Turns out the way that the gain of a passive reflector seems to be measured is: "the ratio of the power density at a distant point due to the passive repeater to the power density which would exist at the same point" if the repeater were replaced by a matched antenna (or basically nothing at all).
So basically it's a measure of how much better the signal is when you add the reflector, and that's why it can achieve such high gains: because the signals traveling so far are already being atmospherically attenuated by hundreds of dB. Maybe that's not new information to others.
Anyways, cool stuff. Sometimes the best solutions are the simplest.
http://www.gbppr.net/splat/Passive-Repeater-Engineering.pdf#...
https://blog.ucs.org/david-wright/the-moon-and-nuclear-war-9...
Thanks too relatively modern digital modes this doesn't need too much transmission power.
On the upper GHz bands with dishes they even manage to do reliable FM chats. But that requires a lot of gain and active steering of the dish.
I’m pretty sure that I have connected line of sight to cell sites at much longer ranges than that.
Or maybe I’m remembering an old pre-gsm TDMA phone?
https://en.wikipedia.org/wiki/Project_Echo
Radio astronomy was an accidental offshoot of this project: they noticed the reflected microwave signals from space came back with some extra noise...
Perhaps you're conflating Project Echo with Karl Janksy's Bell Labs research in the 1930s? Radio astronomy's "birth" is probably best set to when Jansky detected radio emission from the Milky Way in 1932-1933 while trying to identify the source of noise in wireless telephone transmissions.
Grote Reber picked up radio astronomy in the pre-war (WWII) years and then the advancement of RF technology for radar during WWII led to some further discoveries (e.g., radio emission from the Sun). After WWII, radio astronomy initially had good participation from radar folks.
Many national radio observatories were already in existence before Project Echo. Jodrell Bank Observatory (UK) was established in 1945. ASTRON (Netherlands Institute for Radio Astronomy) was founded in 1949. The US's Green Bank Observatory was created in 1956 and this led to the creation of the National Radio Astronomy Observatory in 1959. Parkes, in Australia, was completed by 1961.
Radio astronomy was well under way before Project Echo.
For the sake of simplicity I have used "passive repeater" here to refer to microwave reflectors only, but the same term is also used for arrangements of two antennas connected back-to-back. These are much more common in VHF/UHF than in the microwave, although microwave passive repeaters of two parabolic antennas have been used in limited cases.
No, that's not correct at all. The Tube Alloys project[0] was the key, codified in the Quebec Agreement[1], giving the USA access to UK nuclear weapons research.
[0] https://en.wikipedia.org/wiki/Tube_Alloys [1] https://en.wikipedia.org/wiki/Quebec_Agreement
It is an item of some irritation to me that many people think the USA was the nation which started nuclear weapons development first. "In July 1940, Britain had offered to give the United States access to its research, and the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as advanced."
I could probably rewrite the above sentence to improve it, but I focused on the magnetron because it was seen to be of special significance at the time (directly addressed issues that US efforts at e.g. MIT Radiation Laboratory were struggling with) and that there's an interesting story surrounding the couriering of the "most secret" magnetron to the US (it was briefly lost). These were the early days of "classified" as a concept and consistent techniques around safeguarding classified matter hadn't been developed, so the magnetron plays an interesting role there as well (along with documents on a number of topics, but I believe the magnetron was the only "physical artifact" brought by the Tizard mission).
https://www.reddit.com/r/cellmapper/comments/1h4i1cl/passive...
jauntywundrkind•5mo ago
https://www.jpl.nasa.gov/news/giant-radar-antenna-reflector-...
ziofill•5mo ago
isoprophlex•5mo ago
admash•5mo ago
Alternately, complex orbital tracks may result in irregular accumulation of multiple scans, with double scanning only finally being achieved after 12 days.
evil-olive•5mo ago
> In a first, the satellite combines two synthetic aperture radar (SAR) systems: an L-band system that can see through clouds and forest canopy, and an S-band system that can see through clouds as well but is more sensitive to light vegetation and moisture in snow.