Update README.md

README.md

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-# gps-sdr-sim
+# GPS-SDR-SIM
+
+GPS-SDR-SIM generates GPS baseband signal data streams, which can be converted 
+to RF using software-defined radio (SDR) platforms, such as 
+[bladeRF](http://nuand.com/), HackRF, and USRP.
+
+### Windows build instructions
+
+1. Start Visual Studio.
+2. Create an empty project for a console application.
+3. On the Solution Explorer at right, add "gpssim.c" to the Souce Files folder.
+4. Select "Release" in Solution Configurations drop-down list.
+5. Open the Property Pages dialog box and expand the Configuration Properties.
+6. Expand the C/C++ node and select the Language property page.
+7. Enable the OpenMP Support (/openmp).
+8. Build the solution.
+
+### Generating the GPS signal file
+
+A user-defined trajectory can be defined in a CSV file, which contains 
+the Earth-centered Earth-fixed (ECEF) user positions at 10Hz.
+
+The user specifies the GPS satellite constellation through a GPS broadcast 
+ephemeris file. The daily GPS broadcast ephemers file (brdc) is a merge of the
+indiviual site navigation files into one. The archive for the daily file is:
+
+[ftp://cddis.gsfc.nasa.gov/gnss/data/daily/](ftp://cddis.gsfc.nasa.gov/gnss/data/daily/)
+
+These files are then used to generate the simulated pseudorante and
+Doppler for the GPS satellites in view. This simulated range data is 
+then used to generate the digitized I/Q samples for the GPS signal.
+For example;
+
+```
+> gps-sdr-sim brdc3540.14n circle.vsv gpssim.bin
+```
+
+### Transmitting the samples with bladeRF
+
+The TX port of the bladeRF is connected to the GPS receiver under
+test through a DC block and a fixed 50dB attenuator.
+
+The simulated GPS signal file, named "gpssim.bin", can be loaded
+into bladeRF for playback as shown below:
+
+```
+set frequency 1575.42M
+set samplerate 4M
+set bandwidth 2.5M
+set txvga1 -25
+cal lms
+cal dc tx
+tx config file=gpssim.bin format=bin
+tx start
+```
+
+### License
+
+Copyright © 2015 Takuji Ebinuma  
+Distributed under the [MIT License](http://www.opensource.org/licenses/mit-license.php).