What doesn't GPS do?

Global Positioning System or GPS, is a technology that can give your accurate position anywhere on earth (latitude/longitude). To avail of this technology, you need a special GPS receiver that can receive signals from satellites.

It works anywhere on the planet where you can receive satellite signals. You will need a clear view of the sky for the GPS to work. It will not work inside buildings, underground or even in a forest.

The Global Positioning System is owned and operated by United States Department of Defence, but it is freely available for the use of anyone in the world. Other countries have developed similar Satellite Navigation Systems of their own. For example, the European Union is developing a system called Galileo, while Russia has an operational system called GLONASS. Many modern receivers are capable of using signals from all these systems.

A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, a precise orbit for the satellite sending the message (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac).

These signals travel at the speed of light. The receiver uses the arrival time of each message to measure the distance to each satellite, from which it determines the position of the receiver. The resulting coordinates are then converted to more user-friendly forms such as latitude and longitude, or the location on a map, before being displayed to the user.

Although four satellites are required for normal operation, fewer satellites may do the job in special cases. If one variable is already known (for example, a plane knows its altitude), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as re-using the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give degraded answers when fewer than four satellites are visible.

As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a non-uniform arrangement. Such an arrangement was shown to improve the reliability and availability of the system, relative to a uniform system, when multiple satellites fail.

GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices. They can cost a few thousand rupees to lakhs, depending on the use.

In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user.

A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay.

To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about one percent of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate at the speed of light, this represents an error of about three metres.

Position accuracy can be improved by using the higher-chiprate P(Y) signal. Assuming the same one percent bit time accuracy, the high-frequency P(Y) signal results in an accuracy of about 30 centimetres.

Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitise the receiver, making acquiring and tracking the satellite signals difficult or impossible.

Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field.

In automotive GPS receivers, metallic features in windshields, such as defrosters, or car window tinting films can act as a barrier, degrading reception just inside the car.

Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well-documented case, the entire harbour of Moss Landing was unable to receive GPS signals due to unintentional jamming caused by malfunctioning TV antenna preamplifiers.

Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight. In 2002, a detailed description of how to build a short- range GPS L1 C/A jammer was published in Phrack, an online magazine.

The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.
The military applications of GPS span many purposes:

Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies. The GPS-receivers used by commanders and soldiers are called the Commander's Digital Assistant and the Soldier's Digital Assistant, respectively.

Target tracking: Various military weapons' systems use GPS to track potential ground and air targets before they are flagged as hostile. These weapon systems pass GPS co-ordinates of targets to precision-guided munitions to allow them to engage the targets accurately. Military aircraft, particularly those used in air-to-ground roles use GPS to find targets.

Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions. Artillery projectiles with embedded GPS receivers able to withstand accelerations of 12,000G have been developed for use in 155 mm howitzers.

Search and Rescue: Downed pilots can be located faster if they have a GPS receiver.

Reconnaissance and Map Creation: The military uses GPS extensively to aid mapping and reconnaissance.

The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an Electro-Magnetic Pulse (EMP) sensor (W-sensor), which form a major portion of the United States Nuclear Detonation Detection System.

Civilian uses:
Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS: absolute location, relative movement, and time transfer.

You can make accurate maps by yourself using a simple GPS device! Say you like to go trekking in mountains and want to map out the trails. Just carry the receiver with you and it will record your path. You can then download and process this data using any of the software and make a map of the area.

You can fit a GPS device in your car and can track where your car has been, how much distance you travelled etc. For real-time positioning, you would need a 'transmitter' too. GPS devices cum transmitters are also available in small sizes, which can be fitted into a shoe or a watch. These are popular to keep a watch on children. These devices would automatically send you a message when your child goes outside a defined perimeter, or allow you to know where your child is at any time.

GPS has become very popular in India among fleet management companies. GPS allows real-time vehicle tracking which enable transportation industry and delivery services. GPS can be effectively used in almost any business that relies on precise location information. Surveying, field data collection, yellow pages, tourism, infrastructure, communication -- GPS has applications in all these areas.

Most of you would have also heard of A-GPS or Assisted GPS. A-GPS enhances the start-up performance of a GPS satellite-based positioning system. It is used extensively with GPS-capable cellular phones, as its development was accelerated by the US FCC's 911 mandate making the location of a cell phone available to emergency call dispatchers.

Conventional GPS has difficulty providing reliable positions in poor signal conditions. For example, when surrounded by tall buildings (resulting in multipath), or when the satellite signals are weakened when a GPS device is indoors or under trees. Some newer receivers are better at handling these situations.

In addition, when first turned on in these conditions, some non-assisted GPS units may not be able to download the almanac and ephemeris information from the GPS satellites, rendering them unable to function until a clear signal can be received continuously for up to one minute.

An A-GPS receiver can address these problems in several ways, using an assistance server:

• The assistance server can locate the phone roughly by the  cell site it is connected to on the cellular network.
  
• The assistance server has a good satellite signal, and lots of computation power, so it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, and then inform the cell phone or emergency services of the cell phone's position.
  
• It can supply orbital data for the GPS satellites to the cell phone, enabling it to lock to the satellites when it otherwise could not, and autonomously calculate its position.
  
• Simply capturing a brief snapshot of the GPS signal, with approximate time, for the server to later process into a position.
  
• By having accurate, surveyed coordinates for the cell site towers, it can have better knowledge of ionospheric conditions and other errors affecting the GPS signal than the cell phone alone, enabling more precise calculation of position.

A typical A-GPS-enabled cell phone will use an Internet connection to contact the assistance server. Alternatively, it may use standard non-assisted GPS, which is slower and less accurate, but does not lead to network charges for data traffic, which can be considerable.  Some A-GPS solutions do not have the option of falling back to conventional GPS.

Most of the new high-end multimedia phones offer GPS with some pre-loaded map software. Some of them offer A-GPS too.  Technically, those with A-GPS are better since they are more failsafe.