Echo Strength Predictions
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. Steven J. Ostro
PHONE: (818) 354-3173 |
300-233 | FAX: (818) 354-9476
Jet Propulsion Laboratory | email: ostro@echo.jpl.nasa.gov
Pasadena, CA 91109-8099
Asteroid Radar Astronomy
