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ECEF smaller than earth radius

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By definition ECEF coordinates should be a Cartesian system centered at the earth center of mass. So how can a satellite in Aerospace Blockset simulation (R2019) have coordinates that are smaller than the earth radius? (eg, -5.7*10^6, -1.7*10^6, 3.15*10^6.)

Accepted Answer

Collin Russo
Collin Russo on 20 Sep 2019
Hi Judy, can you clarify where you see those values being used in the the Aerospace Blockset?
Those values are close to the initial conditions of the CubeSat Simulation Library "CubeSat Simulation Project" template, but are missing significant digits. The actual values used are [-5762639.8158 -1682742.0789 3156028.3952] :
sqrt(sum([-5762639.8158 -1682742.0789 3156028.3952].^2)) = 6.782341320079139e+06
while Earth's equatorial radius is ~6378.137km (https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)
6782341 - 6378137 = 404204
This ~404km altitude corresponds with the average altitude of the International Space Station, which was the reference orbit for this example.
Hope that helps,
Collin

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