27 April 2024

a difficult signal

Sometimes it may happens to come across signals for which - unless of know it a priori - it is difficult to correctly define the used modulation;  it's the case of the so-called "semi-modes", where some FSK modulations at certain conditions have the shape of phase manipulation just because such signals possessing both PSK and FSK issues. Such "dualism" is spread enough and concerns tightly connected modulations as (G)MSK and OQPSK as well as CPFSK and SDPSK. A signal sent me by a friend of mine just falls into this category.
Let's get some parameters of the signal such as bandwidth (Bw), baud rate (Br), and shift (Sh): as from Figure 1

Bw = 19000 Hz
Br = 16000 Bd
Sh =  8000 Hz

Fig. 1 - main parameters of the signal

Looking at Figure 2, from "Signals Analyzer - radioscanner.ru" [1], MSK modulation has a bandwidth of about 1.5*Br, GMSK Bw is lesser than this value (in it’s limit is very close to theoretical Br), and SDPSK Bw is more than 1.5*Br. Well, this signal has the value of Sh exactly = Br/2 while Bw is < 1.5*Br: so, judging by these results, it could be a GMSK signal.

Fig. 2 - difference between MSK and GMSK

Another specific feature of the so-called "semi-modes" is the spectrum of their second harmonic: looking at Figure 3, the second harmonic has two very clear and defined lines and the spacing between these lines is equal to Br. As from [1], this is the necessary condition for their identification, but not the sufficient one: indeed, also both SDPSK and OQPSK modes exhibit two spectral lines in the second harmonic. Please note that the carrier in the fourth degree is very weakly expressed, sometimes it is practically invisible at all.

Fig. 3

That said, as shown in Figure 4, some equalization/autocorrelation is necessary to bring out the carrier (a) so that the SA demodulator PLL can lock onto it: this way you have a clearer view of phase plane and constellations. The 4-ary constellation (b) and its transitions pattern (c) rule out the OQPSK mode (and GMSK too) since it should show an 8-ary like constellation but w/out zero-crossing transitions. The relative/differential view (Diff=1) show instead a two-state mode (d,e).
The above considerations suggest SDPSK (Simmetrical Diferential PSK) modulation, just like the one used for Orbcomm series sats [2]. Moreover, note the "Offset mode detected" warning that means a relative phase shift keying, aka offset keying! Indeed SDPSK is equivalent to π/2 DBPSK or PSK2 with phase rotation: ie, as shown by the transitions in absolute mode (c), SDPSK assumes that the phase is rotated by +π/2 for bit “0” and by -π/2 for bit “1” thus there is not a 180° turn.

Fig. 4 - phase plane and constellation of the signal being analyzed

So, while the mathematical relations among Bw/Br/Sh point to a GMSK modulation, phase plane and constellations seem to point to a SDPSK (or even CPFSK) modulation: the relative phase planes and constellations are shown in Figure 5 (the SDPSK and CPFSK signals are synthesized).

Fig. 5 - phase planes and constellations of  (synthesized) SDPSK and CPFSK signals

However, the comparison between the phase detector results shows a behavior more similar to a GMSK signal (Figure 6).

Fig. 6 - phase detector results (CPFSK, SDPSK, our signal)

Since such kinds of signals can be demodulated also as FSK, I tried both the SA universal PSK and  FSK demodulaors: the resulting bitstreams are shown in Figure 8, as you can see they are the same (the 15-bit period is due to the initial preamble).

Fig. 7 - SA universal PSK and MFSK demodulators

Fig. 8 - bistreams after PSK and FSK demodulations 

To conclude, "There is a lot of information that proves that semi-modes are practically the same from a mathematical point of view but at the receiver side there is no a reliable and easy method to discern the exact type of modulation. However, if the signal has a good quality, there are some clues that can help tip the balance one way or another... even if they could be not the conclusive" my friend AngazU says.

https://disk.yandex.com/d/WwBwL6tD_CTwLw  (.wav signals and bistreams)

[1] http://signals.radioscanner.ru/info/item68/
[2] http://signals.radioscanner.ru/base/signal16/

19 April 2024

French AF AWACS comms using MS-110A

I recently recorded some transmissions of the French Air Force (FAF), more precisely traffic between the Boeing E-3F Sentry 202 aircraft (ALE address 202E3F) & its home, ie the E-3F Main Operating Base located at BA702 Avord airport (ALE address MOBE3F). These transmissions were recorded on the frequency of 6745.0 KHz/USB and almost all follow the same "format" visible in Figure 1: MS-141A used for automatic link setup (2G-ALE) followed by MS-110A segments used for sending data in (I think) ARQ mode.

Fig. 1 - an Air-To-Ground transmission from aircraft 202 & MOB ground base

The analysis of the bitstreams points out some interesting characteristics:

a) Figure 2 shows that three type of "messages" are used: longer messages such as 1 & 4 of Figure 1 - unlike the others - do not carry data and therefore they could have a sync or signaling function for the receiving side, say a "control" type messages? This type of message is present in all the transmissions I recorded and precedes the real "data" message which in its turn is followed by what I think be an "ACK" type message (for clarity, the bistream in the left part of Figure 2 is time-edited).

b) all message types (control, data, ACK) end with the same string 0x69D3D226 (I added the trailing "00" 100101101100101101001011011001[00] bits to get an hex string).

Fig. 2

c) all data messages have the same length and consist of initial phasing sequence followed by 92 bytes of data (Figure 3). This fixed format could mean sending standard or pre-formatted messages, however it is difficult to venture anything about the content of the messages other than the fact that it is largely Air-To-Ground traffic (from aircraft 202 to MOB).

Fig. 3 - "data" type messages

In particular, a Ground-To-Air transmission (from MOB to aircraft 202) turned out to be composed only of the short ACK type messages that apparently make no sense (Figs 4,5), a possible explanation is the failure to receive the messages sent by the aircraft... but it's just a my guess.

Fig. 4 - a Ground-To-Air transmission

Fig. 5 - bitstreams related to MS-110A transmissions of Fig. 4


Armée de l'Air (French Air Force) Boeing E-3F Sentry 202


15 April 2024

unid datalink protocol(s) over a PSK8 ST and STANAG-4539 (2)

I had the opportunity to record other transmissions on 3712.70 KHz/USB and - also following the comment of my friend KarapuZ - I can state with reasonable certainty that the waveforms analyzed in the previous post [1] come from Thales equipment. 
As mentioned, both Thales and L3Harris use the GMSK-MFSK8 waveform to handle HF links but the L3Harris bitmap/bitstream have a very recognizable pattern that is not present in the bursts recorded today (Figs 1,2): therefore, the GMSK-MFSK-8 signal is the Thales Systeme-3000 "Skymaster ALE", used in TRC-3500 and TRC-3600/TRC-3700 series radios (HF 3000 family).

Fig. 1 - Thales Systeme-3000 GMFKS+MFSK8

Fig. 2 - Thales Systeme-3000 GMFKS: bitstream after differential decoding and 50ms bitmap

As you see in Figure 2, I used the OQPSK "view" to demodulate the preamble of the Skyaster ALE signal: however, the differential decoding clearly show a 2-state keying (precisely GMFSK) that can be demodulated also using the "classic" FSK approach (Figure 3).

Fig. 3 - use of the SA MFSK dem

For what concerns the two PSK8 Serial Tone waveforms A & B [1], they also could be proprietary ones (Thales); indeed, quoting TRC-3600 datasheet: "Thanks to its digital advanced technology, the TRC 3600 offers new embedded services: secure high data rate and digital voice transmissions. It integrates a high data rate, multiwaveform, single tone modem (from 75 to 5400 bps) and a vocoder (800 - 2400 bps) associated to a high security digital COMSEC chip". 

The data link protocol could be the digital voice vocoder (new MELP/LPC10), given the similarity of the bitstream with its L3Harris analogue, but that is just an unconfirmed hypothesis of mine.

Fig. 4 - bitmaps of the two PSK8 ST waveforms 


 [1] http://i56578-swl.blogspot.com/2024/04/unid-datalink-protocols-over-psk8-st.html

9 April 2024

unid datalink protocol(s) over a PSK8 ST and STANAG-4539 (Thales? L3Harris?)

A few days ago I came across some transmissions that caught my attention for at least three good reasons:
1. the frequency used, i.e. 7312.7 KHz/USB, within the 42 meter broadcast band;
2. the use of different traffic waveforms, ie PSK8 serial Tone and STANAG-4539, in ARQ and non-ARQ modes. The ARQ mode is easily recognizable by the difference between the frequencies of the subcarriers (around 50Hz) of data and ACK segments (1);  

Fig. 1 - different traffic waveforms

3. the use of a waveform composed of GMFSK-2000Bd + MFSK8 for the link setup procedure (Figure 2): as far as I know, this particular waveform is used by both Thales and Harris. Notice that the MFSK8 part is 188-141A compatible (125Bd & 250Hz separation between the 8 tones) but use a diferrent tone library.

Fig. 2 - GMFSK + MSK8 link setup waveform

I had already met such transmissions, noting how the system was able to simultaneously demodulate 2/3 waveforms (or more if we consider the ALE exchanges) even during the same logical link. In these recordings a single waveform (PSK8 ST or S-4539) is mostly used and I took advantage of this to study the characteristics of the used datalink protocol; a protocol which - in my opinion and according to my analysis - turns out to be proprietary and quite complex.

PSK8 Serial Tone
Figure 3 shows the 8-ary constellation (states and transitions) as well as the rasters of the two PSK8 modulated waveforms A and B. In the phase states, and especially looking at the transitions, one can easily notice the presence of a PSK2 modulation which is certainly used for the synchronization sequences visible in the bitmaps below. As usual, the resulting PSK2 symbols are then mapped and scrambled to appear, on-air, as a PSK8 costellation. Bot the the waveforms have an ACF of 106.6 ms that makes a 256 PSK8 symbols frame at the modulation rate of 2400Bd. However, although of the same length, two different framings are adopted, in particular the Type B waveform uses a framing similar in composition to that of STANAG-4285. 
Fig. 3 - constellation and bitmaps of the PSK8 Serial Tone waveform

It is important to note both in the bitmaps of Figure 3 and in the demodulated bistream of Figure 4 (related to the Type A waveform) the presence of "regular" and similar patterns, as well as the "invariance" of the symbols of the synchronization sequences. In my opinion such patterns and sequences could indicate the use of a uncoded mode and even no interleaving (or 1 frame length interleaver), furthermore the length of the scrambler should coincide with that of the frame (256 symbols) or at least it should be initialized at the beginning of each frame (2).

Fig. 4 - PSK8 ST type "A" waveform: demodulated bitstream

Examining the symbols of the synchronization sequences offers further food for thought. In Type A waveform, the use of PSK2 modulation is confirmed by the 2-state transitions in the sync sequence, the latter consisting of a pseudorandom sequence of 31 symbols that is repeated twice for a total of 62 symbols (Figure 5).
Fig. 5 - sync sequence symbols, PSK8 type "A" waveform

Two state-transitions are also visible in the sync sequence of Type B waveform (Figure 6). Given its similarity to the S-4285 framing, the synchronization sequence consists of 80 symbols and it too is a pseudorandom sequence of length 31, which is repeated periodically within the 80-symbol window (2 periods of length 31 plus the first 18 symbols of another period).

Fig. 6 - sync sequence symbols, PSK8 type "B" waveform

The most interesting thing, apart from the state values which may be due to both the scrambler and the the possible phase-offset errors of the SA PSK demodulator (3), is that both the two Types of waveforms use the same 31-symbol sync sequence: indeed, as can be seen in Figure 7, the 2-state transitions are the same. Perhaps the length of the sync sequence is used by the receiving modem to figure out which of the two waveforms is incoming, but it's just a my guess.

Fig. 7 - sync sequence symbols, PSK8 type "B" and "A" waveforms

Since the Type B waveform has the same framing as S-4285, an S-4285 decoder recognizes the Type B waveform samples (100% confidence) but since the synchronization sequences are different (see the 2-state transitions in Figure 8) it does not successfully engage any sub-modes.
Fig. 8 - comparison between sync sequences of PSK8 Type "B"  and STANAG-4285

As per STANAG-4539, both the QAM16 and PSK8 waveforms have the same 287-symbol framing (119.6ms ACF, 2400Bd) although the user data rate is different: 6400bps and 3200bps respectively for QAM16 and PSK8.

Fig. 9 - constellations and bitmaps of STANAG-4539 QAM16 and PSK8 waveforms

If in the case of PSK8 ST it was only possible to analyze the symbols after demodulation, in the case of STANAG-4539 it is possible to decode the signals and then analyze the composition of the upper layer datalink protocol(s).
Figure 10 shows a detail of a bitstream obtained after removing the S-4539 QAM16 overhead and consists of 96-byte (768 bits) Protocol Data Units (PDUs), each PDU consisting of 3 bytes header followed by 93 bytes of data:
1st byte: a ID/value field, in this sample: 0x09 (LSB first)
2nd byte: down-counter field (LSB first)
3rd byte: up-counter field (LSB first)
As one can see looking at the values of the two counters in Figure 10, the sample consists of 55 PDUs, numbered from 0 (00000000) to 54 (00110110).

Fig. 10 - headers and part fo bitstream after S-4539 QAM16 decoding
The same fields' structure can be found in the PDUs extracted from a sample of STANAG-4539 PSK8 (Figure 11). Since the half of the user data rate (3200bps Vs 6400bps), each PDU consists of 48 bytes: 3 bytes for the header fields followed by 45 bytes of data. It's interesting to see that a change in the first field (from 01001000 (36) to 00001000 (8)) occurs when the down-counter field restarts its value after reaching the 0: curiously, the up-counter does not "reset" but continues its counting.
It's worth noting that that patterns highlighted in the bitstream of Figure 4 (and the bitmaps of Figure 3) are most likely the two counter fields of Figure 11: if so, both tPSK8 ST and S-4539 traffic waveform transport the same datalink PDUs.
Fig. 11 - headers and part fo bitstream after S-4539 PSK8 decoding
Even more interesting. After removing the 3 bytes of the headers, I reshaped the stream into a 128 bit scheme (16 bytes), i.e. to the most probable value of its period, and I noticed the repetition of the string 0x3CF04F; so I synced the stream on this value (Figure 12), fixing a minimum length of 128 bits. The result highlights the presence of 45 PDUs of a "secondary" datalink protocol where each PDU has an header consisting of 4 bytes and a minimum length of 16 bytes (128 bits), the maximum is over 600 bytes (I was not able to establish it accurately):
bytes 1-3: a ID/value field, [001111001111000001001111] 0x3CF04 (LSB first)
4th byte: up-counter field (LSB first)
Fig. 12 - the emerging "secondary" datalink protocol PDUs

This may be a hasty statement, but it seems that the "secondary" datalink protocol PDUs > 16 bytes length are fragmented into small segments and then incapsulated into the 45/95 bytes payload of the "primary" datalink protocol PDUs. By the way, at least in these samples, the "secondary" PDUs have been found only in the primary PDUs which have the first byte of the hedaer equal to 0x48, maybe just a mere coincidence (Figure 13,14).

Fig. 13

Fig. 14

Since the lack of clear-text callsigns it's impossible to id the user, we may speculate just some guess about the manufacturer of the used devices:
- as far as I know both Thales and L3Harris make use of the GMSK-MFSK8 waveform to manage HF links: unfortunately the GMFSK signals portions are too short to allow the analysis of the bitmaps (L3Harris GMFSK has a well recognizable pattern [1]);

L3Harris typical pattern in GMFSK-MFSK8 signals

- the patterns highlighted in Figures 3,4 are very similar to the ones visible in the demodulated bitmaps of PSK8 Voice Digital waveform (L3Harris VD mode) [2];

L3Harris VD mode bitstream
- from Harris RF-5800 datasheet "L3Harris VD mode also allows data to be sent...both data and voice are secured with Citadel encryption" [2]: well, I did not find the Citadel characteristic pattern within the decoded bitstream, even if they could be plain-text transmissions.
So: Thales? L3Harris? either of them? ...hints and comments are welcome.
(to be continued)
(1)50Hz difference between 1800Hz sub-carriers

(2) FEC encoding and interleaving should provide time separation between contiguous values.

(3) SA is a signal analyzer and not a decoder, therefore its phase-plane demodulator does not sync  any particular protocol, as it happens for example in STANAG-4285 "suited" decoders. Working with phase keyed signals, the SA phane-plane demodulator produces right interpretations and views (number of phases, angles, modulation speed, carrier frequency,...) but it may return wrong demodulated streams due to the possible phase-offset errors.