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Characterizing Small, Dim Near-Earth Asteroids with Coherent Doppler Ladar

Characterizing Small, Dim Near-Earth Asteroids with Coherent Doppler Ladar. Bijan Nemati bijan.nemati@jpl.nasa.gov M . Shao, C. Zhai , S. Turyshev Jet Propulsion Laboratory, California Institute of Technology 9/30/2013. Introduction. Passive (sunlight). NEO trajectory.

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Characterizing Small, Dim Near-Earth Asteroids with Coherent Doppler Ladar

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  1. CharacterizingSmall, Dim Near-Earth Asteroids with Coherent Doppler Ladar Bijan Nematibijan.nemati@jpl.nasa.gov M. Shao, C. Zhai, S. Turyshev Jet Propulsion Laboratory, California Institute of Technology 9/30/2013 Asteroid Initiative Idea Synthesis Workshop

  2. Introduction Passive (sunlight) NEO trajectory Active(Laser) NEO facility mass determinationusing size and extended astrometry Earth Combined Imaging via ISAL at < 1 LD moon Size determinationvia Incoherent Imaging Detection & Astrometry via Synth Tracking at 40 LD • By equipping a 4 meter-class, ground-based telescope with laser active illumination and adaptive optics we can have a facility that can comprehensively investigate asteroids down to a few meters in size, capable of: • Detection, Astrometry, Precise Orbit, Spin rate (out to 40 LD) • with Synthetic Tracking sensitive out to 10m NEO @ 40 LD (3.6 m tel, 2 min) • Size Estimation • with Incoherent LADAR • Imaging • with Inverse Synthetic Aperture LADAR (ISAL) – Coherent Doppler Laser Radar • Mass and density estimation • from Astrometry, over weeks Asteroid Initiative Idea Synthesis Workshop

  3. Implementation • Telescope • 4 m class telescope • A single telescope can be used to both transmit and receive (monstatic operation) • Because fast switching between transmit and receive mode is possible with a laser • Laser system • 1 kW laser at 2 um (coherent) for (imaging, size) • chirp system capable of ~20 GHz/s • 5 kW laser at 1 um (Incoh., AM mod.) for (range,size) • Adaptive optics • Diffraction limitted transmitter • phase matching of LO and return • Acquistion, Tracking, and Astrometry System • High speed camera and GPU for “synthetic tracking” • Synthetic tracking enables precision astrometry NEO track frame 1 AMOSS 3.6 m TelescopeAFRL Maui, HI frame 2 frame 3 SyntheticTracking JPL is in collaboration with USAF on an ISAL project with their 3.6 m telecope frame 4 shift/add velocity vector Asteroid Initiative Idea Synthesis Workshop

  4. Operational Scenario • Discover • Detection (10 m NEO out to 40 LD, further for larger NEO’s) • Using synthetic tracking, detect small asteroids down to < 10 m in size at a rate of tens per night (>100X current rate) • Astrometry to get precise orbits (< 10 mas) • Put the beam on the target • Determine target orbit so it is not subsequently “lost” • Photometry to get spin rate • Estimate Size • Actively illuminate to get size estimate • Visibility drop in chirped AM modulated Incoherent LADAR • Doppler Broadening with Coherent LADAR • Image • ISAL in chirped mode (at < 1 LD) • Measure Mass and Density • via photon pressure, using Astrometry + Size Asteroid Initiative Idea Synthesis Workshop

  5. NEA far away (40 LD) – Synthetic Tracking NEO track frame 1 frame 2 frame 3 SyntheticTracking frame 4 shift/add velocity vector • Detection • Synthetic Tracking allows detection of 10 m object at 40 LD • Astrometry • The beam size, from diffraction, is ~ 2um/4m ~ 100 mas • To put the most power on the target, need to point to ~ 20mas • We have recently demonstrated 7 mas relative astrometry of 19 mag asteroid • GAIA will get everything brighter than 20 mag to < 1 mas on its first release • Once GAIA numbers are available our relative astrometry becomes absolute • absolute astrometry necessary for orbits, to keep track of the asteroid long term Asteroid Initiative Idea Synthesis Workshop

  6. NEA at 12 LD : AM Incoherent LADAR 1 • Higher-power (order 5 kW) laser can also be used, for incoherent LADAR • On the detector side a photon counting detector would be used • The laser is amplitude modulated • Single AM frequency to get radial velocity • Chirp to get range and size • look for a drop in the visibility to sense the size • Estimates show size determination possible at 25% with 5kW laser in 5 hrs from 3.6 m telescope Asteroid Initiative Idea Synthesis Workshop

  7. NEO near (< 1 LD) : full ISAL imaging Measured Phasor (amp, phase) Range Reflectivity(image) Doppler (cross-range) • Send a chirped laser beam with the telescope • Collect the return from the target • use a large telescope to collect more returned power • Mix with a “Local Oscillator” • get heterodyne gain • Fourier-analyze the mixed signal • width from Doppler broadening gives size • chirped version (when SNR is high) gives image Asteroid Initiative Idea Synthesis Workshop

  8. Peformance on 10 m class asteroid • Assume a 5 kW laser from a monostatic 3.6 m facility • Size estimation starts becoming possible at 12 LD (25% ) • At 1 LD imaging becomes possiblewith 1 kW coherent LADAR: • the expected rate of return photons per pixel, assuming an albedo of 0.2, is about 320 ph/s • In a 5x5 pixel image, the expected SNR in each pixel after 1 hr is about 10. NEO trajectory NEO facility mass determinationusing size and extended astrometry Earth Imaging via ISAL at < 1 LD moon Size determinationvia Incoherent Imaging Detection & Astrometry via Synth Tracking at 40 LD Asteroid Initiative Idea Synthesis Workshop

  9. Getting the NEA mass Solar radiation • Sustained observation over several weeks with astrometry at the few mas level allows us to measure the motion of the asteroid due to solar radiation pressure • Astrometry measures the acceleration • and hence the area to mass ratio • ISAL measures the area (via Doppler broadening) • Together these give the mass • Example in backup • 4 mas astrometry over 5 wks of 10 m asteroid with 10 LD impact parameter yields 10% estimate of its masswith synthetic tracking Asteroid Initiative Idea Synthesis Workshop

  10. Technology Status Target BC A/D Target signal target arm PR Freq/Chirp Monitor LO arm RECEIVER LO FREQUENCY MONITOR B 70 GHz chirp in 0.1 sec as measured by freq. monitor LO delay delay AOM TRANSMITTER A/D 5 MHz 5-pole LC AOM XMIT AOM 90% AOM Tunable Laser 1% 99% 10% return from a ball target (+/- chirps) RCVR A Filters prior to ADC range 1 kHz RC C We are developing ISAL technology currently at JPL We are able to produce a 100 GHz chirp in 100 ms (1 THz/s) We have achived first light on a ball target We are working near the photon limit Asteroid Initiative Idea Synthesis Workshop

  11. Backup Asteroid Initiative Idea Synthesis Workshop

  12. NEA < 4 LD : Size via Coherent Doppler mean  speed spread  size • Using a large telescope, transmit a pure tone (no chirp) • Measure Doppler and Doppler spread • Shift the LO frequency to offset the mixed signal off of DC • Doppler mean gives the relative velocity • Doppler spread (+ photometric ) gives the size of the NEA • 1% at 2LD in 1 hr with 1 kw coherent laser Asteroid Initiative Idea Synthesis Workshop

  13. Example: mass of 10 m asteroid at 4 LD Asteroid Initiative Idea Synthesis Workshop

  14. Size Determination – I Asteroid Initiative Idea Synthesis Workshop

  15. Size Determination – II Asteroid Initiative Idea Synthesis Workshop

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