**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 http://www.gnuradio.org. The software comes with tools for data acquisition, performing FFTs to generate timestamped spectrograms (waterfall plots), and analysis, to extract and analyse Doppler curves.
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:
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
END
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.
The output spectrograms can be viewed and analysed using `rfplot`.
