The ISS has a camera mounted streaming live video from space (HDEV). From time to time, there is a blue screen visible, which is noted to be during times of "loss of signal". What exactly defines a "loss of signal"? Does the ISS have to be in line-of-sight contact with a ground station (which does not seem to the correct answer) or a different satellite? How is the ground communication done for this live video feed?


The ISS communicates with ground assets through a robust network including ground stations and geostationary satellites (TDRSS). However, due to line of sight constraints there are brief gaps in coverage.

(edit - expansion)

These gaps can be seen (at least during the Shuttle era) by looking at the Flight Plan. I've cut-n-pasted the relevant parts of one page from the linked STS-131 Flight Plan to give an idea of how often comm outages occur. Take a look at the link to see what a full page of the Plan looks like. Basically I've cut out all the activity information.

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These timeline sections of the Plan show the GMT and Mission Elapsed time at the top, the Day/Night cycles, the orbit count, etc. (The top values are for the shuttle, the bottom ones are for the ISS).

The lines labeled TDRS W (west), E (east), and Z (Zone of Exclusion) show horizontal black bars when the associated Tracking and Data Relay Satellite (TDRS) is in line-of-sight with the shuttle's comm system.

The lines labeled TDRS AVAIL show when any of the satellites are available to the ISS comm system. The gold bar is Ku-band (video) availability, the green bar is S-band (voice and data) availability.

This graph shows the brief periods during which there is loss of voice or video comm due to line-of-sight constraints with the TDRS system.

For more info on the ISS comm system see my answer to this question: Do antennae on the ISS have to constantly move to maintain data links?

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    $\begingroup$ Incidentally, it's remarkable how if you google "ISS Loss of Signal" you get mostly UFO garbage. $\endgroup$ Oct 12 '15 at 15:54
  • $\begingroup$ Because of not finding any useful information I asked this question here. But then, do these "loss of signal" periods happen in always the same regions over earth? Or does it change? Using your answer, I would expect the loss-areas to be in approx. always the same regions over earth... $\endgroup$
    – Alex
    Oct 13 '15 at 6:55
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    $\begingroup$ @OrganicMarble "..if you google "ISS Loss of Signal" you get mostly UFO garbage.." I was hoping the answer was going to be 'when it crosses the Bermuda Triangle'.. ;) $\endgroup$ Oct 13 '15 at 13:24
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    $\begingroup$ During times of "Loss of Signal"there should be no comm link! Why is there audio when no video, shouldn't it be the same comm sat? $\endgroup$
    – user14664
    Jun 5 '16 at 0:13
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    $\begingroup$ @Bagobolts the comm system is complicated but video is normally carried by the Ku-band system and voice is normally carried by the S-band system. Different antennas on the ISS and comsat, different systems. $\endgroup$ Jun 5 '16 at 12:07

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