Keeping on a short leash

It occurs to me that the combination of three technologies may solve the problem of dogs that go walkabout. The technologies: Global Positioning System (GPS), mobile phones and ultrasound; the idea: put a GPS reader, mobile phone and ultrasound emitter into a dog collar. The collar is then programmed (remotely of course) to emit the ultrasound, which is uncomfortable to dogs, when the dog moves outside the range of a certain GPS coordinate. The ultrasound would be emitted at increasing volume as the dog moved further from the coordinate, encouraging it to stay within the perimeter. The GPS coordinate can be either a fixed one, such as your house, or a mobile one, i.e. your GPS/Mobile dog tracking device that you take walking with you. The walker-held device would having a direction/distance meter to help find the wandering dog. As GPS is being implemented in many mobile phones, the walker-end device could just be software that utilises the mobile and GPS technology on the phone.