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Why 6 Satellites?

Discussion in 'Phantom 2 Vision + Discussion' started by denofr, Jun 3, 2014.

  1. denofr

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    The Phantom is programmed to switch to ATTI mode if it doesn't lock onto 6 satellites. My question is, why 6? My Phantom almost always has at least 3 on startup it then takes anywhere from 30 secs to never to get to 6 and up. Rarely do I have more than 8. So why doesn't DJI let us set the satellite lock number manually, maybe having 4 as the minimum? If you had GPS lock with only 4 satellites you would rarely have a flyaway and most flights would be far more stable. I'm an old retired Coastguardsman and years ago there was LORAN and rarely if ever did anyone have more than 3 towers to triangulate their position. It just seems that 6 is overkill and quite frankly in some cases dangerous.
     
  2. kgarrison

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    3 is not enough as if you lose one, your copter is lost. Six provides a reasonable balance of redundancy and I fail to see how more would be dangerous (please explain). We aren't working with high quality GPS components here so there has to be a good safety margin. I RARELY get "only 6", I typically get 11-13.
     
  3. denofr

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    Sorry I wasn't clear. The Phantom is set to switch to ATTI mode if it loses the 6th satellite, meaning you now have 5 or less satellites, when that happens your copter is not automatically returning to home. Wouldn't you rather it RTH with only 4? BTW where are you located where you get 13? If I was getting 13 I probably wouldn't even be thinking about this but that is not the case. Like I said on a good day, make that a very good day, I have 8, often I'm struggling to get 6 and then it will bounce back and forth 5-6 but it always has 4-5.
     
  4. CarlJ

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    I always get 13 and you're right, I've never thought about it.

    wow, your post cheered me right up today :)
     
  5. kgarrison

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    I am in central Colorado
     
  6. xgeek

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    The number of satellites visible in the sky varies during the day even at the same location.

    A quick forecast scan from my location shows anywhere from 6 to 12 visible over the next 12 hours. As well as the number of satellites the PDOP (Dilution of precision) is also important (I.E separation of the satellites you are locked onto)
     

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  7. GreenMarine

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    Come on mate... You can't just show that cool app without telling us what it is ;)
     
  8. xgeek

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    Oh it's GPS plan for the iPhone
     
  9. lizard

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    Better question could be why not 22 satellites, PV2+ have so much interference onboard, DJI needs to assure, the reception is at least somehow working, so the 6 is required in precautionary manner.
    See my post there : http://www.phantompilots.com/viewtopic.php?f=27&t=17007
    Whole PV2+ GPS reception is actually screwed by the parasitic RF radiation from poorly shielded and poorly designed camera subsystem.
     
  10. denofr

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    OK, I'm in. How do we fix this? BTW I'm still not conceding my point. Why let the bird fly away because you only have 5 sats?
     
  11. lizard

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    As soon as there is 3D GPS fix or even 2D, there shall be no issues in GPS mode or RTH. Basically, you'll need 4 sats locked in for 3D fix and 3 for 2D fix. But here DJI wants to go for sure, so this is why they set 6 as minimum.
    Actually the problem is on the other side - 6 shall be no problem at all, shall there be no interference issue. In open space it is matter of 15-30 seconds for cold start (with GPS having clock and local battery backup in place) to get lock on 12 satellites. Unfortunately not with the PV2+ due to reasons I have outlined in the other post. It also appears some birds have more or less interference, what could be both quality and engineering problem.
     
  12. mikeve

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    If you remember high school math, to solve for 3 unknowns, you need 3 equations, 4 and 4, etc. To find your 3D position, you actually have 4 unknowns: Lat, Lon, Alt and time. The clock in the receiver needs to be synched to the satellite system within nano seconds (billionths) and that time difference between the relatively inaccurate receiver clock and the satellite system is the 4th unknown.

    Once you have 4 sats, you should be able to get a position but geometry now comes into play. If, when you look at the sky plot, the 4 sats are lined up, the equations won't solve properly and you need a 5th in another part of the sky to even get a 2D fix. Throw in a hill, building or a couple of trees and you've now lost one to blocking.

    Statistical analysis shows that, with the current constellation of 24 (there are in-orbit spares - there are 24 different orbital positions), 6 is the minimum required to get a good, reliable 3D fix.
     
  13. lizard

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    It is more complicated than it appears. Indeed, you need only 4 satellites for 3D GPS fix.
    In fact, having more correlators locked in to more satellites, using standard least squares method and more complex statistical methods plus additional data from WAAS/SBAS satellites you can further improve precision.
    But than again, shall the GPS reception of PV2+ have enough signal to noise, you'll get plenty of satellites locked in within 30 seconds.
     
  14. yawnalot29

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    I think for satellite lock, you always need # of dimension + 1. The +1 is for "time".
     
  15. Dadcat

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    Thanks a lot for that. I downloaded it and it tells me that if I try to fly at 1:45 this afternoon, I won't have GPS available at all because there are only 5 satellites visible at that time.

    A few minutes before that there are 6, I'll get a good home point set, fly off 1000 feet or so and my Vision+ will be a tiny dot in the sky. I can't see what direction it's pointed, heck, I can barely see what direction it's moving. The compass in my Nexus 7 isn't worth the powder to blow it to hell so radar on the vision app is no help. Now let's say there is a 10 mile per hour crosswind.

    At this point I haven't done anything that violates any of Dji's recommendations. Now at 1:45pm, suddenly there are only 5 satellites visible. What happens?

    1) Vision+ goes into ATTI. It acts like it has no clue where it is.
    2) Vision app now says distance is "N/A" but still reports altitude.
    3) "Find My Phantom Vision" just shows that last position when there were 6 satellites.
    4) Home Lock won't work.
    5) RTH won't work.
    6) Vision+ is going 10 mph whichever way the wind is blowing.

    The only way the Phantom isn't blowing away at 10 mph is if I can use the video feed to navigate my way back. I better be really good at this because in couple of minutes it may blow out of control range or wifi range.

    I experienced this last night. I waited forever to get 6 satellites and a home position set. I'd start the motors, look down and distance was showing "N/A". Only 5 satellites. Wait longer until there are 6 and take off. Fly around for awhile until the thing figures out the magnetic declination, and then position hold won't work anymore. I look down and there are only 5 satellites again and the bird is in ATTI. The wind was only 1 or 2 mph, but if it was 10 or 15 (still within Dji's recommendations) and I was looking at the screen instead of the Phantom, that thing would have been out of sight over the trees within seconds.

    I suspect that this is the Number 1 cause of "fly aways". I have done many hundreds of flights with R/C helis and quadcopters over the last couple of years. I might be able to recover from the scenario I mentioned, but it seems like the majority of these things are being sold to people with little experience and no sense of directional orientation. The chances of them recovering are pretty low.

    Okay, here's the point: It makes a lot of sense to require 6 satellites before the Phantom establishes a home point. That's probably the minimum for a 10 foot or so radius home point.

    It makes absolutely no sense for the Phantom to go into a Non-GPS mode at 5 satellites while it's in the air. With only 3 satellites you'll almost always have positional accuracy within 100 feet or so. That's good enough to get your quad back within visual control range with Homelock or RTH. But you're certain to get "fly aways" with 5 satellites the way it's set now.
     
  16. xgeek

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    By default GPS Plan has the elevation mask set for 15 degrees. You may want to raise or lower that depending on your take off location (I have set mine to 10 degrees as I am in an open location)
     
  17. mikeve

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    Most GPS receivers have the mask set at 5 degrees which is to say "ignore this sat if it's elevation is less than 5". 10 might be good to use in GPS Plan for you on the ground but 5 might be more representative of what the Phantom sees at 100' up.

    The only reliable source is what the Phantom actually sees
     
  18. xgeek

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    I would rather err on the side of caution. If I have it set for 10 and I get 8 sats on the ground then the chances are I will get more in the air. It works for me but other can set the elevation mask to whatever suits them.
     
  19. Dadcat

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    That makes sense. I need to spend more than a couple of minutes with that app.

    Where I was flying last night was about a 100 yard square park with 100 foot tall trees on 3 sides. I've always been able to get 7 or 8 satellites there on the ground and typically 1 or 2 more in the air. But there were 2 occasions last night when it dropped to less than 6 while at least 75 feet up. So 5 satellites was the truth as far as the Phantom knew.

    It was an education for me. I didn't know it would ever see that few in the air, and what the result of that would be. Now I know, but I'll bet there are very few others that know it.

    I'll say it again. If there had been a 10 mile per hour wind instead of 1 or 2, I might have lost it.
     
  20. Dalite

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    Excellent discussion and quite informative.

    To further add woe to the occasion, the USAF advises tht the GPS plan can only guarentee 4 birds visible in all places at all times.

    Additionally, I haven't found a DJI GPS board with a good backup battery; the few I have sampled were quite past unusable. UBlox sets 1.8 VDC as the minimum needed to keep almanac data and RTC (real time clock) calibration info stored when the main battery is not powering the circuit. The mere presence of a dead battery can cause the GPS performance to suffer to point of lower performance level than not using the battery backup option.

    If you use a GoPro camera, it generates enough electrical hash to cause loss of sat lock if powered on after the required 6 have been acquired. They can be reacquired with it powered on, at the expense of start up time.

    The higher you are, the further the GPS antenna's view of the horizon. Time between AOS and LOS is increased (AOS = Acquisition of Signal. LOS = Loss of Signal). These sats are in mid earth orbit. They will appear and disappear from your "view" as a function of the antenna having a relatively clear path to the horizon. They may not have the needed 5 degree over horizon angle, but signal integrity is improved in free space.

    It is what it is. Knowing what you are working with and the reasonable expectations of it's performance will help keep them flying under your command. NAZA giveth and NAZA taketh away... :eek: