The 5Hz mode works as a rate multiplier for the value you set in the NMEA rate. For example, if you set a message to refresh every 1s in normal mode, in 5Hz mode it will refresh every 1/5 seconds. Set to every 5 seconds in normal mode of time will give 5/5 = one message per second in 5Hz mode.
Here are some commands to send to activate this mode. Refer to the NMEA and OSP manuals for details about the parameters and change them as needed for your application:
Procedure to activate: NMEA and OSP Modes
1. NMEA command MID103 ($PSRF103,00,6,00,0*23)
2. OSP command MID136 (A0 A2 00 0E 88 00 00 04 04 00 00 00 00 00 00 00 0F 02 00 A1 B0 B3 )
Note that the 5Hz mode works only in navigation mode, once a GPS fix has been obtained. Until it gets a fix, the receiver will work in 1Hz mode.