Located next to the Sector 30 gate at the end of the linac is SLAC’s newly designated GPS master control base station known as M40. The actual control point is located on top of a 60 cm diameter concrete pillar with the top about 3 m off the ground. A wooden deck surrounding the concrete cylinder provides access. The actual receiver is a Leica System 530 Base Receiver that is housed in a weatherproof box attached to the deck. The AT-504 choke ring antenna is attached to the top of the pillar using a Kern forced centering mount.

This control monument was chosen for several reasons:

- Central SLAC location

- Monument within controlled area of SLAC

- Amount of visible sky is large enabling better GPS geometry

- RTK radio-modem coverage of SLAC nearly perfect

- Permanent power access and potential network connectivity

- Easy access for monitoring the equipment

- Pillar and deck are in good condition