The problem with a microchip that is GPS tracker is you are now talking active RFID and constant RF emissions .... I would worry about cancer in that case. I am not sure I would want a device constantly on my dog's neck either. We do use GPS collars for SAR work and they, too, rely on radio transmission of data (right around VHF 145-150 band) but that is only for short time use, not constantly.
NAPWDA Certified Cadaver Dog
Waiting at the Bridge (italics
=GSDs) (hemangiosarcoma=blue):Grim , Cyra, Toby
, Rainbow, Linus, Oscar
, Arlo & Waggles