Inspired by the radar detector thread but totally different: Would it be possible/practical to have a radar emitter on civilian cars that scans 360 degrees to spot cars beyond visual range – either as a traffic aid or to detect cars speeding towards you (e.g. cops or people wanting to pass)? Would mounting a marine radar on a car work?
Radar is not magic, it cannot detect cars with any real reliability beyond visual range. At least, not without military-grade signal processing that is at a minimum export-controlled. So detecting cops is not really a viable application.
That said, plenty of cars already have radar systems. Mine has an active cruise control radar in the front, that will automatically maintain a set distance from the car in front of me (as well as sound an audible alarm, even with cruise control turned off, if it detects something in front me that it thinks I am going to hit). It also has radar emitters in the mirrors, that alert if you try to change lanes when there is a car next to you, and emitters in the rear bumper that tell you if there is a car approaching from the side (useful when backing out of parking spaces and driveways). It also has some kind of system that automatically turns your high beams on and off, but I think that uses some kind of a camera, not radar.
All of this was part of a $1,000 option package. I didn’t really want it, but it was all the dealer had in stock. I’ve been pleasantly surprised with how useful yet unobtrusive these functions are in practice, and I am an attentive driver that dislikes electronic “nannies” in general.
Sorry, perhaps “beyond visual range” wasn’t the best term; I just meant “far enough out where you can spot and determine the speed of oncoming cars before you can visually identify them”.
Still no?
(And I understand there are assistive radar technologies like you describe; this is something different)
Yeah, I misunderstood your question anyway - I interpreted “beyond visual range” to mean “not in line-of-sight”.
There is no technical reason why someone couldn’t build a radar like you describe. Radar returns from cars would be easy to spot because they would be moving relative to the scenery. It would be hard to tell a cop car from any other car, though, unless it was actively emitting radar (which an ordinary radar detector would pick up anyway). And I can’t think of any other practical reason for wanting to detect cars that far away.
A very sophisticated radar system could probably give you a blurry image of the cop car. Now we are talking about very high-end stuff.
An example image of what you can get off a moving target with a SAR is at
Don’t know what kind of car you have but the mirrors might/probably use cameras, and the parking sensors are probably ultrasonic not radar.
The cruise control is probably radar and collision avoidance might use that or use cameras and laser beams.
(windshields with freakin laser beams. :D)
That wasn’t your fault. I used the wrong term.
There’s a guy who built one for $250, but what does “synthetic aperture radar” even mean?
The “synthetic” is a clear giveaway–the cake is a lie.
You will note the guy is an MIT professor, who has not accounted for the value of the few hundred hours of his time he likely spent on this in his $250 figure. 
It is not the radar hardware itself, but the algorithms that are complicated.
In general, the resolution you can achieve with an imaging system is a function of the size of the “antenna”. A smaller antenna/aperture will result in poorer resolution.
“Synthetic aperture” means you use the motion of the antenna relative to the target to simulate a much larger antenna, using a great deal of computer processing of your radar returns to combine the returns from many different distances/angles into a single static, higher-resolution image.
So-called ISAR systems (inverse synthetic aperture radar) use a fixed radar but a moving target to achieve the same result.
Any such system in a car, used to image other moving cars, would be something of a hybrid of both approaches.
There is such a thing, but you’ll note that the radar dish in this case is mounted on a 10 meter tall mast. Unless you are only going to operate on completely flat terrain with no cover or obstacles it’s hard to see what the radar could do for you that would be beyond what your own eyes could do.
There’s a reason why radar is typically found either on planes on stuff that’s tracking planes.
Google has some autonomous automobiles - they use laser and radar to scan for obstacles and traffic. Takes up a fair bit of space in the car, though, and has a mast on the roof.
The issues are things like clutter and shielding. You don’t get much additional benefit (the radar is still line of sight, and does not have the range of a mk1 eyeball unless it is dark). Also, presenting the radar data in a way that does not distract the driver is a big problem - there is a reason for dedicated Radar Operators in most contexts.
Limited use of radar for ranging and speed measures without presenting too much additional distraction/information overload to the driver is a better use of the technology.