diff --git a/README.md b/README.md
index 094b378..75e4ae7 100644
--- a/README.md
+++ b/README.md
@@ -18,11 +18,12 @@ to RF using software-defined radio (SDR) platforms, such as
 $ gcc gpssim.c -lm -O3 -o gps-sdr-sim
 ```
 
-## Realtime
+## Realtime by Gnuradio
 
 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 (as default, it can be changed by.option -n).
+
 See tcp.grc as a example.
 
 ### Generating the GPS signal file
@@ -76,7 +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
+  -n <port>        Use TCP connection to Gnuradio TCP-Source for realtime simulation. (Default 1234).
 ```
 
 The user motion can be specified in either dynamic or static mode:
@@ -93,6 +94,12 @@ The user motion can be specified in either dynamic or static mode:
 > gps-sdr-sim -e brdc3540.14n -l 30.286502,120.032669,100
 ```
 
+Use Gnuradio to realtime simulate 3000s:
+
+```
+> gps-sdr-sim -e brdc3540.14n -l 30.286502,120.032669,100 -n 1234 -d 3000
+```
+
 ### Transmitting the samples
 
 The TX port of a particular SDR platform is connected to the GPS receiver 
diff --git a/gpssim.c b/gpssim.c
index fa114be..f4165b5 100644
--- a/gpssim.c
+++ b/gpssim.c
@@ -1661,7 +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 TCP-Source for\n realtime simulation and port(default 1234)\n",
+		"  -n <port>        Use TCP connect to Gnuradio TCP-Source for\n realtime simulation. (default 1234)\n",
 		(USER_MOTION_SIZE)/10.0,STATIC_MAX_DURATION);
 
 	return;
@@ -1867,7 +1867,7 @@ int main(int argc, char *argv[])
 		llh[2] = 10.0;
 	}
 
-	if (duration<0.0 || (duration>((double)USER_MOTION_SIZE)/10.0 && !staticLocationMode) || (duration>STATIC_MAX_DURATION && staticLocationMode))
+	if (duration<0.0 || (duration>((double)USER_MOTION_SIZE)/10.0 && !staticLocationMode) || (duration>STATIC_MAX_DURATION && staticLocationMode)&&usesocket==false)
 	{
 		printf("ERROR: Invalid duration.\n");
 		exit(1);
diff --git a/tcp.grc b/tcp.grc
index 2361b06..dee5c85 100644
--- a/tcp.grc
+++ b/tcp.grc
@@ -124,7 +124,7 @@
     </param>
     <param>
       <key>_coordinate</key>
-      <value>(272, 200)</value>
+      <value>(280, 200)</value>
     </param>
     <param>
       <key>_rotation</key>
@@ -152,7 +152,7 @@
     </param>
     <param>
       <key>port</key>
-      <value>2345</value>
+      <value>1234</value>
     </param>
     <param>
       <key>vlen</key>
@@ -1071,7 +1071,7 @@
     </param>
     <param>
       <key>args</key>
-      <value>hackrf=81320f</value>
+      <value>hackrf</value>
     </param>
     <param>
       <key>_enabled</key>
diff --git a/top_block.py b/top_block.py
index 49ae3d6..5551b95 100644
--- a/top_block.py
+++ b/top_block.py
@@ -2,7 +2,7 @@
 ##################################################
 # GNU Radio Python Flow Graph
 # Title: Top Block
-# Generated: Sun Apr 23 01:40:34 2017
+# Generated: Sun Apr 23 02:13:05 2017
 ##################################################
 
 if __name__ == '__main__':
@@ -60,7 +60,7 @@ class top_block(gr.top_block, Qt.QWidget):
         ##################################################
         # Blocks
         ##################################################
-        self.osmosdr_sink_0 = osmosdr.sink( args="numchan=" + str(1) + " " + "hackrf=81320f" )
+        self.osmosdr_sink_0 = osmosdr.sink( args="numchan=" + str(1) + " " + "hackrf" )
         self.osmosdr_sink_0.set_sample_rate(samp_rate)
         self.osmosdr_sink_0.set_center_freq(100e6, 0)
         self.osmosdr_sink_0.set_freq_corr(0, 0)