diff --git a/.gitignore b/.gitignore
index 299e6c0..08869a8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -41,3 +41,6 @@ gps-sdr-sim-lut
 
 # Netbeans project folder
 nbproject/*
+
+# Last used USER_MOTION_SIZE variable
+.user-motion-size
diff --git a/Makefile b/Makefile
index 5b01636..9dbb889 100644
--- a/Makefile
+++ b/Makefile
@@ -6,15 +6,32 @@ all: gps-sdr-sim
 SHELL=/bin/bash
 CC=gcc
 CFLAGS=-O3 -Wall -D_FILE_OFFSET_BITS=64
+ifdef USER_MOTION_SIZE
+CFLAGS+=-DUSER_MOTION_SIZE=$(USER_MOTION_SIZE)
+endif
 LDFLAGS=-lm
 
 gps-sdr-sim: gpssim.o
 	${CC} $< ${LDFLAGS} -o $@
 
+gpssim.o: .user-motion-size
+
+.user-motion-size: .FORCE
+	@if [ -f .user-motion-size ]; then \
+		if [ "`cat .user-motion-size`" != "$(USER_MOTION_SIZE)" ]; then \
+			echo "Updating .user-motion-size"; \
+			echo "$(USER_MOTION_SIZE)" >| .user-motion-size; \
+		fi; \
+	else \
+		echo "$(USER_MOTION_SIZE)" > .user-motion-size; \
+	fi;
+
 clean:
-	rm -f gpssim.o gps-sdr-sim *.bin
+	rm -f gpssim.o gps-sdr-sim *.bin .user-motion-size
 
 time: gps-sdr-sim
 	time ./gps-sdr-sim -e brdc3540.14n -u circle.csv -b 1
 	time ./gps-sdr-sim -e brdc3540.14n -u circle.csv -b 8
 	time ./gps-sdr-sim -e brdc3540.14n -u circle.csv -b 16
+
+.FORCE:
diff --git a/README.md b/README.md
index 62a038c..06b03c6 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,22 @@ to RF using software-defined radio (SDR) platforms, such as
 $ gcc gpssim.c -lm -O3 -o gps-sdr-sim
 ```
 
+### Using bigger user motion files
+
+In order to use user motion files with more than 30000 samples (at 10Hz), the `USER_MOTION_SIZE`
+variable can be set to the maximum time of the user motion file in seconds. It is advisable to do
+this using make so gps-sdr-bin can update the size when needed. e.g:
+
+```
+$ make USER_MOTION_SIZE=4000
+```
+
+This variable can also be set when compiling directly with GCC:
+
+```
+$ gcc gpssim.c -lm -O3 -o gps-sdr-sim -DUSER_MOTION_SIZE=4000
+```
+
 ### Generating the GPS signal file
 
 A user-defined trajectory can be specified in either a CSV file, which contains 
diff --git a/gpssim.h b/gpssim.h
index 4bf7ce3..be86905 100644
--- a/gpssim.h
+++ b/gpssim.h
@@ -16,7 +16,9 @@
 #define MAX_CHAN (16)
 
 /*! \brief Maximum number of user motion points */
+#ifndef USER_MOTION_SIZE
 #define USER_MOTION_SIZE (3000) // max duration at 10Hz
+#endif
 
 /*! \brief Maximum duration for static mode*/
 #define STATIC_MAX_DURATION (86400) // second