diff --git a/SGDP4.cpp b/SGDP4.cpp
index 0cbba17..19293b4 100644
--- a/SGDP4.cpp
+++ b/SGDP4.cpp
@@ -49,6 +49,7 @@ void SGDP4::SetConstants(EnumConstants constants) {
             throw new SatelliteException("Unrecognised constant value");
             break;
     }
+    constants_.S = constants_.AE + 78.0  / constants_.XKMPER;
     constants_.CK2 = 0.5 * constants_.XJ2 * constants_.AE * constants_.AE;
     constants_.CK4 = -0.375 * constants_.XJ4 * constants_.AE * constants_.AE * constants_.AE * constants_.AE;
     constants_.QOMS2T = pow((120.0 - 78.0) * constants_.AE / constants_.XKMPER, 4.0);