SDIO GPS receiver
From TreoCentral
In addition to regular SD cards for storage, you can also use the SDIO card slot for a GPS receiver. You want to look for the ones that has the good chipset in it such as "SiRF Star III" as it's quite sensitive and will pick up the GPS signal quickly. You'll also want to check that it is available for your particular Treo whether running PalmOS of Windows Mobile.
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Where to Buy
SDIO GPS receivers can be found in the TreoCentral Store
