Echo Strength Predictions


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Update on 2009 September 3:

Due to a hard drive crash, most of these plots are not 
currently available, but we have located copies of
two and posted them:

Arecibo   +18 degrees
Goldstone -20 degrees

Both figures show objects out to a distance of 0.5 AU.
We intend to rebuild this site as time permits, so 
please be patient.

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The following plots show predictions of the single-date signal-to-noise ratio (SNR) for echoes from asteroids at Arecibo and Goldstone, as a function of the target's declination, distance, and absolute visual magnitude (converted to diameter by assuming an S-class optical albedo). Assumptions include a 10% radar albedo, an equatorial view, a 4-hour rotation period, and optimal values for telescope system parameters.

The number of pixels with useful dynamic range that can be placed on a target is comparable to the SNR.

The following plots show more detailed views of the region where the distance is less than 0.1 AU:

The following plots show the SNR for larger targets

Why is radar NOT a useful technique for searching for near-Earth asteroids (NEAs)?

The answer is that the parameter space (time delay, Doppler frequency, and direction as functions of time) is simply too huge. Arecibo and Goldstone achieve the sensitivity needed for NEA observations via their large size and hence large antenna gain, which produces extremely narrow beams. These telescopes need a pointing ephemeris good to half an arc minute and a reasonably accurate Doppler ephemeris to be able to detect a known NEA. The phrase "functions of time" is critical, because radar waveforms mandate that radar detection involves time exposures, and hence demand that the target be tracked not just along its sky path but also along its time-delay (range) and Doppler-frequency (radial velocity) paths. This is why a decent ephemeris for an object is needed if there is any hope of detecting the object.

  Dr. Lance A. M. Benner        
  PHONE:  (818) 354-7412      |
  MS 300-233                  |   FAX:    (818) 354-9476
  Jet Propulsion Laboratory   |   email:  lance.benner@jpl.nasa.gov
  Pasadena, CA 91109-8099

Asteroid Radar Astronomy