Hello, The Alpha and Astro dog collars come "tethered" to specific Garmin handheld GPS devices which are designed to work in tandem with the dog collar. It sounds like either you're considering purchasing one of these devices, or possible have one and are looking for a replacement handheld GPS to track them? Please elaborate if you need further assistance and we can proceed.
If looking for a replacement handheld, the Astro is no longer available as new, but can be purchased used at the link below:
Alternatively the Alpha handheld is available as brand new (current model) at the following link:
The Alpha and Astro collar series can only be tracked using the handheld that is "paired" to the devices, and Garmin does not have any subs that will pair. Keep in mind also that the Alpha can be paired to up to 20 dog collars at one time.
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