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  • The end of the kilogram we knew or at least how the metrologists knew it.

    Discussion in 'Suggest a forum topic' started by rafaelherrera, Jan 12, 2019.

    1. rafaelherrera

      rafaelherrera Member EngineeringClicks Expert

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      The truth is that it is very interesting. And I have a doubt regarding the fountain atomic clocks:

      In the GPS satellites that are currently working, cesium and rubidium clocks are still used because the precision of the fountain atomic clocks is not so relevant as to make the change, in spite of being more precise, or because making the change could generate a considerable error in relation to the distance of the earth?
       
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    3. rafaelherrera

      rafaelherrera Member EngineeringClicks Expert

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      The DGPS and the user's location is simply amazing.
       
    4. john12

      john12 Well-Known Member EngineeringClicks Expert

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      I did a few hundred hours in the military on various different aircraft (don't have a PPL now though - haven't flown for quite a long time and it's very expensive to keep it current) when I was younger.
      I was great experience because it was before my formal engineering training but I still got to learn a load of technical information (how jet engines and loads of other things work etc.)
       
    5. GoodCat

      GoodCat Well-Known Member EngineeringClicks Expert

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      By the nature of my business I am connected with GPS systems

      On the very self, the GPS system is very accurate and the main source of error is not time synchronization or bad weather conditions and so on. For localization of 4 satellites, your position is updated with an accuracy of up to 1 centimeter. It does not matter at all whether your watch is synchronized in your phone or what weather conditions, weather conditions should allow receiving a signal from at least 4 satellites. Since there is a possibility that the signal from one of the satellites may disappear, the navigation applications start to work when 5 satellites are visible, the 5th satellite is a backup in case of loss of communication with one of the 4.

      The main navigation error lies in the corrections of geophysical data, namely the lack of these corrections on your phone and causing the detection error of several meters.

      Since the geomagnetic poles of the planet are shifting for navigation, corrections are necessary. This is done as follows:

      A base station is set - this is a GPS receiver that has good satellite visibility. The base station is stationary at one point and is the 5 point of reference along with 4 satellites. This geometry allows you to make corrections to the coordinates of geolocation and using a fixed base, the error of detecting your phone by geolocation will be a couple of centimeters instead of the usual 1-3 meters.

      If the mobile operators on their communication towers had GPS base stations, then your phone could use these bases and the accuracy of the GPS coordinate increased significantly. In practice, this is not done, and the user naturally will not himself unfold the base point for accurate geolocation. This means that for geolocation of phones, tablets, laptops and other devices, the corrections are not used because you do not take into account these corrections, you have about 5-10 times the coordinate detection error.
       
    6. john12

      john12 Well-Known Member EngineeringClicks Expert

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      Is there a reason that they don't use the connections? I heard that the US military used to intentionally 'scramble' the signal (not scramble, but reduce the accuracy) for non-military users but maybe they have turned that feature off now?
       
    7. MSHOfficial

      MSHOfficial Well-Known Member EngineeringClicks Expert

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      So if we have data from 4 satellites we can get 4D position right? 4th dim being time, and that’s the time that our phone is sync-ed to? But why is it necessary to have 4 satellites? If we have 3 we should be good to go for 3D positioning, and usually GPS like google maps does not show elevation (right?) so 3rd dim being time we are good to go right?

      Also the time sync errors, like the relativistic errors don’t matter in the GPS system? I was talking about the time sync with the NAVSTAR GPS satellites with the earth clocks, they do not need to be in sync to be able to provide accurate information about the position of the receiver?
       
    8. john12

      john12 Well-Known Member EngineeringClicks Expert

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      My phone gives elevation data but I'm not sure if Google Maps uses it. I know there's been a few times where Google thinks I'm on the road below or above where I actually am (with raised highways).

      There's a really interesting app called 'GPS Status' that shows you all of your GPS data - including how many satellites you're connected to and how strong their signals are.
      Sometimes if you're outside, in a clear area you can be getting data from ten or more satellites.
       
    9. MSHOfficial

      MSHOfficial Well-Known Member EngineeringClicks Expert

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      I don’t think phones can give elevation data, you might be seeing the data from the GPS being presented to you as elevation. I mean I don’t think there is any sensor that measures elevation in a phone. But elevation angle yes, there are gyros in the phone that do so.

      Google maps shows you are under the bridge or above only when you are navigating, google maps knows that because they know the route, so it knows there is a bridge and you are navigating on the route under it. So it shows you are under the bridge. But for example what I am talking about is, if you are on the 4th floor of a building, google maps doesn’t show you are on the fourth floor. It just shows in 2D that you are in that building, right? So that’s why I think it doesn’t measure the altitude of your receiver.

      Oh I would like to check out that app actually. Sounds like an interesting one. I have this other app that shows all the sensors reading in your phone. You can actually connect it to matlab and if you are connected to the same vpn it will keep sending the data to matlab and you can use it to find your currant location using mapping codes in matlab.
       
    10. john12

      john12 Well-Known Member EngineeringClicks Expert

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      Yeah, the elevation data comes from the GPS data, not directly from a sensor in the phone, but the phone itself does know your elevation. I think you're correct about Google Maps not using this data though.
       

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