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.
If you have a GPS in a vehicle, it's better to wait for the unit to get a fix before driving off. Receiving ephemeris data for a satellite takes 30 seconds. If you momentarily interrupt the signal during that time the GPS it could take up to a minute more to get the ephemeris for that satellite as it has to start over. If you drive in an area with tall buildings or other obstructions it may take a long time to get the ephemeris data, for four satellites, that is needed for the first fix.
The accuracy of the position your GPS reports is influenced by a number of factors, such as the positions of the satellites in the sky, atmospheric effects, satellite clock errors and ephemeris errors etc.
GPS units often show on the screen an accuracy figure, e. EPE on Garmin units. Since accurate and INaccurate clocks, and thus the INaccuracy, is constant in our measurements, there can be only one correction value that reduces the volume of intersection of 4 spheres to a single point of intersection. That value represents the time INaccuracy. The fourth satellite is there just to increase accuracy to a point where it would be useable.
Although, with 3D Trilateration this is not necessary to calculate a location. GPS although requires this because of the accuracy issue. To consider atmospheric delay you need to compare the delays of two signals sent at different frequencies from the same satellite or compare the readings of the same signal seen from two different locations "differential GPS". Modern GPS systems correlate the two encrypted military signals at frequencies L1 and L2 to obtain this information. While I was part of a 3 man team which spent 2 years in the early 90's developing the first non-military differential GPS stations in the South West of England, we came across some extra-ordinary questions.
To explain this, it is best to start with a terrestrial radio navigation system. Take one signal from a fixed known point Station 1 on the beach and beam it at a ship at sea. The ship knows for how long the beam has been traveling and the exact location of Station 1 - it knows this because the time the beam left the fixed point is imprinted on the transmitted signal - e.
Take another known point Station2 from which started a signal at the same time 'A'seconds - but Station2 is on a different known point which gives Range2. From Range2 you know that your ship lies along Range1. Do the same with a 3rd Station and you get an intersection of all 3 Ranges. But they do not intersect perfectly This is due to atmospherics, interference, propagation delays which affect all radio waves.
Now take all of your Stations and stick them in Space as GPS and your ship is positioned somewhere inside a 4 sided 3D triangle of error which is slightly curved on all sides. Answer is here : in 2D you need 2 hyperbola 3 satelites in 3D you need 3 hyperboloid 4 satelites Desmond Schmidt is right.
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Create a free Team What is Teams? Learn more. Why does GPS positioning require four satellites? Ask Question. Asked 10 years, 3 months ago. Active 5 years, 1 month ago. Viewed k times. Improve this question. We do NOT need to calculate x, y, and z. We need to calculate x, y, z and time. See starblue s answer for why. Add a comment. Active Oldest Votes. Just a graphic to add to M'vy's answer.
Math: Remember when you were a kid, in math class, and the teacher said " T o solve for [x] unknowns, we need [x] equations "? Well, that's the case with GPS.
We need to determine four unknowns: position x. Each satellite provides the information for one equation. So, we know the positions of the satellites X, Y, Z and we know the uncorrected distance to each satellite r The uncorrected distance is the speed of light multiplied by the uncorrected signal travel time. In this example, I've used the ISS, but the same principle applies to you sitting in your car. Visual: Each GPS satellite transmits a signal that includes pseudorandom code and the state vector of the satellite.
The same signal is generated by the GPS receiver. The third satellite reduces the choice to two possible points bottom left. Finally, the forth satellite helps calculate a timing and location correction and selects one of the remaining two points as your position bottom right. Even when GPS is leveraging more than 4 satellites, it is still doing trilateration, as opposed to multilateration, which GPS does not use.
Multilateration should not be confused with trilateration, which uses distances or absolute measurements of time-of-flight from three or more sites , or with triangulation, which uses the measurement of absolute angles.
Both of these systems are also commonly used with radio navigation systems; trilateration is the basis of GPS. The major reasons why you need a fourth satellite is for timing corrections.
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