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Update README.md

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Piotr Krysik 2014-12-06 18:40:40 +01:00
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@ -48,7 +48,7 @@ local_blocks_path=/usr/local/share/gnuradio/grc/blocks:/usr/share/gnuradio/grc/b
Usage
=====
There are many possible applications of *gr-gsm*. At this moment there is one application that is ready out of the box. It is improved replacement of the old Airprobe - the program that lets you receive and decode GSM control messages from timeslot 0 on the broadcasting channel of a BTS. After installation of *gr-gsm* there are two python executables that will be installed:
There are many possible applications of *gr-gsm*. At this moment there is one application that is ready out of the box. It is improved replacement of the old *Airprobe* - the program that lets you receive and decode GSM control messages from *timeslot 0* on the broadcasting channel of a BTS. After installation of *gr-gsm* there are two python executables that will be installed:
-```airprobe_rtlsdr.py```,
@ -60,7 +60,8 @@ This program uses cheap RTL-SDR receivers as the source of the signal. It can be
```
airprobe_rtlsdr.py
```
The window of the program contains amplitude spectrum of the signal drawn in realtime. The central frequency of the signal can be changed by moving fc slider. The GSM signal has bandwidth of around 200kHz. By looking for constant hills on the spectrum of such width you can find a GSM broadcasting channel. By setting the fc slider to a carrier frequency of a broadcasting channel the program should immediately print content of subsequent messages on the standard output. If it doesn't happen set ppm slider into different positions. The slider is responsible for setting devices clock offset correction. If the clock offset is too large the clock offset correction algorithm that is implemented in the program won't work. There is intentionally added upper of allowable clock offset - it was done in order to avoid adaptation of the algorithm to neighbour channels that would inevitably lead to instability. You can use the value set later by passing it as argument of the program:
The window of the program contains amplitude spectrum of the signal drawn in realtime. The central frequency of the signal can be changed by moving *fc* slider. The GSM signal has bandwidth of around *200kHz*. By looking for constant hills on the spectrum of such width you can find a GSM broadcasting channel. By setting the *fc* slider to a carrier frequency of a broadcasting channel the program should immediately print content of subsequent messages on the standard output.
If it doesn't happen set *ppm* slider into different positions. The slider is responsible for setting devices clock offset correction. If the clock offset is too large the clock offset correction algorithm that is implemented in the program won't work. There is intentionally added upper of allowable clock offset - it was done in order to avoid adaptation of the algorithm to neighbour channels that would inevitably lead to instability. You can use the value set later by passing it as argument of the program:
```
airprobe_rtlsdr.py -p <correction>
```
@ -97,7 +98,7 @@ https://ask.wireshark.org/questions/7976/wireshark-setup-linux-for-nonroot-user
Videos
======
Short presentation of airprobe'like application of *gr-gsm*:
Short presentation of *Airprobe*'like application of *gr-gsm*:
https://www.youtube.com/watch?v=Eofnb7zr8QE
Credits