Short answer for you here Pete and a longer explanation in a new thread.
http://micronavigation.com/forum/index.php?topic=467.0When I was working with the different SAR (Search & Rescue) teams, in various locations across Japan, I turned off SBAS on my Garmin handheld and instead utilised HotFix.
Brief backgroundGarmin’s original expertise was based upon designing and building aircraft satnav devices, which is why their handheld satnav devices for use on land carry some of the legacy from their aircraft models.
SBAS (Satellite Based Augmentation Systems) has been of limited use for handheld satnav devices, although this situation is changing as a larger footprint across the surface of the Earth is created by different Nation States making their SBAS available. In reality, development such as Predictive Ephemeris (Garmin’s proprietary name is HotFix) has improved accuracy better than SBAS in handheld satnav devices.
So the facility to turn off SBAS on handheld was probably more important, which is why Garmin gives users the choice of the different systems.
My understanding is that satnav receivers will use the correction information, supplied by any of these SBAS satellites, if the Nation State(s) has made the code available and this currently includes WAAS/EGNOS and MSAS.