What is more accurate? GPS or radar?

[Bonnewagon]

If you are trying to calibrate your speedometer so it is as perfect as you can get it, what method do you use? Some say GPS is more accurate but even the US GOV says GPS is only 95% accurate, and other things can interfere. I say those courtesy radar set-ups near schools are more accurate. You know them 'Speed limit is XX and your speed is___' . By me here in Shity City there is suppose to be a 5mph grace but I have seen tickets issued by as little as 2mph over. And because the speedo was off they had no idea. I actually am so old school I use the 'get a buddy to drive 60mph with his flashers on and compare my speedo to his', method.

 

[SS_Malibu]

I think GPS would be more accurate than the standard X or Ka band radar detectors. Laser detectors are probably more accurate than X and Ka bands as well. GPS typically use three (or more) satellites to lock on to a target or location. When I upgraded my gauges to Autometer I opted for the GPS speedo and it seems to run pretty consist with my Garmin when I tested it.

 

[Built6spdMCSS]

Satellite signal is running speed of light, so is RF on the ground. Shorter distance on the ground. Laser is also the same speed so take your pick on what would be more accurate.

 

[Bonnewagon]

I know years ago the GPS signal was not very accurate for civilian use. I remember BOATUS lobbying the government to improve accuracy so EPIRB devices would be able to pinpoint a distress location. We tried using a cell phone GPS to test propellers and found it was not very useful. Maybe it is a lot better now? Anyway, what is the better cenario? "Your Honor my cell phone GPS says my speedometer is more accurate than your radar so my ticket should be dismissed". OR: "Your honor I tested and calibrated my speedometer to your courtesy radar set-up in front of the school. I know my speedometer is correct according to your radar machine. My ticket must be a mistake".

 

[Built6spdMCSS]

I have enough tickets to know their stuff was working to test triple digits...

 

[Supercharged111]

I can't speak to radar, but us mere plebes only get GPS's coarse acquisition code, not the warhead through a window code that the military uses. Accurate within something like 8 feet I think? Still amazing for birds in mid earth orbit. You need a MINIMUM of 4 birds to get velocity, often times there are a dozen in sight. M code is supposed to be available to us mortals, but it's gonna be a while before enough satellites are in orbit with it to be useful.

 

[edfiero]

Go to straight, flat stretch of Interstate. Set the cruise at 60mph. Take your phone, turn on the stop watch.
Time yourself driving between the mile posts.
Time greater than 1:00min your speed-o is reading to fast
Time less than 1:00, you speed-o is reading too low

 

[rfpowerdude]

Depends on number of satellites acquired and how long they have been acquired. Ground radar is accurate as long as you're the only one in its signal path. LIDAR is def the way to measure the most accurately every time.

I would say using the courtesy radars will be pretty spot on if you are the only one approaching or receding. Otherwise, a well acquired GPS will average out the slight inaccuracies over time and be pretty damned spot on.

 

[Supercharged111]

It's the placement of the satellites, not the overall quantity. As stated 4 is the minimum and if there were only 4 available, the ideal placement would be one directly overhead and the other 3 120 degrees apart like a triangle. Those 3 give you position and #4 gives you velocity.

 

[Built6spdMCSS]

I can't speak to radar, but us mere plebes only get GPS's coarse acquisition code, not the warhead through a window code that the military uses. Accurate within something like 8 feet I think? Still amazing for birds in mid earth orbit. You need a MINIMUM of 4 birds to get velocity, often times there are a dozen in sight. M code is supposed to be available to us mortals, but it's gonna be a while before enough satellites are in orbit with it to be useful.

You know what I do for work, stuff we use is accurate in inches. I laughed when they told me that when I first started, then they showed me.

You won't have that on the civi market so I'm still going with radar or laser on ground because signal transmission time.