Why the results differ between pyephem sublat and STK LLA？

Question

import ephem
line1 = "WorldView 4"
line2 = '1 41848U 16067A   20074.82580014 -.00000094  00000-0  00000+0 0  9999'
line3 = "2 41848  97.8435 176.0145 0008992 126.7749 233.3766 15.40359612182266"
iss = ephem.readtle(line1, line2, line3)
iss.compute('2020/3/15 04:00:00.000')
issE = iss.elevation.real/1000
print("%s,%s,%s"%(iss.sublat,iss.sublong,issE))

82:05:50.5,-140:07:20.6,453.627375

but in STK LLA report，the result is：

15 Mar 2020 04:00:00.000 82:08:46.1243 N 140:07:21.4394 W 453.625591.
Especially at certain latitudes, the difference becomes larger

Asked By: andylau

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Source

Answer 1

The STK offers definitions of each field:

https://help.agi.com/stk/11.0.1/Subsystems/dataProviders/Content/Satellite~LLA_State~Fixed.htm

It does not compute true geographic latitude. Instead it simply computes the angle between the satellite and the Earth’s xy plane:

Lat The detic latitude (i.e., angle between the detic subpoint's surface normal vector and the XY plane.)

Geographic latitude and detic latitude only agree at the equator, where both are 0°, and at the pole where both are 90°. You can learn more about the difference here:

https://en.wikipedia.org/wiki/Latitude#Geodetic_and_geocentric_latitudes

Answered By: Brandon Rhodes

Why the results differ between pyephem sublat and STK LLA？

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