ConstantRefraction does not really need to implement Comparable.

src/main/java/org/orekit/rugged/refraction/ConstantRefractionLayer.java

@@ -19,8 +19,9 @@ package org.orekit.rugged.refraction;
 /**
  * Class that represents a constant refraction layer to be used with {@link MultiLayerModel}.
  * @author Sergio Esteves
+ * @since 2.0
  */
-public class ConstantRefractionLayer implements Comparable<ConstantRefractionLayer> {
+public class ConstantRefractionLayer {
 
     /** lowest altitude of this layer. */
     private final Double lowestAltitude;
@@ -45,30 +46,4 @@ public class ConstantRefractionLayer implements Comparable<ConstantRefractionLay
         return refractiveIndex;
     }
 
-    @Override
-    public int compareTo(final ConstantRefractionLayer o) {
-        return lowestAltitude.compareTo(o.lowestAltitude);
-    }
-
-    @Override
-    public boolean equals(final Object o) {
-        if (this == o) return true;
-        if (o == null || getClass() != o.getClass()) return false;
-
-        final ConstantRefractionLayer that = (ConstantRefractionLayer) o;
-
-        if (Double.compare(that.refractiveIndex, refractiveIndex) != 0) return false;
-        return lowestAltitude != null ? lowestAltitude.equals(that.lowestAltitude) : that.lowestAltitude == null;
-
-    }
-
-    @Override
-    public int hashCode() {
-        int result;
-        long temp;
-        result = lowestAltitude != null ? lowestAltitude.hashCode() : 0;
-        temp = Double.doubleToLongBits(refractiveIndex);
-        result = 31 * result + (int) (temp ^ (temp >>> 32));
-        return result;
-    }
 }

src/main/java/org/orekit/rugged/refraction/MultiLayerModel.java

@@ -33,6 +33,7 @@ import java.util.List;
 /**
  * Multi layer model for atmospheric refraction.
  * @author Sergio Esteves, Luc Maisonobe
+ * @since 2.0
  */
 public class MultiLayerModel implements AtmosphericRefraction {
 
@@ -73,13 +74,14 @@ public class MultiLayerModel implements AtmosphericRefraction {
 
     /** Simple constructor.
      * @param ellipsoid the ellipsoid to be used.
-     * @param refractionLayers the refraction layers to be used with this model.
+     * @param refractionLayers the refraction layers to be used with this model (layers can be in any order).
      */
     public MultiLayerModel(final ExtendedEllipsoid ellipsoid, final List<ConstantRefractionLayer> refractionLayers) {
         this.ellipsoid = ellipsoid;
-        this.refractionLayers = refractionLayers;
-        Collections.sort(this.refractionLayers, Collections.<ConstantRefractionLayer>reverseOrder());
-        atmosphereLowestAltitude = refractionLayers.get(refractionLayers.size() - 1).getLowestAltitude();
+        this.refractionLayers = new ArrayList<>(refractionLayers);
+        Collections.sort(this.refractionLayers,
+                         (l1, l2) -> Double.compare(l2.getLowestAltitude(), l1.getLowestAltitude()));
+        atmosphereLowestAltitude = this.refractionLayers.get(this.refractionLayers.size() - 1).getLowestAltitude();
     }
 
     /** {@inheritDoc} */