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strf is the satellite tracking toolkit for radio observations (RF). The software is designed to allow tracking of satellites from radio observations, using Doppler curves to identify satellites and/or determine their orbits.

The software is designed for linux operating systems, and will work with most software defined radios (SDRs), certainly those that are supported by The software comes with tools for data acquisition, performing FFTs to generate timestamped spectrograms (waterfall plots), and analysis, to extract and analyse Doppler curves.


  • For Ubuntu systems or similar.
    • Install dependencies: sudo apt install git make gcc pgplot5 gfortran libpng-dev libx11-dev libgsl-dev libfftw3-dev dos2unix
    • Clone repository: git clone
    • Compile: cd strf; make
    • Install (in /usr/local): sudo make install


  • You will need to set the following environment variables in your login file to run strf.
    • ST_DATADIR path to strf directory (e.g. $HOME/software/strf)
    • ST_TLEDIR path to TLE directory (e.g. $HOME/tle)
    • ST_COSPAR COSPAR site number (add to site location to $ST_DATADIR/data/sites.txt)
    • ST_LOGIN login info (of the form ST_LOGIN="identity=username&password=password")
  • Run tleupdate to download latest TLEs.
  • You should install NTP support on the system and configure time/date to automatically synchronize to time servers.


The main use of strf is to acquire IQ data from SDRs and produce time stamped spectrograms with the rffft application. rffft will perform Fast Fourier Transforms on the input data to a user defined number of spectral channels (via the -c command line option), and integrate/average these to a user defined integration length (via the -t command line option). The output will be a *.bin file which contains a 256 byte human readable header (which can be inspected with head -c256), followed by a binary array of floating point numbers representing the power in the spectral channels. This is an example of the 256 byte header:

UTC_START    2018-01-12T15:59:13.524
FREQ         2244000000.000000 Hz
BW           4000000.000000 Hz
LENGTH       0.998922 s
NCHAN        40000
NSUB         60

The header keywords are mostly self explanatory, though the NSUB keyword specifies that this single bin file contains 60 spectra.

rffft can read from a previously recorded IQ recording, but is usually operated in realtime mode by reading IQ data from a so-called named pipe or fifo (first in, first out). Here, the SDR writes IQ data to a fifo (instead of a file), and rffft reads the samples from the fifo. Using an airspy as an example, it could be configured as follows:

mkfifo fifo
rffft -i fifo -f 101e6 -s 2.5e6 &
airspy_rx -a 1 -f 101 -t 2 -r fifo

Here, we first make the fifo mkfifo fifo, then start rffft to read from the fifo (-i option), with a 101MHz center frequency (-f option) and a 2.5MHz sample rate (-s option). The & puts this command in the background. Finally, we start obtaining IQ data from the airspy with airspy_rx in the 2.5MHz sampling mode (-a 1) at the same frequency (-f 101, in MHz), with the 2.5MHz sample rate (-t 2) and writing the samples to the fifo (-r fifo). Similar scripts can be made with other SDRs, and otherwise with gnuradio flow graphs where the output file sink is a fifo.

Alternatively, when no input filename is given (with the -i option), rffft will read from stdin so it is possible to directly pipe an SDR receiver's application into rffft.

With an RTL-SDR:

rtl_sdr -g 29 -f 97400000 -s 2048000 - | ./rffft -f 97400000 -s 2048000 -F char

Here we use the RTL-SDR receiver with rtl_sdr with a gain of 29dB (-g 29), a center frequency of 97.4MHz (-f 97400000, in Hz) and a samplerate of 2.048MS/s (-s 2048000, in S/s). Note the trailing dash (-) in the rtl_sdr command to tell it to write to stdout instead of a file so it can be piped (|) through rffft. The same center frequency and samplerate are given to rffft. As rtl_sdr outputs data as 8 bits, -F char is required to tell rffft the format of the data.

With a HackRF:

hackrf_transfer -l 24 -g 32 -f 97400000 -s 8000000 -r - | ./rffft -f 97400000 -s 8000000 -F char -c 100

Here we use the HackRF receiver with hackrf_transfer with a lna gain of 24dB (-l 24), an IF gain of 32dB (-g 32), a center frequency of 97.4MHz (-f 97400000, in Hz) and a samplerate of 8MS/s (-s 8000000, in S/s). The output file is given as stdout (-r -). Again the same frequency and samplerate are given to rffft and as hackrf_transfer also outputs 8 bit data -F char is also required for rffft.

The output spectrograms can be viewed and analysed using rfplot.


Radio Frequency Satellite Tracking





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