518kHz Navtex Reception in Shetland

518kHz Navtex Reception in Shetland

An investigation into the comparative signal strengths of 518kHz Navtex transmissions, as received in Clousta, Shetland, May 2012.

Receiver Location

Greenmeadow, Clousta, Shetland ZE2 9LX

60° 17.31´N 1° 25.50´W

Equipment used

Antenna : Wellbrook ALA1530S+ (Serial Number 1433) wideband, amplified loop mounted approximately 1m above ground with the broadside nulls oriented at 90/270 degrees so as not to impact on the antenna response in the directions of interest. The manufacturer provided an estimate for the antenna factor of 1dB at 500kHz.

Distribution splitter : home made hybrid splitter with nominal 6dB insertion loss to each port.

Receiver : Microtelecom Perseus direct sampling Software Defined Radio (Serial Number 03348)

PC : Intel D510 mini-ITX motherboard, 4GB RAM

Software : Navtex decoders - “YaND” and “Frisnit Navtex decoder”, Perseus receiver control software - v4.0b, Perseus recording scheduler – YaPS

Method

The Perseus SDR is capable of recording the directly sampled IQ data representing the RF spectrum, with a bandwidth determined by the chosen sample rate. This data is saved as a WAV file and can then be replayed later by the Perseus software which then behaves as if receiving this section of RF spectrum live. This allows repeated measurement at different RF frequencies within the recorded band, demodulation with different demodulators, adjustment of demodulator bandwidth etc.

A schedule was created to automatically record a 5 minute period, with a bandwidth of 100kHz centred on 518kHz timed to coincide with the published transmission timeslots of the target Navtex transmitters. The recordings are each approximately 200MB in size.

The image above shows the replay of a recording made at 05:00 – 05:05UTC on May 2nd 2012. The signal displayed is that of Cullercoats / GCC.

Run Number One : May 1 st - 2 nd 2012

For the first 24 hour run the recordings were scheduled to capture the timeslots of:

Cullercoats [G]

Malin Head [Q]

Portpatrick [O]

Niton [E] & [K]

This was to compare the performance of Cullercoats and Malin Head with the other two UK based NAVTEX transmitter sites.

Run Number Two : May 2 nd - 3 rd 2012

For the second 24 hour run the recordings were scheduled to capture the timeslots of:

Torshavn [D]

Orlandet [N]

Rogaland [L]

This second run, recording 3 Scandinavian stations, was to gather data on 3 foreign stations with almost complete sea-paths between themselves and the receiver location, as a comparison to the paths from Portpatrick, Malin Head and Niton which have large land-masses over substantial sections of the groundwave path.

Run Number Three: May 5 th - 6 th 2012

A final 24 hour run, recording all of the stations concerned once again. In the first run it was noted that some stations (eg GPK) may not have been active during some of the daytime slots, and this 3rd run was an attempt to gather some missing data on daytime signals, as well as to increase the amount of data gathered on Cullercoats and Malin Head, the main subjects of the study.

5 minute recordings were made of all 6 daily transmissions in timeslots:

[D], [E], [G], [K], [L], [N], [O], [Q]

Once the recordings had been stored, each one was then re-played using the Perseus software, and the following data was noted:

Signal power in dBm

Adjacent channel noise power (dBm), in a 300Hz RBW at a frequency 1kHz below the 518kHz Navtex transmission. The 300Hz bandwidth is chosen to match the specifications in ITU-R M.1467.

Decodability of the NAVTEX and where possible the Message ID being transmitted at the time of the measurements.

These were entered into a spreadsheet in order to automatically calculate the following parameters :

Signal to Noise ratio

Equivalent antenna signal voltage (uV and dBuV)

Equivalent incident electric field strength (uV/m and dBuV/m)

Path Prediction based on ITU-R 368-10

The ITU “GRWAVE” software was used to create an estimation of expected field strength, using the following parameters:

sigma (conductivity) = 5 S/m

epslon (relative permittivity) = 70

These represent the ground conditions for a sea-path

The software produces a table of field strengths to be expected at various distances given a transmitter EMRP of 1kW. Allowance must be made when comparing the predictions with real off-air measurements for the actual TX powers, and for the TX antenna efficiencies, but at present these aren't known to me.

******GRWAVE (RELEASE 2 AT 23/10/1985 )*************COPYRIGHT (C) GEC PLC 1985 *********CCIR Personal Computer Version 1989 Study Group 5 IWP5/1

GRWAVE COMPUTES FIELD STRENGTH-DISTANCE VARIATIONSFOR A HOMOGENEOUS CURVED EARTH WITH EXPONENTIALLYDECREASING REFRACTIVE INDEX

ATMOSPHERIC CONSTANTS REFRACTIVITY =315.00 (N-UNITS) SCALE HEIGHT = 7.350 KM

GROUND CONSTANTS RELATIVE PERMITTIVITY = 70.000 CONDUCTIVITY =5.0000D+00 SIEMENS/METRE

VERTICAL POLARISATION

MINIMUM DISTANCE = 20.000 KILOMETRES MAXIMUM DISTANCE = 1100.000 KILOMETRES DSTEP = 20.000 KILOMETRES

FREQUENCY = .518 MHZ

TRANSMITTER HEIGHT = 10.0 METRES RECIEVER HEIGHT = 10.0 METRES

DISTANCE FIELD STRENGTH BASIC TRANSMISSION LOSS KM DB(UV/M) DB (F) 20.00 83.39 52.86 40.00 77.25 59.00 60.00 73.63 62.62 80.00 70.94 65.31 100.00 68.78 67.47 120.00 66.95 69.30 140.00 65.35 70.90 160.00 63.91 72.34 180.00 62.58 73.67 200.00 61.35 74.90 220.00 60.19 76.06 240.00 59.08 77.17 260.00 58.03 78.22 280.00 57.01 79.24 300.00 56.02 80.23 320.00 55.06 81.19 340.00 54.12 82.13 360.00 53.20 83.05 380.00 52.30 83.95 400.00 51.41 84.84 420.00 50.54 85.71 440.00 49.67 86.58 460.00 48.82 87.43 480.00 47.98 88.27 500.00 47.14 89.11 520.00 46.31 89.94 540.00 45.49 90.76 560.00 44.67 91.58 580.00 43.86 92.39 600.00 43.06 93.19 620.00 42.26 93.99 640.00 41.46 94.79 660.00 40.67 95.58 680.00 39.89 96.36 700.00 39.10 97.15 720.00 38.32 97.93 740.00 37.54 98.71 760.00 36.77 99.48 780.00 36.00 100.25 800.00 35.23 101.02 820.00 34.47 101.78 840.00 33.70 102.55 860.00 32.94 103.31 880.00 32.18 104.07 900.00 31.43 104.82

920.00 30.67 105.58 940.00 29.92 106.33 960.00 29.17 107.08 980.00 28.43 107.82 1000.00 27.68 108.57 1020.00 26.93 109.32 1040.00 26.19 110.06 1060.00 25.45 110.80 1080.00 24.71 111.54 1100.00 23.97 112.28Stop - Program terminated.

Noise considerations

The environment in the vicinity of the receive site used for these tests is rural and has low levels of man-made noise. ITU-R P.372-10 was consulted to find a representative figure for the external noise factor (Fa) for this receiving site. The Environmental Category “quiet rural” was chosen. Calculation of the value of Fa for a “quiet rural” location is given later, using the ITU NOISEDAT software.

In order to estimate the likely S/N which could be achieved on board a ship, where the local man-made noise would be higher, it would be necessary to adjust the measured S/N ratios from this receiving study by an amount equal to the difference in the external noise factors of the 2 environments.

An estimate of the amount of extra external noise for the ship's environment may be made by examining ITU-R M.1467-1, in which it is stated that the figure of “-142dBW (at 3MHz)” for “naval topside noise”, should be used in predictions of coverage of GMDSS shore based transmitters.

The ITU programs contained in the NOISEDAT suite can be used to give statistical predictions of external noise levels. The value of Fa was calculated using NOIS1.EXE, for a “topside noise” of -142dBW at 3MHz, and a signal frequency of 518kHz.

An extract of the results is shown below, for the period “Spring” covering the time of these tests.

LAT = 60.00, LONG = -1.00, Ship SPRING, FMHZ = .518, 3 MHZ MANMADE NOISE = -142 DBW

--MEDIAN NOISE VALUES, FA(DB)-- STATISTICAL VALUES IN DB OVERALL NOISE TIME BLOCK ATMO GAL MANMADE OVERALL DL DU SL SM SU

0000-0400 75.2 58.6 83.1 84.6 6.6 9.2 1.3 3.9 1.4 0400-0800 49.0 58.6 83.1 83.2 6.0 9.7 1.5 5.4 1.5 0800-1200 41.0 58.6 83.1 83.1 6.0 9.7 1.5 5.4 1.5 1200-1600 48.8 58.6 83.1 82.5 6.0 10.2 1.8 6.3 1.7 1600-2000 51.7 58.6 83.1 78.5 6.7 12.7 4.3 15.5 8.1 2000-2400 77.8 58.6 83.1 85.3 6.5 9.0 1.5 3.3 1.4

Ignoring, for this simple treatment, the statistical variations, we see that the Overall FA(DB) figure for Springtime is, on average, 83dB. Man-made noise dominates the overall Noise Factor, since it's considerably higher than the natural noise (atmospheric and galactic) and the overall noise factor is reasonably consistent throughout the day, varying between 85.3dB and 78.5dB (a 6.8dB variation).

Below is the same NOIS1.EXE calculation for a “quiet rural” environment. In this case the man-made noise is lower, and the overall noise factor varies more with changes in atmospheric noise through the day. The variation is between 53.1dB and 78.0dB (a 24.9dB variation)

For the daytime period a value for FA(DB) of around 62dB is seen, which agrees with the published graphical representation given in ITU-R 372.

LAT = 60.00, LONG = -1.00, Home SPRING, FMHZ = .518, QUIET RURAL NOISE

--MEDIAN NOISE VALUES, FA(DB)-- STATISTICAL VALUES IN DB OVERALL NOISE TIME BLOCK ATMO GAL MANMADE OVERALL DL DU SL SM SU

0000-0400 75.2 58.6 61.8 75.5 9.8 10.6 2.7 3.6 2.7 0400-0800 49.0 58.6 61.8 57.1 10.8 14.1 10.8 16.0 10.4 0800-1200 41.0 58.6 61.8 62.6 5.1 9.6 1.9 4.5 1.3 1200-1600 48.8 58.6 61.8 52.4 10.9 17.3 32.7 46.7 38.9 1600-2000 51.7 58.6 61.8 53.1 14.9 18.5 35.0 40.0 34.9 2000-2400 77.8 58.6 61.8 78.0 9.1 10.2 3.1 3.9 2.9

This, then, may indicate that for a ship-born NAVTEX receiver the S/N might be degraded by at least 21dB (83dB – 62dB), and possibly more if the statistical variations are included, when compared to the same received field strengths seen at this remote “quiet rural” location.

Further tests on board a representative vessel at sea would be needed to determine more accurately the prevailing noise environment.

Off-Air Measurement Results

The tabulated results from each recording are shown summarized in the next few pages.

Station : Cullercoats / GCC [G]

Location : 55° 04´N 1° 28´W

Distance to receiver : 579km / 313nm

ITU-R 368 Sea-path predicted Field Strength : 44dBuV/m

Run 1 1 & 2 May 2012

Time RSL / dBm Noise (300Hz) / dBm

S/N / dB Estimated Field Strength / dBuV/m

Message ID Decode quality

01:00:00 -81.6 -111.7 32.3 32.4 GE81 good

05:00:00 -88.5 -111.1 25.5 25.5 GA12 good

09:00:00 -85.1 -112.0 29.1 28.9 GE82 good

13:00:00 -85.2 -111.9 28.9 28.8 GH06 good

17:00:00 -85.8 -112.8 29.2 28.2 GE83 good

21:00:00 -95.7 -113.1 19.6 18.3 GE80 good

Run 3 5 & 6 May 2102




01:00:00 -82.0 -113.4 31.4 32.0 GH07 good

05:00:00 -88.2 -114.0 25.8 25.8 GA42 good

09:00:00 -85.3 -113.7 28.4 28.7 GE94 good

13:00:00 -84.7 -114.9 30.2 29.3 GB78 good

17:00:00 -85.4 -115.4 30.0 28.6 GA18 good

21:00:00 -81.9 -112.6 30.8 32.2 GE92 good

Station : Malin Head / EJM [Q]

Location : 55° 22´N 7° 21´W



Run 1 1 & 2 May 2012




02:40:00 #

06:40:00 -105.8 -112.9 9.3 8.2 QE43 good

10:40:00 -101.4 -113.0 13.8 12.6 $ good

14:40:00 #

18:40:00 -102.2 -113.0 13.0 11.8 $ good

22:40:00 -82.4 -111.5 31.3 33.8 $ good

Notes:

# Netherlands [P] continued transmitting through the 5 minute recorded segment of Slot [Q]

$ Malin Head had already started its message before the recording period and the message ID was missed.

Run 3 5 & 6 May 2102




02:40:00 #

06:40:00 -103.3 -113.1 9.8 10.7 QE65 good

10:40:00 -104.1 -114.0 9.9 9.9 $ good

14:40:00

18:40:00 -104.0 -112.8 8.8 10.0 $ errors

22:40:00 -95.0 -110.5 15.5 19.0 $ good

Notes:

# Netherlands [P] continued into slot [Q] followed by message ID QA44 from Split Radio/9AS in Croatia at 02:41. No signal from Malin Head

$ Malin Head had already started its message before the recording period and the message ID was missed.

I am informed by a former Malin Head Radio Officer that the early transmission timings of EJM are deliberate, to prevent the inhibiting of the EJM NAVTEX TX caused by reception of the co-timed transmission from 9AS/Split Radio in Croatia.

Station : Portpatrick / GPK [O]

Location : 54° 51´N 5° 07´W



Run 1 1 & 2 May 2012




02:20:00 -71.1 -111.8 42.9 42.9 OA05 good

06:20:00 -104.2 -113.2 11.2 9.8 OE44 good

10:20:00

14:20:00

18:20:00

22:20:00

Run 3 5 & 6 May 2102




02:20:00 -88.4 -113.9 25.5 25.6 OH07 good

06:20:00 -103.9 -114.2 10.3 10.1 OE56 good

10:20:00 -103.8 -113.7 9.9 10.2 OA06 good

14:20:00 -104.1 -115.6 11.5 9.9 OA09 good

18:20:00 -104.5 -112.8 8.3 9.5 OE54 good

22:20:00 -74.7 -113.9 39.2 39.3 OA06 good

Station : Niton / GNI [E]

Location : 50° 35´N 1° 38´W



Run 1 1 & 2 May 2012




00:40:00 -84.3 -111.9 29.8 29.7 EE56 good

04:40:00

08:40:00 -108.4 -112.3 6.1 5.6 EE57 #

12:40:00

16:40:00

20:40:00 -96.5 -112.8 18.5 17.5 EA41 good

Note : # very weak signal. Poor decode using “Frisnit” decoder, but 100% decode using YaND decoder, which is more capable in weak signal conditions.

Run 3 5 & 6 May 2102




00:40:00 -78.0 -112.9 34.9 36.0 EE68 good

04:40:00

08:40:00 -110.6 -113.5 2.9 3.4 EE69 errors

12:40:00 -111.5 -115.1 3.6 2.5 nil

16:40:00 -112.8 -115.2 2.4 1.2 nil

20:40:00 -101.7 -113.5 11.8 12.3 EE67 good

Station : Niton / GNI [K]

Location : 50° 35´N 1° 38´W



Run 1 1 & 2 May 2012




01:40:00 -88.3 -111.9 25.8 25.7 KA45 good

05:40:00

09:40:00

13:40:00

17:40:00 -111.0 -112.8 4.0 3.0 #

21:40:00 -92.5 -111.1 20.8 21.5 KA45 good

Note:# Very weak signal. No decode at all with “Frisnit” decoder. High error rate in the decode using YaND. Some fragments of text decoded correctly.

Run 3 5 & 6 May 2102




01:40:00 -93.7 -1113.5 19.8 20.3 KA47 good

05:40:00

09:40:00 -112.3 -114.3 2.0 1.7 nil

13:40:00 -112.4 -114.5 2.1 1.6 nil

17:40:00

21:40:00 -105.5 -113.5 8.0 8.5 KA47 good

Station : Torshavn / OXJ [D]

Location : 62° 00´N 6° 47´W



Run 2 2 & 3 May 2012




00:30:00 -73.9 -106.8 35.1 40.1 DA28 good

04:30:00 -74.6 -111.1 38.7 39.4 DA28 good

08:30:00 -74.7 -111.9 39.4 39.3 DA28 good

12:30:00 -74.3 -111.6 39.5 39.7 DA28 good

16:30:00 -74.9 -112.7 40.0 39.1 DA28 good

20:30:00 -76.1 -111.2 37.3 37.9 DA28 good

Run 3 5 & 6 May 2102




00:30:00 -82.9 -112.4 29.5 31.1 DA28 good

04:30:00 -75.9 -113.7 37.8 38.1 DA28 good

08:30:00 -76.4 -112.7 36.3 37.6 DA28 good

12:30:00 -76.9 -115.1 38.2 37.1 DA28 good

16:30:00 -76.3 -114.9 38.6 37.7 DA28 good

20:30:00 -77.7 -112.3 34.6 36.3 DA28 good

Station : Orlandet / LFO [N]

Location : 63° 40´N 9° 33´E



Run 2 2 & 3 May 2012




02:10:00 -96.0 -109.3 15.5 18.0 # good

06:10:00

10:10:00

14:10:00 -99.8 -112.0 14.4 14.2 NA86 good

18:10:00 -98.9 -107.8 11.1 15.1 NA88 good

22:10:00 -83.4 -102.8 21.6 30.6 NA80 good

Note : # Orlandet started the message before its scheduled timeslot, so initial ID header was missed

Run 3 5 & 6 May 2102




02:10:00 -89.9 -113.7 14.8 15.1 # good

06:10:00 -99.9 -113.6 13.7 14.1 NA43 good

10:10:00 -100.1 -114.0 13.9 13.9 NA71 good

14:10:00 -99.6 -114.9 15.3 14.4 # good

18:10:00 -99.9 -114.9 15.0 14.1 NA43 good

22:10:00

Note:# Orlandet started the message before its scheduled timeslot, so initial ID header was missed

Station : Rogaland / LGQ [L]

Location : 58° 39´N 5° 36´E



Run 2 2 & 3 May 2012




01:50:00 -83.0 -108.0 27.2 31.0 LE93 good

05:50:00 -87.8 -113.2 27.6 26.2 LA88 good

09:50:00 -87.3 -112.0 26.9 26.7 LE94 good

13:50:00 -86.6 -112.0 27.6 27.4 LE94 good

17:50:00 -88.9 -111.9 25.2 25.1 LA95 good

21:50:00 -83.9 -107.8 26.1 30.1 LA88 good

Run 3 5 & 6 May 2102




01:50:00 #

05:50:00 -89.7 -113.2 23.5 24.3 LA88 good

09:50:00 -89.9 -113.6 23.7 24.1 LA88 good

13:50:00 -89.0 -114.6 25.6 25.0 LE98 good

17:50:00 -89.8 -113.4 23.6 24.3 LA70 good

21:50:00 -91.1 -113.6 22.5 22.9 LB37 good

Note:# Message IDs ME99 and MA25 received at RSL -77.6dBm, probable source = Jeloya/Norway transmitting in the wrong timeslot.

Conclusions

Cullercoats has a published range of 270nm, over which distance it is likely that, under most atmospheric conditions, for most marine installations, the received signal to noise ratio will be high enough to allow reliable decoding. The limiting value of S/N ratio is 8dB in a 300Hz bandwidth, according to ITU-R M.1467-1.

My reception tests, covering a small sample of transmissions from Cullercoats indicates that at this location, a distance of 313nm from Cullercoats, reliable decodes are made of all transmissions, day and night. The average S/N ratio from 12 samples was 28.4dB. The average estimated field strength was 28.2dBuV/m.

The location used for these tests is a very quiet RF environment. It could be expected that a ship may have a local external noise level of around 20dB higher, leading to reduced S/N ratios. Reducing the average S/N ratio observed in these tests by 20dB would lead to an average of 8dB. This is the published limit which defines minimum received quality.

Considering extending the required coverage outwards to 62 degrees North would increase the range by at least another 100nm from the location used for these tests. ITU-R 386 estimations would indicate that this would further reduce the received field strength by around 8dB.

From the previous example GRWAVE output:

Predicted Field strength at 780km (approx 420nm) is 36dBuV/m:

Predicted Field strength at 580km (approx 310nm) is 44dBuV/m

There would be no problem for a receiver, in an environment similar to that used for these measurements, to decode NAVTEX under these circumstances. It is likely though that this won't be the case for receivers located in vessels having higher external noise factors, such as described in ITU-R M.1467-1.

The presence of land masses has a marked negative effect on the field strength of signals, as can be seen from the measurements of Malin Head, Portpatrick and Niton.

Portpatrick registered an average daytime S/N ratio of around 10dB, as did Malin Head. Niton was unusable in daytime. The extra distance, with a considerable section of overland path, has the effect of reducing the groundwave field strength of Niton to a level at or below the local noise floor.

For vessels located to the west of Shetland it is likely that the additional ground losses caused by the land mass of northern Scotland would rapidly attenuate the signal the further west the vessel travelled. There is a considerable area of sea, screened from both Cullercoats and Malin Head, situated to the west of Shetland and lying to the south of 62N.

Signals from Rogaland and Torshavn were reliable day and night – Torshavn had S/N ratios of over 30dB at all times, and Rogaland had average S/N ratio of 25dB. Torshavn would make a better prospect for NAVTEX coverage for vessels around Shetland and outside the accepted coverage area of Cullercoats.

Comparing measured field strengths with predicted values showed an error of 15dB for both Cullercoats and Torshavn while Rogaland and Orlandet showed a discrepancy of around 25dB. The sources of these errors could include: inaccurate Antenna Factor for RX antenna, failure to account for the local ground effects in the area surrounding the receive antenna, excess feeder/splitter loss in the receive system, lower TX power and lower TX antenna efficiency leading to an EMRP of significantly less than 1kW, higher ground losses over the path than accounted for in the ITU-R 368 prediction. The truth is likely to be a combination of all of these factors. It would be interesting to know the actual transmitted EMRP of each station in the study. I have no details of the transmit antenna dimensions, grounding system losses, tuning system losses etc. from which to estimate the station EMRP.

While it is certain that the groundwave signal from Cullercoats provides a good, usable NAVTEX service in Shetland at the location used for these tests, extra study is required before it could be accepted that the coverage is adequate for real-world marine installations.

John Pumford-GreenMay 6th 2012

518kHz Navtex Reception in Shetland

Documents

Transcript of 518kHz Navtex Reception in Shetland