So I'm coding up some initial orbit determination stuff in MATLAB (for personal education; I love this stuff), and I am using 'Methods of Orbit Determination' by Escobal as a guide. I already got Vallado's code to work, now I'm just comparing other methods. I'm having an issue with one of his equations, though. Obviously, the method is angles-only, but one of the givens Escobal requires is H_i, and in his nomenclature section he says that H is the "Altitude of satellite measured normal to adopted ellipsoid". So its the altitude, which is an unknown since we don't have the orbit. I don't even have range data. Am I interpreting this part wrong? The reason I care about that variable is that my values for the X, Y, and Z vectors are off, which gives me a small slatn range value, which gives me the incorrect final value. the set of equations is: G1(i) = a_e/sqrt(1-(2*f-f^2)*sin(phi)^2) + H ; G2(i) = (1-f)^2*a_e/sqrt(1-(2*f-f^2)*sin(phi)^2) + H ; X(i) = -G1(i)*cosd(phi)*cosd(theta(i)) ; Y(i) = -G1(i)*cosd(phi)*sind(theta(i)) ; Z(i) = -G2(i)*sind(phi) ; Any ideas/help would be greatly appreciated.