III. GPS SATELLITE-END TRACKING PROCESS
In order to track an object accurately on the earth, a minimum of 3 satellites are required as seen in Fig.3. Each satellite broadcasts two signals; one at frequency L1 (1575.42MHz) and other at frequency L2 (1227.6MHz). A unique satellite code, known as the Course/Acquisition code, is used to differentiate between the satellites. This C/A code is of 1023bits, which modulates L1 signal every 1ms at 1.023MHz. Assimilating the process, the satellite gathers a 1000Hz data and transmits it towards the receiver on the earth. This is known as the ’navigation message’ that is required by the receiver on earth to resolve the geographic position of the object. The contents of this navigation message sent at 50bps, by each satellite are; ”This is satellite ’X’ reporting, & my location is ’Y’, sending the information at time ’Z’. Along with this data, each satellite transmits some orbital parameters known as ephemeris and almanac data explained in section IV.
Further the frequency planning, C/A code generation, modulation and transmission technique deployed in satellite payload are discussed.