1. Dear Andrea, 
Regarding the discussion at the closed SWT, we would like to pose the following question to flight dynamics to clarify the situation on lander - orbiter visibility and hence science capability following landing. This was based on the discussion that came from the more general point on trajectory flexibility on timescales shorter than the LTP, where we learned that flexibility is possible via minor updates which were in exceptional cases and the following levels of flexibility could be facilitated:
–     change slightly the close fly-by radius
–     change the close fly-by time by less than 1 hour i.e. max 30 deg in comet longitude 
Scenario:
The #1 landing site is known/decided upon 60 days before landing. LTP3 and LTP 4 deliveries come after this point in time (referring to ANNEX_5_SWT35_MOC_status, close 8 attached to for convenience) and so science observations and planning are based on the rank#1landing position.
It is assumed that part of the landing site selection criteria includes optimisation of post landing lander-orbiter visibility.
CONSERT operations require visibility between Lander and Orbiter < 25° and distance <=30km for
synchronization and then the passage on the opposite side for measurements.
Overall lander RF communication has broader visibility conditions (angle <60° at larger distances).

2. Questions
1/ Given the lander error ellipse of 1 km, are nominal operations able to ensure lander - orbiter visibility requirements as mentioned above ? 
2/ Would the flexibility values mentioned above (fly by altitude and fly by timing) be applicable to modifying the trajectory for optimising the landing site - orbiter visibility? For example, does the 30 degrees provide enough margin to cover the landing ellipse?
3/ In the case of 2 , is such flexibility considered exceptional or part of the nominal post-landing operations.

3. The constraint on the attitude pointing is: the angle a between Rosetta –Lander and Rosetta +z at the orbit should not be larger than amax=39- arctg(rcomet/(R-rcomet)), where rcomet and R are respectively the comet and the otbit radius. This gives 24° for the orbit at 10 km and of course larger angle at R>10km.
The distance from Comet should be less than 30km. In fact this condition will be reevaluated after the FSS. For FSS the orbits at < 10 (the lowest as possible) km would be the best one.
The footprint velocity (from the lander to S/C) on the surface should be less than 1m/s. This constraint could be violated just before and after the visibility period.
The maximum distance from lander to surface section (ground track of intersection of the surface with the line connecting Lander and Orbiter) should be as large as possible and larger than 3km (the best would be this distance equal to diameter of the comet) in order to explore the maximum interior of the comet.
The surface section trace should correspond, as close as possible, to the great circle on which the lander landed. The S/C should pass as best as possible on the opposite side of the comet (line: S/C-center of the comet-Lander)
The Consert operations has to start and should finish when the direct visibility between Lander and Orbiter exists. This is to start with the synchronization of the experiment and finish with the calibration. This calibration will increase our confidence in the data however in principle is not necessary.

4. ANSWERS from RMOC:
1)[RMOC] This is something that RSGS and Lander team can compute themselves depending on the Landing site and trajectory they select. 
These requirements are not applicable for LTP#3 (see RO-LAN-RD-1100). 
The 25° angle is not necessary for FSS as Consert will be synchronized before separations.
However the conditions concerning measurements on the other side of the comet should be fulfilled. (Consert doc dec 2012)
For LTP#4 the trajectory requests are under SGS responsibility. 
The SGS can construct trajectories that fulfil nominally the requirement. 
However, due to landing dispersion (and to lesser extend to orbiter position error) it cannot be ensured that in actual operations the orbiter will be within 25 deg of the lander Z axis. 
Assuming that the lander position and orientation can be reconstructed post landing (which is questionable), this information can be used by SGS for later LTPs. 
It can mean that Consert will be not able to operate after FSS! …
However we are developing software that should permit to find good period for operations but this means the flexibility in orbit phasing and in programming of operations (PTR).

5. 2) [RMOC] See above for what concerns the dispersion. 
A fly-by arc can be requested for LTP#4, such that the orbiter passes nominally over the Lander Z axis. 
Assuming that the lander position and orientation can be reconstructed post landing, 
it would be possible to guarantee the 25 deg constraints by adjusting the fly-by timing and the time of the synchronisation operation.  ok
3) [RMOC] Exceptional. Such exceptional operation shall not be accepted unless the position and orientation of the lander has been reliably and unambiguously reconstructed. OK
4) [RMOC] Within the limits agreed it is fine, beyond that it is a mission re-definiton i.e. re-run LTP. 
It is up to the mission manager to call for it. 
Consequences (delays, interruptions, etc.) have to be accepted. 

6. ANSWERS FROM RSGS
(1) The main applicable requirement for the following discussion is RD , Long Term Science, of RO-LAN-RD-1100, which states that links are required ">90 min every 24 hours on average. Exceptions will be defined (e.g. CONSERT orbits)."
OK work in progress with RLGS and SGS
(2) For the current LTP products, a landing site on the equator is assumed, which, to the best of our knowledge, is a case for which frequent and long communication windows can be expected.
OK wait and see the real comet…
For the early SGS-designed LTPs we note the following, bottom line being that their design is such that communication opportunities are maximised:
(3) Far Flybys: the design of far flybys is mainly rectangular, with the longer sides of the “rectangles” being oriented latitudinally.  For a lander located on the equator, this clearly maximises visibility and communication opportunities, independent of other parameters. 

7. (4) Close Flybys: early in the mission they are deliberately put into the orbit plane of the comet, thus providing good communication opportunities.
OK and tuning and observation for CONSERT.
(5) Terminator orbits: in this case, there are not many degrees of freedom that can be tweaked.  However, early terminator orbits have a phase of days, which would co-incide with the longest period of non-communication, see RD : “In case periods without Lander communication opportunities for more than 60hrs are identified this must be flagged to the Lander project. Depending on the specific circumstances this can be then be approved by the Lander project."
(6) Given all this, I think that orbit design generally has taken into account communication opportunities with the lander and optimised for them, given the current state of knowledge.
(7) Additionally, I would point out that I regard the original answers given by Andrea as correct and fully support them.
(8) i understand that the Flight Dynamics Team at SONC is working and has worked on more detailed simulations.  The have also had presentations at past meetings on this topic and might be able to provide quantitative answers as well, should you desire them.