AP_RTC: added a millisecond jitter correction function

libraries/AP_RTC/JitterCorrection.cpp

@@ -87,3 +87,14 @@ uint64_t JitterCorrection::correct_offboard_timestamp_usec(uint64_t offboard_use
     
     return uint64_t(estimate_us);
 }
+
+/*
+  correct an offboard timestamp in microseconds into a local
+  timestamp, removing timing jitter caused by the transport. 
+
+  Return a value in milliseconds since boot in the local time domain
+ */
+uint32_t JitterCorrection::correct_offboard_timestamp_msec(uint32_t offboard_ms, uint32_t local_ms)
+{
+    return correct_offboard_timestamp_usec(offboard_ms*1000ULL, local_ms*1000ULL) / 1000ULL;
+}

libraries/AP_RTC/JitterCorrection.h

@@ -13,6 +13,10 @@ public:
     // timestamp. See JitterCorrection.cpp for details
     uint64_t correct_offboard_timestamp_usec(uint64_t offboard_usec, uint64_t local_usec);
 
+    // correct an offboard timestamp to a jitter-free local
+    // timestamp. See JitterCorrection.cpp for details
+    uint32_t correct_offboard_timestamp_msec(uint32_t offboard_ms, uint32_t local_ms);
+    
 private:
     const uint16_t max_lag_ms;
     const uint16_t convergence_loops;