You'll need to use Emacs and PGP to encrypt your coordinates and then post the message on blockchain. It's free, doesn't require a non-free device and everyone can see the transaction history!
Hey I do use that;) But actually I was talking about a peer-to-peer ride selling and ride wanting matching system that cuts out the middle man. I use Uber but I cann't help but think the drivers should get a larger share of the fare.
GNUber