Satellite Navigation Using GPS

The NAVSTAR Global Positioning System

The NAVSTAR Global Positioning System,usually called GPS, consists of three components: a space segment of GPS satellites, acontrol segment that monitors and operatesthose satellites and a user segment that employsGPS receivers to observe and recordtransmissions from the satellites and performposition, velocity, attitude and time calculations.

The GPS Space Segment

The space segment is based on three-axisstabilized satellites orbiting in near-circularorbits with a period of half a sidereal day andan inclination of 55 degrees. There are six orbitalplanes, each of them with four satellites.This constellation provides global coveragewith more than four satellites in view at alltimes.

The significance of the visibility of at least four satellites is that the GPS system is intendedto allow instantaneous real time determinationof the user position (3 variables) and thetime of the fix (one more variable). Previouspositioning systems, like the methods used inthe Transit and Tsikada systems, were basedon the processing of several passes of data(requiring hours to days) and did not providethe instantaneous solutions that GPS (orGLONASS) offers.

The GPS satellites carry very stable atomicclocks that are used to derive the ranging signals.The basic signal for civil use, L1, has afrequency of 1575.42 MHz and it is modulatedwith a Clear Acquisition (C/A) PseudoRandom Noise (PRN) code at 1.023 MHz thatis different for every satellite.

The signal isalso modulated with a 10.23 MHz Precise (P)code that is usually encrypted and only availableto authorized users. On top of this thereis a 50-bit-per-second modulation which isused to transmit the satellite ephemerides(predicted orbit and clock) and other information.

Authorized users have also access tothe Precise code on a second frequency L2,that allows users to correct for ionosphericpropagation delays. Some receivers are ableto measure the delay between the signal inthe L1 frequency and the L2 frequency withoutaccess to the P code. There are plans toadd in future satellites another frequency forcivil users so they can easily correct for ionosphericdelays.

The GPS Control Segment

The GPS control segment tracks and monitorsthe signal from the GPS space segmentand estimates the orbits and clock behaviourof the satellites. This information is uploadedto the satellites so it can be transmitted to users.

The GPS User Segment

The GPS user segment can perform two basicmeasurements of the GPS signals. It cancompare the C/A or P code that it is receivingwith a locally generated copy in order tocompute the transmission delay between thesatellite and the receiver. This measurementis called pseudorange. Pseudoranges to fouror more satellites can be used to determinethe position of the user once the position ofthe GPS satellites has been obtained usingthe ephemerides of the navigation message.

The second and more precise method is toobtain the difference in phase between thereceived carrier signal and a receiver generatedsignal at the same frequency. Thismeasurement is known as the carrier phaseobservable and it can reach millimetre precision,but it lacks the accuracy of the pseudorangebecause the phase when the trackingis started can only be known with an ambiguityof an unknown number of times the carrierwavelength (about 19 cm for L1)