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Posts posted by PakledHostage

  1. On 8/30/2021 at 6:01 AM, YNM said:

    (in reality I think you need a 4th one due to 3-dimensional sphere rather than circles that it produces). 

    Three is enough. Extra signals allow for an estimate of the error (by comparing solutions from the different sets of three), and also allow for fine tuning of the GPS receiver's internal clock (by tweaking the time to minimize the error).

    The computation of position using three distances from three known points is called trilateration. It is how my old Kerbal GPS mod (Figaro) does it. In that mod, just like in the real GPS system, extra combinations of three are used to estimate the error.

    (A relevant link for anyone who's interested: https://gisgeography.com/trilateration-triangulation-gps/)

  2. On 6/17/2021 at 1:39 PM, mikegarrison said:

    I work in an industry where, everywhere in the world, airplane altitudes are measured in feet and speed is measured in nautical miles per hour.


    Not everywhere. Many EFIS displays even have a button to allow the crew to switch between metric and imperial altitudes, for use when operating in airspace of countries that use metric altitudes.

    That, and the International Nautical Mile (the one that counts) is defined as EXACTLY 1852 metres.

  3. I caught a glimpse of this during the tribute montage at the end of the briefing that I linked to above. I'm surprised it didn't make bigger headlines (it impresses me, anyway):

    The cliff face on the left (The Eiger's North Face in Switzerland) that the red Perseverance logo is projected onto is over 5000 vertical feet high.  All three peaks that have images projected onto them (The Eiger, Jungfrau and Mönch) exceed 13,000 feet (~4000m) in height. This would have been quite the sight to see!


  4. 12 hours ago, JoeSchmuckatelli said:


    Speaking of nerds... I went back and refined my estimate of Perseverance's entry trajectory.  I initially went with the simplifying assumption that the path between data points was linear (rather than hyperbolic) and that the speed over that distance was just the average of the speeds at the two data points. Unfortunately, the solution is quite sensitive to the angular distance between the data points used in the system of equations, so the small error introduced with that assumption turns out to be significant. That assumption did provide a good seed value for a couple of rounds of iterations, however.

    To make a long story short, I refined the solution by computing the actual distance along the hyperbolic curve between the two data points, and computing the actual average speed over that distance. I then adjusted the angular distance between the two points iteratively, so that the distance traveled was correct for the speed and elapsed time between the points. The solution converged quickly. It only took two iterations to settle onto a result that was consistent, all around. I also used the most widely spaced values from the mission control video that were still consistent with the vis-viva equation, in order to minimize the error that results from not knowing the exact elapsed time between the data points. (I used the video's elapsed time at each data point as my time measurement, so I was probably only accurate to +- a second or so.)

    Anyhow, the new estimate was that:

    The periapsis point of the hyperbolic trajectory was about 76 km below the Martian surface
    The hyperbolic eccentricity "e" was about 1.327
    The velocity at periapsis (in the absence of pesky atmosphere and rocks) would have been about 5.473 km/sec
    The entry interface angle (at the  entry interface time cited during the landing) was -14.6 degrees

    And just for completeness, I plotted it.  The heavy read line represents the Martian surface, the dotted red line the Martian atmosphere, and the blue solid line is the entry trajectory. The two pluses show the computed locations of the data points that I used for the estimate.




  5. 1 hour ago, mikegarrison said:

    This is known as active noise control, and it really only works in certain circumstances. The problem is that phase matters.

    Mostly this is used for headphones, because the distance between the headphone and the eardrum is very controlled.

    I recall that Bombardier tried active noise cancellation in the cabin of the Dash-8 Q400 aircraft, but it struck me more as a gimmick... and we're getting off topic.

  6. So I nerded out yesterday evening and attemped to compute the hyperbolic trajectory that Perseverance followed to the entry interface. I used the data that was shown on mission control's readouts in the mission control livestream. I ended up having to make some simplifying assumptions but I arrived at an eccentricity of about 1.3 and a periapsis point about 130km below the Martian surface. The entry interface angle worked out to about 18 degrees. That's a bit steeper than the value of ~15 degrees that I heard quoted during the livestream. The data from the early part of the entry in the livestreams was still consistent with the vis-viva equation (i.e. aerobraking wasn't having a significant effect yet), so I don't think my numbers will be too far off. But it would be interesting to know the actual values. Anyone have any idea what they might have been?


    7 hours ago, tater said:


    Next press conference is at 10:00 Pacific Time today. Hopefully they'll show them then?


  7. 3 hours ago, YNM said:

     Plus given that they need the skycrane to hover on one spot is I think close enough (if not more impressive) than landing on the ground

    Interestingly, Al Chen, JPL's Entry Descent and Landing team lead said yesterday during the news conference that it is a misconception that the sky crane hovers. He said that it only actually hovers for about a second.


    Edit: I should add that I'm really excited to eventually see the footage from the EDL cameras. We're all probably imagining how the landing looks, but I expect that the real thing is a lot more dynamic and scary. For example, I didn't realize what a "sucide burn" landing the Apollo landings were until I watched the documentary "Apollo 11" with it's overlaid altitude and speed data... This landing will be eye opening and impressive as heck.

  8. 2 hours ago, cubinator said:

    The transfer speed from a solar orbit is faster than orbital speed - but by aerobraking from the Sun you can lose all your velocity all the way down to the surface with just the atmosphere. Entering orbit first requires firing engines, but it helps reduce the intensity of atmospheric entry.

    Interestingly, Perseverance's atmospheric entry speed, direct from its transfer orbit, will be on the order of 5 km/s, which is substantially less than atmospheric entry speed from LEO. Of course that doesn't mean Mars atmospheric entry is easy...


    I'm not just here to be pedantic, though...

    Does anyone know how they're acquiring the video data from the cameras on the backshell and sky crane? I understand that the video will be stored on Perseverance until they have the bandwidth to downlink it to Earth some weeks or months after landing, but they'd obviously need to stream the video data directly from those cameras to Perseverance during the landing. The backshell and sky crane won't be around anymore once Perseverance is on the surface. 

  9. I'll revive this thread with my image from this evening of the Jupiter/Saturn conjunction, rather than start a new one. (Just because there's a lot of good stuff in here that deserves another look.)


    I shot it through a 600mm lense at 1/200, f6.3,ISO200.  I then brightened Saturn by about a stop and dimmed Jupiter by about a stop using a linear gradient in Lightroom.

  10. 1 hour ago, SOXBLOX said:

    Anyways, always nice to see our space junk flying back by.

    One day, the charred remnants of a Tesla Roadster may crash through the roof of some feudal lord's keep, and puzzled, they'll treat it as a gift from the gods...

  11. 2 hours ago, Dragon01 said:

    Spectroscopy could tell us something about it, but it might be hard to tell it apart from a hunk of ferrous rock.

    In one of the articles, they also talk about the effect of light pressure on its trajectory. If it is just a hunk of rock, its trajectory won't be affected as much by light pressure as it would if it's a much less dense spent Centaur stage. 

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