A user's GPS-enabled phone sends data to the server at regular intervals. These points are used to create an accurate, real-time map that the other party can easily see.
The system detects when two people are a certain distance of one another. When the two users are a certain distance apart, an SMS message is sent to both parties.
A user can wrap up client work or finish what they are doing right up until the moment the other party gets there. I don’t have to waste time dealing with uncertainty. It’s the equivalent of “on my way” and “here”, the two most common co-location ‘drags’.