[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

*Subject*: Re: [amsat-bb] Trakstar and satellite latitude.*From*: Phil Karn <karn@xxxxxxxx>*Date*: Tue, 17 Apr 2001 17:18:02 -0700*In-reply-to*: <auto-000011499543@warpspeed.megalink.net> (wejones@megalink.net)

Bill, You're seeing the difference between "geoCENTRic" and "geoDETic" latitude. Geocentric latitude is what you get when you compute arcsin ( z/sqrt(X^2+Y^2+Z^2)) It represents the angle, measured at the center of the earth, between your location and the equatorial plane. But the word "latitude" usually means geodetic latitude, not geocentric latitude. They differ because the earth is not a perfect sphere. To a good first approximation, it's an oblate spheroid. The earth is oblate for a simple reason: it spins. The actual shape of the earth is much more complex, but the oblate spheroid ("ellipsoid") is such a good approximation to reality that it is a primary reference in GPS and many other mapping and positioning applications. (Try searching Yahoo for "WGS84 ellipsoid"). Geodetic latitude is defined as the angle, with respect to the polar axis, of the tangent to the ellipsoid that runs through the location in question. If you draw this out, you'll see that this tangent is not quite perpendicular to the line running from the center of the earth to the tangent. The only exceptions are at the poles and on the equator, where geodetic and geocentric latitudes are the same. At all other points, the geodetic latitude is somewhat greater than the geocentric latitude. Geodetic latitude is (approximately) what you get when you measure your latitude by the stars. That's why it's the usual meaning of "latitude" for maps and such. Here's another way to look at it. Consider a plumb bob hanging freely at your location. You might think that it points directly at the center of the earth. But it doesn't, unless you're on the equator or on one of the poles. Due to the centrifugal force of the rotating earth, your plumb bob points slightly south (if you're in the northern hemisphere) or slightly north (southern hemisphere) of the true direction to the center of the earth. This deflection is (ignoring local gravitational anomalies) equal to the difference between your geodetic and geocentric latitudes. Phil ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: Trakstar and satellite latitude.***From:*Bill Jones

**Re: Trakstar and satellite latitude.***From:*Jeff Davis

**References**:**Trakstar and satellite latitude.***From:*Bill Jones

- Prev by Date:
**Help with Station Drivers** - Next by Date:
**Re: 145.800 packet** - Prev by thread:
**Trakstar and satellite latitude.** - Next by thread:
**Re: Trakstar and satellite latitude.** - Index(es):