GPS tracking devices are a great means of tracking your dog in field events and if you have a large yard area, but I wouldn't use them as a sole means of tracking for your dog. Pet GPS tracking devices are run off of batteries, so the batteries can die; they also only have a certain distance from the home that they'll track, so once the dog has surpassed that distance, they no longer track properly. Also, because they are a little bulky, they can be easily spotted and removed by a passerby, or they can get stuck on something and possibly slip off.
Although, there are so many reasons as to why they may not work, they are a good investment for those with large yards and those who use their dogs in field events.
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Dog GPS Tracking Devices