GPS for Dummies.pdf - Engineering Surveyor

52Part II: All About GPSFigure 3-2:The constellationofNAVSTARsatellitesand theirorbits.GPS satellites don’t just help you stay found. All GPS satellites since 1980carry NUDET sensors. No, this isn’t some high-tech pornography-detectionsystem. NUDET is an acronym for NUclear DETonation; GPS satellites havesensors to detect nuclear-weapon explosions, assess the threat of nuclearattack, and help evaluate nuclear strike damage.The solar-powered GPS satellites have a limited life span (around 10 years).When they start to fail, spares are activated or new satellites are sent intoorbit to replace the old ones. This gives the government a chance to upgradethe GPS system by putting hardware with new features into space.GPS radio signalsGPS satellites transmit two types of radio signals: C/A-code and P-code.Briefly, here are the uses and differences of these two types of signals.Coarse Acquisition (C/A-code)Coarse Acquisition (C/A-code) is the type of signal that consumer GPS unitsreceive. C/A-code is sent on the L1 band at a frequency of 1575.42 MHz.C/A broadcasts are known as the Standard Positioning Service (SPS).C/A-code is less accurate than P-code (see the following section) and is easierfor U.S. military forces to jam and spoof (broadcast false signals to make areceiver think it’s somewhere else when it’s really not).The advantage of C/A-code is that it’s quicker to use for acquiring satellitesand getting an initial position fix. Some military P-code receivers first trackon the C/A-code and then switch over to P-code.