How do LoJack receivers work?

[fog]

I stumbled across the LoJack frequency a while back, and have a pretty good understanding of how the system in general works -- central transmitters will blast out 'turn-on codes' to the units in stolen cars, which begin transmitting a data burst with a unique ID in the hopes of a DF-equipped cop car finding them.

I passed a Boston cruiser equipped with LoJack the other day, and got to thinking... It's just four omni whips on the roof. I'd think they'd want directional antennas, since the signal strength at each of the four omnis, all within a couple feet of each other on the car's roof, would be all but identical. How can you calculate a bearing from four adjacent omnis?

[davidmc36]

Originally Posted by fog

I stumbled across the LoJack frequency a while back, and have a pretty good understanding of how the system in general works -- central transmitters will blast out 'turn-on codes' to the units in stolen cars, which begin transmitting a data burst with a unique ID in the hopes of a DF-equipped cop car finding them.

I passed a Boston cruiser equipped with LoJack the other day, and got to thinking... It's just four omni whips on the roof. I'd think they'd want directional antennas, since the signal strength at each of the four omnis, all within a couple feet of each other on the car's roof, would be all but identical. How can you calculate a bearing from four adjacent omnis?

Go down to page six here:

http://home.computer.net/~pcara/docs/pcud0406.pdf

It explains how they virtually rotate the four antennas and use the doppler effect to get the direction.

[mikepdx]

A demo on YouTube of how LoJack works:
YouTube - LoJack Tracking a Stolen Vehicle

[kb2vxa]

The antennas don't "virtually rotate", the phase differences correlate to time delay and direction, that's the Doppler effect. (See goniometer.)