GPS Technology

Introduction:

GPS stands for Global Positioning System, the complete satellite navigation system that till now has no parallel. The system comprises more than two-dozen GPS satellites. The function of GPS is to broadcast precise timing signals through radio frequencies, which, in turn are taken up by GPS receivers. The signals thus received accurately determine the location of the monitored object(s) according to the longitudes and latitudes, irrespective of weather conditions, time and location. All these factors put together renders GPS a vital global utility. Modern navigation on land, sea, and air depends largely on the technology; it has also become an indispensable tool for cartography and land surveys. A distinct feature of the GPS is an extremely precise time reference, without which, telecommunications and certain scientific researches (e.g. seismography) wouldn’t have traversed the distance that they have till now.

The Concept:

The design that’s based on the ground-based radio navigation systems (e.g. LORAN of the early 1940s) is an outcome triggered by Soviet Union’s launching of the first Sputnik. Sputnik's radio transmissions fell victim to the Doppler Effect that made the signal frequencies diminish with distance and vice-versa; the phenomenon brought forth the realization that the exact location of the satellite can be pointed out along its orbit by measuring the Doppler distortion. The theory, however, holds true even when it’s reversed i.e. knowing the satellite's position also makes clear the position of the watchers on Earth.

History:

Credits go to the United States Navy for using the first satellite navigation system; Transit, as it was named, used a constellation of five satellites to provide a navigational fix approximately once every hour. The year was 1960; seven years later, with the help of the Timation satellite, US Navy succeeded in placing accurate clocks in space. Till today, it is the technology that drives the GPS system. But the true saga initiated in the 1970s, when the ground-based Omega Navigation System came into existence; it was the first worldwide radio navigation system that functioned on signal phase comparison.

February of 1978 saw the advent of the first experimental Block-I GPS satellite manufactured by Rockwell International; Lockheed Martin currently handles the production of the same. Eight years later, ten more experimental Block-I satellites came into existence and December 1993 saw the concept being transformed to reality. The following year saw a total of 24 satellites in orbit.

Technicalities:

The entire function of GPS involves five logical steps, which are based on the triangulation principle. Triangulation is defined as a trigonometric method of determining the position of a fixed point from the angles to it from two fixed points a known distance apart. A GPS receiver measures distance by calculating the traveling time of the radio signals, which in turn comprise three different types of data forming the primary navigation signals: Almanac (Orbital position data for each GPS satellite), Ephemeris (Precise orbital position and clock data for each GPS satellite) and a pseudorandom code (an I.D. code that identifies the satellite transmitting information). The radio signals are low powered ones, designated L1 and L2; while the L1 frequency (1575.42 MHz in the UHF band) is intended for civilian purposes and travel by line of sight, L2 frequency (1227.6 MHz) carries only the P code used for the precise positioning service (PPS).

The two forms of accurate clock information broadcasted by the satellites are the Code Acquisition code (C/A) and the Phase code (P-code). While C/A consists of a 1,023 bit long pseudo-random code broadcast at 1.023 MHz that repeats every millisecond and is mostly used for civilian navigation, the P-code is broadcasted at 10.23 MHz and repeats once per week. P-code is also subjected to encryption; this is done to prevent spoofing (an attempt to gain access to a system by posing as an authorized user) by encrypting the P-code into Y-code. Decryption requires certain units with a valid decryption key. Thus, the process takes place in the following way: NM + C/A + P(Y) => primary radio channel, L1. However, the P(Y) signal can also be broadcasted singly on the L2 channel.

How GPS functions:

GPS receivers can accurately calculate the distance between themselves and the satellites by measuring the time delay and multiplying it with the speed of light. The signals sent by the satellite(s) also contain information about the position of the satellite. Thus, comparing the positions and range, the receiver locates its current situation.

Application:

GPS tracking systems determine the location of anything under the sun; it can be a vehicle, a person or a pet. It records the position of the subject at regular intervals and creates a track file. Thus, apart from the hi-tech military and commercial purposes, GPS found its usage in aviation as well as in civilian purposes like route-finders and keeping track of pets and children.