Nigel, you seem to be talking about measuring the actual servo pulses which the receiver outputs on each of the servo channels. As you say, this is a rather easy thing to do, you just time the pulse widths to get servo position.
Harvey is talking about reading the PPM signal, which would be an internal encoded signal containing all the same info, just in a PPM format - ie, the actual data that is coming over the radio link, and is then decoded/reencoded to the servo pulses on each channel. This format does not really sound extremely complex to interpret with a microcontroller either (you're just timing the delay between each of a number of fixed-width pulses, instead of timing the width of a bunch of fixed-delay pulses), and would mean you'd only have to monitor one signal, instead of multiple signals (one for each servo channel) but would require hacking into the reciever and finding this signal among the traces.
I haven't looked too deeply at the real details of the PPM scheme, but I believe that essentially you just time the duration between each of the pulses in the string, and each of the inter-pulse delays corresponds to one of the channel positions. When the sticks are centered, there will be a certain delay. If the delay for a given channel is higher or lower than its 'centered' value, you know which way to move. (probably better use a threshold so the stick doesn't have to be PRECISELY centered to get the motors to stop) In addition, because the time delays represent positions, it shouldn't be hard to run your servo motors at a speed proportional to how far the stick was moved, giving you better control, if you wanted.
HarveyH42 said:
This is kind kind of weird, controlling a digital device, with an analog signal. Not a normal application, but could turn out to be very useful for other things later on.
Yet, if you stop and think about it, what's inside the servo is a digital circuit controlling the motor, using a the voltage of a potentiometer attached to the servo output shaft as feedback. so really, it's an analog input controlling an analog output, with a digital interface in between just because it's suitable for radio transmission.