GPS segments

The GPS system can be broadly divided into three segments : the space segment the control segment and the user segment.

The space segment

 is composed of satellites the constellation consists at the time of writing of 29 satellites including spares the satellites are in almost circular orbits at a height of 20 200 km above the Earth or about three times the radius of the Earth and with orbit times of just under 12 hours The six orbital planes are equally spaced and are inclined at 55 to the equator individual satellites may appear for up to five hours above the horizon.

The GPS satellite weigh when in final orbit approxim

ately 850 kg the design life of the satellites is 7.5 years but they carry 10 years’ worth ofpropulsion consumables two sun seeking single degree of

The original planned GPS constellation:24 satellites in 6 orbital planes at 55 inclination and 20 200 km altitude with 12 hour orbits (courtesy Leica Geosystems) 

the control segment

freedom solar arrays which together cover over 7 m provide the electrical power Power is retained during eclipse period by three nickel cadmium batteries reaction wheels control the orientation and position of the satellite in space thermal control louvers layered insulation and thermostatically controlled heaters control the temperature of this large satellite the satellite is built with a rigid body of aluminium boarded honeycomb panels the satellite may be navigated to a very limited extent in space with small hydrazine jets there are two small trim thrusters and 20 even smaller attitude control thusters antennae transmit ensure precise timing.

As far as the user is concerned each GPS satellite broadcasts on two L band carrier frequencies Ll= carry two codes known as the P code or SPS (Standard Positioning Service ) ( Figure 9.3 ) the C/A code has a chipping rate which is a rate of phase modulation of 1.023 × 10 Multiplied by the speed of light each bit is then 193 m long and the whole code about 300 km by contrast the P code chips at 10.23 × 10 bits / sec.

key to selective access to the GPS system

 

and repeats every 267 days although each satellite only uses a seven day segment of the P code will appear as Pseudo Random Noise ( PRN ) this means that it is relatively easy for the user’s equipment to obtain lock onto the C/A code since it is short simple and repeats 1000 times a second without knowledge of the P code it is impossible in practice to obtain lock because the P code is so long and complex this is the key to selective access to the GPS system only those users approved by the US DoD will be able to use the P code a 50 Hz data stream that contains the following information further modulates each code: 

- The satellite ephemeris i e its position in space with respect to time. 

- Parameters for computing corrections to the satellite clock.

-The Hand Over Word ( HOW ) for time synchronization that allows the user with access to the P code to transfer from the C/A to P code. 

-Information on other satellite of the constellation including status and ephemerides 

The satellite navigation message which describes the satellite positions is uploaded to the satellites by the Operational Control Segment ( OCS ) the OCS operates as three elements.

- Monitor Station at Ascension Island Diego Garcia Kwajalein and Hawaii. 

- A master control station at Colorado Springs USA. 

- An upload station at Vandenberg Air Force Base USA.

the user segment

The monitor stations are remote unmanned stations each with a GPS receiver a clock meteorological sensors data processor and communications their functions are to observe the broadcast satellite navigation message and the satellite clock errors and drifts The data is automatically gathered and processed by each monitor station and is transmitted to the master control station by comparing the data from the various monitor stations the master control station can compute the errors in the current navigation messages and satellite clocks and so can compute updated navigation messages for further satellite transmission to the satellites by the ground antenna the monitor stations then receive the updated navigation messages from the satellites and so the data transmission and processing circle is complete.

The master control station

is also connected to the time standard of the US Naval Observatory in Washington DC in this way satellite time can be synchronized and data relating it to universal time transmitted other data regularly updated are the parameters defining the ionosphere to facilitate the computation of refraction corrections to the distances measured the user segment consists essentially of a portable receiver / processor with power supply and an  omnidirectional antenna the processor is basically a microcomputer containing all the software for processing the field data.