diff --git a/firmware/application/app_ais.cpp b/firmware/application/app_ais.cpp
index 09f81a5d..df5b36f1 100644
--- a/firmware/application/app_ais.cpp
+++ b/firmware/application/app_ais.cpp
@@ -224,7 +224,11 @@ uint32_t Packet::message_id() const {
 	return field_.read(0, 6);
 }
 
-uint32_t Packet::source_id() const {
+MMSI Packet::user_id() const {
+	return field_.read(8, 30);
+}
+
+MMSI Packet::source_id() const {
 	return field_.read(8, 30);
 }
 
diff --git a/firmware/application/app_ais.hpp b/firmware/application/app_ais.hpp
index ffca3443..b5739ce2 100644
--- a/firmware/application/app_ais.hpp
+++ b/firmware/application/app_ais.hpp
@@ -56,6 +56,8 @@ struct DateTime {
 using Latitude = int32_t;
 using Longitude = int32_t;
 
+using MMSI = uint32_t;
+
 class Packet {
 public:
 	constexpr Packet(
@@ -72,7 +74,8 @@ public:
 	bool is_valid() const;
 
 	uint32_t message_id() const;
-	uint32_t source_id() const;
+	MMSI user_id() const;
+	MMSI source_id() const;
 
 	uint32_t read(const size_t start_bit, const size_t length) const;