If you can be arsed to figure it out, it's possible to sms your gps or cell network location using tasker or locale. You can trigger the sms texts via incoming text (ie location on demand), timer (eg every hour), or by location (eg when at a control). Saves battery compared to 3g services.
Unfortunately, when I used it on the Seething 600, almost nowhere had even a 2g signal, and my wife got v. cross that I'd not checked in. I didn't know until I got back to civilisation when a brace of increasing anxious then angry texts poured in at once.
I now try to avoid promising any contact.