fDot_and_gDot.m

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function [fdot, gdot] = fDot_and_gDot(x, r, ro, a)
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%{
This function calculates the time derivatives of the
Lagrange f and g coefficients.
mu - the gravitational parameter (km^3/s^2)
a - reciprocal of the semimajor axis (1/km)
ro - the radial position at time to (km)
t - the time elapsed since initial state vector (s)
r - the radial position after time t (km)
x - the universal anomaly after time t (km^0.5)
fdot - time derivative of the Lagrange f coefficient (1/s)
gdot - time derivative of the Lagrange g coefficient (dimensionless)
User M-functions required: stumpC, stumpS
%}
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global mu
z = a*x^2;
%...Equation 3.69c:
fdot = sqrt(mu)/r/ro*(z*stumpS(z) - 1)*x;
%...Equation 3.69d:
gdot = 1 - x^2/r*stumpC(z);
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