diff --git a/src/main/java/org/orekit/files/ccsds/ndm/cdm/CdmData.java b/src/main/java/org/orekit/files/ccsds/ndm/cdm/CdmData.java
index b158c457bceaace1f8322e4075c6227cb342d419..b6b3cb707f08c1ccd9cce97fe3cb7902bc67792c 100644
--- a/src/main/java/org/orekit/files/ccsds/ndm/cdm/CdmData.java
+++ b/src/main/java/org/orekit/files/ccsds/ndm/cdm/CdmData.java
@@ -104,6 +104,8 @@ public class CdmData implements Data {
     }
 
     /** Get the covariance matrix logical block.
+     * <p> The RTN Covariance Matrix is provided in the 9×9 Lower Triangular Form. All parameters of the 6×6 position/velocity submatrix
+     * are mandatory. The remaining elements will return NaN if not provided. </p>
      * @return covariance matrix block
      */
     public RTNCovariance getRTNCovarianceBlock() {
diff --git a/src/main/java/org/orekit/files/ccsds/ndm/cdm/RTNCovariance.java b/src/main/java/org/orekit/files/ccsds/ndm/cdm/RTNCovariance.java
index d602d3a89cefe0eded56f036b4b8d8cb58006a57..a827c972334e3450c6fc398ef7947fd439a2132d 100644
--- a/src/main/java/org/orekit/files/ccsds/ndm/cdm/RTNCovariance.java
+++ b/src/main/java/org/orekit/files/ccsds/ndm/cdm/RTNCovariance.java
@@ -25,6 +25,8 @@ import org.orekit.files.ccsds.section.CommentsContainer;
  * attribute which can be acces with getRTNCovariaxMatrix method. Beware that
  * there are thus 2 ways to modify the RTN covariance : setC... ( setCrr,
  * setCtr ...) which should be prioritized and getRTNCovariaxMatrix.setEntry(row, col, value).
+ * <p> The RTN Covariance Matrix is provided in the 9×9 Lower Triangular Form. All parameters of the 6×6 position/velocity submatrix
+ * are mandatory. The remaining elements will return NaN if not provided. </p>
  * @author Melina Vanel
  * @since 11.2
  */
@@ -36,6 +38,8 @@ public class RTNCovariance extends CommentsContainer {
     /** Simple constructor. To update matrix value there are 2 ways to modify the RTN
      * covariance : setC... ( setCrr, setCtr ...) which should be prioritized and
      * getRTNCovariaxMatrix.setEntry(row, col, value).
+     * <p> The RTN Covariance Matrix is provided in the 9×9 Lower Triangular Form. All parameters of the 6×6 position/velocity submatrix
+     * are mandatory. The remaining elements will return NaN if not provided. </p>
      */
     public RTNCovariance() {
         covarianceMatrix = MatrixUtils.createRealMatrix(9, 9);
@@ -90,6 +94,8 @@ public class RTNCovariance extends CommentsContainer {
 
     /**
      * Get the RTN covariance matrix.
+     * <p> The RTN Covariance Matrix is provided in the 9×9 Lower Triangular Form. All parameters of the 6×6 position/velocity submatrix
+     * are mandatory. The remaining elements will return NaN if not provided. </p>
      * @return the RTN covariance matrix
      */
     public RealMatrix getRTNCovarianceMatrix() {