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C NMEA 0183
Exception Behavior
When no position fix is available, some of the data fields in the NMEA messages will
be blank. A blank field has no characters between the commas. There are three
general cases when no fix is available: at power-up without back-up data on SRAM
(cold start); at power-up with without back-up data on SRAM (warm start); and when
the GPS signal is temporarily blocked. These three cases have different NMEA
output behavior in the Copernicus GPS Receiver. This section describes the behavior
for the current product. The specification for this behavior may change in future
products.
Power-up with No Back-up Data on SRAM
In this case, no previous fix is available in battery-backed memory. If the output
message list and output rate has been customized (using TSIP command packet
0x7A) and stored in Flash memory, then at power-up the receiver will output the
messages according to the customized setting. Otherwise, GGA and VTG messages
are output every second. Before fixes are available, the message fields will be empty.
Power-up with Back-up Data on SRAM
In this case, a previous fix is available in battery-backed memory at power-up. If the
output message list and output rate has been customized (using TSIP command
packet 0x7A) and stored in Flash memory, then at power-up the receiver will output
the messages according to the customized setting. Otherwise, GGA and VTG
messages are output every second. Before fixes are available, the message fields will
be empty except for the Time field, assuming the back-up battery power is present so
that time can be tracked continuously by the RTC (Real Time Clock).
Interruption of GPS Signal
If the GPS signal is interrupted temporarily, the NMEA will continue to be output
according to the user-specified message list and output rate. Position and velocity
fields will be blank until the next fix, but most other fields will be filled.
216 Copernicus GPS Receiver