diff --git a/SGDP4.cpp b/SGDP4.cpp
index 921354b..c911819 100644
--- a/SGDP4.cpp
+++ b/SGDP4.cpp
@@ -354,7 +354,7 @@ void SGDP4::FindPositionSDP4(Eci& eci, double tsince) {
 
 }
 
-void SGDP4::FindPositionSGP4(Eci& eci, double tsince) {
+void SGDP4::FindPositionSGP4(Eci& eci, double tsince) const {
 
     /*
      * the final values
@@ -419,7 +419,7 @@ void SGDP4::CalculateFinalPositionVelocity(Eci& eci, const double& tsince, const
         const double& a, const double& omega, const double& xl, const double& xnode,
         const double& xincl, const double& xlcof, const double& aycof,
         const double& x3thm1, const double& x1mth2, const double& x7thm1,
-        const double& cosio, const double& sinio) {
+        const double& cosio, const double& sinio) const {
 
     double temp;
     double temp1;
@@ -1057,7 +1057,7 @@ void SGDP4::DeepSpacePeriodics(const double& t, double& em,
  * deep space secular effects
  */
 void SGDP4::DeepSpaceSecular(const double& t, double& xll, double& omgasm,
-        double& xnodes, double& em, double& xinc, double& xn) {
+        double& xnodes, double& em, double& xinc, double& xn) const {
 
     static const double THDT = 4.37526908801129966e-3;
 
@@ -1193,7 +1193,7 @@ void SGDP4::DeepSpaceCalcDotTerms(double& xndot, double& xnddt, double& xldot) c
 /*
  * deep space integrator for time period of delt
  */
-void SGDP4::DeepSpaceCalcIntegrator(const double& delt, const double& step2, double& xndot, double& xnddt, double& xldot) {
+void SGDP4::DeepSpaceCalcIntegrator(const double& delt, const double& step2, double& xndot, double& xnddt, double& xldot) const {
 
     /*
      * calculate dot terms
diff --git a/SGDP4.h b/SGDP4.h
index 957cca4..575101c 100644
--- a/SGDP4.h
+++ b/SGDP4.h
@@ -29,16 +29,16 @@ private:
     void DeepSpacePeriodics(const double& t, double& em, double& xinc,
             double& omgasm, double& xnodes, double& xll);
     void DeepSpaceSecular(const double& t, double& xll, double& omgasm,
-            double& xnodes, double& em, double& xinc, double& xn);
+            double& xnodes, double& em, double& xinc, double& xn) const;
     void FindPositionSDP4(Eci& eci, double tsince);
-    void FindPositionSGP4(Eci& eci, double tsince);
+    void FindPositionSGP4(Eci& eci, double tsince) const;
     void CalculateFinalPositionVelocity(Eci& eci, const double& tsince, const double& e,
             const double& a, const double& omega, const double& xl, const double& xnode,
             const double& xincl, const double& xlcof, const double& aycof,
             const double& x3thm1, const double& x1mth2, const double& x7thm1,
-            const double& cosio, const double& sinio);
+            const double& cosio, const double& sinio) const;
     void DeepSpaceCalcDotTerms(double& xndot, double& xnddt, double& xldot) const;
-    void DeepSpaceCalcIntegrator(const double& delt, const double& step2, double& xndot, double& xnddt, double& xldot);
+    void DeepSpaceCalcIntegrator(const double& delt, const double& step2, double& xndot, double& xnddt, double& xldot) const;
 
     bool first_run_;
 
@@ -247,12 +247,17 @@ private:
     double d_fasx4_;
     double d_fasx6_;
 
+    /*
+     * integrator constants
+     */
     double d_xfact_;
-
     double d_xlamo_;
-    double d_xli_;
-    double d_xni_;
-    double d_atime_;
+    /*
+     * integrator values
+     */
+    mutable double d_xli_;
+    mutable double d_xni_;
+    mutable double d_atime_;
 
     struct Constants {
         double AE;