How Does The Power Save Work On The Vista?

[mudhuggers]

If you set SAVE power mode to ON to save power with the Garmin etrex Vista, does it still track satellites? If you set power save mode to off does it suck batteries bad?

[peter]

The Garmin power-save mode works by shutting off the radio receiver electronics for short periods of time so they won't use as much power. Once the unit has gotten a satellite lock, the processor checks if the latest position is reasonably predictable from the previous ones (i.e. you're either standing still or moving at a fairly constant speed). If it is, then the receiver circuits are turned off briefly and then turned back on again to get a new position every five seconds. Having the receiver circuitry turned off in between position fixes saves about 20 - 30% of the power.

 

If the unit ever finds that the new position is significantly different than was expected based on the previous fix and speed, then the receiver circuits are left on continuously until the motion is again predictable. So the power saving won't be nearly as significant if you're in an area with poor reception or if you're rapidly changing your speed and direction of travel.

 

Some Garmin units (not sure about the Vista) also appear change the 'mask' used to determine the satellites for which the receiver should listen. By setting a higher mask angle, the unit won't listen for satellites very close to the horizon. That reduces the number of receiver channels that need to be powered up at the five second intervals.

 

Note that the discontinuous reception during power-save mode is incompatible with WAAS which needs to get a continuous stream of correction data, so power-save can't be used if you want WAAS accuracy. Power-save is also not a good idea in situations with poor reception. It won't save as much power, and, even worse, the unit may find itself unable to reestablish a lock at the end of one of the five-second 'sleep' intervals.