Course, Governments/Military have overall control over civilian use of the Birds! or certainly use to have, whether that’s still the case I’m unsure!
Ken.. G0ORH Sent from my iPad > On 24 Jan 2018, at 17:21, Fred Jensen <[email protected]> wrote: > > The elevation changes seen with civilian GPS are the result of two fairly > fundamental issues: > > 1. A GPS receiver doesn't really "know" where actual sea level is, it uses a > mathematical model. Since the earth is ellipsoidal-ish rather than spherical > on a large scale, and since there are smaller local variations, the model is > a trade-off between accuracy and complexity. On State Route 37 just north of > San Francisco, you are immediately adjacent to San Pablo Bay and can see that > you're about 10 ft above it. My Garmin GPS pegs my elevation at 61 ft. > > 2. The second is "dilution of precision," or DOP. The relative positions of > the satellites in the sky change constantly, and finding your coordinates is > a process of finding the intersection point of lines of position [LOP] from > multiple satellites. When the satellites are scattered all over the sky, > many of the LOP's intersect at large angles [90 deg is ideal] and the > intersection is well defined. If the satellites are more or less lined up in > one area of the sky, the LOP's tend to intersect at smaller angles and the > exact intersection point is less well defined. This is Horizontal DOP or > HDOP. > > Unfortunately, when it comes to elevation [VDOP], roughly half the satellites > are below the horizon at any given time, there are thus fewer LOP's to work > with, and the probability of finding large-angle LOP's is inherently lower. > Hence, VDOP is usually [almost always] greater than HDOP. > > Thus endeth GPS 101. > > I also think they are communicating using some service on the Iridium > constellation which is probably expensive, and the ship is only making around > 10 kts or so, so more frequent updates wouldn't indicate much. Given the > overall expense of this expedition however, communications costs are probably > below the round-off. [:-) > > 73, > > Fred ["Skip"] K6DGW > Sparks NV DM09dn > Washoe County > >> On 1/24/2018 5:52 AM, Bayard Coolidge, N1HO via Elecraft wrote: >> The elevation discrepancies are normal - I see the same effect whendriving >> around down here in South Florida, randomly dropping to 10-20feet below sea >> level. The civilian GPS systems are not configured forextreme elevation >> accuracy, particularly when close to sea level. >> >> The main web site's position reporting may be outdated because, I think,that >> they're relaying their position via satellite (Iridium, perhaps?) whichcan >> get pretty expensive. Given the length of the journey, it's probablynot >> worthwhile providing hour-by-hour position updates. They'll get therewhen >> they get there, and they'll get on the air after they get things set up. >> 73, >> Brandy, N1HO (EL96xh) >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
