U-BLOX GPS chip ANTRAIS 4 and above can run in several power modes:
0: Continuous Tracking Mode, about 3.5 mA
1: Fix Now -- sleep mode about 130 uA

Fix Now mode can be enabled by ubx binary message CFG-RXM. Detailed
parameters are configured by CFG-FXN.

One of the problem is how to activate (wake up) from sleep mode. I've read
several message lists discussing this issue, it seems no way to wake up
through serial port.

I've tried make GPS chip goes into sleep mode several days ago, but can't
wakeup it.
Last night I found good references through google:

"When waking up TIM-Lx with RxD1, RxD2 from sleep- or backup state send a
number (at least 8) of
0xFF characters to wake up the serial communication module otherwise the
first bytes may get lost. To
request a position fix a position request message must be sent via serial
port after waking up the
ANTARIS GPS Receiver."

   5.3.3 Change operation mode

"Please add header with dummy data for any command when the GPS module is in
sleep mode. This is required for wake-up in sleep mode. The dummy data in
hexadecimal is “0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF”."

I've test this with ubx binary protocol:
1) enable FixNow (sleep mode) by CFG-RXM, configure CFG-FXN
2) periodically poll NAV-POSLLH, NAV-VELNED, NAV-SVINFO. Before writing each
poll request, write 
   unsigned char array {0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF, 
   /* note:  first line: dummy data, second line: RXM-POSREQ */
   then sleep 1 second;
3) toggle between "Continuous Tracking Mode" and "FixNow mode" by call
CFG-RXM (if in FixNow mode, first write dummy data and RXM-POSREQ)

With MON-SCHED, the average GPS CPU load in "Continuous Tracking Mode" is
about 35%, regardless of send_rate, max nav SVs, 2d/2d nav mode. No
performance test data for FixNow mode for now.

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