On 2002-03-01 08:38, slickrick wrote:
First you need to understand what GPS is, Iím not going to get into it here, there is enough knowledge on the web (some good, some bad) that people can read off line.
>Luckily I do understand what it is and how it works.
Once you understand what it is and how it functions you know that there is faults in the system, granted much less since the government stopped scrambling the single using Dithering, however, Doppler-Shift does still exist and therefore you need something designed from the ground up to take this into effect. The only system I know off that was designed to measure speed utilizing GPS and takes all known variables into account is VBOX by RaceLogic. More on VBOX can be found here:
>The advantage of the vbox is not it's accuracy so much as it's rapid speed calculation update. They claim that it uses a survey grade GPS engine but that has no bearing on speed, just position.
>There are 2 signals available by GPS, PPS and SPS which stand for Precise or Standard positioning system. PPS (Survey grade) requires reception of the L1 and L2 signals while regular GPS only receive L1 signals. I don't know if the vbox truly utilizes the PPS signal or not.
There is a lot to take into consideration when using GPS to determine your speed; it isnít as simple as determining position A, race really fast to position B and measuring your time with a stop watch and using basic math to come up with a number.
>You are correct; sort of. GPS doesn't measure speed using time over distance calculations but instead it calculates the doppler shift of the received signals. It does this very accurately because each satellite caries a Cesium beam time standard (very accurate). This Cesium standard not only keeps the satellites accurate but also keeps your GPS accurate; essentially your GPS receives it's own timing corrections from the satellite which is the heart of it's ability to accurately measure speed by calculating doppler shift.
As previously stated there is the Doppler-Shift to take into consideration as well as some of the issues below:
>I'm not sure what you're trying to say here but doppler is how speed is measured by GPS. Radar uses doppler too but, as I just explainedr, is missing one big advantage of GPS - self calibration.
Dithering - while no longer an issue from the signal being sent, a lot of GPS systems where made to ďadjustĒ for it and are now more inaccurate then before due to it not having to adjust any longer.
>I think you mean SA (Selective Availability). SA applies a time bias to the L1 (SPS) signal on each satellite. Well, it used to as it's been turned off for 2 years.
>Not true about older GPS units being built to adjust to SA. That's the first time I ever heard that rumor; interesting though.
Differential GPS - A technique to improve GPS accuracy that uses pseudorange errors measured at a known location to improve the measurements made by other GPS receivers within the same general geographic area, without it your less accurate.
>Has nothing to do with speed measuring. Besides, D-GPS isn't really widely used as far as I can tell. PPS is more commonly used.
Dilution of Precision - An indicator of satellite geometry for a unique constellation of satellites used to determine a position. Positions tagged with a higher DOP value generally constitute poorer measurement results than those tagged with lower DOP.
>More accurately, GDOP which is Geometric Dilution Of Precision. Yes, the correlation between you and the satellites does have an impact on the accuracy of measurements. But, since all modern GPS receivers have 12 channel parallel receivers and you can easily see 8 to 12 satellites at a time on open terrain this is not a valid consideration for speed measurements.
Kinematic positioning - Kinematic positioning refers to applications in which the position of a non-stationary object (automobile, ship, bicycle) is determined. Just using a plain GPS and a stopwatch isnít sufficient.
>Uhh, ok. And this has to do with what?
Multi-channel receiver - A GPS receiver that can simultaneously track more than one satellite signal. There are 24 Satellites used for the GPS system used by the majority in the USA, however, there is another system implemented by the Soviet Union named Global Navigation Satellite System or GLONASS for short. The more channels the more accurate you GPS.
>All GPS receivers are multichannel otherwise they wouldn't work. What you are trying to say is that there are sequential and parallel multichannel receivers. ALL GPS receivers in the past 4 or so years have been 12 channel parallel receivers. This simply means that it receive 12 satellites at once as opposed to sequencing through them one by one. I doubt this has any real bearing on speed measurements.
>More channels do not necessarily mean more accuracy. There is really never more than 12 satellites visable to you at any given time.
>Also, there are 28 GPS satellites in orbit, not 24. 24 are operational and 4 are in-orbit spares or in maintenance right now.
Multipath error - Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. This is usually caused by one path being bounced or reflected. Such as by Microwave Towers, Power lines, Power Plants ETC.
>Yes, this can account for up to a whopping .5 meters of distance innacuracy. Again, this has little to no bearing on speed measurements.
While GPS systems are highly accurate for a stationary object such as a hiker in the mountains or a missile base, once that object is in motion it is a whole other ball game.
>Sorry, wrong again. GPS in usable everyday terms is more accurate for speed measurements than location measurements. A consumer grade GPS is accurate to about 30 meters in distance and .1 MPH in velocity.
I welcome a rebuttal if you feel up to it.
>I do and I did. By the way, did you cut and past this out of a "GPS for dummies" book?