HETE Operations

1
HETE OperationsLessons Learned

• Roland Vanderspek, MKI
• September 1, 2005

2
Overview

• Overview of HETE Operations
• Application to Eclairs

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HETE Operations

• Spacecraft in 600 km equatorial orbit
• Commanding done from any of three Primary Ground
  Stations (PGS) Cayenne, Kwajalein, Singapore
• Uplink and downlink are completely automated
• Duty Scientist (DS) is responsible for
  responding to spacecraft anomalies and burst
  events (one week at a time).

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HETE Instrumentation

• Spacecraft bus includes basics (power, RF) and
  attitude control (momentum wheel, three
  orthogonal torque coils).
• Science instruments are Fregate (wide-field NaI
  scintillators, 6-400 keV), WXM (wide-field X-ray
  imager (2-25 keV), SXC (wide-field CCD imager
  (2-10 keV).
• Spacecraft aspect is calculated using two 5x5
  optical imagers coaligned with the instruments.
  Aspect is calculated every 1.0 seconds.
  Precision of aspect is 5 arcseconds.

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Automated Operations

• Spacecraft commands are generated automatically
  simple shell scripts define the parameters of the
  observations (start and stop times, instrument
  configurations, etc.)
• Basic satellite and instrument housekeeping data
  are reduced by automated scripts. Abnormal
  situations (e.g. low voltages, anomalous
  attitude) are flagged and the DS is notified (by
  email or beeper, depending on severity).

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Burst Operations (I)

• Fregate (or WXM) burst trigger leads to full
  on-board localization analysis and downlink of
  full set of high-resolution burst data from all
  instruments (time-tagged photons, detailed
  optical aspect). On-board burst localization
  analyses typically complete within 30 seconds
• Burst localization calculated on board is
  broadcast over the VHF antenna to the Burst Alert
  Network the coordinates are then relayed to the
  GRB Coordinates Network (GCN) at GSFC via MIT.

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Burst Operations (II)

• High-resolution burst data are reduced by
  automated scripts shortly after downlink.
• HETE scientists, alerted by beeper, perform
  manual analyses of the downlinked data improved
  localizations are typically available within 60
  minutes.
• Discussion of manual analyses occurs via the HETE
  chat line (hchat).

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Flight Operations

• Spacecraft pointing is nominally anti-solar.
• Exceptions are handled by nodding the
  spacecraft away from antisolar by up to 40
• Exceptions include
• Full moon avoidance (monthly)
• Sco X-1 avoidance (April-July)
• Galactic bulge (May-August)

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Lessons Learned from HETE (I)

• Flight software development should be done under
  conditions as close to flight as possible
  (software uplink, data downlink, inter-processor
  communications).
• Each HETE instrument team was given a copy of the
  flight processor hardware (pizza box
  spacecraft processor instrument processor) for
  software development.
• A hardware copy or high-fidelity software
  simulator of the UTS (and EGCU?) is needed.

10
Lessons Learned from HETE (II)

• Each instrument must be end-to-end and GSE
  testable, both in the lab and on the spacecraft.
• This means
• Laboratory tests need high-fidelity version of
  the flight computer (pizza box)
• Instruments should be equipped with
  eavesdropping connections (for GSE)
• If possible, instruments should be operable at
  room temperature (for on-the-rocket tests).

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Lessons Learned from HETE (III)

• HETE flight software was developed in US (MIT
  Los Alamos), Japan, and France. The Fregate
  WXM transputer software was developed at Los
  Alamos, but required input from Japan and France
  inadequate communications between teams caused
  problems and introduced delays.
• Flight software modules should be as distinct as
  possible.
• Software interfaces should be defined clearly and
  strictly adhered to.

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HETE Lessons Learned (IV)

• HETE SXC ground analysis software relies on full
  information about spacecraft pointing during the
  burst, i.e., spacecraft aspect every 1.0 seconds.
• The performance of the ESXC flight software
  depends on the frequency of delivery and the
  precision of attitude information (plus
  temperatures) from the spacecraft.
• Good thermal/mechanical model of the spacecraft
  is necessary to calculate an aspect error budget.
• More temperature sensors is better than fewer
  temperature sensors.

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HETE Lessons Learned (V)

• HETE SXC mylar/aluminum Optical Blocking Filter
  (OBF) was destroyed by atomic oxygen on orbit
  Beryllium CCD covers have been unaffected by
  orbital debris.
• ESXC will use a single Be cover over the CCDs.

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HETE Lessons Learned (VI)

• The primary limitation in the precision of the
  HETE SXC is the knowledge of the SXC pointing.
• HETE spacecraft aspect is known to a precision of
  5 at 1 Hz. The aspect cameras are co-mounted to
  the SXC focal plane
• SXC localizations of bright, on-axis,
  orbit-midnight sources have 20 accuracy
  however, the inability to measure the systematic
  thermal distortions of the SXCOPT system have
  led to uncertainties in SXC localizations of 80
  radius.
• Precise spacecraft aspect and knowledge of the
  spatial relation between the star trackers and
  ESXC pointing are essential for precise burst
  localization.

15
Lessons Learned (VII)

• Reliable communication between team members is
  essential during burst analyses.
• The HETE team uses a text-based chat line
  during time-critical burst analyses. Team
  members in Japan, US, Europe can discuss burst
  details in real time with no delay (only the
  those who cannot type quickly suffer). The text
  of the chat is stored in the HETE data archive
  for reference.

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Lessons Learned (VIII)

• Sco X-1 and the galactic bulge sources are very
  bright, resulting in higher background rates from
  April through August this affects instrument
  sensitivity and downlink mass.
• HETE uses the nod capability to move the FOV of
  the WXM away from Sco and the bulge sources.
• E-SXC may also ignore CCD columns which are
  contaminated with Sco X-1 photons this requires
  aspect knowledge in the E-SXC.

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Lessons Learned (IX)

• Estimates of PGS contact durations which assume
  that contact begins at AOS and ends at LOS so not
  properly account for signal acquisition times
  nominal 10-minute contacts are actually 8- or
  9-minute contacts

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Questions for Eclairs (and some answers)

• How many star cameras are there, and how far are
  they from the ESXC? Two, about one meter.
• What is the PSD of spacecraft stability?
  .004/.3s, .0075/1s, .01/5s, more answers are
  expected.
• Flight localization analysis requires steady flow
  of spacecraft aspect what rate can we expect,
  what accuracy? 4 Hz, .02 (?)
• Can we nod? I think Bertrand said yes.
• Will it be possible to have a flight processor
  simulator, hardware of software?