The application can be found for free download in Google Play. Search for "richard hunt tracker" or go to https://play.google.com/store/apps/details?id=com.mycloudbase.tracker
The phone only needs to have a GPS, you don't even need a data contract if you only want to send text messages.
Tracker is working on handsets from Android 2.0 through to the very latest Android 8 and Android Go. Specific handsets that are known good and tested with Tracker v8.12 are as follows:
* The altitude reported by this phone is to the WGS84 Geoid (EGM96) and not WGS84 Ellipsoid as specified by Android. It will consequently report around 45 - 50m out in the UK
** The GPS time reports local time and not UTC (GMT) as specified by Android. Change timezone to workaround.
The following have an issue in that they kill Tracker whilst it is running: