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The BBC's US wants giant radar in UK to track space objects says:

Lt Col Jack Walker of the US Space Force told the BBC the US was "in discussion" with the UK about putting the radars "possibly in Scotland or further south".

He said the site would house "anywhere from 10 to 15 parabolic antenna (large satellite dishes) for tracking and four to six for transmitting" and cover an area of about 1km square.

Each radar dish will be 15 metres in diameter.

Col Walker said the purpose of the system would be to "detect and track targets which could potentially be threats to our high-value assets".

and this answer to How can we install a radar on radio telescopes like FAST or GMRT? in Astronomy SE discusses a similar-sounding cluster of mixed transmitting and receiving dishes for "China's near-Earth asteroids radar system" and the phasing of the dishes as discussed in the paper it links to.

Radar surveillance is heating up!

Question: Does transmitting from a few dishes significantly improve the performance of radar surveillance from Earth at GEO and beyond? If so, how exactly?

Related:

uhoh
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Multiple dishes can operate as a phased array to produce a narrower beam, but due to the thinned array curse, can not produce a brighter beam than any one of them could if it had the same total transmit power.

However, such an array can scan the beam electronically far more quickly than a dish could move, which might allow more rapid surveys of a patch of sky, or different types of observations that depend on such rapid scanning. Also, the transmit dishes may not operate as a phased array, but be used to illuminate multiple different regions. An array with 4 transmitters and at least as many receive dishes could potentially be split up to continuously track 4 different objects in completely different parts of the sky, or to continue investigating a newly discovered object while the bulk of the array continues with a survey.

In short, it's a tradeoff. Investing the same resources in a single dish might allow brighter illumination and longer radar range/sensitivity to smaller objects, at a cost in flexibility.

Christopher James Huff
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  • A single dish can produce an array of beams using a focal plane array of feed horns. This is done in both receive 1, 2 and transmit 3 modes and could be multiplexed in frequency or in timing to avoid adjacent channel cross-talk if necessary or the beams can be computationally combined similarly to ASKAP. – uhoh Jul 18 '21 at 22:27
  • "Thinned array curse" solves The problem had a specific recognized name that I can't recall... though I'll need to change "receive" to "transmit" in that paragraph unless it applies in both directions (which it seems it might). – uhoh Jul 18 '21 at 22:31
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    @uhoh it is a general truth about electromagnetism that antennas receive in exactly the same way they transmit, in what is usually called the reciprocity theorem. – Ryan C Nov 18 '21 at 07:18
  • @RyanC yes indeed, but I'm referring to the curse rather than the radiation pattern. If there's a curse reciprocity theorem I look forward to reading about it! – uhoh Nov 18 '21 at 08:25
  • @RyanC the reason there could be some asymmetry is in the ways a sparse array could be used differently between radio astronomers and radar engineers. In addition to different targets, sources of noise and signal processing schemes, the radar folks will be working more of a near field (e.g. 10 km baseline, 400 km altitude) while it's strictly far-field for the astronomers. So I'm going to remain noncommittal for now on this. – uhoh Nov 18 '21 at 08:41
  • The more I think about this the more interesting it becomes! If two transmitting dishes are several km apart they will excite a complex-shaped LEO spacecraft somewhat differently because of the different angles. The "curse" in its simplest form applies to really far-field patterns. I can see that it's going to take me a long time to work this whole problem, which is a lot bigger than one question can cover. It will be my own personal "rabbit hole". But in short, the reciprocity theorem is extremely helpful, and perhaps could even be moved up into the answer. Thanks! – uhoh Jan 09 '22 at 17:59