GPS for Dummies.pdf - Engineering Surveyor

Chapter 5: Selecting and Getting Started with a GPS Receiver95Initializing Your GPS ReceiverYour GPS now has power, so it’s ready to go, right? Well, almost. After you putbatteries in your GPS receiver and turn it on for the first time, don’t expect itto instantly display your location. A GPS receiver first needs to go through aninitialization process before it can tell you where you are. The type of initializationand the amount of time it takes depends on what information the GPSreceiver has previously received from the satellites and when.The process is mostly all automatic, and you don’t need to do much as yourGPS receiver starts up and begins to acquire satellites. Your GPS user manualmay contain model-specific initialization information.To initialize a new GPS receiver, take it outside to someplace that has an unobstructedview of the sky (such as a large field or a park) and turn on the power.(You did install the batteries first, right?). After the start-up screen displays,the receiver will begin trying to acquire satellites.It can take anywhere from 5–30 minutes for the GPS receiver to gather enoughsatellite data to get a position fix for the first time (usually more toward the 5minutes end of the scale). Don’t worry; your GPS receiver isn’t going to be thisslow all the time. After the GPS receiver is first initialized, it usually only takes15–45 seconds to lock on to the satellites when you turn it on in the future.GPS receiver initialization nitty-grittyYou really don’t need to know this technical informationto operate your GPS receiver, but to startacquiring satellites to get an accurate location fix,a GPS receiver needs the following satellite data: A current almanac (rough positions of all thesatellites in orbit) The GPS receiver’s current location The current date and time Ephemeris data (precise position of individualsatellites)If some or all the data is missing or out-of-date,the GPS receiver needs to get updated informationfrom the satellites before it can accurately fixa current position. The types of data that are outof-dateor missing determine how long the GPSreceiver takes to initialize. If the GPS receiver isbrand new, out of the box, several hundred milesaway from where it was last used, or has beenstored for a prolonged period of time, initializationwill take longer.