almanac” data which contains the approximate positions of the satellites in space. Every GPS satellite continuously sends out this information for receivers to pick up in which each receiver then stores this information so it can plot the orbit and location of each satellite it detects. The second data the GPS receiver receives is the corrected and exact position data which is also called the “ephemeris” data. The “ephemeris” data is only valid for 4 – 6 hours so constant updates from the last GPS segment comes into play. The Control Segment is the facility that tracks every GPS satellite in space making sure that the updated location data of each satellite is sent out to the GPS receivers.
With this data, the GPS receiver now knows where each satellite is. The next thing that it has to know is its distance from each of the satellites. Remember that the location of the satellites have already been determined. Now each satellite sends out a “pseudo-random code” along with its status message. This “pseudo-random code” is a timing code which is generated by both the GPS satellite and GPS receiver. This GPS receiver compares the two codes to determine the difference in timing. This difference in timing is multiplied by the speed of light to get the distance.
Each distance measurement is also dependent on the GPS receiver’s clock. Because the GPS receiver’s clock is not as accurate as the atomic clock that synchronizes the GPS satellites, further correction for the distance information is needed. To get a more accurate reading, the position of the GPS receiver can then be calculated by intersecting distances from multiple satellites. Three satellites are required to determine a 2-dimensional position and four or more are necessary for to determine a 3-dimensional position.
While GPS sounds like a wonderful thing, current GPS receivers are not useable in areas where GPS signals from the satellites do not reach. These are underground structures, tunnels, underwater, or in heavily fortified facilities that can block GPS satellite transmissions. Even some modern concrete and steel buildings block the signal.
GPS accuracy depends on the GPS receiver used. Some receivers lack error-correcting capabilities, but most are typically accurate to within about 15 meters from the actual location. Some of the latest receivers may come with some form of error-correcting capabilities such as Differential GPS (DGPS), which brings the accuracy to within 5 meters or Wide Area Augmentation System (WAAS) which can raise accuracy to within than 3 meters.
In the case of navigation and logging GPS products, that sample rate is especially important. A Navigation product needs to take a sample at least every few seconds to accurately locate the receiver and provide correct navigation. However, in tracker (logging) products, they need to take a sample reading every second to accurately measure speed. Location, and travel direction are easy, but accurate speed measurements require more precise sample readings from the GPS satellites. A one second sample can accurately measure speed to +/- 1 MPH/1KPH, but some trackers take a sample only every 6-20 seconds which can induce significant error.
Differential GPS (or DGPS) uses a second stationary GPS receiver for correcting the measurements of the mobile GPS receiver. If the position of the stationary receiver has been surveyed accurately, (i.e. by means of a long wave transmitter) a correction signal can be sent. It is then received and analyzed by a receiver connected to the mobile GPS.
WAAS (Wide Area Augmentation System) has been operational in the United States since 1999 and has been available for portable GPS receivers since 2001. WAAS consists of approximately 25 ground stations that control GPS signals along with two reference stations. These reference stations collect the data from ground stations, which also calculate correction data. This data contains corrections for the satellite orbits, clock drift and signal delay of the satellites caused by the Earth’s ionosphere and troposphere. The data is sent to the receivers via geostationary satellites.
Unlike Differential GPS, the reception of WAAS requires no additional receivers. In order for a GPS receiver to utilize WAAS, it needs only the software that supports the reception of WAAS correction signals. However, for it to work the GPS receiver needs a clear line of sight for one of the geostationary satellites.
Today’s GPS devices are no longer just navigational aides. In-car navigation systems often have a plethora of other functions such as radio, TV-tuners, DVD players, in-car computers and much more. There are even GPS-enabled mobile phones that can work as GPS trackers, giving out its location when needed. There are GPS devices that feature FRS/GMRS 2-way radios, allowing the user to locate other FRS/GMRS radios within its radio range displaying their precise location on screen. There are PDAs and smartphones that come with built in GPS capability. There are also wristwatches for sports enthusiasts that come with GPS data loggers. USB GPS mice (a.k.a. Dongles) are small GPS modules which turn Notebook PCs and PDAs into GPS navigational or tracking devices. There are GPS mice that can be attached to compatible digital cameras, recording the exact location where every photo was taken.
How much do you pay for GPS service? Nothing! The Presidential Executive Order of March 29, 1996 states that the US will offer GPS Standard Positioning Service for peaceful civil, commercial and scientific use on a continuous, worldwide basis, free of direct user fees.
Glossary:
Space Segment: consists of 24 operational satellites in six orbital planes, four satellites per orbital plane. The satellites are spaced in semi-synchronous (approximately 12-hour) orbits. At any given time, a minimum of 6 satellites will be in view from anywhere in the world. As the satellites orbit they continuously broadcast position and time data back to earth, which is in turn received by a GPS receiver.
Control Segment: primarily consists of a Master Control Station at Schriever Air Force Base in Colorado Springs, Colorado, along with five monitor stations and three ground antennas at various locations throughout the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The information is then sent to the Master Control Station and analyzed. Any discrepancy is corrected and then transmitted to each satellite via the ground antennas.
User Segment: consists of receivers that allow land, sea, or airborne users to receive the GPS satellite broadcasts in order to compute their precise position, velocity and time. GPS receivers measure their distance from a group of satellites in space to determine their exact position on earth. Each GPS satellite transmits an accurate position and time signal; the user’s receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the relative distance to the satellite. Measurements collected simultaneously from four satellites are processed to solve for position, velocity and local time.
Contributed by: Carlo Guerrero, TigerDirect News Correspondent
Source: United States Air Force, Los Angeles Air Force Base, NASA
Resources: GPS Products @ TigerDirect.com
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