I know it is 19 meters.....but from what Ilistened to, it sounds like a digital pager transmission.
If one has ever punched in a radio frequency for a alpha-numeric pager and listened ( for me a VHF freq ), that is exactly what it sounds like to me.
The radio transmittions is almost....if not continous in spacing.
I am not saying it is a paging system.....it may or may not be. I am just saying that with my previous monitoring of VHF paging frequencies ( I was bored, and working the "main gate" - so I experimented ) it sounds EXACTLY like a alpha-numeric pager broadcast.
FF, you got me thinking here, it does sound rather what I remember hearing from some paging systems. I am not much of a VHF/UHF monitor (not a lot around here) and I do tend to look at every signal from an HF angle first. So I took a fresh look at it (the recording) from the direction of a pager signal, assuming I had been biased in my first assessment.
The short answer:
I still say, with almost no doubt, this is an OTHR, I just can't ID which specific one (not unusual, you generally can't specify the majority of the HF radars seen other than maybe country of origin, although a few are easily identifiable). If the recording had been made in SSB mode I could be even more sure, by confirming a chirp rate, something a pager would not have. Since it appears to have been made in AM mode that is not possible to determine.
The longer answer (skip it if you don't want a minor tome):
Why not a pager?
First is frequency, I don't think I am aware of any pagers working below about 27 MHz on a regular basis. Of course, it is possible that the OP missed a digit and meant the RX was on 154.800 MHz. And it is also possible that the receiver used was experiencing imaging issues, and a VHF signal was being presented at HF frequencies.
The OP identified which remote he was using (Barcelona node on Globaltuners) so it was easy enough to jump on that node and tune up 154.800 MHz to see if any pagers were present. None heard in 45 minutes of monitoring that freq.
Image / overloading was still a possibility. Those kinds of issues tend to be persistent to a location and hardware, meaning if it happened once it is probably a common occurrence. So again on the Barcelona node I tuned to 15480 kHz. I checked it several times for up to 45 minutes per, no sign of a similar signal. Not proof, but possibly an indicator that it was not overload or imaging.
Next, most two tone pagers I am aware of have a small gap between the first tone and the second tone. This signal does not have such a gap.
Most two tone pagers I am aware of use higher tone frequencies. This transmission always starts (in the 14 complete examples presented in the recording) with a 100 Hz tone and that is followed immediately by second tone of around 30 Hz.
Looking closely at each burst it makes a lot of sense as a radar. Keep in mind we only have 14 complete bursts and 1 partial burst in the recording. But a cycle is seen.
The bursts occur on average slightly less than 5 seconds apart, start to start. This burst timing is variable with actual revisit times ranging between 2.75 seconds and 9.19 seconds.
Each burst starts with 20 fast chirps, at a 100 Hz rate. This means one chirp every 10 millisecond, or 200 millisecond of time. After the end of the 20th sweep the waveform transitions to a lower rate sweep, these lower rate chirps are always sent 64 times. This lower rate occurs on one of five fixed values. Arbitrarily assigning the number 1 to the first event heard on the recording, then number 1 cycle is a sweep every 31.2 millisecond, number 2 is a sweep every 29.9 millisecond, number 3 is a sweep every 32.8 millisecond, number 4 is a sweep every 28.6 millisecond, and number 5 is a sweep every 34.7 millisecond.
Throughout the recording the cycle stays the same, #1, #2, #3, #4, #5, repeat. Since each burst is well defined, 20 fast chirps always sent at 100 Hz followed by 64 slower chirps always sent at one of 5 fixed rates, the length of each of these 5 different cycles is always the same (cycles #1 are always the same length, cycles #2 a different length from #1, but all #2's are the same, etc). In the recording each is sent 3 times (the first number 1 is a partial, the recording starts after the burst has started).
This stepped and varying sweep rate is the FMCW radar version of a staggered PRF or staggered PRI in the pulse radar world. It has advantages in both interference reduction and in killing blind speeds.
The two tone transmission (or two sweep rate transmission) also has an advantage in the radar world. Higher repetition rates, like the first 200 msec of each of these bursts, results in higher average power returned from a target, and a higher probability of detection for smaller radar cross section targets. The faster chirp rate of these faster sweeps (assuming the same swept width for fast and slow sweeps) also increases range resolution (the ability to separate two targets close to each other in range). However such faster sweep rates with an unencoded transmission mathematically limits maximum unambiguous range. In this case, 100 Hz, it might make detection more probable, and smaller targets more likely to be detected, but for any target beyond 1500 km the range becomes uncertain. You can detect it, but you do not know, for sure, what the range is. For example a target detected as at 1000 km might indeed be at 1000 km, or it might be at some multiple of 1000 km, like 2000, 3000, or 4000 km. This is called an "nth time around target".
The addition of the approximate 30 Hz portion of the waveform pushes the unambiguous range out to roughly 5000 km. So in the ambiguous range 1000 km example above we could tell, for sure, that the target was instead at an unambiguous 3000 km (for example). The fast portion of the waveform increases detection probability and range resolution, the slower portion of the waveform increases unambiguous range while the varying sweep rates decrease susceptibility to noise and jamming while also eliminating radar blind speeds.
A nicely put together radar waveform.
T!
