There is nothing wrong with using single sensor, STS also relies on single air speed signal. The key is to limit control authority so that elevator still can overpower stab in fault condition
From the Bloomberg article:"During the crashes of Lion Air and Ethiopian Airlines planes that killed 346 people, investigators suspect, the MCAS system pushed the planes into uncontrollable dives because of bad data from a single sensor.
That design violated basic principles of redundancy for generations of Boeing engineers, and the company apparently never tested to see how the software would respond, Lemme said. "It was a stunning fail. A lot of people should have thought of this problem - not one person - and asked about it", he said."
There is nothing wrong with a single sensor?! I believe I don't understand you. A single sensor is what cost 346 people their lives.
The single sensor STS thing seems to come up periodically as some sort of justification for changing to a single sensor MCAS or avoiding having a two sensor MCAS post V0.0. So I thought I'd have a look at STS a bit deeper to try to understand it. Surprisingly, there seems to be little information out there especially regarding technical detail.
I found it very difficult to understand what STS is actually doing and why. However, I felt in good company when reading back on some of the other forums to find that many 737 pilots don't quite understand what it is doing or why either. Some of these were just constantly concerned STS appeared to work against their intentions, some said they just ignored it under the assumption it was doing something for a reason and at least one said he purposely countermanded STS whenever it operated, presumably by blipping the thumb switch.
As near as I can get to it, STS is intended to maintain the most efficient and stable pitch and neutral stick force for a given Computed Airspeed, so if you are at a ~constant speed and everything else is equal it will stick at a stabiliser position it has chosen. During acceleration or deceleration, it will adjust the trim to try to maintain the previous speed measurement, and the next iteration the previous speed measurement ad-infinitum that is what gives the impression of acting against the pilot because it is effectively slugging the accel/decal with AND or ANU and contrary stick force. It would also appear that STS cannot move the stabiliser beyond a maximum of ~6deg of travel from 0 in ANU direction at the maximum Computed Airspeed it is operational.
So, I would guess, if the Airspeed sensor fails low within valid range (bearing in mind Computed Airspeed is also slugged by other parameters within the ADIRU and note that AOA is just converted to digital effectively raw) could, at high actual Airspeed, pull the Stabiliser in AND from 6deg to 0 or conversely fails high, within a valid range, at low actual Airspeed could pull the stabiliser up to 6deg ANU (at the appropriate rates) also bearing in mind, if it fails high beyond the STS operating range, STS will be inhibited. I am not sure how the degrees of travel relates to the actual trim measurement expressed in units but I would have though 0 degrees in this context would be trim setting in units at T/O ~5units?
However, as I understand it (no detail I could find), the STS algorithm is also moderated by Thrust and - Vertical Speed - I.e. 'G' (where have we heard that before). I make a presumption that STS will assume for high thrust that acceleration or deceleration in the case of low thrust is intended and so moderate its trim correction accordingly and similarly will moderate trim correction ANU at higher +G or AND for higher -G. These are protections that appear to be missing from MCAS V1.0. Of course, over correction by STS can also be countermanded by pulling the yoke through the ANU or AND microswitches and only one of these is present for MCAS. Or Thumb switch available for both.
In conclusion, my view would be that single sensor STS is an over simplification and can not be used as justification for single sensor MCAS. I could be completely wrong of course, if so I am sure someone will put me right.