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The BBC article InSight Diary: The silence of space tells an interesting story and has an interesting audio recording of InSight's seismometer signals sped up to audio frequency, made while in presumably silent deep space on its way to Mars. Mysterious vibrations/sounds were recorded, and the article tells the story of their surprising discovery and mystery. It's worth the read.

These vibrations were detected in the two horizontal directions, by SP2 and SP3. Vertically oriented SP1 can not measure in microgravity because the Mars gravity-cancelling spring has no gravity to compensate it and so pushes the accelerometer to its limit.

If zero acceleration is too far out of the range of Mars' 3.71 m/s^2 acceleration, then Earth's 9.81 m/s^2 gravity would be way too large to test this accelerometer.

Question: Was the full 3D seismometer tested in Earth's gravity somehow? Perhaps dropped off the side of a building with a tether to simulate 0.38 g acceleration?

below: One of the microseismometer sensors, carved from a single piece of silicon 25mm square. Source

enter image description here

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    $\begingroup$ This reminded me of the Bremen drop tower which may have nothing to do with InSight's seismometers $\endgroup$ – Jack Nov 25 '18 at 9:18
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    $\begingroup$ @Jack I think I've seen that one linked in another comment recently, perhaps by (at)Hobbes but I can't find it now. It could be a different one. There's also this one: en.wikipedia.org/wiki/Zero_Gravity_Research_Facility $\endgroup$ – uhoh Nov 25 '18 at 9:28
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    $\begingroup$ @Jack Found it! It's a different one; Einstein-Elevator of the Hannover Institute of Technology space.stackexchange.com/a/31846/12102 $\endgroup$ – uhoh Nov 25 '18 at 9:45
  • $\begingroup$ Not really worth writing an answer around it: These devices are not sensitive to any force perpendicular to their axis of measurement. So it's sufficient to just tilt it until the resulting force along the measurement axis drops to the desired value. You can test always two of the sensors at the same time, so there is no need to run a full-system test with all three of them. $\endgroup$ – asdfex Feb 5 at 18:07
  • $\begingroup$ @asdfex they are not supposed to be in a perfect world, but imperfections in manufacture and design can result in some weak sensitivity or cross-talk or even higher order non-linear effects. Remember these are intended to measure 1E-09 m/s^2 in a 3 m/s^2 static field. So while testing you should not presume perfection but instead be thorough and rigorous, and do the test right. $\endgroup$ – uhoh Feb 5 at 23:11
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SEIS: Insight’s Seismic Experiment for Internal Structure of Mars has a truly gobsmacking amount of information all about Insight's seismometers.

Testing facilities mentioned include:

  • Pinion Flat Observatory (PFO)
  • the IPGP 'Observatoire de Saint Maur' facility
  • the Kinemetrics test vault in Acton, Southern California

as mentioned by commenter @asdfex above, tilting the sensor within Earth's gravity to simulate Mars' gravity was used:

A non-flight (but similar) model vertical-axis SP was field tested at ambient temperature, inclined to match Mars gravity, over six days in the Kinemetrics test vault in Acton, Southern California.

For the actual sensors actually installed into the actual spacecraft, it seems like, among other tests, they just turned them on at various times and waited to pick up some seismic events:

The mission schedule was compatible with only a few days of passive seismic monitoring at the different stages of the VBBs integration, both prior to their integrations in the sphere and after. Nevertheless, several earthquakes were observed by the Flight sensors during testing activities, including cold tests. Figure 14 shows two such earthquakes...

For those interested in the topic, there is much much much more information in the report linked above.

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  • $\begingroup$ That's quite a tome, thank you for the link and the conclusive answer! $\endgroup$ – uhoh Sep 5 at 0:13

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