There a few things that differentiate the two. For one, the Terrafix I/O can float whereas the Microfix can't. However, the Microfix is much smaller and lighter which also makes it more expensive (reducing the size and weight while improving the performance usually makes something more pricey). The Microfix has SiRF GPS technology whereas the Terrifix I/O doesn't, thus the GPS will be quicker at acquiring satellites and will be able to get through wooded areas and low visibility terrain better. It's also much easier to operate the Microfix in an emergency due to only having to push one button for a second. The Terrafix requires the user to push X2 buttons at the same time for one second.

nothing its just smaller about 20% smaller. So you pay more money. It depends what size means to you both are really nice items.

If the UID is not affixed to the back of the unit, you'll likely need to call ACR and have them do a serial lookup for the unit's UID number. You can reach them at 1-800-4320-ACR or by online form at http://www.acrelectronics.com/form.htm

This is not an avalanche beacon, but a personal locator beacon used in times of emergency. Unlike an avy beacon, the user must push a button to transmit the signal after extending the antenna (avy beacons, once on, are always transmitting and receiving but only work in about a 40-60 meter radius). The antenna must also not be covered by water or snow in order to transmit. If you are in avalanche country I would first get a partner, understand the dangers involved by educating yourself on slopes angles, types of snow, what to do in an avalanche, etc., and then purchase an avalanche beacon (understand they only work two at a time; one will do you no good). The PLB is used if there are life-threatening injuries after everyone has been recovered. This could include trauma sustained from debris, hypothermia, fractures, shock, etc. A PLB can also be used in all 4 seasons and if you venture out solo, it will work on it's own. Education is key when in dangerous backcountry conditions. Please learn about all the vital info needed before going out!

From previous answer: "1. Even without the GPS the beacon uses a secondary 121.5 MHz frequency and will pinpoint you to within 2.3 nautical miles. Where the with GPS it is accurate to 110 yards."Correction about the above statement: As of Feb 01 2009 the Search and Rescue (SAR) Satellites will no longer monitor the 121.5MHz frequency. Only the 406MHz frequency will be monitored. This being said, the LEOSAR satellites can still determine your position with the 406MHz frequency via Doppler Shift even without GPS data. HOWEVER, the position obtained through Doppler Shift is nowhere near as precise as GPS positioning so you REALLY want to have GPS. This will narrow down the search range and make it easier/faster to find you. Furthermore, determining your position via Doppler Shift takes longer as you need to wait for one of the LEOSAR satellites to fly over you. With GPS the signal is picked up by the GEOSAR satellites which cover most of the globe, with the exception of polar regions (See http://www.cospas-sarsat.org/Status/geoCov.htm ), at all times. All the more reason to buy a unit with GPS on board. See:http://www.cospas-sarsat.org/FirstPage/121.5PhaseOut.htm for more details. The 121.5 MHz frequency will still be used as a SAR homing frequency. SAR planes and helicopters are equipped with 121.5 MHz receivers and will use that signal to pinpoint your location once they are close. Beacons MUST be registered. This is so the SAR authorities can call your emergency contacts to ensure the beacon activation is not a false alert. Beacons can be registered for free at: www.beaconregistration.noaa.gov/ for the USAwww.beacons.nss.gc.ca for Canadaor by mail or fax.If size/portability are an issue the ACR MicroFix 406 is the smallest unit. That's the one I am considering getting. Hope this helps. Play safe.

The critical function of the ACR MicroFix is really to make sure the person is rescued when all other options fail. Because this is it's mandate, the battery has to be absolutely fresh at all times in case the person using it has to rely on the unit to work flawlessly...If it included a typical GPS, this would compromise the batteries which would make it less than reliable in emergency situations.The best option would be two units, a dedicated GPS and an ACR PLB for emergencies. This will ensure the person will have what he/she needs to get through most situations, as long as he/she understands how to operate both units and that both are working properly.

No, not at all. There are products intended for those situations.The SPOT offers 911 type emergency services, check it out:http://www.backcountry.com/store/SOT0001/SPOT-Satellite-Messenger.html