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Satnav speed only works correctly if you are travelling in a straight line, and unless you have a specially calibrated GPS, (usually for aircraft use), if you are on a level road.
When you go round a corner, the satnav measures the straight line distance between two points over a set time, and calculates the speed based on that. So it tends to under-read.
snip
As an extreme, if you drove off a cliff and went straight down, the satnav would say you had zero speed, since you aren't moving over the ground.
quite possibly untrue, since it can measure its speed relative to the satellites, it doesnt need to do distance/time calculations. How its implemented isa matter for the developers but AFAIK speed output is likely to be implemented in the chip using dopller calculations and thus speed measurements do not need to involve distance at all.
Like MrZipy I drive a C4 and find the speedo is about 1 mph fast most of the time. Speed limiter and cruise control is a boon especially with specs. That large digital speedo in the middle of the dash is a boon for the older generation (me) whose eyes don't adjust to near and far distances so readily anymore.
Joined: Mar 04, 2006 Posts: 119 Location: West Suffolk
Posted: Fri Mar 17, 2006 10:38 pm Post subject:
I've got a picasso and I couldn't agree more.
The big display is wonderfull!
As for accuracy I've noticed that the picasso is showing about +2
And my Focus is showing about +4.
JD _________________ TTG 300 Died the death of a cracked screen
Navman F20 USB Fell off
TTG 520 Third time lucky?
Bloody Superb PGPSW Camera Database.
Not much else!
Joined: Mar 02, 2006 Posts: 384 Location: Bedford, England
Posted: Sat Mar 18, 2006 12:41 pm Post subject:
quite possibly untrue, since it can measure its speed relative to the satellites, it doesnt need to do distance/time calculations. How its implemented isa matter for the developers but AFAIK speed output is likely to be implemented in the chip using dopller calculations and thus speed measurements do not need to involve distance at all.
Tw[/quote]
Sorry to correct you, but GPS doesn't work like doppler. With a single satellite, you get information that you are somewhere on the edge of a ball of space at a known distance from the satellite. 2 satellites will intersect the balls, placing you on the circumference of the circle of intersection. The third satellite places you on one of 2 points on the circle. Usually that is enough, since one of the points can normally be ignored. A fourth satellite, brings you to a single point in space. The receiver identifies that spot, waits a set time, identifies your new location and works out the speed/time/distance equation. In some aviation GPS, the receiver can work in 3D and calculate speed in any direction. In vehicle GPS, the receiver normally only works in 2D, and can only calculate speed on an assumed level surface.
For doppler shift to be any use, there has to be a measurement of the frequency of the signal, which is then compared to the reference frequency. Differences are then identified as speed. For this you need to have a measureable speed towards or away from the transmitter, or there is no doppler shift. Since the satellite is looking down on the vehicle, the movement towards or away from the satellite is infinitesimal, and doppler shift cannot be discerned.
The global positioning system (GPS) is able to determine the location of anyone or anything which has a receiver. The complete global positioning system consists of 29 satellites and a GPS receiver which can decode signals from the satellites. The GPS receiver is what we can purchase in stores here on Earth. Each GPS satellite transmits radio signals which contain the current time. The GPS receiver uses the time stamp to determine how long it takes the signal to travel between the satellite and the receiver. In this manner the receiver can determine the distance to each satellite. Once the distance is known between the receiver and several satellites, the location of the receiver can be constructed. In addition, the Global Positioning System can determine the velocity of the receiver by calculating the distance between two locations, and how long it takes for the receiver to travel between the two locations
"As well as calculating its position, a GPS receiver can also determine its velocity by measuring the Doppler shift: the difference between the expected frequency of the satellite signal and the actual frequency of the incoming signal." _________________ Gone fishing!
"As well as calculating its position, a GPS receiver can also determine its velocity by measuring the Doppler shift: the difference between the expected frequency of the satellite signal and the actual frequency of the incoming signal."
Sorry guys. you're plain wrong. Let's just for a minute assume that it does measure the doppler effect. Doppler shift occurs when there is a change in distance from the transmitter to the receiver. As the receiver moves towards the transmitter, the frequency is increased. As it moves away from the transmitter, the frequency is decreased. The change in frequency is translatable into speed. If you maintain a constant distance from the transmitter, there is no doppler shift. (Used by aircraft to defeat enemy doppler radar. Fly around the transmitter at a constant distance and you cannot be seen)
Don't know who Trimble is but they're talking rubbish. Even with a doppler radar, simple frequency shift cannot give direction, as they claim. Unless you are transmitting and receiving a directional beam, it has no way of knowing if the frequency shift is because you are travelling slowly directly towards/away from, it, or faster, but at an angle. The doppler shift is identical.
Satnav velocity works on the basis of measuring locations and times and then doing the calculation.
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
Posted: Sun Mar 19, 2006 5:26 pm Post subject:
The GPS data is only updated at one second intervals, so its of very little use for true 0 to 60 tests, as the cars speed will be changing faster than the GPS can update, I only rely on speed information on straight level roads, even then its only to double check how far out the speedo is (hire cars on a regular basis) - Mike
Joined: Nov 17, 2004 Posts: 368 Location: Kettering, Northants, England
Posted: Sun Mar 19, 2006 9:48 pm Post subject:
me being a complet nere might agree with that -- as the speed on the TOMTOM does lack behind that of the speedo... yo can be doung 20 mph on the speedo and only 10 on Tom... only for a few seconds - true. _________________ Happy to meet -- Sorry to part -- Happy to meet again
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Sun Mar 19, 2006 10:56 pm Post subject:
TomDavison wrote:
Don't know who Trimble is but they're talking rubbish.
Why don't you go and do a google for Trimble and see who they are before you make a statement like that.
TomDavison wrote:
Satnav velocity works on the basis of measuring locations and times and then doing the calculation.
How do you know this?
I presumed that this was how they worked too, but when I did some reading about it and a number of people say that the speed is measured by doppler because the position jumps around too much to get an accurate speed reading. _________________ Gone fishing!
Don't know who Trimble is but they're talking rubbish.
Why don't you go and do a google for Trimble and see who they are before you make a statement like that.
TomDavison wrote:
Satnav velocity works on the basis of measuring locations and times and then doing the calculation.
How do you know this?
I presumed that this was how they worked too, but when I did some reading about it and a number of people say that the speed is measured by doppler because the position jumps around too much to get an accurate speed reading.
Plus, it won't be doppler from a single satellite used to determine the speed, it will be averaged from all the ones the unit has a lock on. But you make agood point, if you have a GPS unit that doesnt lock you to a road, you'll see that your position jumps about a fair bit. Without WAAS, maybe by 20 to 30 metres at a time. even with WAAS it will vary a fair bit, certainly enough to make speed differences vary dramatically from one moment to the next. Using doppler would eliminate most of that. So, take your dopplered speed from say 6 satellites, and 'averaging' it is going to give you much more accuracy.
Joined: Mar 02, 2006 Posts: 384 Location: Bedford, England
Posted: Mon Mar 20, 2006 10:46 am Post subject:
First my background. I spent 26 years in the RAF working on military radars, and part of that was the operation of dopper radars and how to defeat them.
When a signal is transmitted at a known frequency, and received by a stationary object, the frequency is the same. If the distance between the two is changing, the frequency is apparently changed. If you move towards the transmitter it is increased, and if you move away from the transmitter it is decreased. If you are moving, but remain at a fixed distance from the transmitter (Going around in a circle, or flying at a tangent), there is no frequency shift. In fact, this is how we defeat doppler radar. By flying around doppler radar locations at a fixed distance, you can avoid detection. Your flightpath becomes a series of arcs around radar locations, but you avoid being seen.
Lets assume you are driving directly towards the satellite at 60mph. Doppler shift would give an accurate figure. As soon as you turn through 90 degrees you would be driving at a tangent, and there would be no doppler shift, therefore you would read zero speed. If you were measuring doppler shift from a variety of satellites, you would get readings which would vary between your true speed and zero. Averaging these would therefore always give a figure below your correct speed, and approaching half of your correct speed as the number of readings increase
I said, lets assume, because in practice, you are almost never driving directly towards or away from a satellite. This would only occur if the satellite appeared on the horizon, directly in front or behind you. If the satellite is anywhere else, doppler speed calculations would under-read.
Doppler radar is used on GATSO because the target is driving directly towards or away from the transmitter. Drive directly across the beam as fast as you wish and you'll never get flashed, because the doppler shift doesn't occur and your speed is read as zero.
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