Black=Ground when armed?

I just got a dual zone radar sensor, and not sure if I understand what the ground means. Does this mean that it is grounded only when the alarm is triggered, or just hook it to a chassis ground? It is an Omega brand, and I'm installing it on a no name alarm. It has 4 wires  red=12 volt, black=?, blue=hard alarm and green=prewarn. Also the shock sensor is only 3 wires.  I know I have to hook thru  my shock sensor(with diodes) because I have no place on the module to plug it in. Can anyone tell me what this black wire instructions mean? Thanks for any help/advise.

That black wire should be grounded.   That black wire is the ground input for the radar sensor.

You can ground that directly to chassis.  If you do it this way, then the radar sensor is constantly "ON" 24 hours a day , but will only trigger the alarm when armed. 

But I would recommend that you ground it to the "ground-when armed" of the alarm.   This wire turns to ground as soon as you arm your alarm.    This way the radar sensor is only "ON"  when needed.    I personally think thta this way your radar sensor has a much greater life expectancy. 

Greetings 

Thatis what I was thinking, but I wanted to be sure. Do you think I could hook it in with the shock sensor with diodes? My only question, if so, would be which way to put the diode, and I'm sure i want to isolate the different sensors.

assuming its sending a negative trigger to the alarm...., the cathodes would face the sensors, and the anodes would face the alarm.

Can someone confirm this, I'm going to hook this up to the shock sensor:

 I connect the green and blue(one diode on each, cathode side towards the brain) the red 12 volt to a constant hot. And black (ground when armed) to the black on the shock sensor cathode side towards the brain side of exterior trigger?

https://www.velocitymotorsport.com/images/install_notes/155.jpg