BUG: accuracy of sgp4 orbits

[jklenzing]

Describe the bug
When generating an orbit with sgp4, the expected altitude may be off by up to 10s of km for LEO orbits.

To Reproduce

import pysat
sat = pysat.Instrument('missions', 'sgp4', inclination=5, alt_periapsis=400)
sat.load(2019, 1)

Expected behavior
The expected orbit is circular at 400 km altitude. The mean altitude and geodetic altitude are low.

Screenshots

Desktop (please complete the following information):

• OS: Mac OS X 11.6.5
• Version: python 3.8.11
• pysatMissions rc 0.3.0

Additional context
Potential sources

• mismatch of gravity / earth mass constants in pysatMissions, sgp4, geospacepy
• mismatch of earth radius assumptions in packages
• keplerian orbit calculations do not consider an ellipsoidal earth
• usage of WGS72

[jklenzing]

After running tests with skyfield support (see #106), the resulting lat / lon / alt match within 5-6 decimal places. The location of the error probably lies in the assumptions in the methods.orbits code.