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NASA JPL page https://echo.jpl.nasa.gov/asteroids/1999KW4/1999KW4_planning.2019.html says of the radar observations during the close approach of asteroid 1994 KW4 (in part):

Update on 2019 May 22: A functional klystron will not be available at DSS-14 so monostatic observations will not occur at Goldstone. We have scheduled time at DSS-13 instead and have requested time at the Green Bank Telescope to receive.

In 2019, the asteroid will approach from the south, and the first day of visibility also coincides with the closest approach (May 25). Thereafter the asteroid will move north and the SNRs will rapidly diminish as the asteroid moves away from Earth. Due to the southern declinations, 1999 KW4 is observable at Goldstone four days before it enters the beam at Arecibo.

1999 KW4 has the least accessible delta-V for a spacecraft mission of any known binary near-Earth asteroid due to its heliocentric inclination of 39 degrees, eccentricity of 0.7, and semimajor axis of 0.64 au.

This object will be the target of an extensive observing campaign supported by NASA's Planetary Defense Coordination Office.

1999 KW4 has been classified as a "Potentially Hazardous Asteroid" by the Minor Planet Center.

Questions:

  1. What are "monostatic observations"
  2. Why does the non-availability of a functional klystron at DSS-14 prevent them
  3. How exactly will DSS-13 be used in its place
  4. Why does the use of DSS-13 instead of DSS-14 then require the Green Bank Telescope to receive?
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  1. Monostatic just means the transmitter and receiver are in the same place (DSS-14), as opposed to bistatic radar where they are separate (DSS-13 transmitter, Green Bank receiver).

  2. No klystron means they don't have a functioning radar transmitter.

  3. DSS-13 will be used as the transmitter, while Green Bank will receive the radar echo.

  4. Monostatic radar requires a system to isolate the powerful transmitter from the sensitive receiver. You don't want to blast the receiver with waves straight from the transmitter.

The decision to use Green Bank may be because DSS-13 does not have such an isolation circuit.

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