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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



AIS CSDTMA – what is it ?

AIS Class B, as specified within IEC-62287-1, in fact operates slightly differently in the way transmissions are controlled it does not use SOTMA it uses CSDTMA .

Bob Lee, Technical Sales Consultant at Software Radio Technology plc – says

…”Class B vessels using CSTDMA (Carrier Sense Time Division Multiple Access) mainly send fixed length telegrams using time slots not used by Class A vessels. Class A vessels reserve a particular time slot (SOTDMA) and negotiate the use of that time slot with other Class A vessels within radio range.

Class B vessels use any time slot as and when it is available. If no slot is available and a Class B vessel wants to transmit it simply misses out that ‘go’ and tries again after a pre-set time.

Its ability to provide a service therefore degrades gracefully and the user is informed if three potential transmission slots are missed by a warning signal. As soon as time slots become available the Class B unit picks up the reporting pace and goes back into the standard reporting timing.

So the environment is dynamic and depends on such issues as:
– How many Class A and Class B vessels are within radio range of each other
– What the Class A vessels are doing (moored, under way, etc)
– What the mix is of Class A and Class B vessels at that location …”


AIS Class B – Whats the difference

AIS Class B was specified as a much less expensive, limited range and limited feature sub-set of the original Class A.

Class B has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to every 10 sec. for Class A)

So Class B:-

  • Does not transmit the vessel’s IMO number or call sign
  • Does not transmit ETA or destination
  • Does not transmit navigational status
  • Is only required to receive, not transmit, text safety messages
  • Is only required to receive, not transmit, application identifiers (binary messages)
  • Does not transmit rate of turn information
  • Does not transmit maximum present static draught

Andy Norris of the University of Nottingham, writes in the Journal Of Navigation…

“The CSTDMA Class B system has been designed to prevent overloading of the AIS VHF data link. This is briefly reviewed but there are a number of other aspects that need to be considered. These include: the increased garbling of Class B messages compared to those of Class A; the problems accruing from the low update rate of Class B information; the increase in display information that will need to be managed; and the possible increase in inappropriate manoeuvres of leisure craft caused by misplaced reliance on AIS. As a result of the investigation the paper highlights the fact that Class B users must not assume that their own presence, in the form of Class B transmissions, will be particularly visible on the bridge of many SOLAS vessels. This will continue to be the case for many years into the future, until such vessels are mandated to carry radar with AIS target overlay capability….”


AIS – which VHF band ?

A commercial vessels AIS unit has two dedicated VHF receivers and one transmitter to which the International Telecommunications Union (ITU) has allocated two dedicated frequencies, these are 161.975 MHz and 162.025 MHz (VHF 87B & 88B), however they may not be available in every part of the world.

In the United States for example, VHF 87A & 87B were recently allocated for AIS communications instead.

In addition the transponder unit includes a GPS system (GLONASS or GPS) to determine the ship’s own position and for time synchronization, as well as the means for displaying the received information from other AIS equipped ships.