Intelecommunications,communications blackouts are
Uptime being a key goal of mostcommunications networks, power supplies andbackup generators are typically used to ensure high-reliability power.
Wireless networks may be subject toradio jamming; wired networks can be physically severed.Network design can also play a role in maintaining communications reliability; depending on the constraints in building afiber-optic network, aself-healing ring topology may be used.
The communications blackouts that affect spacecraftre-entering the Earth's atmosphere, which are also known asradio blackouts,ionization blackouts, orreentry blackouts, are caused by an envelope of ionized air around the craft, created by the heat from the compression of the atmosphere by the craft. The ionized air interferes with radio signals. For theMercury,Gemini, andApollo spacecraft, such communications blackouts lasted for several minutes.[1]Gemini 2, for example, endured such a blackout for four minutes, beginning at 9 minutes 5 seconds into the descent.[2]
For Apollo missions, the communications blackout was approximately three minutes long.[3] ForApollo 16, for example, pre-advisory data (PAD) for re-entry listed the expected times for re-entry communications blackout to be from 0 minutes 16 seconds after entry interface to 3 minutes 33 seconds after entry interface (a total of 3 minutes 17 seconds).[4] For theApollo 13 mission, the blackout was much longer than normal because the flight path of the spacecraft was unexpectedly at a much shallower angle than normal.[4] According to the mission log maintained byGene Kranz, the Apollo 13 re-entry blackout lasted around 6 minutes, beginning at 142:39 and ending at 142:45, and was 1 minute 27 seconds longer than had been predicted.[5]
Communications blackouts for re-entry are not solely confined to entry into Earth's atmosphere. They apply to entry into any atmosphere where such ionization occurs around a craft. TheMars Pathfinder endured a 30-second communications blackout as it entered Mars' atmosphere, for example. TheHuygens probe endured a communications blackout as it entered the atmosphere ofTitan.[1]
Until the creation of theTracking and Data Relay Satellite System (TDRSS), theSpace Shuttle endured a 30-minute blackout. The TDRSS allowed the Shuttle to communicate by relay with aTracking and Data Relay Satellite during re-entry, through a "hole" in the ionized air envelope at the tail end of the craft, created by the Shuttle's shape.[1]
Radio blackouts on Earth caused bysolar flares are measured by theNational Oceanic and Atmospheric Administration on a scale from R1 (minor) to R5 (extreme).[1][6][7]
Communications can also be lost when theSun is blocking or behind one station in the same line of sight;Sun outages periodically interrupt communications withgeosynchronous satellites. It is also a common problem for interplanetaryspace missions.
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22.