HETE2 VHF Network Status: Implications for ECLAIRS

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HETE-2 VHF Network Status Implications for
ECLAIRS

• George Ricker
• MIT Kavli Institute for Astrophysics and Space
  Research
• Contributors John Doty, Joel Villasenor, Peter
  Ford, Geoff Crew

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HETE-2 VHF Specifications
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HETE-2 Spacecraft Transmitter
VHF amplifier
Radio system weighs lt 1kg
Whip antenna
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HETE-2 Burst Alert Network

• A network of 14 stations, all connected via the
  internet
• Delay from each station dependent on internet
  connectivity, typically seconds from
  transmission
• Each station is autonomous, low maintenance
• Station components are low cost lt 2000/station
• Coverage is overlapping (gt95), providing
  redundancy
• Installation can be accomplished within a day
• Spares pool is maintained at MIT

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HETE Ground Station System Diagram
HETE VHF
Quadrifilar antenna
Low Noise Pre-amplifier
Receiver (Drake R8, AR3030)
DC Block
Cable run
Audio line (Signal)
RS-232 (Doppler tracking)
Computer
Internet
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HETE-2 VHF Station Equipment
Quadrifilar antenna low-noise pre-amplifier
PC (monitor, keyboard) Radio receiver
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HETE-2 Ground Station Locations
Maui
Marquesas/ Hiva Deux
Natal
Singapore
Palau
Ascension
Malindi/ Malindue
Kwaj-PGS/ Kwaj-806
Galapagos
Kiribati
Bohol
Bangalore
Gabon
Cayenne
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VHF Network Efficiency for lt10 s Alert Delays

• Efficiency is based on continuous, 24 hour
  monitoring
• Efficiency is gt90 for lt10 min alert delays
  100 for lt20 min delays

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Network Current Status (4-7 April 2005)
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VHF Station locations
Original 14 sites
augmented with 15 more
HETE i2 deg
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ECLAIRS Suggested VHF Sites
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Simulation ECLAIRS Downlinks to 29 VHF Sites
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ECLAIRS Downlink and Orbit Determination

• In spacecraft, use stable oscillator
• Accuracy 1108 or better
• Drive millisecond clock
• Insert clock values synchronously into downlink
  packet headers
• On-board GPS receiver unnecessary
• Retain 14 HETE VHF Burst Alert Stations
• Use existing 137 MHz antennae
• Use existing control computers
• Use existing Internet connections
• Add 15 new HETE VHF Burst Alert Stations
• Replace existing VHF receivers with
  pre-amplifiers and computer-controlled digital
  receivers to time-tag telemetry packets and
  derive one-way Doppler shifts
• Add GPS-derived ground receivers to
• Time-tag received data packets
• Determine downlink carrier frequency
• Improve over HETE NTP synch method

137 MHz
ECLAIRS
Pre-amp
To theInternet
Digital receiver
GPS Time Frequency

• NORAD tracking (approx. weekly) accurate to 20
  km
• 1-way Doppler tracking ( 3 KHz at 137 MHz)
  accurate to lt 1 km
• Burst timing accurate to 0.8 msec

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ECLAIRS Suggested Ground Station Design

• Quadrifilar VHF Antenna 300
• already installed at all HETE-2 ground stations
• Pre-amplifier with 20 dB gain at 137 MHz 200
• Digital Receiver, e.g., Ettus Research USRP 500
• 4 coherent channels
• coherent both with carrier and with bit
  modulation
• 800 µsec time resolution if using h2 modulation
  index
• uses open hardware and software standards
• compatible with existing HETE-2 software
• uses existing Internet connections
• GPS receiver, e.g., HP 58540A 1850
• provides both standard frequency and time
• Control computer 500
• use low cost Emachine (w. Linux) from HETE-2
• runs existing HETE-2 packet extraction software
• runs existing HETE-2 Doppler tracking software

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Summary ECLAIRS VHF Network

• Utilize HETE-2 flight and ground hardware
  heritage
• Retain HETE VHF frequency allocation for ECLAIRS
• No flight GPS is needed. VHF system alone can
• Provide absolute timing needed for science goals
• Derive accurate orbital elements
• Augment the network from 14 to 29 stations to
  provide coverage for i20 degree orbit
• Utilize low cost, next generation receivers
• Can raise bandwidth to 600 bps if needed
• More compact standard interfaces
• Open source hardware and software
• Eliminates problems from vendors discontinuing
  models
• Network Costs 30 x 3.5K 100K
• Propose VHF Network as part of US mission
  contribution (fabrication, integration,
  testing, maintenance)

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Option Low Cost SXC for ECLAIRS?

• Science Drivers for limited-function SXC
• Provide vernier to reduce CdTe error boxes to
  lt1arcmin
• Important for prompt ground-based IR searches for
  zgt7 GRBs, since few large telescopes have IR
  imagers w. FOV gt 4arcmin (Reason 1K x 1K HgCdTe
  arrays _at_ 0.25arcsec/pixel)
• HETE and Swift results indicate at most 10 of
  ECLAIRS GRBs will be at high-z, or 10 per year
  for ECLAIRS. Since chance that a specific
  ground-based telescope can promptly observe a
  given burst is 10, would need 10 telescopes
  with Dgt3 meters which can image the ECLAIRS error
  box w.o. rastering. Error boxes of 1-2 arcmin
  would assure all high-z candidates were observed
  in IR.
• Explore 0.5-2 keV prompt emission in XRFs, XRRs,
  and GRBs

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Low Cost SXC for ECLAIRS-cont

• Candidate SXC for ECLAIRS
• 2sr FOV, coaligned with CdTe
• Single X and single Y camera, just like HETE,
  would provide lt1 locations for 50 of
  CdTe-localized events (50 per year in vernier
  mode)
• Per camera Dims 10cm x 10cm x 10cm Mass 1kg
  Power 2W
• Low cost
• Mechanical design from HETE
• Low power electronics from MIT DARPA program
• High likelihood of funding
• US-funded program cost would be less than a
  balloon or sounding rocket
• Non-NASA funding sources are possible