The almanac contains information about the status of the satellites and approximate orbital information. The GPS receiver uses the almanac to calculate which satellites are currently visible. The almanac is not accurate enough to let the GPS receiver get a fix. If the GPS receiver is new, or has not been used for some time, it may need 15 minutes or so to receive a current almanac. In older GPS receivers, an almanac is required to acquire the satellites, but many newer models are able to acquire the satellites without waiting for the almanac.
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. Privacy policy. This page was last modified on February 22, For the desktop version of this page, rotate your device to landscape.
Skip to content GPS. GOV Official U. Space Segment The space segment consists of a nominal constellation of 24 operating satellites that transmit one-way signals that give the current GPS satellite position and time.
Learn more Control Segment The control segment consists of worldwide monitor and control stations that maintain the satellites in their proper orbits through occasional command maneuvers, and adjust the satellite clocks. It tracks the GPS satellites, uploads updated navigational data, and maintains health and status of the satellite constellation. The GPS satellites are not in a geostationary orbit, but rise and set two times per day.
Each satellite broadcasts radio waves towards Earth that contain information regarding its position and time. We can receive this information by using special receivers, called GPS receivers , which can detect and decode this information. By combining signals transmitted by several satellites and received simultaneously, a GPS receiver can calculate its position on the Earth i.
There are more sophisticated receivers that can be used to determine position with an accuracy of a few millimeters. What does is mean to determine our "position" on the Earth? The usual method is to refer to a terrestrial position i. Therefore, most GPS receivers will displaying their current latitude and longitiude. The usual format for displaying this information is in degrees and minutes. There are degrees in a complete circle, and 60 minutes in one degree.
The minutes are usually displayed as a decimal number, like The latitude is measured relative to the equator. If a location is in the Northern Hemisphere, the latitude will be followed or preceded by the letter N. If a location is in the Southern Hemisphere, the latitude will be followed or preceded by the letter S. Sometimes no letter is given, and the latitudes in the Southern Hemisphere will be expressed as a negative number. By historical convention, longitude is measured relative to the "Greenwich" or "Prime" Meridian.
If we measure an angle east of the Prime Meridian, we write the letter E preceding or following the longitude. If we measure an angle west of the Prime Meridian, we write the letter W preceding or following the longitude. Sometimes, negative values are used to express longitudes measured west. In the Global Mapping Experiment, we will talk about the "visibility" of a satellite, or when we can "see" a satellite.
By this terminology, we do not mean that we can, with the unaided eye, see the satellite although it is sometimes possible to do so, especially when the Sun glints off it. We use the term "visibility" and "seeing" to mean "to have an unobstructed view of. After they have set, for example, they are below the horizon and therefore "not visible. After they rise, satellites are above the horizon and thus potentially "visible. Sometimes, even after satellites rise, their view is obstructed.
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