Sunday, March 8, 2020

2400 symb/sec bursts with 256 symbol long frames

Interesting PSK-modulated burst signals were picked up recently on a KiwiSDR in the UK:
  • 2400 symb/sec
  • 8-PSK modulation
  • The 1st 6 frames show a substructure of known symbols not present in the following frames

abs(IQ) in 1280 sample frames (12 kHz sampling rate) 


arg(symb) in 256 symbol frames (2400 symb/sec) 






Tuesday, February 18, 2020

2000 symb/sec 8PSK bursts

Today 8PSK bursts with 2000 symb/sec were received on a KiwiSDR in WCNA on 7862 kHz USB (center frequency is 7863.8 kHz).

Starting from the recorded WAV file in I/Q mode, all processing (symbol synchronization, doppler correction, symbol analysis, plotting) was done manually in GNU Octave using signal-analysis for LFSR-detection.

The bursts consist of a BPSK-modulated preamble which is a subsequence of a length-14 LFSR with period 214-1, i.e., a maximal length LFSR. The preamble is followed by 6 720-symbol-long frames, where each frame starts with 72 known 8PSK-modulated symbols, followed by 72 BPSK-modulated unknown symbols and 576 8PSK unknown symbols.

Preamble and frame structure

720 symbol frames

Monday, January 6, 2020

Some HF radar signals

7500 kHz


A number of similar HF radar signals have recently been observed on KiwiSDRs located in Europe on different frequencies.

The pulse repetition rate is 40 Hz (25 ms/pulse).

abs(IQ)

Each pulse consists of a linear chirp and of a pause.

FM demodulation

KiwiSDR TDoA multilateration indicates that these signals come from somewhere in Russia.

KiwiSDR TDoA multilateration

Bursts of what seem to be linear FM chirps have been observed on different frequencies around 8050±~50 kHz.

6390 kHz

Like the signal on 7500 kHz the pulse repetition rate is 40 Hz. However the duty cycle is smaller.

abs(IQ)

FM demodulation


8050 kHz


For this signal, the pulse repetition rate is 96 Hz.

abs(IQ)
Each pulse consists likely of a linear chirp. The spikes in the instantaneous frequency seen below are probably cause by HF propagation effects. Note that these radar-like signals are narrow-band (±1kHz variation in instantaneous frequency, only)

FM demodulation
KiwiSDR TDoA multilateration indicates a position somewhere in the Atlantic Ocean (compatible with the Azores).

KiwiSDR TDoA multilateration

Monday, November 4, 2019

Update on the OQPSK signal from Chicago

This is an update to this and to this blog post.

(Updated 11/5/19)

While before the offset between the "X" and "Y" bit streams was 458,748 bits, recently this offset has changed (but see below) to 65,536=216 bits, i.e., a non-pseudo-random bit stream can be obtained as follows:

  m = 65536;
  b = bitxor(bY(1:end-m),~bX(1+m:end)); # (*)

Recently this signal was observed on 12190 kHz with quite a lot of selective fading, so the coherent demodulation is not perfect.

12190 kHz QQPSK demodulation

The bit stream obtained by using (*) shows a repeated pattern of 7 bits (0011101):

fixed bit patten

It can be easily verified that this bit pattern is generated by an LFSR:
   bn = bn-1 + bn-3 ,   (**)
where "+" denotes XOR. In fact the offset is the same as before. The fact that a LFSR-generated sequence is left after (**) is an artifact of using 1/7 original offset. This also makes the additional descrambling mentioned below obsolete.

At another time the same type of signal was observed on 10790 kHz, this time carrying data. Again the offset between the "X" and the "Y" bit streams was 216 bits. Using the LFSR (**) the resulting bit stream obtained from (*) can be descrambled, and then closely resembles the one found before: there are "frames" of 48-bits and the bits come in pairs, so in one given frames there are 24 "independent" (but see below) bits.

bit stream in terms of 48-bit frames

Treating each line in the above plot as one "symbol" it was found that 16 symbols occur with (about equally) high frequency and all other symbols with very low frequency:

48-bit symbol frequency
For now let's treat the symbols with low frequency as transmission errors (they might not be that).

The 16 symbols, when arranged in order, show an interesting pattern: in each group of four symbols 1) the last group of three bits is repeating, and 2) the first 7 groups of three bits are the same. In the table below the last number in brackets indicates the number of times a given symbol has occurred.


i= 1: 000_100_000_000_000_100_001_000 (497)
i= 2: 000_100_000_000_000_100_001_010 (494)
i= 3: 000_100_000_000_000_100_001_100 (531)
i= 4: 000_100_000_000_000_100_001_111 (487)

i= 5: 000_101_011_011_000_111_111_000 (470)
i= 6: 000_101_011_011_000_111_111_010 (561)
i= 7: 000_101_011_011_000_111_111_100 (536)
i= 8: 000_101_011_011_000_111_111_111 (506)

i= 9: 001_100_011_101_101_010_011_000 (494)
i=10: 001_100_011_101_101_010_011_010 (522)
i=11: 001_100_011_101_101_010_011_100 (495)
i=12: 001_100_011_101_101_010_011_111 (538)

i=13: 010_011_111_001_111_110_110_000 (508)
i=14: 010_011_111_001_111_110_110_010 (495)
i=15: 010_011_111_001_111_110_110_100 (485)
i=16: 010_011_111_001_111_110_110_111 (482)



The sequence of these symbols is not entirely random: the last three bits determine if the following symbol is in the group 13-16, 9-12, 5-8, or, 1-4:

Distributions of symbols following the one indicated in the subplot title.

This might indicate a form of Trellis coding, because not all possible transitions between symbols are allowed.

Any information about this kind of error correction is very much appreciated.




STANAG 4285 from FUX, Reunion

STANAG 4285 CARB (Channel Availability and Receipt Broadcast) messages from FUX, Reunion received on a KiwiSDR located in India:

gr-digitalhf screenshot


ALL DE FUX

VV TESTING

RYRYRYRYRYRYRYRYRYRYRYRY

SGSGSGSGSGSGSGSGSGSGSGS

0 9 8 7 6 5 4 3 2 1   VOYEZ VOUS LE BRICK GEANT QUE J EXAMINE PRES DU GRAND WHARF

INT ZBZ

KKKK

Sunday, October 27, 2019

DGPS TDoA

Recently, some people have observed a DPGS signal on 318 kHz. Existing software displays its location as Shepelevskiy (Leningrad Oblast) ; however KiwiSDR TDoA shows a different likely location.
318 kHz DGPS

Friday, October 11, 2019

4800 symb/sec QPSK signal on 10165 kHz

... noted today on European KiwiSDR. This signal is likely from the same origin as the one described in this blog post and this blog post.

However this time the data stream seems to be pure pseudo-random, at least for all methods of analysis I have tried.

Update 10/12: The signal still carries pseudo-random data

Using GNURadio a perfect constellation diagram can be obtained:

QPSK constellation diagram

... except during times when the signal fades:

QPSK symbols vs. time



The used GNURadio flowgraph, adapted from this tutorial, is shown below.

GNURadio flowgraph