I'm trying to understand the details of the 1972 Viking BLDT test missions (in earth's atmosphere). I found this diagram in a book and the general description that the (four) BLDT (balloon drop) tests were done at altitude of 36 to 43 km in order to simulate the density of Mars' atmosphere.
From the diagram it's clear that the retro rockets of the AV (Viking test vehicle) were fired; actually the text says that only in 3 of the 4 tests this was done. But it's not clear at what altitude the retro rockets were fired. From the diagram it looks like the BLDTs weren't a complete retro-powered landing done as it would later be done in the actual mission e.g. as Wikipedia says about Viking 1
At 1.5 km (0.93 mi) altitude, retrorockets on the lander itself were ignited and, 40 seconds later at about 2.4 m/s (7.9 ft/s), the lander arrived on Mars with a relatively light jolt.
The BLDT tests seem to have fired the retro rockets sending the AV high up again, from which it then descended only with its parachute... and even landed "upside down" if the diagram is correct. What was basically the point of this BLDT test sequence, which seems different from the actual Mars landing? The text doesn't clearly say at what altitudes the retro rockets were fired during these BLDT tests, which might shed light on this.