[X] My Assistant

[ngunn]

I was struck by the first paragraph on p.166 relating to Huygens:

The SSP tilt sensors and the HASI accelerometer both seemed to indicate the probe was tilted by several degrees relative to the local vertical. But the horizon on the DISR images was pretty horizontal. It seemed unlikely that the ground would be sloping so steeply. Were some of the sensors wrong - and if they were, why did they agree with other factors? Maybe the probe had bent out of shape, changing the relative alignment. In fact, it would be impossible to know.

Reading this makes me want to view for myself the surface image oriented as the instruments indicate it should be, rather than on the assumption of a horizontal skyline. Just how implausible would it look? Presumably the imaging team did this, but perhaps that was before we knew how surprisingly high some of the 'pebble banks' near the landing site are. We only see one short section of the skyline after all.

Could Ralph or anyone provide a version of the image rotated accordingly?
EDIT - Or a version of the surface image annotated with the position of the theoretical horizon determined from SSP and HASI?

[rlorenz]

I was struck by the first paragraph on p.166 relating to Huygens:

[i]The SSP tilt sensors and the HASI accelerometer both seemed to indicate the probe was tilted by several degrees relative to the local vertical. But the horizon on the DISR images was pretty horizontal.

Wow, seize upon the least interesting thing from the whole encounter........ The DISR images were studied to
death by Erich Karkoschka - see his Planetary and Space Science paper, from which I quote below

8.3. Attitude of Huygens after landing
The best constraint from DISR about the rest attitude
comes from the comparison of the sky brightness in SLI
images taken before and after landing at similar azimuths,
taking into account the gradual decrease of brightness with
altitude. This yields a pitch after landing of 3  1 and a
roll within a few degrees of zero.
More accurate are measurements by the SSP tilt sensors
(Lorenz, private communication). Since there are concerns
that one of the tilt sensors may have had a constant offset,
we will only use relative tilt data here. For the last 20 SLI
exposures, we measured an average pitch of 0:3 and an
average roll of 0:3. These data are the most reliable
because of the visibility of the horizon in these images. The
average SSP tilt data for these 20 instances are 0:3 for
TIL-X and 7:6 for TIL-Y (Lorenz, private communication).
Considering the directions of the tilt sensors, we used
this relationship between pitch/roll and SSP tilt data to
convert the SSP tilt data measured after landing, which are
2:0 for TIL-X and 8:6 for TIL-Y, into pitch of 3:1
and roll of 0:9. This means DISR is 3:1 looking up and
0:9 rotated clockwise in the viewing direction. These data
are estimated to be accurate to about 0:5. They are
consistent with the data using the sky brightness. In other
words, SSP detected a change of average tilt of 3 at
impact, which corresponds to a decrease of pitch of 3 and
an increase of roll of 1. Since pitch and roll averaged near
zero before landing according to our investigation, they
must have been 3 and 1, respectively, after landing.
Because the horizon on SLI images after landing seems
to be depressed by 1:5, and because the pitch was 3:1,
the horizon was actually raised about 1:6, which indicates
moderate relief south of the landing site. Because the
horizon in SLI images after landing does not seem to be
tilted either way, but the roll was 0:9, the horizon actually
was sloping up toward the left by about 0:9, a very
gentle slope. Images taken before landing suggest higher
hills to the east and lower hills to the west, simply judging
from the contrast of features, which is consistent with such
a slope.

As for rotating images, they are on the PDS.... have at it.

[Stefan]

Wow, seize upon the least interesting thing from the whole encounter........

Historical note: Back then this was considered a very interesting discrepancy leading to heated debates, with the SSP team gradually adjusting to (the uncomfortable) reality.