diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.cpp b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
index 62dd58d847..e7041f07f3 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
@@ -1126,7 +1126,7 @@ bool NavEKF3::use_compass(void) const
 // The sign convention is that a RH physical rotation of the sensor about an axis produces both a positive flow and gyro rate
 // msecFlowMeas is the scheduler time in msec when the optical flow data was received from the sensor.
 // posOffset is the XYZ flow sensor position in the body frame in m
-void NavEKF3::writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, const Vector3f &posOffset)
+void NavEKF3::writeOptFlowMeas(const uint8_t rawFlowQuality, const Vector2f &rawFlowRates, const Vector2f &rawGyroRates, const uint32_t msecFlowMeas, const Vector3f &posOffset)
 {
     if (core) {
         for (uint8_t i=0; i<num_cores; i++) {
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.h b/libraries/AP_NavEKF3/AP_NavEKF3.h
index 40b6ec3ad1..74dfc52a11 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.h
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.h
@@ -206,7 +206,7 @@ public:
     // The sign convention is that a RH physical rotation of the sensor about an axis produces both a positive flow and gyro rate
     // msecFlowMeas is the scheduler time in msec when the optical flow data was received from the sensor.
     // posOffset is the XYZ flow sensor position in the body frame in m
-    void writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, const Vector3f &posOffset);
+    void writeOptFlowMeas(const uint8_t rawFlowQuality, const Vector2f &rawFlowRates, const Vector2f &rawGyroRates, const uint32_t msecFlowMeas, const Vector3f &posOffset);
 
     /*
      * Write body frame linear and angular displacement measurements from a visual odometry sensor
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp b/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp
index 04cdf29bb4..8504d2eb6b 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp
@@ -161,7 +161,7 @@ void NavEKF3_core::writeWheelOdom(float delAng, float delTime, uint32_t timeStam
 
 // write the raw optical flow measurements
 // this needs to be called externally.
-void NavEKF3_core::writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, const Vector3f &posOffset)
+void NavEKF3_core::writeOptFlowMeas(const uint8_t rawFlowQuality, const Vector2f &rawFlowRates, const Vector2f &rawGyroRates, const uint32_t msecFlowMeas, const Vector3f &posOffset)
 {
     // limit update rate to maximum allowed by sensor buffers
     if ((imuSampleTime_ms - flowMeaTime_ms) < frontend->sensorIntervalMin_ms) {
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_core.h b/libraries/AP_NavEKF3/AP_NavEKF3_core.h
index 1384662d3f..3e79c6b7ed 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_core.h
+++ b/libraries/AP_NavEKF3/AP_NavEKF3_core.h
@@ -202,7 +202,7 @@ public:
     // The sign convention is that a RH physical rotation of the sensor about an axis produces both a positive flow and gyro rate
     // msecFlowMeas is the scheduler time in msec when the optical flow data was received from the sensor.
     // posOffset is the XYZ flow sensor position in the body frame in m
-    void writeOptFlowMeas(uint8_t &rawFlowQuality, Vector2f &rawFlowRates, Vector2f &rawGyroRates, uint32_t &msecFlowMeas, const Vector3f &posOffset);
+    void writeOptFlowMeas(const uint8_t rawFlowQuality, const Vector2f &rawFlowRates, const Vector2f &rawGyroRates, const uint32_t msecFlowMeas, const Vector3f &posOffset);
 
     // return data for debugging optical flow fusion
     void getFlowDebug(float &varFlow, float &gndOffset, float &flowInnovX, float &flowInnovY, float &auxInnov, float &HAGL, float &rngInnov, float &range, float &gndOffsetErr) const;