GPS speed accuracy

[OB_ROR]

The Motorcyclist mag/ZX12R thread evolved into a discussion of using a GPS receiver as a better speedometer than the Hayabusa's optimistic unit, or maybe even radar.

Take a look at the Garmin site, www.garmin.com - About GPS link for a good explanation of the technology. The bottom line is that typical civilian GPS units are accurate to within 60-225 feet, according to Garmin.

At 120 mph you are traveling 10,560 ft/min. 60 feet more or less is a difference of .0057 (10,500/10,580), or + or minus 0.7 mph. If the error was 225 feet, the speed would be off by + or - 2.5 mph. The figures work out about the same at 60 mph or 180 mph.

I haven't tried riding with my 2-yr-old Garmin 38 model, which cost $160 when purchased, and takes 10-15 minutes to find itself when started at the same place it was last turned off. New units in that price range now are ready to go in a few seconds and apparently have better receivers, more memory and faster processors. Higher priced units add features like better screen resolution and size, and especially stored databases, none of which is useful for measuring speed.

 

[OB_JohnC]

As I said in the other thread, the faster I'm going the more consistent my GPS III+ seems to be. I see 1 to 2 MPH variance at highway speeds. Which corresponds with the math you've shown.

I'll have to strap it to the bike some day, and see what it says. I hit an indicated 120 on the Busa this weekend, but it didn't feel that fast. I can't tell if it's because the Busa is more stable than other bikes I've owned, or the spedo is just more optimistic.

 

[Guest]

ROR... Thanks. I stand corrected.

 

[nmsu]

Hope this helps. Taken from joe.mehaffey.com

GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.

HOW ACCURATE IS THE SPEED READING?
If you are completely stopped, Selective Availability (the "noise" put into the GPS by intent to prevent military use by unfriendly folks" --See: http://joe.mehaffey.com/sabysam.htm ) causes appearent motion despite your GPS being at a fixed location. Typically this speed is in the range of zero to about 3mph (5kph)
but it can be higher. Until the speed of the GPS receiver gets to perhaps 10mph, the indicated speed is not accurate. As speed increases, accuracy increases until at 60mph (100kph), the speed accuracy is typically
within about +/- 1mph (1.5kph). If SA were NOT present, Garmin (for instance) specifies that the speed accuracy of the GPS receiver itself is within about 0.1mph.

 

[OB_jeffw]

What is even worse, this accuracy changes based on the US Gov schedule. Can't have some one using this to guide missles now can we.

 

[OB_THE MOTORHEAD]

digital gps speedos are available from summit racing.

 

[OB_Utah Rider]

The military is going to end the scrambling of the GPS signal from the satellites. This should make GPS speed and position calculations VERY precise. Looks like we won't need RADAR anymore for calculating top speed. Having a GPS also means you won't have to guess at what speed your going due to speedometer error.

http://www.wired.com/news/politics/0,1283,36021,00.html

 

[Guest]

The accuracy restrictions for civilian use was supposed to end today - or so it was reported. Now everyone with a GPS will have the same military spec accuracy as the government. Anyone besides me ever watch the evening news?

[This message has been edited by Todd (edited 02 May 2000).]

 

[OB_scotty]

It is true. The error factor introduced has been abolished via executive order. They still will have the error integrated into the signal recieved by countries hostile to the USA. So don't rely on the speed of your GPS in Iraq.