35meter Deep Space Antenna

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• 35-meter Deep Space Antenna
• Pointing Calibration System

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Author

• Robert A. Plemel
• SED Systems, a division of Calian Ltd.,
  Saskatoon, Saskatchewan, S7N 3R1, Canada

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Background

• In February, 2003 SED Systems of Saskatoon,
  Canada was awarded a contract by ESA to supply an
  X/Ka-band 35-meter TTC antenna system to be
  installed at Cebreros, Spain
• The pointing error (PE)requirement is 6 mdeg
• SED is developing aPointing CalibrationSystem
  (PCS) to ensurethe PE requirementis met
• The PCS will provideautomated determinationof,
  and compensation for,systematic pointing errors

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Systematic Pointing Error Sources
SPEM Model

• Az and El encoder offsets
• Gravity deformation of the main and subreflector
• Tower tilt
• Az/El Axis non-orthogonality
• RF collimation
• Beam waveguide mirror and feed misalignment
• RF beam squint (polarization and frequency band
  dependent)
• Atmospheric refraction
• Thermal deformation

Real Time
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PCS System Design
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PCS System Design

• The main elements of the PCS are
• Pointing Calibration Computer (PCC)
• Runs the PCS application software to control the
  pointing calibration process
• Ethernet LAN for communication with the FEC (for
  remote control of the PCS)
• IRIG-B time interface
• LAN interfaces to ACU, weather station,
  radiometer, and temperature measurement system
• Pointing Calibration Workstation (PWS)
• Provides the local user with a graphical
  interface for the PCS
• A remote access capability is provided to allow
  the same functions from a remote workstation

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PCS System Design

• Radiometer
• Used to measure system noise temperature
• Separate inputs for each downlink frequency band
  and polarization
• IRIG-B time interface
• Noise adding and total power modes

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PCS System Design
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PCS System Design
Radiometer MC Processor Board
Radiometer RF and Measurement Board
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Antenna Physical Temperature Measurement System

• 250 temperature sensors located on the main
  reflector back-structure, and the subreflector
  quadrapod struts
• Temperature data is usedby the PCS to calculate
  thepointing error due to thermaldistortion of
  the mechanicalstructure
• Update rate 1 minute

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PCS Operating Modes

• Calibration
• SPEM calculation
• Compensation
• PE Measurement
• Noise Temperature Measurement

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Calibration Mode

• Typical calibration for one frequency band, one
  polarization, will take approximately 8 hours
• Approximately 100 PE measurements are taken
• Main elements are
• Scheduler
• PE measurement, including Star tracking
• SPEM curve fit

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Scheduler

• The operator begins by specifying
• Start time and duration of a calibration
• Elevation constraints, if any
• The scheduler automatically selects from a
  library of calibration sources
• Sources have an angular extent of less than 1
  mdeg
• Flux density gt 1.5 Jy in both X- and Ka-band

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Scheduler

• The scheduler automatically builds a measurement
  schedule
• Maximizes the number of PE measurements to be
  made in the calibration period
• Provides near-uniform distribution over the
  hemisphere
• Avoids successive measurements in a small area of
  the sky
• Gives preference to higher flux density sources

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Scheduler
Scheduler Output for 8 hour Calibration (80 PE
Measurements)
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PE Measurement

• PCS calculates the track of the star (accurate to
  0.1 mdeg)
• Uses a grid of Az and El offsets around the
  nominal position of the star
• X-band beamwidth 64 mdeg
• Ka-band beamwidth 17 mdeg

• PCS commands the antenna to follow a trajectory
  through the grid points
• PCC commands the radiometer to measure system
  noise temperature at each grid point

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PE Measurement

• Typical time for one PE measurement is 5 minutes
• A mathematical model is fit to the measured Tsys
  data to determine the location of the RF beam
  relative to the commanded position

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PE Measurement

• Measurement accuracy depends on radio star flux
  density, integration time, and background noise
• Accuracies are similar for X-band and Ka-band

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SPEM Calculation

• Can use 50 to 10,000 PE measurements
• Mathematical model (SPEM) is fit to the PE
  measurements
• Model for DSA2 contains 14 coefficients

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SPEM Calculation
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Compensation Mode

• Normal operating mode of the PCS
• After calibration, the PCC transfers SPEM
  coefficients to the ACU
• ACU applies the systematic corrections to
  commanded antenna positions

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Other PCS Capabilities

• Single PE measurement
• Typically used by engineering staff
• User selects a specific star from the calibrator
  database
• PE measurement is made
• Results provides PE, beamwidth, atmospheric
  noise, and temperature contribution of the radio
  star
• Useful for monitoring antenna performance,
  including downlink noise temperature, G/T, and
  antenna efficiency
• Different PE compensation models can turned ON
  and OFF to measure their relative size

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Other PCS Capabilities

• Mini-cal
• Typically used by mission operators to verify the
  pointing accuracy of the antenna prior to
  critical mission events
• User specifies the duration (say 30 minutes)
• PCS selects an optimum set of stars (typically 3
  to 5) widely distributed in the sky
• PE measurements are made
• Noise Temperature Measurement Mode
• Used by engineering staff to check for
  degradation of the cryogenic LNAs or RF portion
  of the antenna
• Radiometer is used to measure the noise
  temperature at a specific elevation angle and
  azimuth

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Conclusion

• The development of the PCS is well advanced
• Factory testing of the radiometer and PCS
  software scheduled for completion in September
  2004
• Commissioning of the DSA2 antenna under
  construction at Cebreros, Spain is scheduled for
  June 2005
• Based on measurement, simulation, and analysis
  results to date, the PCS is expected to meet its
  operational and performance objectives

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