RE: 150 meter rule.
nonstoprc
The ET GPS specs state position accuracy of 2.5m (8.2 ft) and speed accurace of 0.4 m/s (0.9 mph). I've found the repeatability suitable for pattern use. When using the pressure based altitude sensor the absolute altitudes may not always agree with the GPS data, but the delta is consistent.
The GPS uses dual antenna to accomodate rolling / inverted flight and the pressure system can be affected by pressure changes at the pitot reference port when the airplane is pitching / rolling. So, there is a little noise in the data. With the large volume of data available it's not hard to smooth it so as to see what's going on with the airplane.
For example, in figure M it's real easy to measure the entry / middle / exit altitudes and the max altitude of each leg close enough to see errors and make adjustments in one's flying until the errors disappear. My observation is that large errors recorded with the ET are also noted by judges and the judges precision isn't generally as good as the ET. So - all in all - a useful tool for working on geometry and position. Can't beat a good coach for S & G.