When watching the ISS HD Earth Viewing Experiment, I realized that they turn the cameras off when the ISS enters night. How often are they turned off when its dark? How come they aren't left on?
I believe they are and my network analysis tools show nearly constant transmission (data bandwidth remains the same), even when the image is all black or all gray. According to High Definition Earth Viewing (HDEV) Experiment quick notes on its USTREAM channel:
- Black Image = International Space Station (ISS) is on the night side of the Earth.
- Gray Image = Switching between cameras, or communications with the ISS is not available.
Same is repeated in German on the Columbus Eye pages by the University of Bonn. But it doesn't say anywhere that the cameras are switched off during that time, and I get stream bandwidth in the average of 700 - 800 kB which cannot be down to buffering, since it remains constant for the whole duration of ISS's orbit in Earth's shadow.
According to NASA on High Definition Earth Viewing:
HDEV Design for Operations: The HDEV operates one camera at a time. The HDEV is designed so that when the system is initially powered on, after a 1-2 minute warm up period, the Cameras are turned on one at a time in a repeating cycle. The Forward looking camera is powered first, followed by the Nadir and each aft looking camera, such that the HDEV video “follows” a location on the earth as the ISS passes overhead. This auto-cycle mode of the HDEV does not require any input from ground operators, so the HDEV can be operated any time that the ISS power and data resources are available, without requiring a ground controller present to operate the payload. The only command required, is the initial “power on” command, which is performed by ESA’s Columbus Control Center as schedule by ISS Payload Operations.
Alternately as desired by ground controllers, the HDEV video can be commanded. Ground operators have the choice to change the cycle of the images noted in the auto-cycle mode (either changing which cameras that are powered on, or changing the length of time they are powered on), or, if desired, ground controllers can command a single camera to remain powered on and no auto-cycle to take place.
So here's what I'm thinking is happening;
- Before entering the daylight part of a new ISS orbit, the aft facing camera is activated because the front facing ones would produce too much camera glare (reflections from the station, front glass elements,...) when the beta angle of the Sun w.r.t. the station at dawn would be too direct.
- Aft facing cam doesn't produce any glare most of the times (I've seen it on occasion though, you can see the front lens element, black lens rings and protective glass defects in the reflection of the enclosure's front glass), since the only sources that the sunlight could reflect directly towards the camera are the Soyuz TMA solar panels (currently, two Soyuz spacecraft are docked to the station).
So it appears to me that during a lot of the time in one orbit, there simply isn't anything to see and that's why the screen remains black. For example, here's a screen capture from the aft facing camera when the station was entering the Earth's shadow, with a barely visible arc of Earth's atmospheric glow at dusk on the left hand side of the image:
As you can see, the rest of the image remains uniformly black, even though the camera is apparently on. The remaining question is, why don't we see any lightnings, meteors and light pollution on the surface of the Earth?
For lightnings and meteors, I doubt that the frame rate of these transmissions is fast enough to catch most of them. Remember, there is limited bandwidth available to them, so the streams would be highly compressed, and the cameras are COTS (Commercial Off-The-Shelf) quality. And a sporadic bright artefact that can be seen would more likely be produced by on-sensor noise, especially when the station is exposed to more radiation when over the South Atlantic Anomaly.
As for light pollution, the cameras don't seem to employ any lens filters, like the polarizing filters that could help reduce atmospheric light diffraction and suppress glare, and their angle w.r.t. the atmosphere is simply too shallow, so all the relatively small point sources of light are blurred-out with the rest of the background, and camera sensors are not sensitive enough to pick up such small differences in illumination, or stream compression is set too high and ignores such small difference between the individual frames.
To somewhat easier appreciate image quality of these HDEV cameras, and how much the atmosphere blurs out surface features, here's my own work in trying to improve image quality of a few HDEV snaps, using only simple image post-processing filters (output levels, color normalisation, unsharp mask,...), "before and after" style:
- Central Australia (Simpson Desert National Park):
- Andes at dawn (Peru, Chile, Bolivia, Argentina):
And this is one such "black frame" from one of HDEV cams a few minutes ago, somewhere over South Pacific Ocean, west of Ecuador:
It's probably hard to believe looking at this captured frame on most monitors, but the image uses no less than 69 unique colors and the live stream data rate was in excess of 700 kB/s during the capture. This is the same frame, with highly exaggerated difference between pixels (using color palette stretching):
(Click on any image for a higher resolution, 978 x 550 pixel lossless compression PNG image)
I actually worked on the team that built HDEV (came across this question while googling).
TildalWave's answer is mostly correct, though we leave the cameras on during black (ISS night) to help with dead pixel analysis. The aperture and focus settings are fixed for sunlight viewing to better characterize any radiation caused issues with the sensors (the primary purpose of HDEV). If something is extremely bright and close-ish to the station you might see it, but doubtful. I remember watching it once in a severe thunderstorm system over Brazil and could just barely see tiny specs of lightning, though even then it wasn't in complete night but only partial.
We sometimes change the cycle between cameras/durations to try and capture specific events either on the surface or vehicles docking to the station within view. Nominal ops though is just cycling through the cameras every ten minutes or so. If the payload has to be reset, it will default back to sixty seconds per camera.