GPS Receivers

 

A GPS receiver is a device that determines the geographical location accurately by receiving radio signals from GPS satellites. Initially, the GPS recievers were used by the United States military, but now most receivers are in automobiles and smartphones.  According to market research from the independent analysts, the sales of GPS-enabled GSM/WCDMA handsets is four times more than stand-alone GPS receivers.

Although the GPS was originally intended to be used by the US Department of Defence, in 1980, the U.S. government decided to allow the GPS program to be used by civilians. The satellite data is free and works anywhere in the world.  However, civilians were to be given access to the slightly degraded "Selective Availability" positioning signal.

The world’s first commercial handheld GPS receiver manufactured by Magellan Navigation Inc. was offered for sale for $ 2900 in 1989.  In 2000, the Clinton administration ordered the removal of military use signal restrictions, thus providing full commercial access to the use of the US GPS satellite system.

As GPS navigation systems became more and more widespread and popular, the pricing of such systems began to fall, and their widespread availability steadily increased. Also, several additional manufacturers of these systems, such as Garmin (1991), Benefon (1999), and TomTom (2002) entered the market. Benefon's 1999 entry into the market also presented users with the world's first phone based GPS navigation system. Later, as the smart phone industry developed, a GPS chip eventually became standard equipment for most smart phone manufacturers. To date, ever more popular GPS navigation systems and devices continue to proliferate with newly developed software and hardware applications.

While the American GPS was the first satellite navigation system to be deployed on a fully global scale, and to be made available for commercial use, this is not the only system of its type. Due to military and other concerns, similar global or regional systems have been, or will soon be deployed by Russia, the European Union, China, India, and Japan.

SENSITTIVITY

GPS receivers vary in sensitivity, speed, vulnerability to multipath propagation, and other performance parameters. High Sensitivity GPS receivers use large banks of correlators and digital signal processing to search for GPS signals very quickly. This results in very fast times to first fix when the signals are at their normal levels, for example outdoors. When GPS signals are weak, for example indoors, the extra processing power can be used to integrate weak signals to the point where they can be used to provide a position or timing solution.

GPS signals are already very weak when they arrive at the Earth’s surface. The GPS satellites only transmit 27 W (14.3 dBW) from a distance of 20,200 km in orbit above the Earth. By the time the signals arrive at the user's receiver, they are typically as weak as −160 dBW, equivalent to one tenth of a million-billionth of a watt (100 attowatts). This is well below the thermal noise level in its bandwidth. Outdoors, GPS signals are typically around the −155 dBW level.

Conventional GPS receivers integrate the received GPS signals for the same amount of time as the duration of a complete C/A code cycle which is 1 ms. This results in the ability to acquire and track signals down to around the −160 dBW level. High Sensitivity GPS receivers are able to integrate the incoming signals for up to 1,000 times longer than this and therefore acquire signals up to 1,000 times weaker, resulting in an integration gain of 30dB. A good High Sensitivity GPS receiver can acquire signals down to −185 dBW, and tracking can be continued down to levels approaching −190 dBW.

High Sensitivity GPS can provide positioning in many but not all indoor locations. Signals are either heavily attenuated by the building materials or reflected as in multipath. Given that High Sensitivity GPS receivers may be up to 30 dB more sensitive, this is sufficient to track through 3 layers of dry bricks, or up to 20 cm (8 inches) of steel reinforced concrete for example.

GPS devices may have capabilities such as:

1.      Maps, including streets maps, displayed in human readable format via text or in a graphical format.

2.      Turn-by-turn navigation directions to a human in charge of a vehicle or vessel via text or speech.

3.      Directions fed directly to an autonomous vehicle such as a robotic probe.

4.      Traffic congestion maps (depicting either historical or real time data) and suggested alternative directions.

5.      Information on nearby amenities such as restaurants, fueling stations, and tourist attractions.

GPS devices may be able to provide information on:

1.      The roads or paths available.

2.      Traffic congestion and alternative routes.

3.      Roads or paths that might be taken to get to the destination.

4.      iI some roads are busy (now or historically) the best route to take.

5.      The location of food, banks, hotels, fuel, airports or other places of interests.

6.      The shortest route between the two locations.

7.      The different options to drive on highway or back roads.

Notes & Handouts

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