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VHF – Low or High Power on Transmit?

VHF radios that conform to the ETU spec have two power settings 1 or 25 watts for fixed sets and 1 or 5/6 watts for handhelds. If you were calling for help especially in a MAYDAY situation you would naturally choose high power so that the Coastguard and as many vessels as possible would here your call.

ht640 525 awardBut using high power for routine calling severely restricts the number of people who can use the few channels available at any one time – in other words several 1 watt conversations can take place within a given area but only one 25 watt conversation. Misuse of the high power setting is a bit like walking into a bar and shouting at your friends – eventually everyone will have to do the same and that is counter productive.

Marinas can only reply on 1 watt if they comply with their license, race management uses channels M and M2, and inter ship is channels 6,8,72 and 77 – so it is a good idea to use the right channel and use only low power – that means you should only be using high power on channel 16 – Coastguard.


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What is the Automatic Identification System (AIS)

Courtesy of the excellent US Coasd Guard Navigation Site…


Picture a shipboard radar display, with overlaid electronic chart data, that includes a mark for every significant ship within radio range, each as desired with a velocity vector (indicating speed and heading). Each ship “mark” could reflect the actual size of the ship, with position to GPS or differential GPS accuracy. By “clicking” on a ship mark, you could learn the ship name, course and speed, classification, call sign, registration number, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available. Display information previously available only to modern Vessel Traffic Service operations centers could now be available to every AIS-equipped ship.

With this information, you could call any ship over VHF radiotelephone by name, rather than by “ship off my port bow” or some other imprecise means. Or you could dial it up directly using GMDSS equipment. Or you could send to the ship, or receive from it, short safety-related email messages.

The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation.


Dual Band AIS – Raymarine AIS250

At last Raymarine have produced an AIS receiver – not only that but it combines three other cost saving attributes :

Ramarine AIS250-275-2061. It has a built in multiplexor with 2 inputs and 2 outputs which can all be set to different speeds. Remember the C and E series Raymarine chart plotters only have one NMEA port and that can only be set to one speed. If you dedicate this port to AIS then you have to set it to 38,400bd and if it is dedicated to your normal NMEA instruments it would be set to 4,800bd. This new unit from Raymarine would avoid that choice having to be made and would make the Brookhouse multiplexor that I installed last year redundant since that was primarily used to enable me to run NMEA0183 at 4,800 baud (normal instruments) and at 38,400 baud (AIS signal). Of course the Brookhouse also gives me USB connection for a PC so that is a bonus that the Raymarine AIS does not offer – yet. Cost saving for a UK install – approximately £200. On the otherhand I have greater resilience by using a seperate multiplexor.

2. It has an antenna splitter. This means that you dont need a completely new antenna, antenna mounting, and cable run just to install an AIS. Your main VHF aerial can now be used for AIS, FM, and VHF – Cost saving for a UK install – I would say about £250. Again, I would counsel against using splitters, and going for the greater resilience of a separate VHF aerial for the AIS

3. It is dual band receiving on 161.975 MHz and 162.025 MHz but only one channel at a time (just like my NASA unit – oh well….)


Only gripe – no status lights ! – why dont they learn !


Dual Band AIS – Secas

Secas have come out with a dual band AIS receiver – with status lights ! – a big gripe with the NASA product that I installed last year.

SECAS AIS 150 product 08Full product specifications for the 150 :-

• Lightweight, compact design
• Fast, easy USB connectivity
• Low 1.2 watt power consumption
• Receives data from as far away as 30 nautical miles
• 16-channel GPS, WAAS and EGNOS enabled
• Dual frequency: AIS1 161.975 MHz and AIS2 162.025 MHz simultaneously monitored
• 25 kHz bandwidth
• Baud rate 38400 bps
• EMC certified EN61000-6-3:2001, EN61000-6-1:2001, IEC 60945
• 10-meter antenna cable (maximum cable length of 30 meters is available)
• Receives all AIS VDL messages simultaneously on both channels
• Uses industry standard IEC 61162-2/NMEA 0183 message protocol
• Operating temperature ranges from 5 to 131 degrees Fahrenheit
• Sensitivity to –107 dBm
• GMSK demodulation
• 70 dB adjacent channel rejection
• SafePassageAIS is PC- and Mac-compatible
• SafePassageAIS comes with a serial cable for chart plotter and radar connectivity



Connecting Phillips Navigator MK8 to a MT500 DSC/VHF

Philips MK8After a recent correspondence via this blog, I thought it would be useful to publish the interface information for this connection.

The Philips MK8 has two cables both with shields BUT the shields will only be connected at one end – this is the clue. So at the MK8 end there is a D-SUB 9 pin connector – open it up and have a look at the wiring….it is wired as follows:



Pin 3 – Red – NMEA OUT (to a listener)
Pin 8 – Black – NMEA OUT (to a listener)
Shield – connected to MK8 shield
(DO NOT connect the shield at your Listener end)

Pin 2 – Red – NMEA IN (from a Talker)
Pin 7 – Black – NMEA IN (from a Talker)
Shield – NOT connected to MK8 shield
(DO connect the shield at your Talker end)


Connect to Phillips Mk8
1 Brown – INPUT DC + <<<<———- signal from Pin3 RED wire on MK8
2 Red – INPUT DC – <<<<———- signal from Pin8 BLACK wire on MK8

Connect to PC – Serial cable
3 Orange NMEA OUTPUT +
4 Shield NMEA OUTPUT –

Connect to Flash programmer
5 Green – Clock
6 Blue – Reset
7 Grey – PDI
8 Purple – PDO

Now to the NMEA Protocol

If you have done any research you will have realised by now that the NMEA protocol is really a very poorly implemented “standard” . What you have discovered about the wiring carries on into the programming of the chips that send the data. This standard is the NMEA 0183 protocol – now replaced by NMEA 2000 – which is also badly implemented. Just in case you are asking why – it is because manufacturers have used this confusion to protect market share and make their equipment incompatible on purpose.

NMEA 0183 version 2.0 sentences for GPS all start with $GP and then a 3 letter acronym.

$GP*** Sentence Titles
APA: Autopilot Sentence “A”
APB: Autopilot Sentence “B”
BOD: Bearing, Origin to Destination
BWC: Bearing and Distance to Waypoint, Great Circle
GGA: GPS Fix Data GLL: Geographic Position, Lat/Long
GSA: GPS DOP and Active Satellites
GSV: GPS Satellites in view RMA: Recommend minimum Specific Loran-C Data
RMB: Recommend Minimum Navigation Information
RMC: Recommend Minimum Specific GPS/TRANSIT Data
ROO: Waypoints in Active Route (not in standard)
RTE: Waypoints in Active Route or GOTO
VTG: Track Made Good and Ground Speed
WCV: Waypoint Closure Velocity
WPL: Waypoint Location
XTE: Cross-Track Error, Measured
ZTG: UTC & Time to Destination Waypoint

So you can see that when the MT-500 DSC manual says $GPRMC / GGA they mean that the two sentences above must be being sent by the Phillips MK8.

The Phillips MK8 does comply with NMEA 0183 version 2.0 (January 1992 standard)

It will transmit RMC (Time, date, position, course, and speed data) every 2 seconds.

BUT it will also transmit GGA – Global Positioning System Fix data (Time, position, fix) if you have the MK8 with differential software. This was issued as a software upgrade after the MK8 started shipping.