26 G12 OEM Board & Sensor Reference Manual
green to indicate satellite lock and then red to indicate power status. Each
additional satellite to which the receiver locks on produces an additional green
flash; that is, if the receiver is locked onto seven satellites, the LED flashes green
seven times and red once. A short green flash (.25 sec) indicates the satellite is
locked but not used in position computations; a long green flash (.75 sec)
indicates that ephemeris for that satellite is available. Once the unit is locked to
enough satellites to compute a position (three or more), the duration of the red
flash becomes longer to indicate that positions are being computed.
The G12’s two RS-232 ports (A and B) can receive command messages from an
external control device, send response messages to an external control device
(such as a PC), output data to a separate data logging device, and send or
receive differential corrections from a reference or remote station.
G12 Input Messages
Input messages are comprised of set command messages, query command
messages, and general command messages. These messages comply with the
format defined in the NMEA 0183 standard to the following extent:
• NMEA 0183 ASCII byte strings following a dollar sign ($) character
• Data fields are separated by commas
• Checksum character delimiter and NMEA checksum bytes are recognized
by the G12 but are optional. The hexadecimal checksum is computed by
exclusive OR-ing all of the bytes in the message between, but not
including, the dollar sign ($) and the asterisk (*).
• Messages end with the standard NMEA message terminator characters,
[CRLF] (carriage return/line feed).
Input messages deviate from the NMEA standard as follows:
• Headers are Ashtech proprietary
• Message IDs are Ashtech proprietary
• Message length may exceed 80 characters
All command messages—set or query—can be composed in uppercase or
lowercase characters. All command messages are sent by pressing <Enter>. A
valid set command causes the G12 to return the $PASHR,ACK*3D
(acknowledge) response message. A set command containing a valid $PASHS
header followed by character combinations unrecognized by the G12 causes the
receiver to respond with $PASHR,NAK*30, a “not acknowledge” response
message indicating that the command is invalid. Valid query and messages are
acknowledged by return of the requested information. All invalid query and
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