Berkeley Varitronics Systems Fox Manual page 42

Ttcf= input anywhere across one second period
+ command processing + output anywhere across
one second period following command processing
= 0.5 s + 0.05s + 0.475 s
= 1.025s
Worst Case (Position Fix): Any command
Twcf= input beginning of one second period + output end
of one second period
= 1 s + 1 s
= 2s
NMEA-0183 Format Description
Output of data in NMEA-0183 standard format allows interface via the RS232 port to an electronic navigation instrument that
supports the specific messages that are transmitted. The ONCORE Receiver will support the following NMEA output messages
per the NMEA-0183 Revision 2.0
Specification:
GPGGA CPS Fix Data
GPGLL Geographic Position - Latitude/ Longitude
GPGSA GPS DOP and Active Satellites
GPGSV GPS Satellites in View
GPRMC Recommended Minimum Specific GPS/
TRANSIT Data
GPVTG Track Made Good and Ground Speed
GPZDA Time and Date
You can enable or disable each message output independently and control the update rate at which the information is output.
Once enabled to output a particular message at a pirticular rate, the GI'S Receiver remembers the settings when powered off
and reconfigures itself to the same state when powered up again. All NMEA messages are formatted in sentences that begin
with ASCII $ (hex 24) and end with ASCII <CR><[.F> (hex OD and hex OA). A five-character address occurs after the ASCII $. The
first two characters are the talker ID (which is GP for GPS equipment), and the last three characters are the sentence formatter
or message ID from the table above. Any number of fields and an optional checksum can occur in the sentence as long as the
total number of characters does not exceed 79. Fields within the message are delimited by the ASCII comma. The checksum is
calculated by XORing the 8 data bits of each character in the sentence between, but excluding, the $ and the optional (*) or (CS)
checksum. The high and low nibbles of the checksum byte are sent as ASCII characters. You control the output of the above listed
messages with Motorola NMEA format messages. Input messages are allowed in the NMEA specification, and take the form
$PMOTG *CS<CR><LF>. All input parameters are separated with comma delimiters. The P character identifies the message
as Proprietary format, and the MOT is the manufacturer designator for Motorola Inc.
For the case where more than one output message is scheduled during the same 1 second interval, the GPS Receiver will out-
put all scheduled messages but will attempt to limit the number of bytes transmitted each second to 375 bytes. For the case of
multiple output messages, if the next message to be,sent fits around the 375 byte length goal, then the message will be output.
For example, if messages totaling 334 bytes are scheduled to be sent, and the user requests another 80 byte message, then 414
bytes will actuafly be sent. If the user requests yet another 70 byte message, then its output will not be generated. The order
for priority for transmitting messages is simply alphabetical.
LORAN Emulation Format Description
This particular Output message format is intended to emulate the position status message string from a LORAN receiver. This
allows you to use the GPS receiver to replace the LORAN receiver in embedded positioning system applications. You can request
the LORAN position status message string to be output at any update rate (from 1 second to 1 hour in 1 second increments) and
can operate it in a polled mode where the host can request the receiver to output the position status message upon request. The
selected rate of the output message is remembered between power on-off-on sequences.
Page 41