CanukNortonNut said:
The faster you go with a gps the more accurate they become. when you are standing still the position is pinging around a point. Once you move in a direction and gain speed the gps is spot on.
CNN
I worked on Sat Nav software for four years a few years ago. Your statement is only partially correct.
"Pinging around a point" has a couple causes.
When stationary, a GPS nav system/speedo knows nothing of your heading. This can cause a GPS nav system to "rotate".
There is also a bit of error in civilian based GPS systems. The United States government currently claims 4 meter RMS (7.8 meter 95% Confidence Interval) horizontal accuracy for civilian (SPS) GPS. Vertical accuracy is worse. Mind you, that's the minimum. Some devices/locations reliably (95% of the time or better) can get 3 meter accuracy. 3 meters is just about 10 feet. This can cause a position to "jitter" about a point.
This becomes worse in an urban environment, where you have a lot of high rise buildings. Here, you will get reflections of signals off of the glass buildings, causing systems (with typically smaller GPS antennae chip sizes in mobile devices having worse "reception" and more prone to error vs. inbuilt car systems or airborne systems with larger and more accurate chips) to become confused as there are multiple time-position signals simultaneously for a given satellite. This alos causes "pinging around a point".
This positional accuracy and errors do not change with vehicle speed.
GPS devices are positional speedometers, based on how far the device has moved since the last measurement. The algorithm also uses the doppler shift in the pseudo range signals from the satellites. It should also be noted that the speed reading is normalized, and is not an instant speed.
Speeds are updated at short intervals to maintain accuracy at all times. It uses frequent calculations to determine the vehicle’s speed. For example, using a standard movement per time calculation, if you have covered 80 feet in one second, the GPS device works out and converts that to MPH, which in this case is 55MPH.
Depending on the device, and how it was designed (Note for pete.v: specified), speed update periodicity can be very different. For example, mobile devices (as opposed to in-car Sat Navs) have a strong bias for battery life. In this application, one of the compromises I have seen made is in how many times the software controlling the GPS module asks for an update on position. The fewwer times the GPS module asks, the greater the potential normalization error.
Rohan said:
Can we ask what your speed is there, nothing especially stands out..
3 satellites is enough for a fix.
If some is good, more must be gooder ??
A system needs 3 for a fix in the horizontal plane, and 4 for a 3D fix. Most systems will only pay attention to 3 or 4 satellites at one time (depending on specification - if you have an altimeter function, for example), and these are the first ones "locked on". Again, this is for computational and power savings.