Rosetta Evaluation of Sun Interference During LEOP

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Rosetta Evaluation of Sun Interference During
LEOP

• by
• Attilio Gesmundo, Enrico Vassallo, Marco Lanucara
  
• ESA ESOC
• Robert-Bosch-Str. 5, D-64293, Darmstadt (Germany).

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Introduction

• The Rosetta mission due for launch in January
  2002 has been delayed as precautionary measure
  after the failure of the previous launch with the
  European Ariane 5 launcher Due to that, a new
  mission plan targeting comet Churyumov-Gerasimenko
  (C-G) instead of the initially baselined
  Wirtanen, has been devised.
• For the new trajectory and launch window, it is
  possible that the angular offset between the
  probe and the Sun as seen from the primary ESA
  station at New Norcia (Australia) becomes as
  small as almost 1 deg during the initial days of
  operations.

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• The Sun-Earth-Probe (SEP) angle versus launch
  date is depicted in the following figure. The
  worst case happens if the launch occurs on
  13/03/2004, two days after the launch.

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• In these conditions the possibility to control
  the spacecraft and to receive telemetry from it,
  becomes a critical issue.
• The link scenario for the baseline launch is
  depicted in the following figure (not in scale).
• Figure 1 Nominal launch scenario

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• Launching Rosetta in the opposite direction (as
  depicted in Figure), the uplink reception at the
  spacecraft may be degraded by the Sun noise while
  the downlink is not affected.
• Figure 2 Alternative launch scenario
• Rosetta on its trip to the comet will come back
  to Earth for three fly-bys necessary to target
  the comet.
• If Rosetta were launched according to Figure 2,
  then the telemetry problems would occur three
  times instead of only once during LEOP.
• Therefore, decision was taken to keep the
  baseline launch profile (shown in Figure 1).

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Methods employed for the analytical calculation

• The analysis of the degradation of the antenna
  G/T due to the additional Sun noise has been
  carried out following two different methods
• The first one employs an empiric formula that
  allows evaluating the degradation of the G/T
  considering the antenna gain as a constant around
  the selected offset.
• The second one carries out the integration of the
  Sun noise power over the antenna pattern

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• In the first method the Sun is considered as a
  uniform radiating body (of 0.5 deg extension) and
  the antenna gain is equal to the value at the
  intersection between the antenna pattern and the
  centre of the Sun (see figure).

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Calculation of the Sun contribution to the
antenna noise

• Calculation considering the Sun as a point source
  
• The antenna system temperature including the Sun
  contribution is evaluated considering the
  following formula
• The numerical values in the previous formula are
• Sunsize 5.98 10-5 Sterad
• Gmax . Sensitivity 29-25Log(?) New Norcia
  antenna mask
• Tsun 200000 K (S-band)

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Results of method 1

• The results of Eq. 1 for the Spacecraft to
  Earth-Probe Angle (SEP) ranging from 0 to 10 deg
  can be seen in the following table and figure.

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Calculation using the integration of the Sun disk
over the antenna mask

• This method employs the direct integration of the
  Sun disk over the antenna mask. The thermal
  contribution from the Sun can be modeled as an
  additional antenna noise temperature, calculated
  as follows
• P is a single polarisation attenuation factor
  (0.5 in this case)
• TSUN is the Sun's black body temperature
• G(q,f) is the gain of the antenna
• q, f are angles in spherical polar coordinates

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• The expression of Eq. 2 can be further
  simplified
• ?x?0-?s or 0 which ever is larger
• ?s is the apparent semi-angular extension of the
  Sun, as seen from the Earth, 0.25.
• ?0 is the already introduced SEP angle
• ?i is a function of ? as follows
• Given ?TA, the degradation ? in the G/T is given
  by
• where L is the station loss in linear units
• Tsys is the antenna system temperature (equal to
  55 K for New Norcia station).

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Results of method 2

• Using the antenna mask, the results of these
  computations are shown in the following figure
  and table.

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Results from the measurement campaign at New
Norcia

• The Sun contribution to the total noise has also
  been measured at the New Norcia station. For
  these tests, the antenna was swept past the Sun
  with constant right ascension angle and later
  with constant declination angle thus forming two
  sets of locally orthogonal cuts.
• Measurements of the system noise increase with
  respect to the reference point at 20 deg angular
  offset were performed (carrier only) with a
  spectrum analyzer (Delta Specan noise reading)
  and with the Intermediate Frequency Modem System
  IFMS (this includes delta noise estimation with
  the main chain IFMS A and delta noise estimation
  with the redundant chain IFMS B) moreover
  measurement of Es/No degradation observed while
  having a telemetry data flow test on-going by
  reading the demodulator's (IFMS) noise estimates
  (delta Es/No.)

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Results from measurement campaign
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Considerations after the measurement campaign

• It can be seen that the maximum observed noise
  increase at 1 deg offset is 10.5 dB.
• Given that the measurement error is about 1 dB,
  the expected degradation ranges from 11.5 dB to
  9.5 dB, very close to the estimates obtained with
  the two different analytical approximations.
• Such measurements also prove that the actual
  antenna gain plot is very close to the antenna
  mask at least at 1 deg offset.

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Link budgets evaluation

• Starting from the G/T degradation figures
  calculated and measured, new link budgets have
  been computed in order to evaluate the maximum
  allowable telemetry bit rate and the maximum PLL
  bandwidth usable by the IFMS (ESA Ground
  receiver).
• During LEOP the mission is designed for a
  telemetry rate of 5461 symbol-per-second or
  2380.53 bps (information bit-rate). to establish
  and maintain lock on Rosetta's signal.
• The link budgets evaluated are related to the
  above cases as follows
• Nominal case (according to the measurements
  results). G/T degradation 10.5 dB
• Best case (according to the results of Table 2).
  G/T degradation 9.3 dB
• Worst case (according to Table 1). G/T
  degradation 11.80 dB.

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Link budgets (Results of calculation with extra
degradation due to the Sun)
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Link budgets (Results of calculation with extra
degradation due to the Sun) 2
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Link budgets (Results of calculation with extra
degradation due to the Sun) 3
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IFMS testing with additional noise input

• The ESAs station demodulator (IFMS) is specified
  to operate with noise density levels at its input
  ranging from -140 to -120 dBm/Hz.
• The standard set-up of the New Norcia station is
  such than at 1-deg SEP angle the noise density
  level would be -113.9 dBm/Hz
• Therefore, a short verification test was run to
  see if the IFMS could operate normally under
  these conditions.
• The test was done at the 'nominal' Rosetta LEOP
  rate of 5461 sps and encompassed also the station
  frame synchronizer and decoder (called CDS-2A).
• The system worked with noise density levels
  considerably higher than the IFMS specifications
  as given in the following table (pre-set Es/No of
  approximately 6.0 dB).

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IFMS testing with additional noise input (results)

• For noise density levels as high as -108 dBm/Hz
  the end-to-end degradation was less than 0.5 dB.
• In order to be sure that at -113.9 dBm/Hz ? 1 dB
  no additional radio loss is generated by the Sun
  noise, it has been proposed to reduce the down
  converter gain to the minimum value (10 dB
  reduction) such that the noise density level at
  the IFMS is -130 dBm/Hz in nominal conditions
  with no Sun irradiation.
• In such case, even for a 12 dB noise increase the
  resulting noise density of -118 dBm/Hz will not
  result in additional loss on top of the G/T
  reduction.

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Conclusions

• Two different analytical methods have been used
  to estimate the noise level increase at the New
  Norcia station due to the Sun illumination of the
  antenna during the initial LEOP days of Rosetta.
• A measurement campaign was also carried out with
  measurements ending up in between the two
  analytical numbers.
• Although the ESAs demodulator (IFMS) is
  specified to work with noise density levels below
  120 dBm/Hz, tests have demonstrated that it
  continues to work at least under Rosetta specific
  conditions up to 108 dBm/Hz with an additional
  radio loss of 0.5 dB.
• By offsetting the down converter gain by 10 dB,
  the noise level is such that even with the Sun
  irradiation no additional radio loss has to be
  accounted for.
• Finally, link budgets for the case of Sun at
  1-deg offset in S-band at New Norcia have been
  computed showing that the IFMS is able to
  demodulate the 5.4 ksps telemetry baselined for
  LEOP (and the emergency 21 sps rate) with no
  additional loss.