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You just gave me a thought. GPS mode is designed to grab as many satelites as possible to ensure stability in any direction or hovering. As you indicated, you can do high speed with satelites turned off and still obtain stable flight. I wonder if the P4 cuts the number of satelites to 4 when in "S" mode so that speed is maximized yet just the essential satelites exist? Just a thought.
Speed has nothing to do with satellites. Even in P-GPS mode when you are moving the P3 is tracking satellites but it's effectively in ATTI mode as GPS is not controlling your movement (motors). At that point it's just the accelerometers and gyroscopes helping you keep a level flight as it relates to your stick inputs (forwards, backwards, sideways etc.) just like any drone without GPS.
This is evident as the P3 will drift with the wind during movement even when in P-GPS mode. It's only when you stop while in P-GPS mode that the P3 uses GPS to control your hover in place even in wind. GPS locks your movement in place.
The speed restriction in P-GPS is purely a DJi programming limit. When in GPS limit speed. That limit is not in place with ATTI mode. You can flip to ATTI mode and ride a tailwind well past 40mph but you cannot while in P-GPS mode. In ATTI you are still tracking the same number of satellites it's just when you stop GPS isn't locking your movement while hovering. This is evident as RTH continues to work while in ATTI mode if you have good satellite locks.
S-Mode with the P4 merely allows the P4 to angle at a higher degree (45 degrees) than the P3 was ever able to and DJi removed the 35mph restriction that was there in the P3 in P-GPS mode. So you get to go faster up to a point I imagine. DJi probably is limiting S-Mode speed as well on the high end but until we get our hands on the P4 we won't know what that limit is if any. As far as we can tell S-Mode acts just like P-GPS mode in that when stopped the P4 is GPS locked in a hover. The P4 has ATTI mode as well but we won't know how it works in relation to S-Mode as far as top speed is concerned.