Introduction and Timeline

Nominal Phase Segment 2 refers here to the second two orbits of the NMP, so roughly calendar year 2023, encompassing LTPs 10, 11, 12 and 13. It is the shortest period that can be planned independently of the rest of the mission, i.e. it contains a period during which the SSMM will not empty, so planning decisions made for LTP 10 will have an effect on the scope for operations in LTP 13.

The orbit during these periods is as follows (note these still assume geometrically placed Remote Sensing Windows):

Here we analyse the scope for science operations and data generation during this segment, with the goal of informing the SWT so they can decide the following during the upcoming meeting:

  • Science Priorities for NMP Segment 2
  • Exact RSW times during LTP 10 and/or LTP 11. Approximate (though the more precise the better) RSW times for LTP12/13
  • End date of LTP 10. Assuming no changes are made to the proposed RSW dates, we suggest 20 March, if the first remote sensing window is brought forward we should discuss this with the SWT.
  • Telemetry allocation / generation focus for all of 2023 (see below for details)

Other deadlines for this segment that are not too far in the future are listed here for info:

  • Requests for off-points / rolls campaigns for out of RSW calibrations during LTP 10. Deadline end July for pre-LTP TN.
  • Times during which to avoid  placing Engineering Window for SSMM maintenance in LTP 10. Deadline end July for pre-LTP TN.
  • Requests for high priority science opportunities outside of RSWs for LTP 10, so we can flag them to flight dynamics. Deadline end July for pre-LTP TN.

A Note about Segment 1 (~2022) Planning and Extra Telemetry

The last time we did this exercise, a year ago, ground station performance was still uncertain given ongoing upgrade work at Cebreros and Malargüe. This, together with our not taking into account extra passes during Navigation windows, as well as postponement of launches expected in 2022, led to a significant underestimate of the amount of data we would be able to downlink during this year. This meant significant additional telemetry had to be allocated at short notice once we received the inputs from MOC during the planning of each LTP by the SOWG. This in turn meant that the SWT did not have as much input into the allocation of the total telemetry as we would have liked.

Since then, the upgraded ground station performance has become clearer so we have updated our planning tools to more accurately model the data rates we can expect. For example, we can sustain the maximum downlink rate of ~1 Mbit s-1 as far as 0.7 AU from Earth, whereas before the downlink performance was modelled to decrease starting at 0.4 AU from Earth.

Furthermore, during 2023 there are no navigation windows so the allocation of passes we can expect will not diverge as much from the baseline of 1 pass per day as has happened in the past.

Together, these mean we are much more confident about the modelled downlink this time than we were a year ago, so we don't anticipate there being as much extra telemetry to allocate during long term planning process as previously. Thus the decisions made by the Executive SWT about allocations will be more final than has been true until now.

The Link Between Remote Sensing Window Timing and the Scope for Data Generation

The data that can be generated during a particular period of scientific interest (e.g. a remote sensing window) is the sum of the data that can be downlinked during that window and the data that can be stored in the SSMM for later downlink. The latter is itself a function of how much data is already in the SSMM at the start of the period and crucially, how much data is expected to be stored in the SSMM during the next period of scientific interest, and also how much data can be downlinked in between the two periods. Thus, the ideal case would be two intervals that are well-separated in time and take place during periods of good downlink. The worst case would be two concatenated intervals that take place during a period of bad downlink. Assuming the SSMM is not emptied at any point, any data that are generated in between the intervals (e.g. from a synoptic programme) are data that cannot be generated during the intervals of scientific interest.

Because the downlink rate is highly variable, the first step in assessing the scope an LTP period has for data generation, and the split between RSW and synoptic generation (equally applicable for high rate vs normal rate intervals for in situ instruments) is thus fixing the location of the remote sensing windows.

Proposed Remote Sensing Window Locations

During these early, low-inclination, orbits of the nominal mission phase, the default geometric placement of remote sensing windows doesn't make scientific sense. Based on experience during the first orbit, we've assumed the following, including a 1 day gap between RSWs for flat fields, etc. Note also that Solar Orbiter is moving faster during these orbits than previously, so the scope to move the windows in time while maintaining interesting geometry is reduced.

2023 H1:

  • RSW7: 2023-03-24 (0.47 AU) - 2023-04-02 (0.35 AU); closest approach to the Sun-Earth Line on 2023-03-28 (0.41 AU).
  • RSW8: 2023-04-04 (0.34 AU) - 2023-04-13 (0.30 AU); perihelion (0.29 AU) is on 2023-04-10 (Easter Monday).
  • RSW9: 2023-04-15 (0.31 AU) - 2023-04-24 (0.42 AU).

2023 H2:

Note there is a conjunction that ends on 2023-09-28, so in order to have a few days of observations either side of the perihelion while maintaining a 1 day gap in between RSWs:

  • RSW10: 2023-10-01 (0.33 AU) - 2023-10-10 (0.31 AU); perihelion (0.29 AU) is on 2023-10-07.
  • RSW11: 2023-10-12 (0.31 AU) - 2023-10-21 (0.42 AU).
  • RSW12: 2023-10-23 (0.44 AU) - 2023-11-01 (0.56 AU).

An orbit movie for this segment with the proposed RSW locations is here 

Heliocentric Distance Ranges

These are the dates on which the spacecraft crosses certain heliocentric distance ranges, together with a rough indication of downlink at that time. These will be used to identify the more interesting intervals for higher rate in situ observations.

Distance (AU)DateDownlink

Data Generation Scenario

No assumptions have been made about the science content of the plan, nor of the exact downlink share between instruments. In situ data generation is modelled based on default ~EID-A and "high rate" (~4EID-A) observation definitions as used so far. Remote sensing synoptic data rates are based on those used during LTP 8. RS generation in remote sensing windows is based on generic observation definitions without making any assumptions about e.g. whether data comes from EUI HRI or FSI. No changes to SSMM store sizes have been made. Daily passes are assumed throughout.

The plan was built following these steps:

  1. STIX sending data to the SSMM at a constant 100MiB per week throughout the entire segment.
  2. EPD generating in close mode +600bps throughout the entire segment.
  3. For other remote sensing instruments, Include a synoptic programme outside of RSWs based on synoptics as defined for LTP8.
  4. For other in situ instruments schedule high rates within 0.5 AU, EID-A the rest of the time.
  5. For other remote sensing instruments include 2xEID-A rate during each RSW.
  6. Simulate the plan.
  7. If telemetry allows, increase remote sensing windows by a factor of EID-A. (e.g. from 2xEID-A to 3xEID-A).
  8. If telemetry allows increase IS generation fro EID-A to high rate in the next farthest distance band (e.g. from 0.5 to 0.6 AU).
  9. Simulate and repeat as necessary.

For EUI and PHI flushes are modelled as follows:

  • Assume PHI flushes data taken in the first remote sensing windows steadily until the start of the next 3 remote sensing windows, also synoptics as they are generated. Flushes are daily during RSWs, weekly otherwise.
  • Assume EUI flushes RSW data daily, synoptic data weekly.

The frequency of flushes is somewhat arbitrary. More frequent, smaller flushes would be better, but including these would have made building the plan more time consuming. Less frequent, larger flushes may not work when the SSMM is closer to full.


In summary, with the assumed station coverage, we should be able to support:

  • LTP-8 style RS synoptics outside RSWs
  • EPD & STIX constant rates as described
  • Other IS in high rate whenever we're within 0.8 AU, also beyond 0.8AU in good downlink conditions (this latter is functionally free).
    • EID-A rate: 2023-05-28 - 2023-08-20, high the rest of the time
  • 4x EID-A for other remote sensing in RSWs 7-10
  • 5x EID-A for other remote sensing in RSWs 11 & 12
  • Note PHI is really only constrained by their internal memory during RSWs 10-12, we'll be able to downlink (almost) anything they flush to the SSMM during the interval in which we're close to Earth.

This results in the following SSMM fill profile:

This gives us a peak SSMM fill state of around 70%. We don't want to go further at this point.

This implies the following data latencies (note for EUI, PHI & STIX it's time between flush & downlink, not generation & downlink):

In situ latencies are higher because of the high rate observations extending some weeks after the end of the RSWs into the worse downlink period. No latencies are higher than 90 days.

Some more optimisation of downlink share needs to be done to equalise fill states and latencies per instrument as far as possible.

Data Accounting

The straw man plan analysed here implies the following data generation:

  • IS Generation over the segment: 221270 MiB (287 out of 371 days in high rates)
  • RSW Generation over the segment: 232960 MiB
  • RS Synoptic Generation over the segment: 84625 MiB

Scope for More Data Generation

The above plan essentially fills the downlink, apart from during the very good periods when the spacecraft is close to the Earth:

So the only scope for additional activities is during these periods (i.e. before RSW7 and after RSW 12). Of course, PHI could generate more in the first set of remote sensing windows if they were willing to keep those data internally until after the second set of remote sensing windows.

Available Trade-offs

  • Move data between RSWs. This is mostly limited by SSMM store size.
  • Increase RSW data by switching IS to high rates closer to the Sun.
    • Switching IS to high rate at 0.7 AU instead of 0.8 AU buys approximately 1.1 remote sensing window EIDA volumes, i.e. we could increase one RSW's generation from 4xEIDA to 5xEIDA.
    • Switching IS to high rate at 0.6 AU instead of 0.8 AU buys approximately 2 remote sensing window EIDA volumes
    • Switching IS to high rate at 0.5 AU instead of 0.8 AU buys approximately 2.8 remote sensing window EIDA volumes
  • Run IS high rates at higher heliocentric distances by reducing RSW data.
    • Switching IS to high rate at 0.85 AU instead of 0.8 AU would mean reducing RSW generation by 0.75 of a RSW EIDA volume.
    • Switching IS to high rate at 0.9 AU instead of 0.8 AU would mean reducing RSW generation by 1.75 RSW EIDA volumes.
    • Keeping IS in high rates throughout would mean reducing RSW generation by 4.25 RSW EIDA volumes.
  • Reallocate data between RS synoptics and others.
    • We'd prefer not to do this, if synoptics are always the same ops are simpler. 

Implications for 2023 pass requests

Assuming RSW locations don't change significantly, for H1, we plan to ask for the following:

  • Daily passes in March & April so as not to over-constrain RSW ops but also allow VSTP.
  • Maximise downlink in May & June through extra passes if available.
  • Give up station time in January and February if needed to achieve the above.

A positive outcome would help us do a bit more. An equivalent request will be made for H2 late this year.

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