A manual of the GPS receiver may be found in the "Product Manuals" drawer (close to the Dewar Vacuum Pump). This document gives only the operations to measure latitude, longitude, altitude, and time.

1) Make sure the GPS receiver is off.

2) Connect a Mac null-modem (reversed) serial cable (Sub-D 9-pin plug at one end, DIN 8-pin at the other end) between the GPS receiver (RS-232 socket) and the "Delay-Line" Macintosh (Modem socket). The DIP-switch block on the back of the GPS must have been configured as following (from the left to the right) :

10110100 (0 = down, 1 = up).

3) Boot the Macintosh, run the TTY emulator named "Versa Term PRO" (Path = McIntosh HD::Utilities). Set the serial port parameters with the "Settings" menu as following:

• Baud Rate = 9600.
• Parity = none.
• Data Bit = 8.
• Stop Bit = 1.

4) Turn on the GPS. "Power" and "Fault" LEDs should be on.

5) The Macintosh should now display every second a line beginning by $STATUS. Otherwise, type LOG STATUS and strike "return" key (Important: use uppercase letters only).

6) Among the parameters displayed are "DATA INVALID" and "FAULT", followed by a number. After about 15 to 30 min., "FAULT" is followed by 0000 and the "Fault LED" on the GPS turns off.

7) Type LOG POS and strike "return" key. Now the $STATUS line is followed by a $POS line giving the topographical information about the GPS. The syntax is the following:

$POS, week number, time-of-the-week, direction (either N or S), latitude (degree), latitude (min), latitude (sec), direction (either E or W), longitude (degree), longitude (min), longitude (sec), altitude (in meters, according the WGS-84 ellipsoid earth-model), altitude (in meters, above the mean sea level), position uncertainity (in meters), position figure of merit

The precision depends on the number of satellites tracked. You can check this number in the $STATUS line displayed ("TRACKING x SVs", where x is this number).

8) After about 10-15 hours, the "Data Valid" LED turns on and DATA VALID is displayed in $STATUS. The uncertainity should be 1 meter. We found the following values for the IOTA site:

• Longitude = N 31 deg 41 min 29.49 sec
• Latitude = W 110 deg 53 min 05.72 sec
• Altitude = 2564 m (above the mean sea level)

• The Julian date can be found from the "week number" (WN) and "time-of-the-week" (TOW) using the following formula:

JD=7*WN+TOW/8640000+2444244.5-13/86400
• The Stellar GPS sends a 20 us TTL pulse every second. Between 6 and 9 ms later, data are sent on the serial port (for about 85 ms).