This commit adds a generic IQ receiver supporting
arbitrary output sampling rate. It also allows the
user to disable or enable the Doppler correction applied.
The commit also drops the outdated fm_demod flowgraph.
This commit adds a hierarchical block that performs Doppler
compensation and resampling.
Rather than using directly the available Doppler correction blocks,
based on the user parameters of the incoming sampling rate and the
desired target sampling rate, it applies proper decimation on the signal
so the frequency shift to be applied to a smaller sampling rate,
reducing significantly the CPU resources required. At the previous
architecture (gr-satnogs 1.x.x) we used seperate blocks for the doppler
correction and the LO digital shift, operating at the device sampling rate.
These two blocks, performing almost the same operation, contributed to a
30% CPU usage of the overall application. Now the LO is compensated by
the Doppler correction block, taking into account at the same time the
Doppler drift.
After the digital LO shift, the Doppler corrected signal is passed through
an Polyphase Arbitrary Resampler, to match exactly the sampling rate
requested by the user.
The waterfall file has now a constant sized header of 52 bytes,
so that plotting tools can reconstruct properly the spectrum.
The structure of the header is the following:
- A 32 byte string containing the timestamp in
ISO-8601 format. This timer has microsecond accuracy.
- A 4 byte integer containing the sampling rate
- A 4 byte integer with the FFT size
- A 4 byte integer containing the number of FFT snapshots for one row
at the waterfall
- A 4 byte float with the center frequency of the observation.
- A 4 byte integer indicating the endianness of the rest of the file. If
set to 0 the file continues in Big endian. Otherwise, in little endian.
The change of the endianness is performed to reduce the overhead at the
station.
Note that all contents of the header are in Network Byte order! The rest
of the file is in native byte order, mainly for performance reasons.
Users can use data of the header to determine if their architecture match
the architecture of the host generated the waterfall file and act
accordingly.
The file continues with information regarding the spectral content of the
observation.
Each waterfall line is prepended with a int64_t field indicating the
absolute time in microseconds with respect to the start of the waterfall
data (stored in the corresponding header field).
The spectral content is stored in $FFT$ float values already converted in
dB scale.
The new GNU Radio linkage system seems that has some
issues. There are linkage dependencies that they should
be private on the GNU Radio target.
In addition, find_package(Boost) is also called on the
gnuradio-runtime target. Calling it again in the OOT
module overwrites the existing list of dependencies,
causing many problems.
We are staring a process of dropping Boost and use
the C++11 variants.
In case of scrambling the self synchronizing scrambler ensures
that enough repetitions of the AX.25 flag have been received.
However, this does not hold for the case of non scrambled
transmissions. In this case, we wait for at least two consecutive
AX.25 flags to reduce the false alarms. Experiments have shown
that due to the poor CRC there were many false positive frames.
It is expected however, to miss some transmissions that use only one
AX.25 flag.
Update the obsolete flowgraphs and update the existing with
the new decoding architecture blocks. Client should
not have big problems with the new decoders as we utilizing
the multiformat frame sink that takes care of it. This
is just a work around for testing as we move forward for
fully socket-based communication with the client
To allow easier integration to the new architecture the
multi format file sink supports bot legacy and the new
format of the PMT messages. In poth cases, only the PDU
is extracted. Legacy used the PDU as a BLOB, whereas
now the PDU is stored in base64 format inside a
dictionary, which structure is defined through
the gr::satnogs::metadata() class
Vasilis Tsiligiannis
Manolis Surligas
nickoe
takurx