Sputnik radio transmission audio - url: http://nssdc.gsfc.nasa.gov/planetary/sound/index.html

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This is not the Sputnik-1 signal. There are a number of authentic recordings of the signal that can be found on Sputnik 1 in the external links. Please don't link this into the article on Sputnik-1. It is not Sputnik-2 or 3 either, so nobody really knows what it is. It does appear to be some kind of PDM signal, maybe from a Soviet spacecraft.

This file was selected as the media of the day for 02 February 2014. It was captioned as follows: English: Possible PDM signal labeled as Sputnik by NASA Other languages English: Possible PDM signal labeled as Sputnik by NASA 中文（简体）：NASA判定可能来自史泼尼克的PDM信号