# Why were two simultaneous command transmissions interpreted by Kosmos 57 as an order to begin the descent?

This answer to Encryption in radio system points to the Wikipedia article Kosmos 57 which says:

The unmanned spacecraft was destroyed on its third orbit around Earth. Two ground control stations, one in Klyuchi and the other in Yelizovo, sent simultaneous commands, instead of sequentially as planned, instructing the craft to depressurize its airlock. The craft interpreted this as an order to begin the descent and a propulsion error put the craft into a tumble. Approximately twenty-nine minutes later, the craft's automatic self-destruct function activated. The craft was completely destroyed to prevent sensitive information from literally falling into enemy hands. Over 100 pieces of the spacecraft were tracked, falling into the ocean between 31 March and 6 April 1965.

The two sources cited are:

1. Hall, Rex; Shayler, David (2001). The Rocket Men, Vostok and Voskhod, the First Soviet Manned Spaceflights. Chichester, UK: Praxis Publishing. p. 243
2. https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1965-012A

and the paragraph appears to be nearly a direct copy/paste from that NASA page.

Question: Is it known why two ground control stations sending simultaneous commands instead of sequentially as planned was interpreted as an order to begin the descent, assuming this is indeed what happened?

"A crucial difference on the NASA page is that it says 'The signal accidently (sic) set off the self destruct mechanism" not "depressurize its airlock'." per @organicMarble's comment

• A crucial difference on the NASA page is that it says "The signal accidently (sic) set off the self destruct mechanism" not "depressurize its airlock". – Organic Marble May 2 '20 at 1:25
• The book source is available on Google Books and has a third version...: The wrongly received command fired the engine in a bad direction and half an hour later automatic self destruct took place. – asdfex May 2 '20 at 8:48

Then, in 1965, a would-be-Vostok-2 was running several tests to test a brand-new and never flown before equipment for EVA. That was an unmanned mission to avoid putting cosmonauts at risk because EVA equipment was potentially very dangerous. Mission control would slowly run EVA operations step-by-step and record state of a spacecraft after each step. Unfortunately, they had only a narrow window of opportunity to communicate with spacecraft when it was flying over Soviet territory. Soviet radio equipment was not particularly reliable, so they had a primary station and backup one in case the primary one would fail. A range of those stations was also relatively short, so different stations would be used on different orbits (and some orbits would have no SATCOM at all).

Now let's move to the Kamchatka peninsula that happened to be close to the path of next satellite orbit. Mission control was well into the test program, with EVA dock already deployed on spacecraft and it wanted to remotely open the hatch to EVA dock. To do so they had to transmit command #42. That command originated in main mission control center, but had to be relayed to spacecraft by Kamchatka radio station NIP-6, with another station NIP-7 serving as backup. It was expected that backup station would stay silent, but due to management mistake, both primary and backup stations were active and therefore relayed command #42. But relaying a signal introduces some (small) delay (for example there's a delay of signal propagation over distance), so spacecraft received two slightly shifted copies of command #42.

Now consider that a command #42 was a sequence of impulses that we can write, say 101001. Remember that lack of an impulse is "0" and presence of impulse is "1"? Impulses of second, delayed copy of command #42 were interpreted as "ones" in positions that were originally supposed to be "0". Like this:

   101001
+   101001
=  1111011


So instead of a command 101001 (decoded as "42") spacecraft thought that it was command 1111011 (decoded as "5"). Note that I took those particular bit sequences out of the thin air for illustrative purposes only.

Unfortunately, command #5 happened to be "start spacecraft descend". Executing this program would take spacecraft back to Earth, but since it was positioned on orbit in the wrong position, it would not descend to USSR territory. Soviet designers anticipated the possibility of such scenario (descend of a secret spacecraft in enemy territory) and installed independent self-destruction device that would destroy spacecraft if it attempts to descend outside of the pre-programmed range of valid descend trajectories. Therefore soon after command #42 was send by mission control, communication with satellite suddenly and abruptly ended. Recorded logs later showed that spacecraft relayed confirmation of "command #5 received" and the whole story was eventually reconstructed.

In aftermath of that event later Soviet SATCOMs were updated. However 12.12.1975 another satellite Kosmos-785 was lost when a random error in a normal code sequence was misinterpreted as a self-destruction command.

More on that topic (in Russian) can be found here and here.

Accident with Cosmos-57

https://www.kik-sssr.ru/Evolution_of_KRL.htm

The test of the gateway was successful and the Post-2D station NIP-6 (Yelizovo) issued a command to transfer air to the gateway, and the Post-2D station of the neighboring NIP-7 (Kluchi) duplicated it. As a result of adding two identical commands, another command was formed. This was established by the emergency commission (based on the analysis of TMI urgently delivered to NIIP-5).

As a result, the same command transmitted from two NIPs almost simultaneously overlapped each other in such a way that it was deciphered by the on-board equipment as a descent command, which caused the automatic destruction of the satellite (APO), because in this case, the satellite would have landed outside the territory of the USSR.

By the middle of the day on February 25, the emergency commission unequivocally established that the cause of the APO operation was the issuance by the Kamchatka stations NIP-6 and NIP-7 of the overlap of two identical commands No. 42. Two commands, overlapping each other, were perceived by the airborne decoder as one command No. 5 - "descent".

As a result of the investigation, a technical decision was made to protect critical commands in the radio line.