# Solar Orbiter -- Payload Working Group

## Action Completion Form

***[Action ID Number:]{.underline}*** 10.9

***[Responsible Working Group Member:]{.underline}*** R.A. Harrison,
B.Fleck, L. Harra, J.-F. Hochedez

***[Date of Completion:]{.underline}*** Var 1 -- 24/7/2002; Var 2 --
6/9/2002

Var 3 -- 25/9/2002

***[Action:]{.underline}*** To assess the operation scenario and
operations planning of Solar Orbiter.

***[Report:]{.underline}***

1.  Basic Outline

The Solar Orbiter mission has a basic orbit of 150 days with a nominal
'encounter' period of 30 days. The precise definition of the 'encounter'
is yet to be made but, at this stage we must consider the methods for
Orbiter operation and planning assuming two scenarios, (i) a mission
with scientific observations only during the encounter periods, and (ii)
a mission with two observation modes, i.e. during encounter and for the
rest of the orbit.

This mission does not enjoy continuous, high telemetry contact. Indeed,
during the encounter stage, the high gain antenna is stowed in the
shadow of the spacecraft. We must, therefore, assume no scientific
instrument contact during the 30 day encounter. This demands two things:

-   *The spacecraft (or, indeed, the instruments) must carry sufficient
    on board memory to hold at least 30 days worth of observations at
    the nominal telemetry rates.*

-   *Observation planning for the encounter must be determined and
    pre-programmed before the encounter starts.*

The first bullet implies that at the nominal telemetry rate of the
instruments (74.5 kbit/s), for a period of 30 days, the on-board data
memory had better be of order 2 x 10 ^11^ bits. The Pre-Assessment Study
Report on Solar Orbiter states that a 2.4 x 10^11^ bit (240 G bit)
memory is anticipated. This would be adequate.

2.  Joint Observing Programmes (JOPs)

The instrument observations must be planned using a method similar to
the SOHO Joint Observing Programmes (JOPs) with pre-planned sequences,
which can be stored on board in a deferred command store. A particular
JOP would define the operation of the instrument package, in the pursuit
of a particular scientific question. For example, one might design a JOP
for quiet Sun transient event observation. This would demand that the
remote sensing package be pointed to a quiet Sun area. The EUS
instrument might be run using small-area rasters with a few emission
lines selected, in order to produce rapid cadences. Similarly, the EUI
imager may be requested to make partial field, rapid observations on the
same area. In this way, a complete study is built up and the JOP can be
run through software planning tools to set up the instrument/spacecraft
command sequence for storage in a JOP library. On SOHO, several
instruments have sophisticated planning tools of this type. However, for
Solar Orbiter, since the remote sensing instruments will be operated
together, it would be sensible to have a central planning tool.

Given such a central activity, this is one argument for the need for a
central Solar Orbiter planning and operations facility. This is
discussed later.

For an encounter period, one might expect a number of JOPs to be run;
some may be run over several days, some may require just a few hours.
For example, during one pass, the targets may range from quiet Sun to
coronal hole, with rapid imaging for transient event detection, through
to long exposures for spectral atlas studies.

JOPs should be designed by the user community, much as the SOHO JOPs
are, and should be scheduled ahead of the encounter. They should be
stored in a JOP 'library'.

Given the nature of the mission, we do not have the luxury of repeat
observations. If a JOP fails on SOHO, we can repeat it the next day. [It
is suggested that all JOP sequences be tested in flight during the
non-encounter periods to ensure that the JOP will work during the
encounter passage (when we have no contact), i.e. we must use the
non-encounter periods for test activities.]{.underline} This assumes
that there is contact during the non-encounter periods, which will
require detailed studies of on board memory, telemetry and ground
station use, to ensure that it is feasible with no impact on the
encounter data. The fact that such tests would be sensible is well
illustrated by the example of CDS on SOHO where tests are run on a
proto-type instrument on the ground prior to operation in space; this
allows a thorough analysis of the operation and, in the case or Orbiter
could also supply useful solar data.

There must be a user interface -- perhaps as a Web site -- where the
user community can both request observations and can access information
on planning and, perhaps, the planning tool software. As with SOHO, the
access for the user must be straightforward and open.

Unlike most solar missions, the prime observation periods are short --
only 20% of the orbit. Thus, we may be heavily oversubscribed. This will
require a more formal and rigorous procedure to select and plan
observations than SOHO. It is suggested that there is a call for
proposals for each encounter, and a formal evaluation board. However,
the PI groups must have a role in screening and scheduling incoming
requests.

The selection and scheduling can be done using a central planning and
mission diary facility, which can be little more than a Web site akin to
those used by some SOHO teams. This would allow some discussion and
visible scheduling of observations before the 150 day planning meetings
(see next section).

3.  Planning Meeting Cycle/Operations Facility

There must be a central Solar Orbiter operations facility. The
activities of that facility may cater for a range of operational issues.
At one extreme, the facility may only house the flight/spacecraft
operations team, with no presence from the instrument teams. Solar
Orbiter is not as 'hands on' as SOHO, for example, so this is a possible
option and many scientific planning tasks can be done at home
institutes. On the other hand, such a facility may include an instrument
team presence to cater for all planning meetings, test activities,
scientific evaluation and JOP design and planning; it would certainly be
valuable to maintain an operations facility manned by the instrument
teams. The following discussion outlines the planning and operations
activities without the assumption that the instrument teams must be
present at a central operations facility. The activities are outlined
and possible uses of such a facility are discussed, as are possible
activities from home institutions. From a feasibility point of view,
both options are possible but it is for ESA to decide upon the provision
of a central scientific and operations planning facility based on the
tasks listed in this document.

The principal planning meetings should be held on a 150 day cycle,
possibly held at the PI home institutes in turn, but possibly in the
dedicated operations facility. The instruments are co-pointed, so the
JOP selections should be made in open discussion between the groups, but
consistent with the formal selection process. This is akin to the SOHO
planning meetings but on a much longer time-scale and rather more
formal.

Each instrument team could, in principle, provide commands to uplink for
the coming encounters from their home institutes prior to the encounter.
In this case, they must have real-time access to the spacecraft and
instrument technical data-stream, but this, again, does not demand a
presence at a centrol facility. It does demand good contact with that
centre and it does require that the instrument teams have the ability to
monitor and control their instruments. On the other hand, for security
reasons, as well as for planning co-ordination, ESA may choose to demand
that all commanding and uplinks are limited to activities at the
operations facility.

### We have already mentioned that the JOP planning will be critical to the mission, and must be performed in close collaboration well before the encounters. Face to face planning may be the most efficient way of doing this. In addition, if JOPs can be tested during the non-encounter periods, joint analysis of the operation is required. Also, the formal calls for proposals and the proposal selection and subsequent planning must be done somewhere. Combined with all of this, as well as the 150-day planning meetings, if we require instrument teams to work closely during the lead up to the encounters and during the encounters, we may be able to fully justify a dedicated planning facility. 

The basic scientific outline (JOP schedule, scientific targets) for the
next encounter would be designed at the 150 day planning meeting. This
ought to be held at least 30 days prior to the encounter onset to ensure
that any testing and sequence design can be done well head of time. The
basic pointing should be defined, using projected target areas using the
Sun at the time.

For many targets (coronal holes, quiet Sun) the pointing selected at the
150 day meeting may be fine. However, in the final days before the
encounter there should be a Pointing Review Meeting to refine the
pointing selection and, if necessary, update the pointings given the
state of the Sun. This meeting should not change the JOP structure, just
the pointing.

It is suggested that a basic overall science operations plan could be
designed for the full mission, before launch. This would outline the
basic priorities but include flexibility. It could provide the principal
framework for scheduling the specific JOP activities.

4.  Intelligent Operations

Target selection, as described above, is done using a projected view of
the Sun and, in many cases, this will be fine. However, for some
phenomena, such as active regions or bright points, we will need to
consider options for pointing updates based on on-board measurement.
Again, the JOP structure would not change, but, it should be possible,
for some targets to enable a last minute pointing update based on the
EUI or VIM images. This could only be for limited use but for active
regions and bright points, for example, would seem to be quite possible.
As far as the 150 day planning is concerned, it should be decided at
that time whether of not we need to enable such an update at the start
of a particular scheduled JOP. We must decide at that time whether or
not it is scientifically useful. The precise details of this would have
to be defined elsewhere.

Another 'intelligent' option is the response to 'flags'. It ought to be
possible to store extra JOPs, which are not scheduled but could be run,
using on board decision making, if certain circumstances come about. For
example, if an active region JOP is being run and there is a flare,
could data from the EUI be used to trigger a pre-stored flare JOP? This
option must be considered fully but at a payload level.

This issue must be discussed at mission level. The ability to have a
lookout instrument (e.g. FSI or/and VIM) allows Orbiter to study
rare/intermittent events with negligible (statistical) drawback on other
goals, particularly when on the hidden side of the Sun. Such events
would include flares, but also various CME types, prominence eruptions,
acoustic waves, Moreton/EIT waves, active region morphological
reconfigurations, etc. This concept could extend to the ability to also
trigger special JOPs from ground-based or other space-based (SDO,
STEREO) instruments.

Several issues must be considered here. A thorough study of flag-driven
operations was considered for SOHO, and was implemented. It has never
been used at an instrument-instrument level. We must be sure of the
scientific benefits and the technical feasibility at an early stage and
build it into the Orbiter operations concept. The most basic difference
between the SOHO and Orbiter operations is that Orbiter will be out of
contact for long periods and, thus, a flag operation may be more useful
from a scientific viewpoint; it may also be more risky! Thus, a full
study is required. Having said that, instruments such as CDS on SOHO use
pre-stored JOPs which can interrupt on-going JOPs under certain
circumstances. This has been a well used concept.

As another 'intelligent option', Solar Orbiter on-board processing for
event recognition may have additional benefits: it could allow data
recording to recover pre-event observations. Such a technique could be
used to maximise the scientific return by careful management of the on
board memory and the telemetry. Such an option needs careful study.

5.  The Rest of the Orbit

The non-encounter periods of the orbit are considered to be periods when
data are trickled back to Earth from the encounter observation.

Also, as suggested above, such periods should be used to test JOPs prior
to encounters.

However, good scientific research is perfectly possible during the
non-encounter periods and we must press to enable scientific operation
in those periods. This would include, for example: solar spectral atlas
and irradiance studies with distance, latitude and longitude; full Sun
and CME observations at large Earth-Sun-spacecraft angles; calibration
programmes etc... These would all benefit from observations at any time
during the orbit. Other scientific and PR benefits would include farside
and high-latitude observations of the Sun at any distance, a round-trip
movie of the Sun, deconvolution of (3D) helispheric structure, and
measurement of neutron half lives. This is all in addition to the
operational aspects such as JOP rehearsals.

From a scientific viewpoint, the non-encounter periods must be
exploited, and this is recommended. We must request that the Project
pursue this option. It would require more on board memory and changes to
the downlink philosophy.

Such activities must NOT impact the 30-day encounter scientific
activities. However, in addition, we must assess whether any enhanced
memory or telemetry link would be better used to enhance the encounter
science rather than the non-encounter activities. This must be
discussed.

These periods may be scheduled differently. It is suggested that the
no-encounter periods be more freely scheduled. Instrument teams must
schedule their own independent calibration and test programmes. However,
some JOP testing and scientific observations must be scheduled by
coordination between the PI teams. This need not wait for the 150 day
encounter planning meeting.

During the non-encounter periods, it is assumed that the remote sensing
instruments would be pointed to Sun centre. This needs to be discussed.

**[Conclusion:]{.underline}**

There is a logical planning and operation concept for a mission such as
Solar Orbiter but it includes a number of assumptions and
recommendations, which must be built into the mission from an early
stage. It also requires some discussion about the use of a dedicated
planning facility and of policies with regard to the use of any enhanced
memory or telemetry options. As far as feasibility is concerned,
planning around a 150-day cycle is no problem. We recommend that there
be a thorough study of the operations concept to ensure that the issues
such as pre-mission planning, on--board flags, non-encounter period
exploitation are built into the mission at an early stage.

**[References:]{.underline}**

None