• Description of the Master Control Station SLAC M40
• History of GPS Operations at SLAC
• Data Flow
• Official Coordinates for SLAC M40
• Regional Network Analysis

Description of the Master Control Station SLAC M40

Located next to the Sector 30 gate at the end of the linac is SLAC’s newly designated GPS master control station known as SLAC 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 RS 500 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. To protect the instruments from lightening, a lightening arrester is installed at M40.

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

History of GPS Operations at SLAC

Oct 2001 - Acquired three Leica SR 530 receicvers followed by demonstration and two training sessions Dec 13-17 2001 - First Campaign Jan 2002 - Bernese Sofware was purchased and installed Feb 2002 - Decision to use M40 as Base Station Mar 2002 - The initial antenna mount was changed and the weather radome was removed. Apr 2002 - RTK Mapping begins Jun 4 2002 - RTK presentation Jan 2003 - Leica's RS 500 CORS receiver purchased. Initial tests performed. Feb 2003 - RTK verified using new receiver. Mar 2003 - New M40 box installed to accomodate UPS and RS232-Ethernet converter. Mar 2004 - RTCM Version 2.2 (msg types 1&2&3&18&19&22) used for RTK operations.

Data Flow

The GPS data is recorded using Leica's RS 500 receiver. The following diagram represents the flow of data from the receiver to the final storage

Official Coordinates for SLAC M40

The official coordinates of the Master Control Station refer to the Antenna Reference Point (ARP) of the LEICA AT-504 GPS antenna. The ARP is located at the bottom of the preamplifier as indicated in the antenna diagram:

  LEIAT504
  
                           +-------+                          <-- 0.1393
                         /     +     \                        <-- 0.1282 L2
                        |      +      |                       <-- 0.1093 L1
      +-----------------+-------------+------------------+    <-- 0.1020 TCR
      |                                                  |
      |                                                  |
      |                                                  |
      |                                                  |
    +-+--------------------------------------------------+-+  <-- 0.0380
    +-------------------+-------------+--------------------+  <-- 0.0350 BCR
                        |             |
                        |             |
                        +------x------+                       <-- 0.000  BPA=ARP
  
    <--                      0.3794                      -->
  

     ARP: Antenna Reference Point
     L1 : L1 Phase Center                   L2 : L2 Phase Center
     TCR: Top of Chokering                  BCR: Bottom of Chokering
     TGP: Top of Ground Plane               BGP: Bottom of Ground Plane
     TPA: Top of Preamplifier               BPA: Bottom of Preamplifier
     TOP: Top of Pole                        

  All dimensions are in meters.
            

The coordinates are geocentric XYZ triplets in the International Terrestrial Reference Frame 2000 (ITRF2000) geodetic datum as provided by the International Earth Rotation Service (IERS). They are updated on an as-needed basis.

The current coordinates are also available in different datum definitions and with other representations.

For the datum conversion from ITRF2000 to NAD83 we used the transformation parameters as published by the National Geodetic Survey (NGS). The California State Plane coordinates Northing N and Easting E were computed with NGS's routine spsc83. The orthometric height H was derived from the ellipsoidal height h by using the GEOID99 geoid undulation value of N=-32.512m.

For the current epoch (2002.90) velocity values have been determined (using approximately 1.5 years of data). In the ITRF00 and NAD83 reference frames the velocities take the following values: