0
$\begingroup$

@Hobbes' concise yet thorough answer to the question What are monostatic radar observations, and how will Deep Space Network's DSS-13 be used to observe asteroid 1999 WK4's flyby of Earth? tells us that NASA's Deep Space Network's DSS-14 dish is capable of both transmitting powerful radar signals and receiving the extremely weak reflections from objects in deep space.

This is called monostatic radar.

This radar is a little different than the kind of radar used to detect speeding cars, aircraft, or ships.

The delay-doppler signal may be used to generate images of 1999 WK4 and it's little orbiting moonlet, so it may have some special coding that is sent in a long stream such that the received signal will be pretty complicated. It's not just a "ping!"

Question: How exactly does Deep Space Network's DSS-14 implement monostatic radar for the study of asteroids in deep and cis-lunar space?

Possibly helpful NASA JPL page https://echo.jpl.nasa.gov/asteroids/1999KW4/1999KW4_planning.2019.html


The NBC News article Mile-wide asteroid and its tiny moon to zoom past Earth this weekend; Dubbed 1999 KW4, the "binary" space rock will skim past our planet harmlessly at a distance of 3 million miles. shows the animated GIF below, and links to the Asteroid Tracker page 1999 KW4.

enter image description here

enter image description here

$\endgroup$
2
$\begingroup$

Evidently the DSS-14 does rely on separate periods of transmitting and receiving, but it is able to switch between the transmitting and receiving modes rapidly using a "quasi-optical switch". Per Perez and Bhanji 1997,

Due to the large antenna to target distances, involving round - trip light times of up to several hours, duplexing in this radar system has been achieved until recently by positioning the antenna sub - reflector to focus either on the transmit or receive feedhorn

(in other words, they could transmit a signal, and re-focus the antenna so that the received echoes would come in to the receive feedhorn, and this was a slow process, but that was okay because the echoes would take minutes or hours to arrive), but

Recent emphasis on the observation of near - Earth asteroids, with round - trip light times as short as 30 seconds present a difficulty in that the antenna sub - reflector requires approximately that amount of time to be re - positioned from the transmit to the receive position. Additionally, this causes a great deal of mechanical wear on the positioning mechanism of the 9 meter diameter, 3600 kg sub - reflector due to the high number of transmit / receive cycles that maybe performed while a near - Earth asteroid is visible in the sky.

and that the QOS allows "switch times in the order of one second, allowing radar imaging of asteroids inside a Lunar orbit".

Unfortunately the visibility of the diagrams in the scanned paper is poor, and the explanation in the text is minimal, but it seems apparent to me that the switching is accomplished by means of a set of radio "mirrors" that are moved into the signal path during receive, to move the focus point to the receive feedhorn, and moved out of the path during transmit.

Another observation bulletin for a near-earth asteroid from 2012 notes that

We will have to observe bistatically with DSS-13, because the quasi-optical switch at DSS-14 has been removed for repairs.

which confirms the necessity of the QOS for monostatic imaging of nearby objects.

$\endgroup$
  • 1
    $\begingroup$ I took the liberty of correcting "feedhom" to "feedhorn" several times in the quoted material — an obvious OCR error. $\endgroup$ – hobbs May 28 at 15:43
  • $\begingroup$ Thanks for the speedy answer! The upcoming closest approach of 1999 WK4 will indeed have a round-trip time of about 35 seconds. $\endgroup$ – uhoh May 28 at 16:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.