Of course GSM based trackers would be of no use. Has to be sat tracker, out if which really only the Iridium sat network really works. Spidertracks, Garmin inReach and many other providers build sat trackers that use the Iridium sat network.
We use inReach, with a tracking interval of 1 min. I believe on Spidertracks you can also specify a 1 min interval. For helicopter ops I definitely recommend a shorter interval.
ELT: Analysis of dozens of accident reports show that an ELT only successfully works in about 1/3 of crashes. In the other 2/3 of cases it either doesn’t transmit, or the transmission is not received. (Others above have depicted some scenarios how this can happen. No need for me to repeat.)