diff --git a/README.md b/README.md
index 52b8699..6f199fb 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# GPS-SDR-SIM
+# GPS-SDR-SIM realtime
 
 GPS-SDR-SIM generates GPS baseband signal data streams, which can be converted 
 to RF using software-defined radio (SDR) platforms, such as 
@@ -18,6 +18,14 @@ to RF using software-defined radio (SDR) platforms, such as
 $ gcc gpssim.c -lm -O3 -o gps-sdr-sim
 ```
 
+## Realtime
+
+Use -n option can connect to a TCP source in Gnuradio for realtime simulation.
+
+The tcp source should be set in "Server" mode, listening to port 1234.
+
+See tcp.grc as a example.
+
 ### Generating the GPS signal file
 
 A user-defined trajectory can be specified in either a CSV file, which contains 
@@ -69,6 +77,7 @@ Options:
   -b <iq_bits>     I/Q data format [1/8/16] (default: 16)
   -i               Disable ionospheric delay for spacecraft scenario
   -v               Show details about simulated channels
+  -n               Use tcp connect to gnuradio for realtime simulation
 ```
 
 The user motion can be specified in either dynamic or static mode:
diff --git a/gpssim.c b/gpssim.c
index bad7187..76f10fa 100644
--- a/gpssim.c
+++ b/gpssim.c
@@ -1661,6 +1661,7 @@ void usage(void)
 		"  -b <iq_bits>     I/Q data format [1/8/16] (default: 16)\n"
 		"  -i               Disable ionospheric delay for spacecraft scenario\n"
 		"  -v               Show details about simulated channels\n",
+		"  -n               Use TCP connect to gnuradio for realtime simulation\n",
 		((double)USER_MOTION_SIZE) / 10.0, STATIC_MAX_DURATION);
 
 	return;