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why 6 sats or more?

Discussion in 'General Discussion' started by Kregh, Sep 16, 2013.

  1. Kregh

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    Why does the phantom require 6 sats or more?

    You can tell your position with 3 sats....and your altitude with a barometric sensor...so why 6 sats? Is it a safety envelope thing?

    I am concerned because my satellite app on my droid rarely shows 6 sats constantly overhead....so I guess I should get really good at ATTI and pray that when I need RTH...I will be blessed to have 6 sats
     
  2. Gizmo3000

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    I'd just guess that with 6 you have improved accuracy.
     
  3. UTR

    UTR

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    I'm sure I've seen somewhere in the blurb that the Phantom can track 24 sats that being said I get more than 6 every flight. Your position on the ground can be fixed easily with 3 because you are at a known height it still prefers 4 because 3 can give two locations but to track an object in 3D space accurately you can use 6 or more it's just more accurate. It all depends upon the what geometry is being used and I don't know what used is the Naza it might be using Trilateration as opposed to Triangulation. As I said I don't know but reading online is seems that ground based systems use triangulation which prefers 4 sats and some flight systems use trilateration and the more sats the betters. I would be interesting to find out what the Naza does.

    http://gis.stackexchange.com/questions/ ... satellites

    has a nice easy picture explaining the 4

    Then again I also found this

    There is a complication. To calculate the time the GPS signals took to arrive, the GPS receiver needs to know the time very accurately. The GPS satellites have atomic clocks that keep very precise time, but it's not feasible to equip a GPS receiver with an atomic clock. However, if the GPS receiver uses the signal from a fourth satellite it can solve an equation that lets it determine the exact time, without needing an atomic clock.

    If the GPS receiver is only able to get signals from 3 satellites, you can still get your position, but it will be less accurate. As we noted above, the GPS receiver needs 4 satellites to work out your position in 3-dimensions. If only 3 satellites are available, the GPS receiver can get an approximate position by making the assumption that you are at mean sea level. If you really are at mean sea level, the position will be reasonably accurate. However if you are in the mountains, the 2-D fix could be hundreds of metres off.
     
  4. Driffill

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    Being in range of 3 satellites will allow your GPS to triangulate the approx posistion you are in, here is the confusing part! It actually needs 4 satellites, the 3 that connect to your device and a 4th to keep sync between the satellites!

    With 3 sats in range of your device, if one of the satellites is getting a reflected signal this will alter the time it takes for the signal to arrive, resulting in an incorrect location given! Once you have 4, 5 or more satellites in range, the chances of one signal being reflected and causing errors is lowered, as the GPS device can use any 3 other sats to triangulate its position and ignore the "faulty" data, without having lost its true position.
     
  5. shade emry

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    SImple truth is is that the neo 6 m / q Chip does not have whats called GNSS, or dead reckoning support. more sats improves the accuracy and time for correction, it also acts as a failsafe.