diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.cpp b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
index ba68f3422e..62a6481fb0 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
@@ -1324,14 +1324,15 @@ void NavEKF3::writeEulerYawAngle(float yawAngle, float yawAngleErr, uint32_t tim
  * posErr     : 1-sigma spherical position error (m)
  * angErr     : 1-sigma spherical angle error (rad)
  * timeStamp_ms : system time the measurement was taken, not the time it was received (mSec)
+ * delay_ms   : average delay of external nav system measurements relative to inertial measurements
  * resetTime_ms : system time of the last position reset request (mSec)
  *
 */
-void NavEKF3::writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms)
+void NavEKF3::writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint16_t delay_ms, uint32_t resetTime_ms)
 {
     if (core) {
         for (uint8_t i=0; i<num_cores; i++) {
-            core[i].writeExtNavData(pos, quat, posErr, angErr, timeStamp_ms, resetTime_ms);
+            core[i].writeExtNavData(pos, quat, posErr, angErr, timeStamp_ms, delay_ms, resetTime_ms);
         }
     }
 }
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.h b/libraries/AP_NavEKF3/AP_NavEKF3.h
index a5a2c1687e..6f1530982a 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.h
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.h
@@ -289,10 +289,11 @@ public:
      * posErr     : 1-sigma spherical position error (m)
      * angErr     : 1-sigma spherical angle error (rad)
      * timeStamp_ms : system time the measurement was taken, not the time it was received (mSec)
+     * delay_ms   : average delay of external nav system measurements relative to inertial measurements
      * resetTime_ms : system time of the last position reset request (mSec)
      *
     */
-    void writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms);
+    void writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint16_t delay_ms, uint32_t resetTime_ms);
 
     // called by vehicle code to specify that a takeoff is happening
     // causes the EKF to compensate for expected barometer errors due to ground effect
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp b/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp
index de0e2ca512..1648989e7d 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp
@@ -940,7 +940,7 @@ void NavEKF3_core::writeDefaultAirSpeed(float airspeed)
 *            External Navigation Measurements           *
 ********************************************************/
 
-void NavEKF3_core::writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms)
+void NavEKF3_core::writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint16_t delay_ms, uint32_t resetTime_ms)
 {
     // limit update rate to maximum allowed by sensor buffers and fusion process
     // don't try to write to buffer until the filter has been initialised
@@ -967,8 +967,7 @@ void NavEKF3_core::writeExtNavData(const Vector3f &pos, const Quaternion &quat,
     }
 
     // calculate timestamp
-    const uint32_t extnav_delay_ms = 10;
-    timeStamp_ms = timeStamp_ms - extnav_delay_ms;
+    timeStamp_ms = timeStamp_ms - delay_ms;
     // Correct for the average intersampling delay due to the filter update rate
     timeStamp_ms -= localFilterTimeStep_ms/2;
     // Prevent time delay exceeding age of oldest IMU data in the buffer
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_core.h b/libraries/AP_NavEKF3/AP_NavEKF3_core.h
index 4d6fa874e9..a7213784b8 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_core.h
+++ b/libraries/AP_NavEKF3/AP_NavEKF3_core.h
@@ -295,10 +295,11 @@ public:
     * posErr     : 1-sigma spherical position error (m)
     * angErr     : 1-sigma spherical angle error (rad)
     * timeStamp_ms : system time the measurement was taken, not the time it was received (mSec)
+    * delay_ms   : average delay of external nav system measurements relative to inertial measurements
     * resetTime_ms : system time of the last position reset request (mSec)
     *
     */
-    void writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint32_t resetTime_ms);
+    void writeExtNavData(const Vector3f &pos, const Quaternion &quat, float posErr, float angErr, uint32_t timeStamp_ms, uint16_t delay_ms, uint32_t resetTime_ms);
 
     // called by vehicle code to specify that a takeoff is happening
     // causes the EKF to compensate for expected barometer errors due to ground effect