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Notice: This page contains information for the legacy Phidget21 Library. Phidget21 is out of support. Bugfixes may be considered on a case by case basis. Phidget21 does not support VINT Phidgets, or new USB Phidgets released after 2020. We maintain a selection of legacy devices for sale that are supported in Phidget21. We recommend that new projects be developed against the Phidget22 Library.
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GPS Primer
Introduction
GPSes (Global Positioning System) are a class of devices that provide data on their position on the globe. Typically in the form of longitude and latitude though more advanced units can even display a map showing the user the location.
Principles of Operation
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites in orbit. Each satellite continually transmits messages that include:
- Time of transmission
- Orbital information (the ephemeris)
- General system health and rough orbits of all GPS satellites.
The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. The distances along with the satellites precise locations are used to calculate the location of the receiver unit using a process called trilateration.
In order to calculate accurate results GPS receivers need a steady signal from 4 GPS satellites when nothing is known about the current position or time. If altitude is known (aircraft have an altimeter for example) or if time is known very precisely a receiver may be able to get away with only using 3 satellites though this will give lower accuracy.
Signal Acquisition
Acquiring a satellite fix the first time the device is started up can take 5-6 minutes. Many GPS devices have a battery on board that can enable “hot starts” by remembering the most recent position of a satellite and checking that region first. With a live battery installed GPS signal reacquisition can as fast as 1 second or even better. The speed of hot starts is dependent on how long it has been since the device was last started up. The longer it has been the less effective the hot start will be. Generally they take less than 5 seconds though. If the device has been disconnected for longer than a day or two, the device will likely take the full 5-6 minutes to reacquire.
The ability of a GPS to acquire a signal depends greatly on the environment or type of building you are in. Generally, signal strength indoors is vastly reduced. Commercial/industrial buildings and high rises are much worse than a single story house due partly to size and partly to using steel studs in the walls as opposed to wooden. Moving close to a window should give you a stronger signal. If you are still unable to get a GPS signal, try using another GPS unit at the same location to verify that there are readable satellites in the area.
If you are having trouble getting the device to acquire a position fix and give you a reliable position, try moving outdoors into an open area, where most of the sky is visible; this allows the unit to calculate your current position much faster.
GPS Antenna
A number of variables can affect the overall performance of the GPS antenna: avoid putting the antenna very close to electronics or motors of any sort, as well as power transmitters such as WIFI antennas.
Position Accuracy
Even at the best of times commercial GPS units are not exceptionally accurate, often having an accuracy of several meters. For example, the PhidgetGPS is listed as having a position accuracy of 2.5m CEP (circular error of probability) but that is only in a situation where the antenna is located in a field with no tall buildings around, no clouds in the sky, etc... In a more typical environment you can expect the CEP to be much larger.
