How does the Leo's moving car radar measure speed?

[statusquo]

There has to be some other radar signal for a moving leo's car or bike to detect your speed. His car's speed must be in the equation somewhere.

I've looked at the BMW R1100RT-P patrol bikes with front and rear Stalker Ka units and the only thing that gives them speed is a manual cable to the speedometer head. The Stalker doesn't appear to be wired or cabled into the bike for speed determination at all. That's why I am assuming there is another signal from the radar unit itself that needs to be subtracted from the head-on speed used to get your speed verses his speed (e.g. head-on speed - leo's speed = your speed).

Thinking about it, to hardwire into different vehicle's computer system or mechanical speedometers would make installation more difficult as well. Their car unit must be doing something easier for installation.

The hand-held radar guns wouldn't have the feature, but the vehicle ones must if they are moving.

Right or wrong?

[kpatz]

To oversimplify it a bit, the radar gun measures the patrol car's speed (by measuring the "speed" of the ground/stationary objects as the car moves) as well as the target speed, and takes the difference to calculate the actual target speed.

For example, say the patrol is going 50 MPH, and a car is approaching (in the opposite direction) at 70 MPH. The radar unit will pick up two Doppler tones, one from the 50 MPH motion of the cruiser, and one higher tone from the approaching car (which would be measured as 50 + 70 = 120 mph). The radar then deducts the measured patrol speed (50) from the measured target speed (120) to get the actual target speed (70).

While older radar units could only clock approaching traffic in moving mode, some newer units have "same-lane" capability, where it can clock vehicles traveling the same direction as the cruiser. The principle is the same, except the radar has to determine if the target is moving faster or slower than the patrol (getting closer or further away) and either add or subtract the measured target speed from the patrol speed to get an accurate speed measurement.

Some units can be wired into the speedometer, but even those that do just use it to verify the radar reading.

[spankyaf]

Something to keep in mind if the Leos radar has an error and measures its speed lower than actual guess who's speed ends up higher.

[statusquo]

...For example, say the patrol is going 50 MPH, and a car is approaching (in the opposite direction) at 70 MPH. The radar unit will pick up two Doppler tones, one from the 50 MPH motion of the cruiser, and one higher tone from the approaching car...
Thanks for the info.

Do you know exactly when the Leo's car speed Doppler becomes active?

Is it always active and reporting/calculating his speed, or does it only become active during targeting?

Reason I am asking as I've gotten a really weak Ka band signal when their cruiser is within 100 feet or so of me, if I get closer it goes to full alert very quick. I don't believe he was firing at target or instant-on as his car was stopped and he was outside of it with another stop.

[bauzer71487]

The LEO's speed doppler is only active when the gun is in active mode. If it is off. . . It isn't displaying any speeds - LEO's nor Victims.

[kpatz]

Reason I am asking as I've gotten a really weak Ka band signal when their cruiser is within 100 feet or so of me, if I get closer it goes to full alert very quick. I don't believe he was firing at target or instant-on as his car was stopped and he was outside of it with another stop.In that case he had the gun on but not aimed toward you. Perhaps a handheld gun left in the passenger seat or something.

[maxima lover]

Some units can be wired into the speedometer, but even those that do just use it to verify the radar reading.

I believe some of the units wired to the speedometer will automatically change the radar from stationary to moving mode. For anyone that's used one that doesn't automatically change, you'll quickly realize why it's so great when it does.

[bart99gt]

Something to keep in mind if the Leos radar has an error and measures its speed lower than actual guess who's speed ends up higher.

Usually it is accompanied by a much lower than normal patrol speed too. Like 20 mph off. Unless you're dealing with a very dishonest person operating the radar, I don't think you would have anything to worry about.

[spankyaf]

Something to keep in mind if the Leos radar has an error and measures its speed lower than actual guess who's speed ends up higher.

Usually it is accompanied by a much lower than normal patrol speed too. Like 20 mph off. Unless you're dealing with a very dishonest person operating the radar, I don't think you would have anything to worry about.

"Usually" I can drive a truck through that loophole in court :wink:

[statusquo]

This is the reason I was asking about the second radar signal.

Seems many were returning the new Escort GPS detector due to seemingly low sensitivity and the odd ramp up (i.e. a couple of bars and then full display with the Leo's car in very close proximity).

I had very normal ramp ups in Los Angeles and then this past weekend I had one of those "He's too close to me" situations and was seeing one or two bars (Ka), and then it jumped full. He was maybe 50-100 feet from me stopped, not moving, and out of the car so I know it wasn't instant-on.

I was thinking - probably like most others with the unit - the unit's sensitivity bites and it shouldn't behave like this.

I got an answer from one of the techs at Escort who said the car radar shoots two signals: one basically towards the ground for the cruiser's speed and the other for the victim (er, speeder). Just needed to verify his statement which seemed logical.

The new Escort picks up the lower output/scattered cruiser speedometer-correction Doppler signal and reports it as one/two bars until you are next to it at which point the scatter is more concentrated and it then goes to full alert display. This is at pretty close distance (visually) and is not the same as if the radar was actually being used. If he left the unit on, this LED display operation or jump is normal for the situation.

Just thought it was interesting point. Don't know about it being factual though.

[jimbonzzz]

I got an answer from one of the techs at Escort who said the car radar shoots two signals: one basically towards the ground for the cruiser's speed and the other for the victim (er, speeder). Just needed to verify his statement which seemed logical.

Radar only transmits one signal, then must process the multiple returns for target and patrol car speed. There is never two separate signals being transmitted (except in the case of the Stalker DSR-2X which can transmit with the front and rear antenna at the same time).

EDIT: I believe there was one very old radar unit that might have operated with a weak second signal for the patrol car speed: the Kustom KR-11.

[Suf Daddy]

Jim:

I have a KR-11, how do I find out about the (lo doppler) "patrol" vs the info in the manual? got a test for telling which the KR-11 does?
-Suf Daddy


EDIT: I believe there was one very old radar unit that might have operated with a weak second signal for the patrol car speed: the Kustom KR-11.

[maxima lover]

Like Jim said, it's not like 2 signals. In stationary mode, it senses the strongest target and that is the number that is displayed under target. In moving mode, that strongest target becomes the patrol car speed and the second "strongest" target because the target speed. Then you get into things like fastest target which displays in the fastest target window all while showing the strongest target (patrol speed) and the second strongest target (target speed).

So to sum it up, it's not like one radar beam is shooting at the ground and then he activates a stronger radar beam that checks your speed. it's either on or off.