Is a Reward building unlocked by completing the 4 good Police Car missions to unlock this building, or by waiting the time until Mr. Neil Fairbanks says this City needs a Police Chopper to prone the skies, when you get 12,000 sims. The easier way is by waiting out the time, but if you want a more understanding of getting the lay-down of your city, then do the police a favor and help them with the crimes in the city.
Once you unlocked the building, it unlocks both a Police Van( Also for Large Police Station) and the Police Chopper. Normally if you don't interact with the chopper, it will roam around the city heading to the most crime areas of your city. Once it reaches its location it will shine its spotlight for some time until the police arrives. If multiple choppers are available the crime will lessly spawn on the crime map.
Missions related to:
"Catch the Crook from the Air"- Good police chopper mission
"Disturbing the Peace"- Evil police chopper mission
Cop Gone Bad- Good police van mission.
Escape With the Loot: Evil get away van mission.
Hijack the Train: Evil passenger Train mission.