PCM with the error detection and failsafe memory switched off will suffer the same glitches and unpredictable flight pattern from external interference that a PPM without failsafe will do. It is failsafe that makes the difference and it can be built in to both PCM and PPM.
Harry, the only confusion here is your belief that a failsafe scheme based on PPM and PCM will behave the same way. PCM is binary in nature and error detection can be made within a SINGLE frame (bitsteam of data) to do the same with PPM you will need several frames of data before you can make any assumption of errors. Also a valid PCM frame can be recovered IMMEDIATELY wheras a PPM Signal would have to be consistent for many frames before you can make that assumption. On top of that errors in PPM are timing dependent which can be introduced due to the data even within the RX. This is NOT the case with PCM which is why it is used in almost all other forms of communications that requires low data rate errors. It the Mars landers used PPM they would never had made the news...
Sure you can have fancy software filtering which appears to behave the same in tests but under dynamic conditions it WILL be worse. The reason mnfs are employing these scemes is taht under ideal conditions you can have more resolution and more datachannels that with PCM for the same bandwith required for narrow band systems.
Andre Baird