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I was scanning the local aviation radio frequencies, and came across an odd signal (listen to a recording here). It's at around 110.515MHz, and it seems strong within a 30 mile radius of Shropshire, UK (haven't tried tuning in further afield).

Not sure if it's relevant, but there are a couple of RAF bases nearby (RAF Shawbury and RAF Cosford).

I presume it's a navigation aid, but couldn't find anything listed on that frequency. I was hoping someone would recognise the sound and be able to tell me what equipment is transmitting it (e.g. NDB).

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  • 1
    $\begingroup$ I might have a go at triangulating the source; need to figure out how to build a directional FM antenna... I'm sure Google can help with that! $\endgroup$ – sircompo Jan 29 '15 at 16:30
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    $\begingroup$ It's a clearly modulated signal in your recording. Can you identify the frequencies of the modulation? There must be software to do that. If you know the modulation frequencies that will help. $\endgroup$ – DeltaLima Jan 29 '15 at 23:53
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    $\begingroup$ @DeltaLima: The signal is modulated with peaks at 800 Hz, and with a smaller amplitude at 650 and 950 Hz. It seems there is also something at 120 Hz. See this screenshot out of Sonic Visualizer. (I hope the link will work). To see where this zoomed sample is located within the full file provided, look at the bottom of the image. Also in the spectrum (pink) image, the higher peak is 800 Hz, the two ones coming immediately before and after are the 150 Hz the 150 Hz offsets mentionned. $\endgroup$ – mins Feb 2 '15 at 9:40
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    $\begingroup$ What surprises me is the frequency doesn't follow channel plan for this frequency. From Wikipedia: "The lowest 10 MHz of the band, from 108–117.95 MHz, is split into 200 narrow-band channels of 50 kHz". This would lead to 110.5 or 110.55 Is there a possibility the frequency is not accurate? $\endgroup$ – mins Feb 2 '15 at 19:48
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    $\begingroup$ I've not abandoned this, just haven't had the time to build a directional antenna yet. I'd be interested to hear from anyone around the UK who can check this frequency themselves and advise if they're receiving the same signal. $\endgroup$ – sircompo May 12 '15 at 15:51
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Well, the SDR dongle paid off - mystery solved.

It turns out, the signal was actually at 153.2MHz, but was somehow breaking through into 110.515MHz on my handheld scanner.

The signal originates from a high power pager transmitter using the FLEX protocol. As this is plain text, I decoded some of it using an excellent open source application called PDW, and have listed it below for reference and general interest (sensitive information has been redacted!).

  0074162  23:12:31 30-06-15 FLEX-A    ALPHA   3200   31:Jun 30 23:11 Trim Line 2 L359 Error Check 01 Override 010312 30-JUN-2015 23:10:49
  0000002  23:12:56 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0069
  0121518  23:12:59 30-06-15 FLEX-A    StNUM   3200   31948    
  1734139  23:13:01 30-06-15 FLEX-A    ALPHA   3200   61:Jun 30 23:12 Term B  Page-fail test
  1756286  23:13:24 30-06-15 FLEX-A    ALPHA   3200   PIN 6840 PLS CALL DSCC ON 08457 500***
  0118459  23:13:46 30-06-15 FLEX-A    ALPHA   3200   20150630 23:13 boc1web02.servstream.com: conn CRITICAL ALERT
  0000002  23:14:03 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0070
  1734139  23:14:09 30-06-15 FLEX-A    ALPHA   3200   62:Jun 30 23:13 Term B  Page-fail test
  0076963  23:14:29 30-06-15 FLEX-A    ALPHA   3200   55:Jun 30 23:13 23.13hrs Zone 14 L550 down 15 Mins maint in attendance FROM: GB50-AFENNEL3
  0074162  23:14:46 30-06-15 FLEX-A    ALPHA   3200   32:Jun 30 23:13 Trim Line 2 L359 Error Check 01 Override 010312 30-JUN-2015 23:12:42
  0074179  23:14:48 30-06-15 FLEX-A    ALPHA   3200   19:Jun 30 23:13 Z2A LH Stn 200  Main Call 30-JUN-2015 23:13:22
  0074194  23:14:50 30-06-15 FLEX-A    ALPHA   3200   56:Jun 30 23:13 23.13hrs Zone 14 L550 down 15 Mins maint in attendance FROM: GB50-AFENNEL3 FROM: GB50-AFENNEL3
  0107291  23:14:50 30-06-15 FLEX-A    StNUM   3200   2422085  
  0074076  23:14:50 30-06-15 FLEX-A    ALPHA   3200   60:Jun 30 23:13 Fac 1670 More than 20 I/P on conv 30-JUN-2015 23:11:30
  0086317  23:14:52 30-06-15 FLEX-A    ALPHA   3200   36:Jun 30 23:13 Dodo Panel 23 Entry Movement Alarm 000065 30-JUN-2015 23:12:03
  0000002  23:14:56 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0071
  0067432  23:14:59 30-06-15 FLEX-A    ALPHA   3200   71:Jun 30 23:14 Water Test WATER TEST SUMP LEVEL HIGH 001215 30-JUN-2015 23:14:46
  1754937  23:15:16 30-06-15 FLEX-A    ALPHA   3200   'Adv' 7440298 30/06/2015 23:08:33 tight chest, struggling to breathe, EMAS ETA 11 mn, Priority RED2 Age 59 Incident area - KETTERING ** ****** ** NN16 ***
  0067432  23:15:22 30-06-15 FLEX-A    ALPHA   3200   72:Jun 30 23:14 Skid RETURN PROBLEM CHECK SKID RETURN Tested o.k. 30-JUN-2015 23:12:25
  1734139  23:15:24 30-06-15 FLEX-A    ALPHA   3200   63:Jun 30 23:14 Term B  Page-fail test
  0086317  23:15:44 30-06-15 FLEX-A    ALPHA   3200   39:Jun 30 23:14 Dodo Panel 6 P&F Emergency Stop relay 125KN1 - 400301.08 30-JUN-2015 23:13:32
  1734136  23:15:46 30-06-15 FLEX-A    ALPHA   3200   19:Jun 30 23:14 Term A  Page-fail test
  0067154  23:15:50 30-06-15 FLEX-A    ALPHA   3200   Fr:nocteam Su:Escalate PQV-73575-125 Tx:Auto escalation of ticket (3 hour rule) Ticket: PQV-73575-125 Queue: VelociX Date raised: 2015-06-30 20:11:27 Subject: ANTEL New asset request
  0117556  23:15:54 30-06-15 FLEX-A    ALPHA   3200   PLS CALL DUTY OFFICE
  0000002  23:16:03 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0072
  0111087  23:16:07 30-06-15 FLEX-A    StNUM   3200   71301    
  0118459  23:16:16 30-06-15 FLEX-A    ALPHA   3200   20150630 23:15 boc1backup.siops.net: msgs CRITICAL ALERT
  1734139  23:16:31 30-06-15 FLEX-A    ALPHA   3200   64:Jun 30 23:15 Term B  Page-fail test
  0074208  23:16:37 30-06-15 FLEX-A    ALPHA   3200   69:Jun 30 23:15 Warning : BODY DELIVERY STN 1 OVERCYCLE * BD SKID BD SKID (52)
  1734136  23:16:54 30-06-15 FLEX-A    ALPHA   3200   20:Jun 30 23:16 Term A  Page-fail test
  0074221  23:16:54 30-06-15 FLEX-A    ALPHA   3200   09:Jun 30 23:15 Warning : BODY DELIVERY STN 1 OVERCYCLE * BD SKID BD SKID (53)
  0120075  23:16:56 30-06-15 FLEX-A    ALPHA   3200   PLS CALL 12F ON 42848 2ND CALL
  1734139  23:17:39 30-06-15 FLEX-A    ALPHA   3200   65:Jun 30 23:16 Term B  Page-fail test
  1756806  23:17:41 30-06-15 FLEX-A    ALPHA   3200   PLS CALL ***** ***** - ADDENBROOKES ON 01223 257 ***
  0121619  23:17:42 30-06-15 FLEX-A    ALPHA   3200   P2 TS 3170
  0074162  23:17:46 30-06-15 FLEX-A    ALPHA   3200   34:Jun 30 23:16 Trim Line 2 L359 Error Check 01 Override 010312 30-JUN-2015 23:15:30
  1728604  23:18:05 30-06-15 FLEX-A    ALPHA   3200   41:Jun 30 23:17 ****** ******** 07711 239 ***
  0067432  23:18:07 30-06-15 FLEX-A    ALPHA   3200   73:Jun 30 23:17 Conv 11 Conv 1 Starved Load stn Blocked 30-JUN-2015 23:14:55
  0090980  23:18:07 30-06-15 FLEX-A    ALPHA   3200   30/06/2015 23:15:02 INC 14517765 ETFR Countess of Chester Hospital Chester Cheshire -, THE COUNTESS OF CHESTER HEALTH PARK, CHESTER, CH2 *** Male 25Y ProQA Overrid
  0090980  23:18:09 30-06-15 FLEX-A    ALPHA   3201   [Continued message - Fragment #2]den - Call Escalated GRD 340129 368274
  0067432  23:18:09 30-06-15 FLEX-A    ALPHA   3200   74:Jun 30 23:17 Conv 11 Conv 1 Starved Load stn Blocked 30-JUN-2015 23:14:55
  0067432  23:18:11 30-06-15 FLEX-A    ALPHA   3200   75:Jun 30 23:17 Water Test BOOTH 2 LOW / HI WATER PRES30100230 KU-.2N1u013 (?E4
  0118459  23:18:11 30-06-15 FLEX-A    ALPHA   3200   30 23:17]X[??Xns01.siops.net: dnPyt7xyICAL ALERT
  0114910  23:18:13 30-06-15 FLEX-A    ALPHA   3200   40:Jun 30 23:17 PLS CALL LEAH ON EXT 31505
  0119227  23:18:16 30-06-15 FLEX-A    ALPHA   3200   42:Jun 30 23:17 PLS CALL DUTY NURSE 020 7561 ****
  0120151  23:18:27 30-06-15 FLEX-A    ALPHA   3200   PLS COME TO CAVENDISH UNIT
  0118459  23:18:46 30-06-15 FLEX-A    ALPHA   3200   20150633 27z1b"bocm$n:th4Cp
  1743989  23:18:46 30-06-15 FLEX-A    ALPHA   3201   [Continued message - Fragment #2]
                                                          WN i$*CHER 01226 872 199 REajlFE AT POST OFFICE WONPi OCK AND IS WIDE OPEN*?f
  1756806  23:18:48 30-06-15 FLEX-A    ALPHA   3200   PLS CALL ***** ***** - ADDENBROOKES ON 01223 257 ***
  1734139  23:18:54 30-06-15 FLEX-A    ALPHA   3200   66:Jun 30 23:17 Term B  Page-fail test
  0000002  23:18:56 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0075
  1734136  23:19:16 30-06-15 FLEX-A    ALPHA   3200   22:Jun 30 23:18 Term A  Page-fail test
  0067432  23:19:22 30-06-15 FLEX-A    ALPHA   3200   76:Jun 30 23:18 Conv 11 Conv 1 Starved Load stn Blocked 30-JUN-2015 23:16:09
  0067432  23:19:24 30-06-15 FLEX-A    ALPHA   3200   77:Jun 30 23:18 Conv 11 Conv 1 Starved Load stn Blocked 30-JUN-2015 23:16:09
  0076517  23:19:26 30-06-15 FLEX-A    ALPHA   3200   38:Jun 30 23:18 Conv 21 Check Zone Start Pulse Count 30-JUN-2015 23:19:15
  0114375  23:19:26 30-06-15 FLEX-A    ALPHA   3200   30:Jun 30 23:18 Conv 21 Check Zone Start Pulse Count 30-JUN-2015 23:19:15
  0076956  23:19:27 30-06-15 FLEX-A    ALPHA   3200   97:Jun 30 23:18 Conv 21 Check Zone Start Pulse Count 30-JUN-2015 23:19:15
  1734139  23:20:01 30-06-15 FLEX-A    ALPHA   3200   67:Jun 30 23:19 Term B  Page-fail test
  0000002  23:20:03 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0076
  0074162  23:20:09 30-06-15 FLEX-A    ALPHA   3200   36:Jun 30 23:19 Trim Line 2 L359 Error Check 01 Override 010312 30-JUN-2015 23:18:09
  0114910  23:20:13 30-06-15 FLEX-A    ALPHA   3200   41:Jun 30 23:19 PLS CALL **** ****** ON EXT 34335
  1734136  23:20:31 30-06-15 FLEX-A    ALPHA   3200   23:Jun 30 23:19 Term A  Page-fail test
  0102150  23:20:33 30-06-15 FLEX-A    StNUM   3200   0778
  0000002  23:20:56 30-06-15 FLEX-A    ALPHA   3200   THIS IS A TEST PERIODIC PAGE SEQUENTIAL NUMBER  0077
  1734139  23:21:09 30-06-15 FLEX-A    ALPHA   3200   68:Jun 30 23:20 Term B  Page-fail test
  0076517  23:21:14 30-06-15 FLEX-A    ALPHA   3200   39:Jun 30 23:20 Conv 21 Check Zone Start Pulse Count 30-JUN-2015 23:21:02
  0119626  23:21:18 30-06-15 FLEX-A    ALPHA   3200   34:Jun 30 23:20 Conv 21 Check Zone Start Pulse Count 30-JUN-2015 23:21:02
  0102150  23:21:18 30-06-15 FLEX-A    StNUM   3200   0775899****** 
  0076956  23:21:20 30-06-15 FLEX-A    ALPHA   3200   98:Jun 30 23:20 Conv 21 Check Zone Start Pulse Count 30-JUN-2015 23:21:02
  0200905  23:22:34 30-06-15 POCSAG-3  ALPHA   1200   23:22-98 30/06/2015 23:20:18 INC 14517787 31C02 ************, ELLESMERE PORT, CH65 *** Male 64 Years M PASSING OUT GRD
                                                      340506 375692
  0334410  23:23:50 30-06-15 POCSAG-3  ALPHA    512   23:22-41 C2 Incident Allocated, Locn: *****, GREENACRES CARE CENTRE, TEARN ROAD, PL35JF, , AZ:, CAD:300615005110, ChComp:
  1508907  23:23:51 30-06-15 POCSAG-4  ALPHA    512   CH3A 512
  0273040  23:24:04 30-06-15 POCSAG-4  ALPHA   1200   30/06/2015 CH3A
  0269168  23:24:20 30-06-15 POCSAG-3  ALPHA   1200   23:24-10 30/06/2015 23:22:15 INC 14517792 10D02 ***********, NEWBOLD ROCHDALE ROCHDALE, OL16 *** Female 24 Years
                                                      MIGRAINE - FEELS SICK GRD 390789 412576
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  • $\begingroup$ Also, thanks again to everyone for their input. $\endgroup$ – sircompo Jul 1 '15 at 7:45
  • $\begingroup$ Thank you for posting the follow up! Probably your scanner uses an intermediate frequency which caught the interference. Amazing that you found the origin and the protocol used. $\endgroup$ – DeltaLima Jul 1 '15 at 8:59
  • $\begingroup$ Good work impressive $\endgroup$ – Mark Jul 2 '15 at 1:08
  • $\begingroup$ Well, that's really fantastic! Bravo! I did look at this and this (maybe the SDR you use). $\endgroup$ – mins Aug 8 '15 at 14:14
  • $\begingroup$ Impressive job! What SDR dongle are you using? $\endgroup$ – Caterpillaraoz Sep 26 '17 at 7:11
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If your frequency is accurate then it uses 25 khz spacing. VHF channels can be spaced at 100khz, 50 khz, 25 khz, 12.5 or 8.333 - hence the more decimal places on the dial. I think you might have picked up a marker beacon based on someone elses analysis. Basically there are three tones at 1020 hz / 1300 hz and 3khz that tell a pilot how far he is from the runway. I don't think you would get the signal from 30 miles though, but you never know. Also the 150hz tone can be from the localizer - which will definitely be valid over 30 miles. there are two tones - one at 90 hz and the other at 150 hz. Obviously these are modulated on the carrier frequency, and these are part of the ILS system. I have a feeling you don't have the exact frequency but see if it is stronger at 110.55, if it is then this is the localizer frequency.

The 100khz digit is odd for localizer frequencies.

Hope this helps

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  • $\begingroup$ Thanks for the info. I've just purchased an SDR dongle and will use the waterfall view to see if I can identify the full frequency range on which the signal is transmitted. $\endgroup$ – sircompo Jun 17 '15 at 11:03

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