[aprssig] How gps works

[Steve Noskowicz]

Andrew,

  You got some pretty good explanations, so just a few comments.

   Starting in 1-D, think of a straight track, on which you travel, with a clock at each end.  If you are in the center, you will hear the ticks at the same time.  Move off center and one arrives sooner and you know where you are along the track - assuming you know where the ends/clocks are. 
 
  If you can go up, there is a vertical line in the center where the ticks arrive together, so you need another clock 'up' somewhere to tell the up/down part.

  Then go to 2-D with the clocks on opposite corners (across the diagonal)of a city block and you travel only along the joining streets. You now have two locations where the time difference will be the same - the other two corners being where the ticks arrive simultaneously.  Moving off the streets, there will be hyperbolic curves where the delay is the same (the old Loran).

  With multiple clocks scattered around you can do this calculation for them all and get a better and better location.

  Generally, the wider separated the clocks are in the sky, the better the fix.  If sats you are using are close together, the intersecting lines will be close to parallel (spheres in this case will have close centers) and small errors in the radius (of travel) result in large shifts in the intersection point, so the more disbursed the sats are in the sky, the better the possible fix.  Therefore, in a valley (of any type, mountain or urban) the fix will be less accurate.  The ideal would be sats on the horizons.
  
  I didn't see it in other comments, but the ephemeris data also lets you know which sats (decode keys) to look for.

  I thought the "other" location that GPS gives is out in space, not deep in the earth...and isn't there a number of sats (4) where location reduces to just one point in 3-space?

  The super capacitor is most helpful in keeping the memory data when changing batteries.


WAAS (Wide Area Augmentation), as I understand, is a correction that is provided from known location ground receivers and added to the sat data.

DGPS (Differential) uses a known, ground based 'pseudo-sat' to transmit local, accurate location.


P.S.  There are such super capacitors (about 1/2 inch dia by 1 inch) that there are small R/C air planes powered by them instead of a battery. 


Sent from my Knight-Kit Crystal Set.
-- 
 73, Steve, K9DCI   USN (Vet) MOT (Ret) Ham (Yet)
--
I served during the cold war, so you can continue to be served a cold one.



--- On Sat, 5/5/12, Andrew Rich <vk4tec at tech-software.net> wrote:
> Hello
> 
> I am learning how gps works
>