GPS vs Radar

[charlie12]

I watch my GPS in my truck instead of my speedometer. The GPS shows about 2mph faster at 55mph than my speedometer so to be safe I use the GPS.

 

[Suburbazine]

I guess I may have over looked a few things in trying to over simplify my explanations.
I fully agree about the doppler shift being used in the radar.
I was simply trying to illustrate the use of the speed of light in the calculations of both devices.
I also agree that GPS is all about TIME, from the birds standpoint, and not position, as all gps sats have super accurate atomic clock's onboard.
Thanks for pointing those points out.
Still the most basic point I was attempting to make was that since the cancelation of Selective Availability in 2000, civilian gps accuracy is in the +/- 3 meter range.
Buck8Point

It's a +/- 3m range *average*. The sliding differential (I used the wrong word to describe it) in the digital C/A stream alters the device's calculations in a range from 1m to 25m. "Noise" if you will. Just the average accuracy is high, so you can't really use to to pilot a cruise missile in. P-code data on band L1 (shared with C/A) is much more accurate but subject to availability limitations, including not being available for civilian use. Civilian contractors and industry personnel can use devices with P-code L1 access- like airplane pilots.

P-code on band L2 access is restricted to military guidance systems and is heavily encrypted at all times. The only way I know to access it is to use a modified Motorola Oncore chip- and even then you need to either break or know the transmission key (both of which are quite difficult).

 

[bayoutrigger]

Liedar or Lidar?

 

[MRBULLRED]

Is the GPS in question a W.A.A.S enabled GPS unit? That would say a lot about its accuracy.. I would find it hard to believe his GPS was 15mph off.. whenever I had a speed error on my GPS it wasn't a 10mph fart.. it jumped like from 70 to 400...
I think you guys are arguing over two pretty accurate devices and not looking at human error.. is it maybe possible that about time he seen the lights he may have slowed down a bit???

 

[Kee]

Just to be clear, I was traveling Saturday Morning at 8 am, the weather was great not a cloud in the sky, humidity low and temp was cool. It was a flat straight road, I had my cruise at 70 and the GPS normally reads 1 to 2 mph slower than my speedometer. The speed limit was posted at 65. So I am admitting I was speeding at 68 to 69 mph. The LA State Trooper was coming from the other direction, made a u turn and pulled me over saying I was clocked at 74 mph. I did not fight with him. I did tell him I was sorry for speeding but had my cruise on and didn't think I was going that fast. He was very nice and polite. I had no issues with him.

P.S. I was cc and told him so. He looked at the rig and allowed me to keep it on my person while he wrote the ticket.

 

[charliepapa]

I have probably been here longer than you.

quick question on that; on my calendar, march 2009 came way before february 20011. how do you figure you've "probably been here longer"?

 

[Jed]

Not trying to pick sides, but 'malice' is a noun, not a verb.

LMAO...I think he meant malign..

 

[jguilletjr]

When my speedometer says 71, My GPS says 69. I don't know what your radar says, but the last speeding ticket I got was about 10 years ago. I am not in that big a hurry to get any where, and I work too hard for my money to give it to the local Government. (To me it's more about safety any way)

 

[buttanic]

My wife and I were stopped for speeding by a St John deputy on the shoulder of I-10 a few weeks ago. I was on cruise control at 67 MPH, he claimed he clocked me at 86. As I went by him several cars were flying past me. It was obvious his radar got one of the cars that were passing me but he insisted it was me he clocked. When he saw my St John address he said he would give me a break and not a ticket but added I didn't have to lie to him. That pissed me off big time.
Seems speeding is the only time you are guilty until proven innocent.

 

[Tim67]

I'll say the radar since it is calibrated and tested. GPS also seems to be pretty accurate, but they tend to lag a bit and I'm not sure if they have an accuracy standard they are required to meet.

Yeah, when I compare my GPS to my speedometer, it looks like the GPS tells me how fast I was going a few seconds ago.

 

[Tim67]

Just to be clear, I was traveling Saturday Morning at 8 am, the weather was great not a cloud in the sky, humidity low and temp was cool. It was a flat straight road, I had my cruise at 70 and the GPS normally reads 1 to 2 mph slower than my speedometer. The speed limit was posted at 65. So I am admitting I was speeding at 68 to 69 mph. The LA State Trooper was coming from the other direction, made a u turn and pulled me over saying I was clocked at 74 mph. I did not fight with him. I did tell him I was sorry for speeding but had my cruise on and didn't think I was going that fast. He was very nice and polite. I had no issues with him.

P.S. I was cc and told him so. He looked at the rig and allowed me to keep it on my person while he wrote the ticket.

My last ticket was in 2006, so my memory may not be 100% accurate, but this was the first time I looked at the fees in an itemized way and the lion's share of what I had to pay was court cost; the actual fine was only about $1 per mph over the limit, so 74 mph vs 69 isn't a big deal.