Set start mode
0=Hot start
Without initialization, all data in backup buffer is
valid.
1=Warm start
Without initialization, ephemeris is cleared.
2=Cold start
Without initialization, all data except configurations
in backup buffer is cleared.
3=Factory start
All data in backup buffer is cleared and the receiver
is reset to factory settings.
Checksum, exclusive OR (no start or stop bits) of
all characters in the sentence, including "," and "^"
delimiters, between but not including the "$" and
the "*" delimiters.