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I had to look this up on Wikipedia[1], it can apparently transmit as well as listen the way that most radio telescopes do and in at up to 20 TW too. I had fran

I had to look this up on Wikipedia[1], it can apparently transmit as well as listen the way that most radio telescopes do and in at up to 20 TW too. I had frankly thought the most powerful radars in the world were in the 10s of MW range and this thing has got a dish 305m in diameter compared to the 10-50m I'm used to for radars tracking satellites in orbit (I used to do software for that professionally). I wouldn't have thought doing active radar scans of an astroid out in space was possible given that radar returns fall off as the 4th power of range. But given that power and dish size, given that these are large objects being tracked, and given that they move slowly compared to the radar cone I guess it's possible after all. But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack and almost caused WWIII[2] so I guess there's precedent, even though the Moon is much larger and closer. I'm seeing references to it tracking asteroids in the Apollo group, so relatively nearby[3] and plates which are much larger. I'd guess that even the biggest asteroid in the main belt, 1 Ceres, would be too dim for it to see but I'm curious. [1]https://en.wikipedia.org/wiki/Arecibo_Observatory https://en.wikipedia.org/wiki/Arecibo_Observatory [2]https://blog.ucsusa.org/david-wright/the-moon-and-nuclear-war-904 https://blog.ucsusa.org/david-wright/the-moon-and-nuclear-wa... [3]https://en.wikipedia.org/wiki/Apollo_asteroid https://en.wikipedia.org/wiki/Apollo_asteroid

ISL 6y ago
If anyone's transmitting at TW powers, it is in a short pulse. You might be comparing it with more-continuous systems.
saalweachter 6y ago
> But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack... They are in good company; like 90% of UFO sightings are just people seeing the Moon and mistaking it for something else (or rather, not immediately identifying it, leaving them with impressions like "there was a bright light following me down the road" or "there was a glowing orb behind the trees").
jaycroft 6y ago
Regarding the 20TW from the Wikipedia page, that's referring to the EIRP - effective isotropic radiated power. This is the amount of power that you would need to radiate from a theoretical antenna with a perfectly spherically symmetric power distribution. The Arecibo antenna has a fairly narrow beam. The 20TW EIRP transmitter appears to operate at 2.38GHz. Plugging that frequency and the 270m diameter, a beamwidth calculator gives me a gain of about 75dBi. So, yes, it's like a 20TW transmitter, but only one tiny narrow little slice of it. At 75dBi gain, that's one (10^7.5)th of the total sphere, about 1 part in 30 million. Or, you could say, if the antenna were radiating the same power in every direction as it does in it's peak direction, that would be 30 million times as much power. So, dividing to find the power in that narrow beam, we get "only" about 600kW radiated in the direction it is pointing. Add in cable losses and other inefficiencies, and the power output at the amplifier is probably close to a megawatt.

ISL 6y ago

If anyone's transmitting at TW powers, it is in a short pulse. You might be comparing it with more-continuous systems.

saalweachter 6y ago

> But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack... They are in good company; like 90% of UFO sightings are just people seeing the Moon and mistaking it for something else (or rather, not immediately identifying it, leaving them with impressions like "there was a bright light following me down the road" or "there was a glowing orb behind the trees").

jaycroft 6y ago

Regarding the 20TW from the Wikipedia page, that's referring to the EIRP - effective isotropic radiated power. This is the amount of power that you would need to radiate from a theoretical antenna with a perfectly spherically symmetric power distribution. The Arecibo antenna has a fairly narrow beam. The 20TW EIRP transmitter appears to operate at 2.38GHz. Plugging that frequency and the 270m diameter, a beamwidth calculator gives me a gain of about 75dBi. So, yes, it's like a 20TW transmitter, but only one tiny narrow little slice of it. At 75dBi gain, that's one (10^7.5)th of the total sphere, about 1 part in 30 million. Or, you could say, if the antenna were radiating the same power in every direction as it does in it's peak direction, that would be 30 million times as much power. So, dividing to find the power in that narrow beam, we get "only" about 600kW radiated in the direction it is pointing. Add in cable losses and other inefficiencies, and the power output at the amplifier is probably close to a megawatt.