VEX Science Planning Options

[cndwrld]

Hello-

I wasn't sure how most people would like to see stuff posted. Or if this would interest many people. But in the interests of putting out more information, I created this topic as a place to put information on Venus Express science planning.

If you have any ideas about this, let me know.

Cheers-

Don Merritt

[cndwrld]

VEX Planning and Status on 26 April 2007

I haven't been real consistent about what topic I use to put this information
out there. I'll just put it all in this one from now on.

A couple weeks ago, special operations were done with VIRTIS to make movies
of the South Pole of Venus. For this, the spacecraft telemetry downlink
rate was increased, and the DSN Madrid (DSS-43) station was used to
capture the high volume of data. The TM bit rate is now back to 154 kbps.
The preliminary version of the VIRTIS movies are very cool, but will
require a lot of cleaning up before release.

At DOY 104, Venus Express was orbiting Venus at 169 million km
from the Earth. The one-way signal travel time was 564 sec. Everything
continues to work just fine. However, the spacecraft seems to be getting
into its adult years. We seem to be getting more one-off, weird things
occurring. Things like an alarm going off for one frame of telemetry due
to some weird number, but it doesn't recur and everything is fine. Or
something sticks once, but never repeats itself. Or the data from an
instrument gets some strange hiccup, which self corrects. This sort of
thing is common on spacecraft, which get old in pretty much the same
way we do. It starts with little creaks and unexplained pains. But for
now, everything is fine, we just aren't as young as we used to be.

Below, I try to provide some detail about the observing plan we are about
to finish (MTP013), and the one that is about to start (MTP014). MTP015
has been scheduled and approved, and is waiting for its turn. MTP016
planning will be finished next week. MTP017 planning starts today. MTP018
is currently being looked at for long-term, early analysis.

If I haven't made anything clear that you're curious about, let me know.

MTP013
======
We are currently executing the last week of MTP013, our thirteenth monthly
observing plan. I don't believe I put anything down about what we've been
doing in this MTP.

The definition of science cases is shown down in MTP014 section, and in
earlier posts.

MTP013 is included in the mission phases 6 (until orbit 369) and 7. This
MTP provided good conditions for observations of the night side and
atmospheric sounding in solar occultation geometry (eclipse season 3). Solar
occultation observations will be used to study composition and structure of
the atmosphere above the cloud top. Campaigns of Cases #2 (no Cases 3 are
possible in this MTP) will be used to study composition and dynamics of deep
atmosphere on the night side. Conditions were also favourable for
observations of nightglows to study composition and dynamics of the
thermosphere and search for lightning. Limb observations in forward
scattering geometry (spacecraft in eclipse) will provide good opportunity
to study vertical structure of hazes above the main cloud. Relatively
high downlink rate allow the experiments to use high resolution
operation modes to study small scale features and dynamics. Thermal mapping
of the surface and search for active volcanism will be performed. This
MTP also included an Earth occultation season.

The following observations were done in this MTP:

• solar occultation measurements (Case #6) were given high priority
in order to achieve good latitude coverage. This will cover the themes:
o Cloud layer and haze
o Atmospheric composition
o Atmospheric dynamics
o Atmospheric structure
o Escape processes.

• limb observations (Case #7), to study vertical structure of haze
layers. This will cover the themes:
o Cloud layer and haze
o Atmospheric composition
o Atmospheric dynamics.
For a limb observation, we point the SPICAV instrument at a star, and it then
follows the star as it scans the entrance limb, traverses the planet and then
leaves the disc at the exit limb.

• Plasma and magnetic field measurements on the nightside and on the
dayside, covering the theme Plasma environment/escape processes.

• Cases 2 at apocenter and Cases 1 at pericenter covering basically all
themes.

MTP014
======
We start MTP014 next week. MTP014 is part of the mission phase 7, which
includes an Earth occultation season. Proximity to the Earth creates excellent
conditions for bi-static sounding and radio-occultation experiments that
can reach maximum sounding depth. A very good downlink data rate is
available, allowing the study of the atmosphere with high spatial resolution.
The terminator sector of the planet will be available for observations in
this phase. Thus cloud structure and atmospheric dynamics will be important
goals.

This will take place during the end of the so called COLD SEASON. The
terminator crossing is around the 31st of May.

The time span of the MTP014 operations is the follow:
START: 06 May 2007 (ORB 380)
END: 02 June 2007 (ORB 407)


During this MTP, the flip from HGA1 to HGA 2 will occur on June 1 (orbit 406).

The official ground-observation campaign starts on May 23 (orbit 397). To
our knowledge, the following observations will be performed:
- Wind measurements by IRTF: 18-21 May (orbits 392-394)
- Winds measurements by Kitt Peak: 23 May to 9 June (orbits 397-414)
- Atmospheric composition by AAT: 31 May to 6 June (orbits 405-411)
Due to the complexity of this MTP, no complicated pointings were
considered.

The following observations were planned in this MTP:

• radio-occultations experiments (Case #8) will be conducted, covering
the following science themes: Atmospheric dynamics, atmospheric structure,
atmospheric composition and plasma environment/escape processes.

• stellar occultations (Case #5) are planned. This will cover the
themes:
o Cloud layer and haze
o Atmospheric composition
o Atmospheric dynamics
o Atmospheric structure.
o Atmospheric structure.

Cases 7 or 1 could also be planned instead.

• Plasma and magnetic field measurements covering the theme Plasma
environment/escape processes.

• long Cases 2 at all apocenters, and Cases 1 at
pericenter are considered, covering basically all themes.

The final pointing plan was:

Orb # apocenter Pericenter
science Case science Case
====== ============ ============
380 2 8 Case 1: around pericenter, nadir looking
381 2 8 Case 2: Ascending branch, nadir looking
382 2 8
383 2 5+1 Case 3: from apocenter, nadir looking
384 2 8 Case 5: stellar occultation
385 OCM 2-7 Case 7: stellar occultation through limbs
386 2 8 Case 8: radio science occultation
387 2 5-7-2
388 2 8 OCM: orbit correction maneuver,
389 2 5-7-1 regularly scheduled
390 2 8
391 2 7-1
392 3 8
393 2 7-1
394 2 8
395 2 7-1
396 2 8
397 2 7-1
398 2 8
399 2 8
400 2 8-1
401 2 8-1
402 2 8-1
403 2 7
404 2 8-1
405 - 8-1
406 - 8-1
407 - 7-1

The last 3 weeks of MTP014 (from Orb 386) run in the Quadrature
Period. Technically, the quadrature phase is defined as the period during
which the Sun-Spacecraft-Earth angle is between 75 and 95 degrees, requiring
a swap to High Gain Antenna (HGA) 2 for Earth communications in order to
avoid illuminating faces of the spacecraft not designed for such exposure.
But in our loose usage of late, it has come to refer mainly to the period
when we require changes to the spacecraft operations to avoid excessive
VMC illumination.

Due to revising operating constraints on the VMC camera, in quadrature phases
the default Earth pointing orientation results in unacceptable illumination
of VMC. To avoid that, the nominal Earth attitude guidance was modified
by adding a rotation of the S/C around the HGA axis, such that a constant
+10deg illumination is achieved in the +Y panel (the least sensitive
to continuous heating, of the panels which are not supposed to be heated).
This is actually implemented onboard by commanding
fake ephemeredes (i.e. with non-real S/C and Earth positions). By proper
selection of the fake ephemeredes, the distortion of the Earth pointing
guidance computation on-board achieves Earth pointing with the HGA and
a tilt angle around the HGA axis. The Flight Dynamics team and the Flight
Control Team at ESOC in Darmstadt, Germany, are finalizing their planning
to initiate this. They have, as usual, done excellent work on this, and
no problems are anticipated.

As MTP014 is mainly in COLD season, long Cases 2 will be possible, with cold
Cases 1 until pericenter. The Earth communication phase stop times do
not allow Cases 3 to be done at apocenter. Inertial cold pointings
(Cases 5 and 7) can be done after pericenter. A pericenter change (number 3)
orbit change maneuver (OCM) is scheduled.