The cabin data displays were called "PDADS", which meant something like Passenger Display Automatic Data System (or maybe Passenger Data Automatic Display System) - a bit of an innovation at the time. The -200 was an almost wholly analogue aircraft, and I'm unsure where the speed data came from. Altitude, easy, from the ATC transponder, but we had no readout of groundspeed* in the cockpit. Some of the guys were dab hands at timing the rate of change of the DME to measure groundspeed, but I don't think that fed the PDADS, as it's obviously only accurate when the beacon is on track.
* Further thought gets me to recall that the Omega had a groundspeed readout, and eventually most (all?) of the fleet was so fitted. However, Omega as fitted by BAL was somewhat erratic in its performance. When the Quebecair aircraft were leased in, their Omega behaved impeccably. We then learnt that the BAL installation was cheap and not so cheerful, using the ADF sense aerial to feed the Omega, whereas Quebecair's sets had dedicated aerials.
Last edited by kenparry; 28th Aug 2021 at 13:21.
Reason: Note on Omega.