October 19, Skip to main content. See also : What is GPS? Navy builds Transit system satellites to begin tracking submarines. The first fully operational satellite is launched by the U. Air Force as a part of their Block II program. Department of Defense begins decreasing the accuracy of GPS readings for non-military use, stating fear of adversaries gaining military advantages as the reason behind the decision. This became known as Selective Availability.
GPS today and tomorrow As the capabilities of technology continue to expand, one can only imagine what GPS technology will look like in the future. If you liked this post, let us know! Geotab Team. The Geotab Team write about company news. Disclaimer Geotab's blog posts are intended to provide information and encourage discussion on topics of interest to the telematics community at large. Subscribe to the Geotab Blog Sign up for monthly news and tips from our award-winning fleet management blog.
Republish this article for free Republish. Browse categories Compliance. Data and Analytics. Driver Safety. Electric Vehicles. To get a fix, your GPS receiver requires additional data for each satellite, called the ephemeris. This data gives very precise information about the orbit of each satellite. Your GPS receiver can use the ephemeris data to calculate the location of a satellite to with a metre or two. The ephemeris is updated every 2 hours and is usually valid for 4 hours.
If your GPS receiver has been off for a while, it may take up to several minutes to receive the ephemeris data from each satellite, before it can get a fix. Your GPS will have a screen, like the one on the right, which shows which satellites are in use.
The bar graphs show the strength of the satellites that the GPS has acquired. If the bar is hollow, the GPS is still downloading the ephemeris. The circular plot shows the location of the satellites in the sky - the centre of the circle is overhead.
To get a fix, the GPS receiver needs a valid almanac, initial location, time, and ephemeris data. The terms mean different things to different GPS manufacturers. In this state, the GPS receiver does not have a current almanac, ephemeris, initial position or time.
Older GPS units may take up to an hour to search for satellites, download the almanac and ephemeris data and obtain an initial position, though newer GPS units may require much less than this.
If the GPS receiver has moved several hundred kilometres, its assumptions about which satellites to use will be incorrect and it will have to search for them.
Most units will let you enter an approximate location to speed the process. Warm start - current almanac, initial position, and time are all valid. Ephemeris data is either invalid or only partially valid. Time-to-first-fix is likely to be 30 seconds to 2 minutes depending on satellite availability and the type of GPS receiver. Hot start - if the receiver has been off for, say, less than an hour time-to-first-fix will likely be seconds. What does this all mean in practice? If the GPS has been recently used you should get a fix almost immediately.
If it hasn't, put the GPS outside with a clear view of the sky and have a cup of tea. The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth. Each plane contains four "slots" occupied by baseline satellites. This slot arrangement ensures users can view at least four satellites from virtually any point on the planet. The Space Force normally flies more than 24 GPS satellites to maintain coverage whenever the baseline satellites are serviced or decommissioned.
The extra satellites may increase GPS performance but are not considered part of the core constellation. Three of the 24 slots were expanded, and six satellites were repositioned, so that three of the extra satellites became part of the constellation baseline. As a result, GPS now effectively operates as a slot constellation with improved coverage in most parts of the world. Learn more at spaceforce.
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