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After I watched this video of the docking scene in Interstellar sweded using a light bulb and a fan, I was thinking:

Are there speed limitations with moving sensors, such as measuring a spinning wheel for example?

I know that phones can carry information on the highway and planes can communicate with ATC at very high speed. So I am focusing on a particular protocol : Bluetooth LE

  • does fast moving sensors have an impact on speed measure they are transmitting?
  • do IoT chipsets handle quick distance changes?
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    This was a real problem with the Cassini–Huygens communications, realized after it was under way. – JDługosz Feb 28 '17 at 22:07
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Reliable communications between digital devices requires some degree of signal processing to synchronize the data and the timing (clock). Adding relative motion between the transmitter and receiver can complicate the problem. You are probably aware that relative motion can impart Doppler frequency shifts. This also affects the timing of the bit stream.

A device like your cell phone (or even a spacecraft) has signal processing that can adapt to this type of dynamic condition, generally being able to accommodate a wide range of dynamic conditions. But this additional signal processing takes power to function.

I suspect that if a Bluetooth LE (Low Energy) device can't adapt to relative motion beyond some threshold limit, it's was a conscious design decision to not include that sort of adaptive capability. Power consumption would probably be one reason for that.

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This is a question on basic physics. Provided all parts of your network are moving at the same speed (give or take) then there is no impact of being in a moving reference frame (as we all are on Earth).

For long-range radio protocols, there is a need to account for the round trip delay (transmit/receive/transmit synchronisation), and having one end of the link in motion will have the effect of making the two parts of the delay asymmetric. This means that mobile terminal protocols do need some design consideration to allow for the right sort of guard band treatment.

For the specific case of Bluetooth LE, the range is probably too small for transmission to take place in the presence of a significant velocity offset. Even on a rotating object, the velocity will probably be reasonably constrained compared to the bit timing/propagation delay.

You might get a more detailed/specific answer on EE.SE, but you might also need to be a bit more specific about an application.

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    If you move fast enough the carrier medium has to move as well ;) – Helmar Feb 28 '17 at 18:07
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    Luminiferous aether? – Sean Houlihane Feb 28 '17 at 18:45
  • For very long range radio protocols with significantly different gravity at source/destination I suppose there's also a small amount of carrier frequency difference due to time dilation, e.g. a 2.0 GHz signal sent from earth is seen as about ~1.99999999887 GHz at 30km above the surface. A much more significant difference at e.g. the 20,000 km altitude of, say, a GPS satellite (effect is present even if receiver is not moving relative to transmitter). – Jason C Mar 1 '17 at 5:48
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For a stationary spinning wheel: when the antenna is mounted co-axially onto the hub of the wheel (assuming that the internal, typically folded BT antenna has been replaced with a straight wire antenna - an common hack done to improve BT signal strength), you'd be fine.

For a moving wheel, like at a straight moving car, you will additionally have to transport the receiver in parallel to the transmitter. This is mainly because the distance at which BT LE operates severely limits the useful time to transmit data (devices with ranges of up to 200m have been demonstrated, but are unlikely to appear in the wild).

If your moving wheel is circling around the receiver, you'd be fine again (again with the antenna at the hub).

This is all to prevent Doppler shift.

The frequency bands of BT are only 2MHz apart (channel 2: 2408MHz, channel 3: 2410MHz, ...), so once the frequency shift gets too large, you will run into problems. A transmitter on channel 3 in a car with a speed of at 200km/h (125mph) will appear to a non-moving observer to operate on channel 4 (when getting closer, head on) or channel 2 (when getting straight away). And a nice pitch-bend transition while it's zipping past. As mentioned by Jim, BT was not designed for such scenarios.

Off-topic, but related: LTE ("4G") will stop working at 200km/h.


Edit:

As John Deters pointed out the 200km/h limit is wrong. The fact that cell phones work in airplanes traveling at very high speed does not prove that LTE will work reliably (they can still fall back to 3G or 2G, and high-speed passenger trains and passenger aircraft are nowadays equipped with their own LTE base stations).

However, LTE is usable at speeds well above 200km/h. Test have shown that handovers will work at speeds of up to 500km/h (possibly with noticeable interruptions) and the Doppler effect can be compensated for speeds of up to 600 km/h. Well - these tests were performed at an altitude of 300m which makes this more of an test of LTE in a high-speed train than in a high-speed aircraft.

The current design limits depend on the which of the LTE frequency band is used. 350km/h should work in all frequency bands, while 500km/h are possible for select frequencies.

The performance may suffer greatly if a large number of cell phones use LTE at such high speeds within the same cell (like all the passenger in a train or an airplane, hence the increasing use of LTE base stations/repeaters for trains and airplanes).

  • This math is off by orders of magnitude; the first evidence is that cell phones work just fine in aircraft traveling substantially faster than 200km/h. – John Deters Apr 16 at 16:00

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