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The goal of the planning activity carried out at SOWG 11, which was delegated authority to do so at SWT #21, was to produce a baseline mission level plan for the cruise phase based on a February 2020 launch. This plan was to be realistic, feasible, and, in the absence of new information, suitable for use in flight unchanged.

Location of Remote Sensing Checkout Windows

Context: During Cruise Phase, the mission planning concept foresees regular checkouts for RS payload in order to characterise and calibrate the instruments to get ready for science operations in NMP. As a baseline, week-long checkout windows are foreseen, on average every 6 months. As there is flexibility in the placement of those checkout windows, the SOWG (as mandated by the SWT) had to agree on the most appropriate schedule taking into account mission and platform restrictions, calibration opportunities and instrument specific limitations and opportunities.

As a starting point for the discussion, SOC prepared an analysis of the Feb 2020 Cruise Phase and a first proposal for the placement of checkout windows, documented on SOC Public: Feb 2020 Cruise Phase proposal for RS checkouts. SOC proposed to distribute the available 21 days over 4 checkout windows to allow time for data analysis between checkouts:

  • RSCW1 - 5 days around 15 June 2020 – covers distance ranges  0.52 to  0.51 AU, midpoint of window at 0.52 AU. Daily (8h) downlink ~810 Mbytes
  • RSCW2 - 5 days around 1 Mar 2021 – covers distance ranges  0.54 to  0.57 AU, midpoint of window at 0.56 AU. Daily (8h) downlink ~375 Mbytes
  • RSCW3 - 5 days around 15 Sep 2021 – covers distance ranges  0.58 to  0.59 AU, midpoint of window at 0.59 AU.  Daily (8h) downlink ~1490 Mbytes if data comes down after 15th September
  • RSCW4 - 6 days around 20 Oct 2021, incl. a dress rehearsal for a RS window in NMP (i.e. with VSTP) – covers distance ranges  0.73 to  0.77 AU, midpoint of window at 0.75 AU.  Daily (8h) downlink ~3500 Mbytes

Apart from the necessary turn-around time, the proposed schedule was based on thermal environment, both the spread in T and the representativeness for nominal RS windows, spread in time, and quality of TM downlink (i.e. distance from Earth).

The discussion in the SOW started from this proposal. The following aspects got discussed:

  • PSP coordination: The SOWG brought up that in order to assess the best placement of the RSCW, the coordination with Parker Solar Probe should be assessed as well. SOC argued that although the concern is understood, the SOC is currently not in the position of taking other missions into account in the formal planning process, without a clear mandate from mission management and project scientists. Also, the uncertainty in launch dates currently prevents to do a proper analysis of geometrical opportunities. Therefore SOC proposed to perform the current MLP planning activity taking into account only Solar Orbiter's own needs, and fold in any other mandated coordination in the next (and final) round of Cruise Phase MLP (Feb-Mar 2019).

  • thermal environment: the RS teams in general agreed that the representativeness for nominal operations was an important aspect to keep. Especially perihelion opportunities were valued. After some discussion it was also agreed that it is beneficial to keep an 'extra-cold' window, at farther than representative distance from Sun, to improve the thermal modelling/characterisation of the instruments (thermal effects on dark noise, focus, alignment, ...).
  • EMC cleanliness at first perihelion: the IS teams brought up the high value of the first perihelion for science and PR, and the wish to keep this period as EMC quiet as possible. After discussion with the RS teams it was agreed to shift the RSCW before the perihelion (ending 1 day before) to retain the warm opportunity for RS while preserving the exact perihelion point noise-free for IS. Shifting earlier rather than later has advantages for TM return as the SC is moving away from Earth.
  • far side opportunities: it was argued that far side opportunities close to perihelion should be exploited where possible and therefore it was decided to move the 2nd RSCW closer to the conjunction, i.e. closer to the Sun and slightly farther away from Earth. This would be the 2nd 'warm' opportunity for calibration.
  • spread in time: everybody agreed that the last 2 windows were very close in time and this would prevent a proper analysis of all the data before embarking on the last checkout incl. a dress rehearsal for nominal RS windows with VSTP. For this reason the last, extra-cold window was moved to before the VGAM2. It was pointed out and understood that the TM return in this location would be substantially lower. Therefore it was proposed to shrink this new 3rd window to only 3 days in order to allow for extra-cold calibration without much more. In return, the last RSCW could then be prolonged to 8 days to have time to do a last proper checkout and the VSTP rehearsal. The last, long window got moved to solar distances 0.6-0.63AU to have best representation of cold RS windows in NMP.
    In addition, it was discussed and understood that the new location of windows 2 and 3 would not allow for a turn-around between them (not enough time and too low/slow TM return). As they happen at very different thermal environments, this was not considered a problem.

The finally agreed placement of checkout windows are those:

  • RSCW1: 9 – 14 Jun 2020 (0.52-0.51AU) – 5 days
  • RSCW2: 20 – 25 Feb 2021 (0.5-0.54 AU) – 5 days
  • RSCW3: 21 – 24 Mar 2021 (0.69-0.71AU) – 3 days 
  • RSCW4: 22 – 30 Sep 2021 (0.60-0.63AU) – 8 days, daily passes

Based on the new schedule of RSCWs, SOC updated the plots showing the RSCW placement: (1) as a function of time, wrt distance to Sun and Earth, and (2) the orbit plots in HEE frame, highlighting the geometrical configuration wrt Sun and Earth (see presentation, new RSCW placement starting on slide 47).

The event file, as shown in SOOP kitchen, got updated with the new RSCW times and the pass schedule adapted accordingly (shifting the extra passes for RS downlink close to the RSCWs).

Coordination of Campaigns involving Full Payload

While RS payload will focus during Cruise on instrument characterisation and calibration, during the defined RSCWs, IS payload will ramp up to coordinated science observations. It was discussed whether there are any campaigns that need coordination between all of them.

RPW and MAG teams explained that will perform EMC characterisation during all RSCWs in order to analyse what to expect during nominal RS windows. RS observations do not need to be tweaked for this although it may be beneficial that they do not all switch on at exactly the same time. It was agreed that this level of detail would be handled at LTP.

It was clarified that RS instruments would need a series of SC manoeuvres during checkouts including series of SC rolls to different angles, offpointing mosaics and limb pointings. IS payload is not much affected and so coordination at RS level was deemed sufficient. Also MAG needs calibration rolls. As these fall outside RSCWs, RS will not be affected (see below).

There are some RS campaigns that will need to run outside of the formal RS windows: SoloHI will open its door before RSCW2 and this will be handled as an engineering campaign. Metis needs some calibration campaigns that involve observing particular calibration stars. These may fall outside of RSCW (later, the times were confirmed and added to the plan in SOOP Kitchen).

For the dress rehearsal during RSCW4, the SOWG agreed to schedule a SOOP involving all instruments and target tracking to try out a nominal RSW with VSTP updates. For this purpose, the SOOP L_SMALL_HRES_HCAD_Slow-Wind-Connection was chosen, to be scheduled during 3 days, 26-27-28 Sep 2021.

Opportunities for MAG Calibration Rolls

According to the MAG EIDB M-600 and M-601, A calibration roll of 12 complete spacecraft rotations around the spacecraft X axis, at the highest possible angular velocity, should be carried out at least once per orbit (typically taken by SOC to be per six month planning period for the longer duration orbits). At mission level planning we cannot sensibly choose the exact time for a calibration roll, hence we instead define a 3 day window during which the calibration roll should be scheduled. Opportunities for these have been selected as follows:

  1. 2020-05-22T00:00:00.000Z - 2020-05-25T00:00:00.000Z (heliocentric distance ~0.58 AU)
  2. 2021-01-05T00:00:00.000Z - 2021-01-08T00:00:00.000Z (heliocentric distance ~0.65 AU)
  3. 2021-10-01T00:00:00.000Z - 2021-10-04T00:00:00.000Z (heliocentric distance ~0.64 AU)

Similar heliocentric distances were chosen in preference to a quasi-fixed six month cadence. Additionally, the chosen opportunities are close to perihelion such that sensor offsets can be determined and a new calibration matrix potentially uploaded before the perihelion pass, should data latency for bulk science data be low enough (opportunity 1 only), or the MAG low latency data sufficient for a rough calibration. 

Packet Store Resizing Opportunity

SOC stressed the need for an opportunity to resize the SSMM packet stores towards the end of the cruise. Any data present in the stores would be lost at the point of resizing, so one of the drivers for the plan was to ensure the stores are empty before the resizing opportunity. The opportunity to resize the packet stores need not be taken up if it is deemed that the SSMM configuration used for cruise phase is also suitable for the nominal mission phase.

In SOC's initial proposal the resizing opportunity was scheduled be before the final "dress rehearsal" RSCW, so that the nominal phase confuration oculd be used for the final weeks of cruise, allowing selective downlink procedures to be validated, for example. However once the checkout window positions were finalised, the SOWG agreed to place the resizing opporunity after the final RSCW, mid to late October.

In Situ Science Planning Philosophy

Prior to the meeting, as a starting point, SOC calculated some rough telemetry rates that would not overrun the packet stores and also allow for the packet store resizing opportunity.

The suggested telemetry rate / time split was as follows:

  • 15 May 2020 - 15 July 2020: 100% EIDA
  • 15 July 2020 - 15 September 2020: 50% EIDA
  • 15 September 2021 - 11 November 2021: 100% EIDA

Additionally, during remote sensing checkout windows MAG and RPW can generate at 150% EIDA for EMC characterisation of the Remote sensing payload operating in a more realistic way than during commissioning and the interference campaign.

The in situ teams decided to focus telemetry generation on the periods around perihelion. After some iteration it was found that changing from a higher data rate to a lower data while the spacecraft was further than 0.7 AU from the Sun fit the telemetry constraints well. Higher and lower rate periods are as follows:

  • 15 May 2020 - 15 July 2020: Higher rate
  • 15 July 2020 - 31 December 2020: Lower rate
  • 31 December 2020 - 22 March 2021: Higher rate
  • 22 March 2021 - 13 August 2021: Lower rate
  • 13 August 2021 - End: Higher rate.

A pro-rata EID-A telemetry split between the instruments was not strictly enforced. An attempt was made to use an underrun towards the end of cruise, although this ought to planned in more detail at LTP. Similarly, there is potentially unsused downlink at the very start of cruise, but no attempt was made to schedule extra observations there. Again, this ought to be planned in more detail at LTP once actual telemetry constraints are known.    

Remote Sensing Observations Planning

After splitting up RS and IS teams, RS teams discussed a few campaigns that would need coordination between all of them:

  • 2 RS co-alignment campaigns, mainly tailored for the high-resolution telescopes SPICE, PHI/HRT and EUI/HRI: these will be planned during RSCW1 and 4 and involve limb pointins to N,S,E,W + disc centre. The whole campaign takes about 8 hours, based on SPICE durations of full scans. Note that apart from assigning the windows in which the campaign will take place, no detailed planning of the actual time period was done. Each instrument just added a placeholder for the co-alignment observations.
  • PHI+EUI coordinated offpointing mosaics for flatfielding with FDT and FSI: scheduled in RSCW1, 2, and 4.
  • Rolls for Metis and HI calibrations: RSCW2 + HI-specific during last day of RSCW4

Apart from those campaigns, all teams scheduled the necessary instrument-specific observations in the windows.

For the TM allocations, only guidelines on the available TM volume for all RS instruments were given. Pro-rata allocations were not enforced. Each instrument did however tweak schedule on the overall available TM. This approach worked well.

Link to Planned Observations

The mission level plan can be found in SOOP Kitchen:

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