1ST MORE TEAM MEETING

1
1ST MORE TEAM MEETING

• The spacecraft design and the radio science
  experiment

2
Overview

• Spacecraft Configuration
• MORE Experiment Integration
• More Imbedding in the RF-Subsystem
• Antenna Subsystem and Waveguide Routing
• Initial Budgets
• ISA Experiment Integration
• ISA accommodation
• Transform to COM related measurements
• Microvibration

3
Spacecraft Configuration

• Spacecraft Appendixes
• HGA in Mercury orbit continuously moving while
  there is radio contact to ground
• MGA only moving in contingency cases
• MGA boom fixed after deployment
• Solar array episodically moving in Mercury
  orbit (on average once per 11 days)

4
Communication S/S and MORE(Ka-band equipment)
Ka- band equipment with MORE transponder
is located close to the HGA interface
5
Communication S/S and MORE(X-band equipment)
X- band equipment with DST transponder is
distributed over two panels
6
Antenna Subsystem Options
ASTRIUM Cassegrain Titanium
AAS-I ADE Gregorian CSIC
7
ISA Accommodation

• ISA is accommodated between the fuel tanks close
  to the COM.
• Apart from being close to the COM the
  accommodation location of the ISA instrument must
  also have a low level of micro-vibration
• The dominant sources of continuously present
  microvibrations are the reaction wheels
• Other (quasi) continuously operating mechanisms
  are
• The antenna pointing mechanism
• Internal calibration/pointing mechanisms in the
  instruments MERTIS and PHEBUS

8
MORE Experiment Integration

• TTC System Block Schematic Options
• Preference for SSPA integrated with KAT
• MPO Design Impact on Measurement Performance

9
BC BASELINE ASTRIUM PROPOSAL

• Nominal operation
• HGA antenna used forX Ka band
• Simultaneous operation of
• X band command uplink
• X band data telemetry downlink
• Ka band data telemetry downlink
• Range/Range-Rate measurements on X/X, X/Ka and
  Ka/Ka link
• Contingency Operation
• MGA or LGA antenna(s) used
• X band TTC and ranging

10
SSPA in KaT Alternative

• Integration of a dedicated SSPA and a triplexer
  with the CAT would significantly simplify the
  TTC AIV/AIT procedures.
• The Ka-Transponder functions which essentially
  determine the RSE space segment performance are
  contained in a single assembly
• This would allow to integrate an end-to end
  calibration loop for the ranging delay locally
• Only the less demanding x-band transponder
  functions require an integrated system for end-to
  end performance testing

11
Frequency Plan (1)
12
Frequency Plan (2)
13
Potential conflict with Regulations

• The Ka band TWTA is operated in a nonlinear
  regime.
• With the frequency assignment showing about
  200MHz separation between the KAT carrier and the
  data link carrier second order intermodulation
  products fall into a band with strict limits on
  allowable on-ground power flux density.
• With the measured TWTA performance and
  BepiColombo signal parameters and antenna gain a
  formal violation of the regulations would result
  if KAT and Ka-Data link use the same TWTA

nominal signals
largest intermodulation product
the applicable reference BW for narrow
bandwidth signals is 4 KHz
14
RF-Harness (present baseline)
15
Spacecraft Contribution to Range/Range-Rate
Performance

• Space Segment contribution to Allan Standard
  Deviation of Range-Rate measurement
• Dominated by Ka-Ka link performance
• Ka-Ka link performance is dominated by
  thermoelastic deformation of antenna and
  waveguide harness
• Space Segment contribution to Ranging Measurement
  Calibration Error (timescale 0.5 days) and to
  Equipment aging (timescale 1 year)
• Dominated by transponder performance KAT
  respectively DST
• Objective contribution of spacecraft equipment is
  likely negligible (mechanical pathlength effects,
  etalon (VSWR) effect, TWTA and RFDA group delay
  variation)
• However verification accuracy on component and
  integrated assembly level will lead to a
  remaining uncertainty in the order of the
  specified S/C contribution (0.2 ns)

16
Allan Standard-Deviation
Requirement Description   Requ. Alloc. Perf. Unit
SRD-3908/R End to end performance incl. Ground Segment 14 14.01 12.6 1.0E-15
Assumption (BC-EST-RS-02256-01-10 RSE-3) Overall residual, post calibration contribution of the residual from the estimation of the Propagation Media effect Assumption sa_GS   10.0 10.0 1.0E-15
Assumption (BC-EST-RS-02256-01-10 RSE-4) Overall contribution of the Ground Segment to the Allan Standard Deviation (over 1000 seconds) in the Ka/Ka link Assumption sa_RP   6.0 6.0 1.0E-15
SRD-3080/R The overall contribution of the Space Segment to the Allan Standard Deviation (over 1000 seconds) of the Ka/Ka link Doppler measurements shall be less than 81015 sa_OB_Ka/Ka 8 7.74 4.8 1.0E-15
  Allan standard deviation resulting from on board mechanical deformation (effectively equivalent to measuring relative to an object moving with respect to the spacecraft COM) sa_OB_mech   7.67 4.7 1.0E-15
  Ka/Ka link electrical components contribution to Allan Standard Deviation sa_OB_el_Ka/Ka   1.01 1.0 1.0E-15
SRD-3084/R The contribution of DST (X/X link) to the Allan Standard Deviation (over 1000 seconds) shall be less than 41015 32 9.74 7.6 1.0E-15
  X/X link electrical components contribution to Allan Standard Deviation sa_OB_el_X/X   6.00 6.0 1.0E-15
SRD-3084/R The contribution of DST (X/Ka link) to the Allan Standard Deviation (over 1000 seconds) shall be less than 101015 80 12.60 11.0 1.0E-15
  X/Ka link electrical components contribution to Allan Standard Deviation sa_OB_el_X/Ka   10.00 10.0 1.0E-15
17
Mechanical Contributions toAllan
Standard-Deviation
Contribution Description Allocation Perf. Unit
sa_OB_mech Allan standard deviation resulting from on board mechanical deformation (effectively equivalent to measuring relative to an object moving with respect to the spacecraft COM) 7.67 4.68 1.0E-15
  Total onboard mechanical effects 1.15 0.701 mm
  Waveguide length variation of harness including feed and rotary joints over 1000s (Ka-Ka path) (total length traversed 1 way divided by 2) 0.15 0.145 mm
  Antenna phase centre motion from unit thermoelastic distortion   0.220 mm
  Antenna phase centre motion from thermoelastic distortion at APM interface 1.00 0.070 mm
  Antenna phase centre motion from thermoelastic rotation at APM interface   0.055 mm
  Antenna phase centre motion from APM position uncertainty   0.175 mm
  Antenna phase centre motion from S/C pointing uncertainty   0.036 mm

• Based on preliminary analysis of a configuration
  with the ASTRIUM antenna (Cassegrain with
  Titanium main reflector)

18
Ranging Measurement ErrorAllocation of
allowances to contributing units

• For the Ka/Ka link allocation of the overall
  budget to the components KAT, TWTA and RFDA and
  the need to specify and verify on unit level
  leads to a domination of verification
  uncertainties in the budget.

We expect nevertheless a compliant end to end
performance of the entire chain, however
verification will have to take place with the
integrated chain to avoid adding up of
measurement errors on unit level.
19
ISA Experiment Integration

• Challenges for Acceleration Measurement on the
  MPO
• Pointing control by reaction wheels
• Reaction wheels inject periodic forces which may
  impact ISA measurements as out of band
  accelerations Microvibration
• Propulsion system uses liquid fuel
• COM position is difficult to predict and changes
  as result of fuel consumption and fuel
  displacement sloshing
• Moving appendices
• Solar array (episodically moving)
• HGA continuously moving when ISA measurement is
  requested
• PHEBUS (moving baffle tube)

20
Acceleration Product Generation

• The transformation of ISA measurement data to
  acceleration acting at the S/C COM requires the
  acquisition of ancillary data
• Pointing and its time derivatives
• COM position estimation
• COM calibration
• Correction for moving appendices impact

21
Transform ISA measurements to COM

• With the foreseen performance of the gyroscopes
  the absolute COM distance which could occur with
  the foreseen accommodation constitutes not a
  problem for the correction
• Correction for the moving antenna impact can also
  be performed with negligible residual uncertainty
  on MPO generated acceleration

Even with the least performing gyroscopes
considered, the pointing rate knowledge would
allow up to 0.45m off COM positioning of ISA
without violation of the error limits on MPO
generated acceleration noise.
22
COM motion due to moving antenna

• COM
• position
• Velocity
• Acceleration
• from antenna motion
• Compensating for the effect of antennamotion is
  necessary, however to obtainthe required
  accuracy is not difficult.

23
Acceleration Product Generation

• The COM position is determined by an infrequent
  rocking manoeuvre
• Rotate MPO around two axes (roll and yaw)
• Selected acceleration parameters
• Such that acceleration acting on the fuel is
  minimised
• While MPO acceleration signal from tolerable COM
  uncertainty is still larger than the measurement
  sensitivity (SNR 3..6)
• Typical values ? 3.1mrad s-1 d/dt ? 2 10-5
  rad s-2

24
Challenges ofConversion to COM Acceleration

• The largest uncertainty is wrt. measuring the COM
  position by a calibration manoeuvre without
  inducing an unknown displacement of residual fuel
• Initial simulations of fuel displacement
  (believed to be conservative) indicate that with
  the low acceleration levels of the proposed
  manoeuvre the absolute error caused by the fuel
  displacement can be accommodated in the budget
• A fallback solution would be to correct the
  calibration with pre-calculated tables for the
  fuel displacement
• During the Mercury orbit phase there is very
  little fuel consumption (only for offloading of
  the reaction wheel accumulated momentum)
  therefore COM calibration is only infrequently
  needed

Error mm Comment
Calibration 0.8 See description of calibration above
Fuel displacement 2.4 (0.8 with sloshing estimation) Considering accelerations as resulting from rotational state during calibration and residual fuel attached to fuel management device with 6kg fuel bound in the PMD
Fuel consumption lt0.1 Considering COM calibration every 1.5 month (while MPO orbit is over the terminator)
Total 3.3 (1.7 with sloshing estimation) Although the calibration error is not likely correlated with the sloshing a linear sum has been used to reflect a worst case estimate.
Resulting quasi-sinusoidal error 9.6 10-9(4.9 10-9 ) Requirement 10-8 ms-2
25
Micro Vibration

• With the envisaged accommodation of ISA and
  reaction wheels no specific damping is needed.
• With evolving design calculation of transfer
  functions will be reiterated (such that counter
  measures can be taken should resonances develop)
• The flight models of the wheels will be
  individually characterised, since there is a
  considerable variation in the force spectrum for
  the same type of wheels.

Typical wheel force spectrum
Acceleration amplitude from wheel imbalance.