GPS EQUIPMENT

The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defence.  It is the only fully functional GNSS in the world.  It uses between 24 and 32 medium orbit satellites that transmit precise microwave signals, which allow GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR.

 

GLONASS is the satellite navigation system, developed by the former Soviet Union and now operated for the Russian government by the Russian Space Forces. It is an alternative and complementary to the United States' Global Positioning System (GPS) and the planned Galileo positioning system of the European Union (EU). Development of GLONASS was completed in 1995. Following completion, the system rapidly fell into disrepair with the collapse of the Russian economy. Beginning in 2001, Russia committed to restoring the system, and in recent years has diversified, introducing the Indian government as a partner, and accelerated the program with a goal of restoring global coverage by 2009.

 
A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth.  Each satellite continually transmits messages containing the time the message was sent, precise orbital information (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac).  The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receiver's location.  The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included.  Many GPS units also show derived information such as direction and speed, calculated from position changes.

 

"Normal" satellite navigation receivers compare a pseudorandom signal being sent from the satellite with an internally generated copy of the same signal. Since the signal from the satellite takes time to reach the receiver, the two signals do not "line up" properly; the satellite's copy is delayed in relation to the local copy. By progressively delaying the local copy more and more, the two signals will eventually line up properly. That delay is the time needed for the signal to reach the receiver, and from this the distance from the satellite can be calculated. This technique generally realises accuracies of approx +/- 3 metres.

 

For those tasks that require a greater level of accuracy, methods such as Real Time Kinematic (RTK) or differential correction are required.  With these techniques, the time of travel of the satellite's transmission carrier (and not the messages contained within) is used. It should be noted that satellites transmit their signals on multiple frequencies.  For the more accurate GPS techniques, a more elaborate receiver is required; one that is capable of receiving signals on two frequencies (known as L1 and L2).