Add support for transmission via UHD devices

this is currently only tested with an USRP2, but should
work similarly with other UHD devices.

README.md

@@ -38,7 +38,11 @@ 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.
 
 The bladeRF command line interface requires I/Q pairs stored as signed 
-16-bit integers, while the hackrf_transfer supports signed bytes.
+16-bit integers, while the hackrf_transfer and gps-sdr-sim-uhd.py
+supports signed bytes.
+
+HackRF + bladeRF require 2.6 MHz sample rate, while the USRP2 requires
+2.5 MHz (an even integral decimator of 100 MHz).
 
 ```
 Usage: gps-sdr-sim [options]
@@ -92,6 +96,13 @@ For the HackRF:
 > hackrf_transfer -t gpssim.bin -f 1575420000 -s 2600000 -a 1 -x 0
 ```
 
+For UHD supported devices (tested with USRP2 only):
+
+```
+> gps-sdr-sim-uhd.py -t gpssim.bin -s 2500000 -x 0
+```
+
+
 ### License
 
 Copyright © 2015 Takuji Ebinuma  

gps-sdr-sim-uhd.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python
+# a small script to transmit simulated GPS samples via UHD
+# (C) 2015 by Harald Welte <laforge@gnumonks.org>
+# Licensed under the MIT License (see LICENSE)
+
+from gnuradio import blocks
+from gnuradio import eng_notation
+from gnuradio import gr
+from gnuradio import uhd
+from gnuradio.eng_option import eng_option
+from gnuradio.filter import firdes
+from optparse import OptionParser
+import time
+
+class top_block(gr.top_block):
+
+    def __init__(self, options):
+        gr.top_block.__init__(self, "GPS-SDR-SIM")
+
+        ##################################################
+        # Blocks
+        ##################################################
+        self.uhd_usrp_sink = uhd.usrp_sink(
+        	",".join(("", "")),
+        	uhd.stream_args(
+        		cpu_format="fc32",
+        		channels=range(1),
+        	),
+        )
+        self.uhd_usrp_sink.set_samp_rate(options.sample_rate)
+        self.uhd_usrp_sink.set_center_freq(options.frequency, 0)
+        self.uhd_usrp_sink.set_gain(options.gain, 0)
+
+        # a file source for the file generated by the gps-sdr-sim
+        self.blocks_file_source = blocks.file_source(gr.sizeof_char*1, options.filename, True)
+
+        # convert from signed bytes to short
+        self.blocks_char_to_short = blocks.char_to_short(1)
+
+        # convert from interleaved short to complex values
+        self.blocks_interleaved_short_to_complex = blocks.interleaved_short_to_complex(False, False)
+
+        # establish the connections
+        self.connect((self.blocks_file_source, 0), (self.blocks_char_to_short, 0))
+        self.connect((self.blocks_char_to_short, 0), (self.blocks_interleaved_short_to_complex, 0))
+        self.connect((self.blocks_interleaved_short_to_complex, 0), (self.uhd_usrp_sink, 0))
+
+def get_options():
+    parser = OptionParser(option_class=eng_option)
+    parser.add_option("-x", "--gain", type="eng_float", default=0,
+                      help="set transmitter gain [default=0]")
+    parser.add_option("-f", "--frequency", type="eng_float", default=1575420000,
+                      help="set transmit frequency [default=1575420000]")
+    # On USRP2, the sample rate should lead to an even decimator
+    # based on the 100 MHz clock.  At 2.5 MHz, we end up with 40
+    parser.add_option("-s", "--sample-rate", type="eng_float", default=2500000,
+                      help="set sample rate [default=2500000]")
+    parser.add_option("-t", "--filename", type="string", default="gpssim.bin",
+                      help="set output file name [default=gpssim.bin]")
+
+    (options, args) = parser.parse_args()
+    if len(args) != 0:
+        parser.print_help()
+        raise SystemExit, 1
+
+    return (options)
+
+if __name__ == '__main__':
+    (options) = get_options()
+    tb = top_block(options)
+    tb.start()
+    raw_input('Press Enter to quit: ')
+    tb.stop()
+    tb.wait()