[blantonl]

Excellent point. All cell phone sites have a GPS reference clock source for each provider. The clock source is critical for everything from RF tuning to handoff timing between sites.

You'll see this type of antenna mounted typically right on top of the equipment housing structure. Each one you see will tell you how many providers are there.

http://www.antenna.com/media/GPSL1-TMG-SPI-40NCB-thumb.png

(Note that it is small, about 6 inches in height. And different variations abound, but they are typically painted white and mounted right on the structure for LOS view to the sky)