[ARCHIVED THREAD] - GPS tracking... (Page 1 of 2)
Posted: 5/2/2010 1:48:18 PM EDT
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After reading a post here earlier about someone telling a cop that based on his GPS, he was wrong in claiming he got the driver doing 17 mph over the limit, I did some digging to see how this was possible.
Sure enough, Nuvi's have a hidden program that will track and record your route, then let you play it back showing your speed and location. Not sure if it will work on your particular Nuvi, but it works on my 255W. Get a ticket for being accused of speeding when you know you weren't? Here's a great tool to fight it. Here's a link to how to activate the tracker: Nuvi tracker Once you've recorded your route, you can later go back and replay it...even setting a particular time point. I think I'll be firing up my GPS everytime I drive just to cover my ass! |
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If it works like mine it calculates average speed not a speed at any given moment. When replaying your route, it shows your location and your speed at a given time. That maybe but I'm sure it is just showing an average speed. Could be...I don't know. But it'd be nice to show a judge the playback and say "See, your Honor? I wasn't speeding when the officer said I was!" |
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If it works like mine it calculates average speed not a speed at any given moment. When replaying your route, it shows your location and your speed at a given time. That maybe but I'm sure it is just showing an average speed. Could be...I don't know. But it'd be nice to show a judge the playback and say "See, your Honor? I wasn't speeding when the officer said I was!" Can you show us that your GPS is calibrated or designed to be used as a speed measuring device? What is the factory specs for the measuring of speed? How often does it refresh and update the speed? What was the number of satellites being used to measure the speed? (accuracy drops off with less satellites). Just some thoughts 
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If it works like mine it calculates average speed not a speed at any given moment. When replaying your route, it shows your location and your speed at a given time. That maybe but I'm sure it is just showing an average speed. Could be...I don't know. But it'd be nice to show a judge the playback and say "See, your Honor? I wasn't speeding when the officer said I was!" Can you show us that your GPS is calibrated or designed to be used as a speed measuring device? What is the factory specs for the measuring of speed? How often does it refresh and update the speed? What was the number of satellites being used to measure the speed? (accuracy drops off with less satellites). Just some thoughts I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right I bet you think it is alright though for some piece of shit to stop people by lying and say they were speeding when they weren't though huh!!?? This GPS stuff seems like a good way to MAYBE keep those corrupt bastards in check a little. I would think it would have to put some reasonable doubt about the ticket in play also. |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right You're right...God forbid you should stand up to a cop who's lying about what you did. |
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i wonder if it can back fire on you. what if they noticed you were speeding say 10 miles earlier This... I have a Nuvi and have heard of this, but it seems like a double-edged sword, since almost everyone speeds occasionally. I suppose the difference is we don't usually go 17 miles over. What are they gonna do? Write you a bunch of tickets for 5 over? Besides, they'd have to prove you were driving at that time, right? |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right You're right...God forbid you should stand up to a cop who's lying about what you did. RDP is a very good cop based on his posts in here. I would cut the man some slack. With that said, I have heard of people using GPS logs to prove innocence in speeding cases. I think this would be most useful when a cop picks you out of traffic and wrongly accuses you of speeding when he was actually clocking another vehicle. Also, my Nuvi is more accurate than my truck's speedometer in most cases. It would certainly be able to show you were going under 60 and not 75 like the officer is accusing... |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right You're right...God forbid you should stand up to a cop who's lying about what you did. RDP is a very good cop based on his posts in here. I would cut the man some slack. With that said, I have heard of people using GPS logs to prove innocence in speeding cases. I think this would be most useful when a cop picks you out of traffic and wrongly accuses you of speeding when he was actually clocking another vehicle. Also, my Nuvi is more accurate than my truck's speedometer in most cases. It would certainly be able to show you were going under 60 and not 75 like the officer is accusing... While he may actually be a good cop it doesn't make him look very good when he is being the way he is in this thread. After all a GOOD cop would be ALL FOR someone WRONGLY accused using ANYTHING to prove their innocent!! |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right You're right...God forbid you should stand up to a cop who's lying about what you did. RDP is a very good cop based on his posts in here. I would cut the man some slack. With that said, I have heard of people using GPS logs to prove innocence in speeding cases. I think this would be most useful when a cop picks you out of traffic and wrongly accuses you of speeding when he was actually clocking another vehicle. Also, my Nuvi is more accurate than my truck's speedometer in most cases. It would certainly be able to show you were going under 60 and not 75 like the officer is accusing... While he may actually be a good cop it doesn't make him look very good when he is being the way he is in this thread. After all a GOOD cop would be ALL FOR someone WRONGLY accused using ANYTHING to prove their innocent!! I think he is playing devil's advocate |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right You're right...God forbid you should stand up to a cop who's lying about what you did. RDP is a very good cop based on his posts in here. I would cut the man some slack. With that said, I have heard of people using GPS logs to prove innocence in speeding cases. I think this would be most useful when a cop picks you out of traffic and wrongly accuses you of speeding when he was actually clocking another vehicle. Also, my Nuvi is more accurate than my truck's speedometer in most cases. It would certainly be able to show you were going under 60 and not 75 like the officer is accusing... He may very well be a good cop and I respect him for it. Unfortunately, there are many who aren't. |
I run Instamapper on my Blackberry (works with other phones too), and it has the added bonus of allowing realtime tracking on the computer (m wife could follow me as I trucked across the country ). Best of all it's free.
ETA: It shows your exact speed at the location you look back at, not a trip average speed. |
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I run Instamapper on my Blackberry (works with other phones too), and it has the added bonus of allowing realtime tracking on the computer (m wife could follow me as I trucked across the country ). Best of all it's free.
ETA: It shows your exact speed at the location you look back at, not a trip average speed. GPS doesn't have the capability to give an exact speed. When it displays speed, it's calculating an average based on multiple factors and positions. When you look at a point on your tracks and it gives you a speed, it's a calculated speed based on rate of change in position between multiple points. ETA: I should also point out that the radar units most police use are far less accurate at speed measurement than even a commercial GPS, and they're completely subject to operator error. I've gotten at least one ticket that I know for a fact was operator error on the part of the officer who pulled me over. |
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I run Instamapper on my Blackberry (works with other phones too), and it has the added bonus of allowing realtime tracking on the computer (m wife could follow me as I trucked across the country ). Best of all it's free.
ETA: It shows your exact speed at the location you look back at, not a trip average speed. GPS doesn't have the capability to give an exact speed. When it displays speed, it's calculating an average based on multiple factors and positions. When you look at a point on your tracks and it gives you a speed, it's a calculated speed based on rate of change in position between multiple points. ETA: I should also point out that the radar units most police use are far less accurate at speed measurement than even a commercial GPS, and they're completely subject to operator error. I've gotten at least one ticket that I know for a fact was operator error on the part of the officer who pulled me over. I know that, I guess what I was trying to say is that it can take a reading every 5+ seconds (you set the interval), and I have found it to be extremely accurate. I have no doubt there may be some discrepancies at times, but I don't know how many times my wife would call and say "You are at xxxxxx and going xxxxx MPH." and she was almost always dead on. |
| My GPS is within 2 mph of what my speedo says and tracks pretty consistently between the two. Mine also knows what the speed limit is on most major roads and will note when it changes, usually before I see a sign that shows the change. I would say it is fairly accurate. Don't know if mine does logs or not. |
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Quoted: Quoted: I run Instamapper on my Blackberry (works with other phones too), and it has the added bonus of allowing realtime tracking on the computer (m wife could follow me as I trucked across the country ). Best of all it's free.ETA: It shows your exact speed at the location you look back at, not a trip average speed. GPS doesn't have the capability to give an exact speed. When it displays speed, it's calculating an average based on multiple factors and positions. When you look at a point on your tracks and it gives you a speed, it's a calculated speed based on rate of change in position between multiple points. ETA: I should also point out that the radar units most police use are far less accurate at speed measurement than even a commercial GPS, and they're completely subject to operator error. I've gotten at least one ticket that I know for a fact was operator error on the part of the officer who pulled me over. what do you think speed is? all it is Velocity = Change in Position / Time now i dont know about all GPS' but mine will display the speed based on the change within the last foot or so. |
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I run Instamapper on my Blackberry (works with other phones too), and it has the added bonus of allowing realtime tracking on the computer (m wife could follow me as I trucked across the country ). Best of all it's free.
ETA: It shows your exact speed at the location you look back at, not a trip average speed. GPS doesn't have the capability to give an exact speed. When it displays speed, it's calculating an average based on multiple factors and positions. When you look at a point on your tracks and it gives you a speed, it's a calculated speed based on rate of change in position between multiple points. ETA: I should also point out that the radar units most police use are far less accurate at speed measurement than even a commercial GPS, and they're completely subject to operator error. I've gotten at least one ticket that I know for a fact was operator error on the part of the officer who pulled me over. what do you think speed is? all it is Velocity = Change in Position / Time now i dont know about all GPS' but mine will display the speed based on the change within the last foot or so. No, no, it actually won't. Even if you're carrying a PLGR or a DAGR, it won't. GPS doesn't have the capability to measure to "within the last foot or so". It has the capability to measure within about 30 feet or so, which is what introduces errors into the system. Your car's speedometer is measuring RPM of a calibrated sensor, your GPS is measuring change in measured position, and the measurement of each position can easily be +/-30 feet inaccurate. This also assumes all conditions are perfect in the GPS world. I was out running the other day, and my GPS kept switching where it thought I was –– it was bouncing between my actual postion +/- about 20 feet, and a position about 2km NE of me. Just an anomaly, and a reason you don't trust commercial GPS for precise measurement of anything. BTW, velocity != speed. |
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Quoted: Quoted: if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? |
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I know... how dare we use technology to counter the use of technology by the King's roving revenue agents? You can use technology, but you need to know what your technology is capable of. New cars have a black box. Why not pull that and show what it shows in court? At least a speedo error could be argued or shown it was minimal (depending upon the citation). Using a GPS system that isn't designed to accurately measure speed for the purpose of measuring speed doesn't seem right You're right...God forbid you should stand up to a cop who's lying about what you did. RDP is a very good cop based on his posts in here. I would cut the man some slack. With that said, I have heard of people using GPS logs to prove innocence in speeding cases. I think this would be most useful when a cop picks you out of traffic and wrongly accuses you of speeding when he was actually clocking another vehicle. Also, my Nuvi is more accurate than my truck's speedometer in most cases. It would certainly be able to show you were going under 60 and not 75 like the officer is accusing... While he may actually be a good cop it doesn't make him look very good when he is being the way he is in this thread. After all a GOOD cop would be ALL FOR someone WRONGLY accused using ANYTHING to prove their innocent!! That is not what is being discussed here. Good try to attack me and not the point of the OPs topic. We aren't talking about the other topic, but what a GPS is capable of. I have a Nuvi and see the errors in the speed ALL the time. I am not for lying or fabricating squat. I am showing you some holes in using the GPS as a single defense. Not sure about everyone else, but I MUST know how radar works. It comes up in court all the time for speeding offenses. Just like my training in estimating speed, which is what the radar is confirming. |
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Quoted: Not sure about everyone else, but I MUST know how radar works. It comes up in court all the time for speeding offenses. Just like my training in estimating speed, which is what the radar is confirming. Just how radar works? That's pretty damned simple. Just like how GPS works. How is it different unless you know all the facts like I posted above |
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Not sure about everyone else, but I MUST know how radar works. It comes up in court all the time for speeding offenses. Just like my training in estimating speed, which is what the radar is confirming. Just how radar works? That's pretty damned simple. Just like how GPS works. How is it different unless you know all the facts like I posted above It's simple. Point at car, push button, read speed. How radar works! 
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Quoted: Quoted: Quoted: Not sure about everyone else, but I MUST know how radar works. It comes up in court all the time for speeding offenses. Just like my training in estimating speed, which is what the radar is confirming. Just how radar works? That's pretty damned simple. Just like how GPS works. How is it different unless you know all the facts like I posted above It's simple. Point at car, push button, read speed. How radar works! Haha very true. Earlier people said you needed to know all the ins and out of GPS in order to use it, but radar was good to go. I figured most people don't know all the details. |
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if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? Add range and bearing discrimination to that as well. Outside a very small number of technicians, I've never met anybody who even knows what those terms mean, and they're the most critical factors in properly using any radar. ETA: emphasized a very important word above. If you don't understand the two terms I mentioned above, how they apply to radar operation, and the specifications of your radar in those areas, you don't know how to use that radar. |
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Quoted: Quoted: Quoted: Quoted: if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? Add range and bearing discrimination to that as well. Outside a very small number of technicians, I've never met anybody who even knows what those terms mean, and they're the most critical factors in properly using any radar. There is a ton more. I know several radar techs and have visited radar facilities to see what's going on. It is really freaking cool and something I do not take for granted. I also had to do some coursework involving radars and radar limitations, so I can pick out a few things but the rest is all over my head. |
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Not sure about everyone else, but I MUST know how radar works. It comes up in court all the time for speeding offenses. Just like my training in estimating speed, which is what the radar is confirming. Just how radar works? That's pretty damned simple. Just like how GPS works. How is it different unless you know all the facts like I posted above It's simple. Point at car, push button, read speed. How radar works!
Haha very true. Earlier people said you needed to know all the ins and out of GPS in order to use it, but radar was good to go. I figured most people don't know all the details. Very few people do... Very few people know how GPS works and how it calculates speed as well, as shown earlier in this thread. GPS can be accurate –– but you have to know how to use it properly and when to trust it and when not to trust it. |
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if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? Yes, I know how radar works. I know how to operate a traffic radar unit. I know why it works. I don't repair them or build them. |
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<<< Radar Tech.
In addition to a few other things...
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There is a ton more. I know several radar techs and have visited radar facilities to see what's going on. It is really freaking cool and something I do not take for granted. I also had to do some coursework involving radars and radar limitations, so I can pick out a few things but the rest is all over my head. |
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Quoted: Quoted: Quoted: Quoted: if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? Yes, I know how radar works. I know how to operate a traffic radar unit. I know why it works. I don't repair them or build them. Hell I'm sure even I could figure it out. I know how radar works. So in my hands would the radar still be a reliable tool? I |
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if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? Add range and bearing discrimination to that as well. Outside a very small number of technicians, I've never met anybody who even knows what those terms mean, and they're the most critical factors in properly using any radar. Yes, you have gone too far past the capabilities of police radar. Police radar is simple doppler radar, it measures relative speed of objects. Usually the beam is a directional 12 degree cone. The further from coming directly at the radar antenna the target is, the lower the speed reading is, because it measures relative speed. As far as range............... police radar generally can track 1 or 2 targets. A 12 degree cone, at 2000' feet range is huge. The radar reads out the return from the "best reflector". That can be based on range, speed, size of the object, etc. Most cops understand that intuitively rather quickly. Most will understand that when they are wanting to "radar" a particular target, they need to radar it when it is the best reflector. Your understanding of radar is based on true radar, that is a million times more complicated and capable than speed measring tools. |
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GPS accuracy The State of California vs Scott Peterson, 1st degree murder trial Scott Peterson is accused of murdering his pregnant wife. Scott Peterson's movements were tracked by a GPS tracking device attached to his vehicle, during the trial a GPS expert testified under oath that GPS was accurate to within 30 feet, 80% of the time, at one point the GPS recorded Scott's Ford F-150 traveling at 19,000 mph. Is that accurate enough for you? |
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GPS accuracy The State of California vs Scott Peterson, 1st degree murder trial Scott Peterson is accused of murdering his pregnant wife. Scott Peterson's movements were tracked by a GPS tracking device attached to his vehicle, during the trial a GPS expert testified that GPS was accurate to within 30 feet, 80% of the time, at one point the GPS recorded Scott's Ford F-150 traveling at 19,000 mph. Fairly common. GPS hadn't settled or obtained a valid fix, listed him in Canada or somewhere (just for example), then got a valid fix 5 seconds later in, say, LA. Presto, change in position of a couple of thousand miles in a couple of seconds. Happens all the time with GPS, and you just have to know what positions to throw out if you're trying to get a semi-accurate track. ETA: My GPS said I was running 1.5 minute miles the other day for the same reason. It kept bouncing back and forth between invalid and valid fixes, about 2km apart. |
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if police dont have to know how radar works I dont have to know how GPS works... just sayin.... So you are syaing you do have to know how GPS speed meaurement works. So you're saying you know what spectrum and freq's your radar runs at, wavelength, its output, sampling rate, aperture, gain, and processing factors? Add range and bearing discrimination to that as well. Outside a very small number of technicians, I've never met anybody who even knows what those terms mean, and they're the most critical factors in properly using any radar. Yes, you have gone too far past the capabilities of police radar. Police radar is simple doppler radar, it measures relative speed of objects. Usually the beam is a directional 12 degree cone. The further from coming directly at the radar antenna the target is, the lower the speed reading is, because it measures relative speed. As far as range............... police radar generally can track 1 or 2 targets. A 12 degree cone, at 2000' feet range is huge. The radar reads out the return from the "best reflector". That can be based on range, speed, size of the object, etc. Most cops understand that intuitively rather quickly. Most will understand that when they are wanting to "radar" a particular target, they need to radar it when it is the best reflector. Your understanding of radar is based on true radar, that is a million times more complicated and capable than speed measring tools. It's actually exactly the same technology. You've been given some basic information to try to make it useful to you. If you have two targets in view, inside the radar's range or bearing discrimination capability (which, again, you must know to effectively and accurately operate the device), the radar cannot distinguish between them. It, therefore, cannot determine the true speed of either object. You might get a "close enough" number for speed, assuming the two targets are traveling roughly the same speed, but you won't get an accurate number. Now, I don't know the discrimination capability of police radar. Maybe it's so good that it can perfectly discriminate between cars side by side in different lanes of traffic, travelling at different speeds, or cars separated by 50 feet in distance. |
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It's actually exactly the same technology. You've been given some basic information to try to make it useful to you. If you have two targets in view, inside the radar's range or bearing discrimination capability (which, again, you must know to effectively and accurately operate the device), the radar cannot distinguish between them. It, therefore, cannot determine the true speed of either object. You might get a "close enough" number for speed, assuming the two targets are traveling roughly the same speed, but you won't get an accurate number. Now, I don't know the discrimination capability of police radar. Maybe it's so good that it can perfectly discriminate between cars side by side in different lanes of traffic, travelling at different speeds, or cars separated by 50 feet in distance. The radar antenna sends and recieves continuously. It is a fixed beam. LIke I said, most around here a 12 degree cones. If I remember correctly the same radar builder offers 12, 9, and 6 degree antennas..Other manufacturers may offer other "cones". The beam goes out, doppler shift changes the frequency of the part of the beam that hit the object proportional to it's relative speed. That is reflected back to the radar. You can repeat that several times. there can be multiple returns. The radar will read out the strongest returning signal. You can also get the speeds from 2 cars, in lanes next to each other, if one is travelling faster on many radars, because the radar can readout the fastest speed from the returns, and the most powerful return. (using more advanced radars, that offer that capability, it's been around for about 15 years) Most police radar claims about a 2000' max range. I think you are still thinking the radar is more complicated than it is. Police radar does not determine range of the target. |
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It's actually exactly the same technology. You've been given some basic information to try to make it useful to you. If you have two targets in view, inside the radar's range or bearing discrimination capability (which, again, you must know to effectively and accurately operate the device), the radar cannot distinguish between them. It, therefore, cannot determine the true speed of either object. You might get a "close enough" number for speed, assuming the two targets are traveling roughly the same speed, but you won't get an accurate number. Now, I don't know the discrimination capability of police radar. Maybe it's so good that it can perfectly discriminate between cars side by side in different lanes of traffic, travelling at different speeds, or cars separated by 50 feet in distance. The radar antenna sends and recieves continuously. It is a fixed beam. LIke I said, most around here a 12 degree cones. If I remember correctly the same radar builder offers 12, 9, and 6 degree antennas..Other manufacturers may offer other "cones". The beam goes out, doppler shift changes the frequency of the part of the beam that hit the object proportional to it's relative speed. That is reflected back to the radar. You can repeat that several times. there can be multiple returns. The radar will read out the strongest returning signal. You can also get the speeds from 2 cars, in lanes next to each other, if one is travelling faster on many radars, because the radar can readout the fastest speed from the returns, and the most powerful return. (using more advanced radars, that offer that capability, it's been around for about 15 years) Most police radar claims about a 2000' max range. I think you are still thinking the radar is more complicated than it is. Police radar does not determine range of the target. Yes it does it just doesn't tell you Posted Via AR15.Com Mobile |
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It's actually exactly the same technology. You've been given some basic information to try to make it useful to you. If you have two targets in view, inside the radar's range or bearing discrimination capability (which, again, you must know to effectively and accurately operate the device), the radar cannot distinguish between them. It, therefore, cannot determine the true speed of either object. You might get a "close enough" number for speed, assuming the two targets are traveling roughly the same speed, but you won't get an accurate number. Now, I don't know the discrimination capability of police radar. Maybe it's so good that it can perfectly discriminate between cars side by side in different lanes of traffic, travelling at different speeds, or cars separated by 50 feet in distance. The radar antenna sends and recieves continuously. It is a fixed beam. LIke I said, most around here a 12 degree cones. If I remember correctly the same radar builder offers 12, 9, and 6 degree antennas..Other manufacturers may offer other "cones". The beam goes out, doppler shift changes the frequency of the part of the beam that hit the object proportional to it's relative speed. That is reflected back to the radar. You can repeat that several times. there can be multiple returns. The radar will read out the strongest returning signal. You can also get the speeds from 2 cars, in lanes next to each other, if one is travelling faster on many radars, because the radar can readout the fastest speed from the returns, and the most powerful return. (using more advanced radars, that offer that capability, it's been around for about 15 years) Most police radar claims about a 2000' max range. I think you are still thinking the radar is more complicated than it is. Police radar does not determine range of the target. Thanks for the description of how Doppler works. My 6 year old can give that description of how Dopper works, as I could at his age. Your radar follows all the principles of radar. They don't change, based on how complicated or simple the device is. Your radar does not have the capability to determine which vehicle in view it is returning a speed for, if those vehicles are inside its range and bearing discrimination values (from what I'm reading, the devices you use have no range or bearing discrimination –– that is, no ability to distinguish between multiple objects inside the view of the transmitter at all). If that is the case, it cannot determine, nor can it indicate, which vehicle's speed is being returned in any way. In that respect, yes, I was perhaps assuming your device was more complicated than it is. The strongest return will come from the vehicle with the largest radar cross-section –– not the vehicle traveling the fastest. The vehicle with the largest RCS may not be obvious –– it isn't always determined by the physical size of the vehicle (yes, an 18 wheeler will have a larger RCS than a Smart Car, but the problem isn't always so obvious). The frequency shifts between different returns will interact in ways you cannot predict (I can, but only if I have more information than is physically possible to have available without more test equipment), and the resulting display will not necessarily reflect an accurate description of either vehicle's speed. You don't get two separate returns off two separate vehicles, as far as your radar receiver is concerned –– you get a single, frequency shifted, return, and the computer has to try to determine what's happening in the view of the radar gun. It doesn't track multiple vehicles separately, it doesn't capture multiple speeds for them and then determine which one is going faster. Don't get me wrong –– I'm not saying all police radar is inaccurate. I know that you use it to "confirm" a speed you already believe you know, and if the vehicle you're clocking is the only vehicle in view, and it's not offset beyond the physical limits of the radar to correct, you can get a semi-accurate reading. The officer who pulled me over out of a group of 3 cars, where I was travelling 10mph below the speed limit (accelerating away from a right turn after a red light), and claimed he had me on radar travelling faster than my car was capable of accelerating under those conditions, was using the radar outside its parameters. He pulled me over because the truck in front of me (which had not had to stop, and had a significantly higher RCS than my car) was speeding –– not because I was. |