Written by Maud Barthelemy
20/06/2006
Modified by Maud Barthelemy
29/01/2007
This file describes 10 typical Venus Express modes of observations (called
Òscience casesÓ). They are different in goals, geometry of observations, and
experiments involved.
Case 1: Observations in pericentre.
-----------------------------------
Science goals:
- High spatial resolution spectroscopic and imaging survey of the
Northern hemisphere;
- Moderate spatial resolution observations of low latitudes;
- Plasma and magnetic field observations in the planet vicinity.
Payload operations:
- The +Z looking experiments (PFS, SPICAV, VIRTIS, and VMC) perform
observations during 95 minutes around pericentre that corresponds to
spacecraft altitude of less than 10,000 km.
- ASPERA and MAG operate along the entire orbit.
Cooling of the instruments will be done before entering the pericentre arc if
needed.
Spacecraft operations:
- During the pericentre pass the spacecraft will keep +Z axis (that is
co-aligned with the optical axes of PFS, SPICAV, VIRTIS, and VMC) in
the local nadir direction.
- The Y axis will be kept in orbital plane with velocity face (+Y or ÐY)
selected so that to keep the ÐX wall in the shadow.
Out of pericentre phase the spacecraft will keep the HGA Earth pointing. The
telecommunication phase is preliminary placed between 2 and 11 hours orbital
time (OT) in the visibility of the Cebreros antenna.
Case 2: Off pericenter observations
-----------------------------------
Science goals:
Off-pericentre observations by Venus Express are of key importance for
achieving the mission goals especially in what concerns atmospheric dynamics.
The imaging instruments (VIRTIS and VMC) have angular resolution high enough
to provide high quality observations of the planet even from apocentre. The
main scientific objectives of such observations are the following:
- Global view of the Southern hemisphere;
- Global spectro-imaging of the planet with emphasis on atmospheric
dynamics;
- Plasma and magnetic field investigations from a distance.
Payload operations:
- The +Z looking experiments observe Venus on ascending branch of the
orbit between 15h and 23 h OT from a distance ranging from 66,500 km
(apocentre) and ~10,000 km (beginning of the pericenter arc). VIRTIS and
VMC continuously observe the planet. SPICAV observations consist of 5 min
slots evenly distributed between 15 and 23 hours OT. Broad field of view
of PFS limits observation range to a distance of <25,000 km from the
planet. So in case #2 PFS observes only between 22 and 23 hours.
- ASPERA and MAG operate along the entire orbit.
Spacecraft operations:
- The spacecraft keeps the +Z axis nadir pointed.
- The Y axis is kept normal to the Sun direction (yaw steering) to
optimize batteries charging and to avoid sun illumination on the +/-Y
walls. The ÐX wall (with cryogenic radiators) is always kept in shadow.
From time to time (typically once per hour) the solar arrays will be
re-oriented around Y axis to optimize generated power. This attitude will
also provide comfortable thermal environment for the payload (ASPERA in
particular).
Case 3: Global spectro-imaging from apocentre by VIRTIS
-------------------------------------------------------
Science goals:
VIRTIS global spectral mosaic of Venus to study the chemistry and dynamics of
the southern hemisphere.
Payload operations:
Since the VIRTIS field of view is rather narrow the global mosaic of the
planet from apocenter vicinity (> 60,000 km) would require nine re-pointings
of the spacecraft. Duration of complete VIRTIS mosaic (including spacecraft
re-pointing each of which is assumed to be ~1 min) is estimated as follows:
T = Timaging + Trepointing = 9*640s + 8*60s = 6240s = 1.7 hours.
VMC will take a sequence of supporting images with a time step of about 10
min.
SPICAV is operated in Nadir mode during 5 min for each spacecraft pointing.
This orbit type should be repeated in the next 5-6 orbits in order to
reconstruct the pattern of atmospheric dynamics.
ASPERA and MAG operate during entire orbit.
Spacecraft operations:
- +Z axis is near Nadir pointed.
- the Y axis is kept normal to the Sun direction (yaw steering) to
optimize the SA power and to avoid sun illumination of the Y radiators.
The ÐX face is kept in the shadow. During this session spacecraft
performs 9 re-pointing about 4 degrees each.
Case 4: VeRa bistatic sounding
------------------------------
Science goals:
Study of the surface properties of selected regions by the VeRa active radar
sounding.
Payload operations:
The operation consists in
- pointing the HGA1 to a specified target on the surface of Venus.
- emitting simultaneously in S-band and X-band in carrier only mode (ONE
WAY-mode) with the same transponder.
The reflected microwave beam shall be received at the ground station
(specular reflection). These observations require the 70 meter DSN antenna
because of weakness of the reflected signal.
Preferable conditions for the bi-static sounding are defined as follows:
1) Venus is close to Earth (between quadratures);
2) the spacecraft is close to Venus;
3) signal that reaches the Earth is not corrupted by solar coronal radio
noise.
These requirements result in that the most appropriate season for bi-static
sounding is close to quadrature in June-July 2007.
Bi-static sounding is carried out between 00:30 and 1:45 OT. Selection of
surface targets should be provided by the VeRa experiment. Each observation
will last for ~ 30 min preceded and followed by 15 min ground station
calibration. The other instruments except for ASPERA and MAG will not operate
during the VeRa bi-static sounding.
Spacecraft operations:
- HGA1 local nadir pointing.
- The Y axis is kept close to normal to the Sun direction (yaw steering)
to optimise the SA power and to avoid sun illumination of the Y
radiators. The ÐX face is kept in the shadow.
Case 5: Stellar occultation by SPICAV
-------------------------------------
Stellar occultation technique is an effective tool to study the upper
atmosphere from orbit. It will be applied at Venus for the first time.
Science goals:
Study of the composition and structure of the upper atmosphere of Venus by
SPICAV in stellar occultation geometry.
Payload operations:
Stellar occultation can be observed from any portion of orbit. Although dark
limb in stellar occultation is preferable, the observations of the day limb
are also foreseen. The list of bright stars and corresponding occultation
seasons will be provided by the SPICAV experiment.
The observation includes two stellar occultations at the entrance and exit
limbs. Each occultation phase lasts for 5 min. The entire session starts at
22:15 and ends at 23:15 OT. SPICAV, VIRTIS, and VMC are ON for this period
and take measurements during occultation phases.
Spacecraft operations:
- +Z s/c axis should point to the selected star before entering the limb
and keep inertial pointing during the entire observation session, i.e.
till the end of observations on the exit limb. The +Z axis was assumed
to be in orbital plane.
- The Y axis is kept normal to the Sun direction (yaw steering). The ÐX
wall is kept in shadow.
The angle between +Z axis and sun direction must exceed 30¡ for proper SPICAV
and VIRTIS operations.
Case 6: Solar occultation by SPICAV/SOIR
----------------------------------------
Science goals:
Study of the composition and structure of the upper atmosphere of Venus by
SPICAV/SOIR in solar occultation geometry.
The second possibility to study composition and structure of the upper
atmosphere is to use solar occultation opportunities by SPICAV experiment and
its very high spectral resolution channel SOIR. These observations will be
carried out in certain ÒseasonsÓ.
Payload operations:
Each orbit within occultation season will have two observation phases, one at
eclipse entry and one at exit. The SPICAV/SOIR operates during both phases.
- SPICAV solar port (in the +X/+Y quadrant, 60¡ from +Y axis) points to
the Sun.
- The SPICAV slit (which is parallel to the s/c X axis) should be
parallel to the limb.
ASPERA and MAG will operate during entire orbit. All other experiments are
either OFF or in sleeping mode.
Spacecraft orientation:
The spacecraft keeps inertial orientation SPICAV solar port pointing to the
Sun during the entire eclipse phase. The orientation around the sun axis is
dictated by the SPICAV requirement to have the slit parallel to the limb. (In
other words the limb should be in the XY plane). The s/c +Z axis is south
oriented.
Case 7: Limb observations
-------------------------
Science goals:
Study of the composition and structure of the atmosphere, ionosphere, and
hazes in limb geometry by the +Z looking instruments.
Payload operations:
- The observations will be made close to pericentre.
- The +Z looking instruments will be switched ON before entering the limb
and will observe the entrance limb, the planet, and the exit limb. At the
distance to the limb of about 2000 km, they have the following spatial
resolution: PFS Ð 70km; VIRTIS-H Ð 0.6x1.8km; VIRTIS-M Ð 1.4 km;
VMC Ð 1.4 km.
The observation phase includes entrance and exit limbs and the planet between
them. The whole session lasts from 23:30 till 24:00. During the observation
session PFS, SPICAV, VIRTIS, and VMC operate simultaneously.
- ASPERA and MAG are ON during the entire orbit.
Spacecraft operations:
- Before entering the observation phase the spacecraft will be oriented
so that the +Z axis points to the limb on one side of the planet. The
spacecraft will keep inertial pointing so that the +Z axis would first
scan the entrance limb, then it will traverse the planet, and finally
will leave the disc at the exit limb.
- The X axis is parallel to the limb thus fulfilling the pointing
requirements of SPICAV (slit parallel to the limb) and VIRTIS (slit
perpendicular to the limb). The ÐX face is kept in a shadow. The angle
between the +Z axis and the Sun direction must be higher than 30¡ for
proper SPICAV and VIRTIS operations.
Case 8: Earth radio occultation by VeRa.
----------------------------------------
Science goals:
Study of the fine structure of the neutral atmosphere (density, pressure,
temperature vertical profiles) and ionosphere (electron density) in the Earth
occultation geometry. This type of observations will be possible in specific
seasons.
Payload operations:
The VeRa operations consist of pointing the HGA1 to Earth and emitting
simultaneously in one-way S-band and X-band carrier only mode with the same
transponder. This operation is done during New Norcia and DSN antennae
visibility (Cebreros will be only an X-band station).
The VeRa operations includes :
- one calibration phase (Earth pointing without correction);
- a 1st operating phase with attitude correction profile and, depending
on the detailed orbit geometry, a slew maneuver;
- a slew maneuver;
- a 2nd operating phase with attitude correction profile;
- lastly, another calibration phase PFS, SPICAV, VIRTIS, and VMC are OFF
during the Earth occultation.
ASPERA and MAG are ON entire orbit.
The efficiency of VeRa sounding would be significantly higher at small
distance to the Earth. The use of the large high gain antenna (HGA1) is
mandatory for these observations. Between quadratures, however, this would
result in sun illuminating of the ÐX (ÒcryogenicÓ) wall.
Spacecraft operations:
The spacecraft points the HGA1 to the Earth before entering observation
phase. A specific slew maneuver to compensate for atmospheric refraction and
to keep contact with the ground station is required at both entrance and exit
limbs. Attitude profile during observation is specified by the PI. The s/c +Z
axis points toward the planet so that the +Z looking instruments could
perform observations of the planet in the same session.
Case 9: VeRa sounding of solar corona in conjunctions.
------------------------------------------------------
In conjunctions the radio signals sent by the spacecraft to the Earth
propagate through the solar corona and can be used to study its large scale
structure.
Science goals:
Study of the solar corona properties by measuring fluctuations of the radio
signal propagating in the vicinity of the Sun.
Payload operations:
The operation consists of pointing the HGA1 to Earth, and transmitting
simultaneously an S-band and X-band carrier (telemetry OFF) with the same
transponder working in coherent mode. The uplink is kept in X-band (instead
of S-band) to be compliant with the foreseen TC link onboard monitoring
(TLRAP) which deals only with X-band reception configuration of the TT&C
subsystem. This operation is done during New Norcia visibility (Cebreros is a
X-Only Ground station). Science operation starts at 16:00:00 and ends at
22:00:00.
There are two different configurations:
- Superior conjunction (Earth-Venus-Sun angle < 10 degrees)
- Inferior conjunctions (Earth-Venus-Sun angle > 170 degrees) : for this
particular case, a switching from HGA2 (used in communications) to HGA1
(used for VeRa operations) is necessary for the S-band uplink and
downlink operations.
Instruments on the +Z looking platform are not operating.
ASPERA and MAG are ON entire orbit.
Spacecraft operations:
The spacecraft keeps the HGA1 pointing to the Earth during the observations.
Case 10: Study of the Venus gravity anomalies by VeRa.
------------------------------------------------------
Science goals:
Study of the Venus gravity anomalies.
Payload operations:
The operation consists of pointing the HGA1 to the Earth, and in emitting
simultaneously in S-band and X-band in carrier only mode with the same
transponder working in coherent mode. Science operations start at 23:45:00
and end at 00:15:00.
The +Z looking instruments (PFS, VIRTIS, SPICAV, and VMC) operate
simultaneously when the +Z axis crosses the limbs and the planet.
Spacecraft operations:
The observations are performed close to pericentre with the spacecraft not in
Earth occultation condition. The spacecraft keeps the HGA pointing to the
Earth. The +Z axis points to the planet so that the +Z looking instruments
could perform observations simultaneously with VeRa.
Case N/A
--------
VMC uses the "N/A" value for data products resulting from observation during:
- Earth pointing;
- Wheel Off Loading (WOL);
- Orbit Manoeuvre;