What do the "speed enforced by aircraft" signs mean? How do they actually do this?

This form of traffic enforcement is generally used for open roads and the interstate system.
like the other answer, the speed of an automobile, is checked by a fixed-wing aircraft or a helicopter.

First, a safe location has to be established. a location where there is room to safely stop automobiles. second, a road calculation has to be established from a point "a" street marker to a point "b" street marker. third, a trial run is made, by a police vehicle, to assure accuracy of the distance and calibration of the radar unit. a written record is maintained for court. fourth, catch cars are placed in position to stop the violators. once a violation is observed in the helicopter, using laser radar, the flying officer will notify the catch car officer of the violation. the make, color and speed of the vehicle in violation, is broadcast to the ground officers and the violator is stopped. the officer in the helicopter, earlier had signed the traffic citations. he will testify in court.

This is generally the established standard for running radar from an aircraft.

This is done by small planes and sometimes helicopters that police use to patrol highways sometimes. It's easier than having cars patroling all the time, and planes can more easily keep up with speeding vehicles.

*No they can't pull you over but they can call ground units who will get you all the same - and the plane can clock you too so no sense in denying it...

If you see a prop plane following you for more than 2 minutes, it's like a looming ticket...

This is grammatically incorrect. It should be "Speed Monitored by Radar" or "Speed Monitored by Aircraft". To enforce using radar, they would be microwaving us into submission. hahaha

"The SPEED ENFORCED BY AIRCRAFT (CA Code R48-2) sign may be placed, when requested by the
California Highway Patrol, on sections of highway regularly patrolled by aircraft.
Standard:
The R48-2 (CA Code) sign shall be used for both directions of travel.
Guidance:
The R48-2 (CA Code) sign should be placed at the beginning of the section and spaced at 40 km (25 mi)
intervals."
Source:
http://www.dot.ca.gov/hq/traffops/signtech/mutcdsupp/pdf/CA-Chap2B.pdf
How do they do this (humor):
http://www.renderosity.com/mod/gallery/index.php?image_id=1369810&member

This form of traffic enforcement is generally used for open roads and the interstate system.
like the other answer, the speed of automobiles is checked by a fixed-wing aircraft or a helicopter.

First, a safe location has to be established. l location where there is room to safely stop automobiles. second, a road calculation has to be established from a point "a" street marker to a point "b" street marker. third, a trial run is made, by a police vehicle, to assure accuracy of the distance and calibration of the radar unit. a written record is maintained for court. fourth, catch cars are placed in position to stop the violators. once a violation is observed in the helicopter, using laser radar, the flying officer will notify the catch car officer of the violation. the make, color and speed of the vehicle in violation, is broadcast to the ground officers and the violator is stopped. the officer in the helicopter, earlier had signed the traffic citation. he will testify in court.

This is generally the established standard for running radar from an aircraft.

In California, the CHP uses a fixed-wing aircraft to measure vehicle speed between two, marked points on the ground and determine speed. Our aircraft don't use radar or lidar for this, but a pacing method.

After determing a vehicle(s) speed, the aircraft radios down to unit waiting on an on-ramp up ahead. For example, he'll say, "Red pick up in #1 lane is good for 85 mph...he's passing your location right now." The unit jumps out and gets behind the red pick up in the #1 (or if the subject vehicle moves, the aircraft simply tells the ground unit where he went). The pilot confirms the ground unit is behind the correct vehicle, and he makes the stop. The ground unit is the citing officer and the observer in the plate is the arresting officer.

I hate to say it, but it's a lot of fun to do these types of details, because people are caught speeding long before they see the six gound units hanging out at the on-ramp. They slow to 65 and think they're safe--not knowing they were already seen from above! :-)

The aircraft does not use radar or a laser nor do the pace you. they have two fixed points on the ground ( have you ever seen those white lines that run across the road and then a few yards or feet another white line crossing the road) they use them for time and distance calculation. This is how it works, when you cross the first white line or the first marked point they start a clock and when you pass the second point or white line the stop the clock. Because they know the distance from the first white line or marked point to the second white line or marked point and the time it took you to cross the second point or white line, then they do a time and distance calculation, which show your speed. This method is very accurate and no radar detector can help you. All the aircraft has to do is just circle around in the air and watch and when a speeder is spotted, they call a catch car officer to stop them.