diff --git a/ArduSub/surface_bottom_detector.cpp b/ArduSub/surface_bottom_detector.cpp
index 3ea661d6c6..4523c0266a 100644
--- a/ArduSub/surface_bottom_detector.cpp
+++ b/ArduSub/surface_bottom_detector.cpp
@@ -3,11 +3,6 @@
 
 #include "Sub.h"
 
-#define SURFACE_DEPTH -0.05 // 5cm depends on where the external depth sensor is mounted
-#define DIVE_DEPTH_CONSTRAINT 0.10 // 10cm if we cannot achieve this target, we are trying to dig into the bottom
-#define BOTTOM_DETECTOR_TRIGGER_SEC 1.0
-#define SURFACE_DETECTOR_TRIGGER_SEC 1.0
-
 // counter to verify contact with bottom
 static uint32_t bottom_detector_count = 0;
 static uint32_t surface_detector_count = 0;
@@ -18,66 +13,57 @@ static float start_depth = 0; // the depth when we first hit downward throttle l
 // called at MAIN_LOOP_RATE
 void Sub::update_surface_and_bottom_detector()
 {
-    // update 1hz filtered acceleration
-    Vector3f accel_ef = ahrs.get_accel_ef_blended();
-    accel_ef.z += GRAVITY_MSS;
-    depth_accel_ef_filter.apply(accel_ef, MAIN_LOOP_SECONDS);
+	Vector3f velocity;
+	ahrs.get_velocity_NED(velocity);
 
-	// check that the airframe is not accelerating (not falling or breaking after fast forward flight)
-	bool accel_stationary = (depth_accel_ef_filter.get().length() <= 1.0f);
+	// check that we are not moving up or down
+	bool vel_stationary = velocity.z > -0.05 && velocity.z < 0.05;
 
 	if (ap.depth_sensor_present) { // we can use the external pressure sensor for a very accurate and current measure of our z axis position
 		current_depth = barometer.get_altitude();
 
-
 		set_surfaced(current_depth > SURFACE_DEPTH); // If we are above surface depth, we are surfaced
 
-		// ToDo maybe we can lighten the throttle check to a less extreme value, will depend on buoyancy and effective motor force
-		// it won't make a difference beyond informational purposes for non-auto control modes ie stabilize
-		if (motors.limit.throttle_lower) { // We can't predict where the bottom is, unless we have a sounder
-			if (bottom_detector_count == 0) {
-				start_depth = current_depth;
+		if(motors.limit.throttle_lower && vel_stationary) {
+			// bottom criteria met - increment the counter and check if we've triggered
+			if( bottom_detector_count < ((float)BOTTOM_DETECTOR_TRIGGER_SEC)*MAIN_LOOP_RATE) {
+				bottom_detector_count++;
+			} else {
+				set_bottomed(true);
 			}
 
-			if (current_depth > start_depth - DIVE_DEPTH_CONSTRAINT) { // If we can't descend a short distance at full throttle, we are at the bottom
-				if( bottom_detector_count < ((float)BOTTOM_DETECTOR_TRIGGER_SEC)*MAIN_LOOP_RATE) {
-					bottom_detector_count++;
-				} else {
-					set_bottomed(true);
-				}
-			} else {
-				set_bottomed(false); // We are still moving down
-			}
 		} else {
-			set_bottomed(false); // If we are not trying to descend at lower throttle limit, we are not at the bottom
+			set_bottomed(false);
 		}
 
+	// with no external baro, the only thing we have to go by is a vertical velocity estimate
+	} else if (vel_stationary) {
+
+		if(motors.limit.throttle_upper) {
+			// surface criteria met, increment counter and see if we've triggered
+			if( surface_detector_count < ((float)SURFACE_DETECTOR_TRIGGER_SEC)*MAIN_LOOP_RATE) {
+				surface_detector_count++;
+			} else {
+				set_surfaced(true);
+			}
+
+		} else if(motors.limit.throttle_lower) {
+			// bottom criteria met, increment counter and see if we've triggered
+			if( bottom_detector_count < ((float)BOTTOM_DETECTOR_TRIGGER_SEC)*MAIN_LOOP_RATE) {
+				bottom_detector_count++;
+			} else {
+				set_bottomed(true);
+			}
+
+		} else { // we're not at the limits of throttle, so reset both detectors
+			set_surfaced(false);
+			set_bottomed(false);
+		}
+
+	} else { // we're moving up or down, so reset both detectors
+		set_surfaced(false);
+		set_bottomed(false);
 	}
-//	else if (accel_stationary) {
-//		if(motors.limit.throttle_upper) {
-//			if( surface_detector_count < ((float)SURFACE_DETECTOR_TRIGGER_SEC)*MAIN_LOOP_RATE) {
-//				surface_detector_count++;
-//			} else {
-//				set_surfaced(true);
-//			}
-//		} else if(motors.limit.throttle_lower) {
-//			// landed criteria met - increment the counter and check if we've triggered
-//			if( bottom_detector_count < ((float)BOTTOM_DETECTOR_TRIGGER_SEC)*MAIN_LOOP_RATE) {
-//				bottom_detector_count++;
-//			} else {
-//				set_bottomed(true);
-//			}
-//		} else {
-//			set_surfaced(false);
-//			set_bottomed(false);
-//		}
-//	} else {
-//		// we've sensed movement up or down so reset land_detector
-//		set_surfaced(false);
-//		set_bottomed(false);
-//	}
-
-
 }
 
 void Sub::set_surfaced(bool at_surface) {