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?
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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.
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?
answered Oct 12, 2015 at 15:13
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7$\begingroup$ Incidentally, it's remarkable how if you google "ISS Loss of Signal" you get mostly UFO garbage. $\endgroup$ Commented Oct 12, 2015 at 15:54
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$\begingroup$ 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... $\endgroup$– AlexCommented Oct 13, 2015 at 6:55
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2$\begingroup$ @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'.. ;) $\endgroup$ Commented Oct 13, 2015 at 13:24
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1$\begingroup$ 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? $\endgroup$– user14664Commented Jun 5, 2016 at 0:13
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1$\begingroup$ @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. $\endgroup$ Commented Jun 5, 2016 at 12:07