Solar Radiation Pressure

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Spacecraft orbits are influenced by solar radiation pressure (SRP) emitted from the Sun. The accelerations due to the force of the solar radiation pressure act in the direction away from the Sun, and therefore the effect is seen regardless of the Spacecraft's orientation, so long as the Spacecraft is not in shadow. The magnitude of the applied force is proportional to the mass, cross-sectional area normal to the direction to the Sun, the coefficient of reflectivity (CR) of the Spacecraft, and the inverse of the square of the distance from the Spacecraft to the Sun. These parameters provide flexibility in modeling the solar radiation pressure to match your mission's needs.


By default, SRP modeling is disabled in FreeFlyer, but can be easily enabled as described below.


The formula used to calculate the acceleration due to solar radiation pressure is:





aSRP ≡ Acceleration due to solar radiation pressure

S ≡ Mean solar flux at one astronomical unit (AU), defined as 1,358.0 W/m2(1)

AU ≡ Astronomical Unit (the distance from the Earth to the Sun), defined as 149,597,870.0 km

CR ≡ Coefficient of Reflectivity of the Spacecraft

ASRP ≡ SRP Area (the cross-sectional area incident to the Sun-Earth line)

m ≡ Mass of the Spacecraft

R ≡ Distance from the Spacecraft to the Sun

c ≡ Speed of light, defined as 299,792.458 km/s

U ≡ Unit vector from the Spacecraft to the Sun

ν ≡ Eclipse factor, where:

oν = 0 if the Spacecraft is in shadow (umbra)

oν = 1 if the Spacecraft is in sunlight

o0 < ν < 1 if the Spacecraft is in penumbra(2)


(1) Larson, W. J. and Wertz, J. R. Space Mission Analysis and Design (2nd Ed.), Microcosm Press, El Segundo, CA

(2) Equation 3-59, Page 76 of Wertz, James R., Spacecraft Attitude Determination and Control, Kluwer Academic Publishers, Boston, MA



Enabling SRP Modeling

To enable SRP forces, edit the Spacecraft object, proceed to the Force Model page, and check the "Solar Radiation Pressure" checkbox, as shown below.



To enable or disable Solar Radiation Pressure via FreeFlyer script, use the syntax shown below:


// To enable SRP

(Spacecraft1.Propagator AsType RK89).ForceModel.SRP = 1;


// To disable SRP

(Spacecraft1.Propagator AsType RK89).ForceModel.SRP = 0;


Spacecraft.Accelerations is an array containing the accelerations due to various forces on the Spacecraft and can be used to view the solar radiation pressure forces acting on a Spacecraft. The example below shows how to view force contributions from each source.


For i = 0 to Spacecraft1.Accelerations.Count-1;


      // Report the value of each acceleration component

      Report Spacecraft1.Accelerations[i].Description, Spacecraft1.Accelerations[i].Acceleration;





See Also

ForceModel Properties and Methods

ForceModel.SRP property

Spacecraft.SRPArea property

Solar System settings that affect Spacecraft Propagation

Solar Radiation Pressure Modeled using Other Accelerations Sample Mission Plan