Middle position of the S1 switch does nothing. It's a three position switch that was employed for a function that only requires a 2 position switch. Perhaps done this way to reduce the number of different parts needed to be maintained in inventory.
S1 controls the maximum elevation of the gimbal, with the upper position of the switch named "Angle" and the lower position named "Velocity". The upper position, Angle, reduces the maximum elevation achievable and is a tremendous assist to the user in keeping propellers out of the video/image frame. Use it for most of you lower speed imagery work. Velocity permits the camera to position lens elevation a little higher and is best used when flying at high speeds. Most will use this position but infrequently. If S1 is moved to the middle position there will be no change in camera elevation from the previous position it was in. The switch has to move to either the top or bottom position to effect a change.
As for GPS lock, there is no way to establish state of signal strength or time of acquisition via the ST-16. Even when using the GUI, satellite strength is only displayed in vertical color bars. The taller the bar the better the signal. The ST-16 provides two screen locations to view satellite count, one for the ST-16 and one for the aircraft. The one for the ST-16 is located at the top right corner of the view screen between the battery indicator and the time. The aircraft satellite count is displayed in the most left vertical "aircraft state" column of icons. Our only other gauge of GPS state is the indicator light at the back of the aircraft.
Although not asked for, a word of caution. If contemplating flight inside of a building, do not rely on the displayed number of satellites to determine if you have a GPS lock adequate for flight. Unless you are flying in a fabric construction tent GPS will not be dependable. When inside a structure the number the GPS satellites displayed represents a very weak and unreliable GPS signal.