Operational Orbit
Example of a large amplitude libration orbit around L2 for PLATO. The orbit shape and amplitudes depend on the launch date
PLATO will perform its scientific observations on a free-insertion, large amplitude, eclipse-freelibration orbit around L2. This orbit is unstable and shall be maintained by regular station-keeping manoeuvres every 30 days. The angular size of the libration orbit seen from the Earth is approximately 33° in the ecliptic plane and 25° out of this plane.
From the end of the Transfer Phase to the completion of the nominal science operations phase, PLATO will spend in total 4 years around L2. Three additional months in total are currently allocated to launch, transfer, and commissioning.
Communications with ground
During science operations, a communication session with the nominal ground station will be used several days per week. The largest part of it (>3.5 hours) is dedicated to file-based science and stored housekeeping data transmission at 36 Mbps in K-band and real-time housekeeping data transmission at 26 kbps in X-band via a dual X/K-band high gain antenna. This enables downloading to ground the required 435 Gbits of science data produced daily by the payload. Within this slot, a small window is used for communication setup and ranging and the 16 kbps uplink is ensured via X-band. Outside these nominal communication periods, a low-rate data link will be ensured via low gain antennas in X-band for minimum and contingent telemetry (2 kbps), telecommand (4 kbps).
Antennas used for PLATO (Credit: ESA)
Mission observation strategy
PLATO has a flexible observing approach. Two observing strategies are reflected in the science requirements, long-duration observation of the same field versus shorter coverage of shorter different fields, or step-and-stare. These strategies complement each other and allow for a wide range of different science cases to be addressed. Long-duration pointings would be devoted to surveys for small planets out to the Habitable Zone of solar-like stars. Short pointings would be devoted to shorter-period planet detections and will address a number of different science cases such as galactic exploration.
In its nominal science operations, PLATO’s current baseline observation scenario assumes a Long-duration Observation Phase (LOP) consisting of continuous observations of two sky fields, lasting 2 years each. An alternative scenario would consist of a LOP of three years and a step-and-stare phase (SOP) of one year. The current mission design constraints impose the centre of the LOP fields to be at least above 63 degrees or below -63 degrees in ecliptic latitude.
Schematic comparison of observing approaches. Red area: the Kepler target field. Large areas: size of the PLATO field. A combination of short (orange) and long (blue) duration pointings is able to cover a very large part of the sky. Note that the final locations of long duration and step-and-stare fields will be defined two years before launch and are drawn here for illustration only
Although the nominal science operation duration is four years, the satellite will be built and verified for an in-orbit lifetime of 6.5 years, accommodating consumables for 8 years. Consequently, four years of mission operation extensions are possible conditional upon approval by the SPC, in which LOP sky fields may be re-observed or new LOP sky fields may be added. In addition, step-and-stare pointings may be carried out, lasting 2-5 months each.
In view of the exceptionally fast development of exoplanet science, this reference scenario will be investigated throughout the mission development and adapted to the needs of the community about two years before launch.
In-orbit spacecraft and payload events
During long observations, the spacecraft must maintain the same line-of-sight (LoS) towards one field for up to several years. However, the spacecraft must be periodically re-pointed in order to ensure the solar arrays are pointed towards the Sun. This is achieved by rotating the spacecraft around the LoS by 90° roughly every 3 months.
Spacecraft rotation around payload LoS during one orbit
Due to the constant pointing requirements of PLATO and high science measurement duty cycle, and besides the necessary re-pointings between observation fields, only few manoeuvres and interruptions to the scientific measurements are planned. Nevertheless, some are required to maintain the orbit and control capability of the spacecraft as well as for the calibration needs of the payload.
On-station, the spacecraft must maintain its position on the nominal orbit via station keeping manoeuvres which are foreseen to take place every 30 days. Their amplitudes depend on the residual accelerations induced by the spacecraft and therefore depend on its design.
Since the spacecraft fine control is based on reaction wheels, those will need to be de-saturated on a regular basis. These offloading operations can take place for instance once a month or as often as every few days, depending on the wheels’ design and the control strategy. The daily communications will also require regular antenna re-pointing, which may impact the nominal pointing of the spacecraft.
Even if safe modes are required to be limited by design, those events might take place and some off-nominal operations must be allocated in that respect.
The payload itself requires some calibration operations. In particular, the payload needs to acquire a very high-resolution Point Spread Function (PSF) characterisation of each star for each telescope used in the ground data processing later on, typically to correct for long-term drifts and high-frequency pointing jitter. For this purpose, the spacecraft will perform for a few hours a micro-scanning of the field of interest on a regular basis, at least after every field re-pointing and quarterly slew, potentially more often.
Other calibration needs, such as download of full CCD images and telescopes at the beginning after each re-pointing and slew do not require any spacecraft manoeuvres but will interrupt the scientific measurement for a minimum amount of time. Update of the position of the star imagettes are also foreseen to cope with the large-scale and long-term movements of the star in the focal plane due to kinematic aberration and thermo-elastic for instance, but essentially do not impede the science data acquisition.
The duration of all events described above as well as the time required to reach the science nominal mode are accounted for as downtime of the scientific measurements and the associated gaps are evaluated against the scientific gap requirements. Particularly, following any manoeuvre (e.g. orbit maintenance, re-pointing, slew, micro-scanning) the spacecraft will have to retrieve its thermal and structural stability in order to resume its thermal and fine pointing performances.
At the end of mission eventually, a delta-V of 10 m/s is allocated to the disposal manoeuvres in order to place the spacecraft on a heliocentric orbit.
References
PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017
platomission.com 
