diff --git a/libraries/AP_RTC/AP_RTC.cpp b/libraries/AP_RTC/AP_RTC.cpp
index 5b60e40a5f..3871af21af 100644
--- a/libraries/AP_RTC/AP_RTC.cpp
+++ b/libraries/AP_RTC/AP_RTC.cpp
@@ -13,6 +13,31 @@ const char *AP_RTC::_clock_source_types[] = {
     "NONE",
 };
 
+AP_RTC::AP_RTC()
+{
+    AP_Param::setup_object_defaults(this, var_info);
+    if (_singleton != nullptr) {
+        // it's an error to get here.  But I don't want to include
+        // AP_HAL here
+        return;
+    }
+    _singleton = this;
+}
+
+
+// table of user settable parameters
+const AP_Param::GroupInfo AP_RTC::var_info[] = {
+
+    // @Param: _TYPES
+    // @DisplayName: Allowed sources of RTC time
+    // @Description: Specifies which sources of UTC time will be accepted
+    // @Bitmask: 0:GPS,1:MAVLINK_SYSTEM_TIME,2:HW
+    // @User: Advanced
+    AP_GROUPINFO("_TYPES",  1, AP_RTC, allowed_types, 1),
+
+    AP_GROUPEND
+};
+
 void AP_RTC::set_utc_usec(uint64_t time_utc_usec, source_type type)
 {
     if (type >= rtc_source_type) {
@@ -20,6 +45,11 @@ void AP_RTC::set_utc_usec(uint64_t time_utc_usec, source_type type)
         return;
     }
 
+    // check it's from an allowed sources:
+    if (!(allowed_types & (1<<type))) {
+        return;
+    }
+
     const uint64_t now = AP_HAL::micros64();
     const int64_t tmp = int64_t(time_utc_usec) - int64_t(now);
     if (tmp < rtc_shift) {
diff --git a/libraries/AP_RTC/AP_RTC.h b/libraries/AP_RTC/AP_RTC.h
index 424208bded..4c4954fc5a 100644
--- a/libraries/AP_RTC/AP_RTC.h
+++ b/libraries/AP_RTC/AP_RTC.h
@@ -1,26 +1,26 @@
 #pragma once
 
+#include <AP_Param/AP_Param.h>
+
 #include <stdint.h>
 
 class AP_RTC {
 
 public:
 
-    AP_RTC() {
-        if (_singleton != nullptr) {
-            // it's an error to get here.  But I don't want to include
-            // AP_HAL here
-            return;
-        }
-        _singleton = this;
-    }
+    AP_RTC();
+
+    static const struct AP_Param::GroupInfo var_info[];
+
+    AP_Int8 allowed_types;
 
     // ordering is important in source_type; lower-numbered is
-    // considered a better time source.
+    // considered a better time source.  These values are documented
+    // and used in the parameters!
     enum source_type : uint8_t {
-        SOURCE_GPS,
-        SOURCE_MAVLINK_SYSTEM_TIME,
-        SOURCE_HW,
+        SOURCE_GPS = 0,
+        SOURCE_MAVLINK_SYSTEM_TIME = 1,
+        SOURCE_HW = 2,
         SOURCE_NONE,
     };