04/05/2018

Do I need a dual or single AIS receiver ?

“Why can’t I see ship names?”

One of the most common asked questions we get on our forum is ‘why can’t I see ship names’

The answer is always linked to ‘do they have a dual or single AIS receiver’.

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So, what is the difference between a dual or single AIS receiver?

Boat opposite Vancouver in Canada

The best way to answer the question is to go into the ‘key elements of how an AIS receiver works.

An AIS receiver (Automatic Identification System) will collect signals sent out from other boats – in order to help prevent collisions at sea.

AIS signals provide information of the ships in your area. For example their speed, where they are going (their course) and how to contact them (name, call sign, etc.).

When a boat sends out an AIS signal this can look something like this:

!AIVDM,1,1,,A,13aEOK?P00PD2wVMdLDRhgvL289?,0*26
!AIVDM,1,1,,B,16S`2cPP00a3UF6EKT@2:?vOr0S2,0*00

Your AIS receiver will then de-code that message into useful information

For example

  • Ship static and voyage related data
  • AIS Version
  • Call Sign
  • Vessel Name
  • Ship Type
  • Dimension to Bow, Stern, Port & Starboard (all in m)
  • ETA at destination (MMDDHHMM)
  • Destination
  • etc, etc.

 

So, why won’t you get all the information ?

And what’s the difference between a single & dual AIS Receiver?

The key reason is due to the way in which the AIS receiver collects AIS data.

It’s all about ‘Hopping’AIS channel hopping

When a ship sends out AIS signals it will use 2 VHF radio frequencies :

  • Either at 161.975 MHz (AIS1, or channel 87B)
  • and 162.025 MHz (AIS2, or channel 88B).

 

A single channel receiver can only receive the information from one of these channels at a time. Whereas a dual channel receiver will receive it from both at the same time.

What a single AIS receiver will do is hop between the two AIS channels in turn (alternately), whereas a dual channel AIS receiver will collect them both.

And when a single AIS receiver hops between the two channels it can incur a loss of some parts of the message – for example not picking up the ships name.

It’s important to realise that the key collision preventing element of the AIS system (such as items like speed, position, direction, and flag) are all packed in single sentence AIS messages, which you will collect – even with a single AIS receiver.

Similarly a single AIS receiver will also be less quickly updated than a dual AIS receiver

For example:

  • At rest (or slower than 2 knots) a single one takes 6 minutes compared to 3 minutes for a dual channel one
  • At speed a single one takes 1 minutes compared to 30 seconds for a dual channel.

 

In practical use this means that (if there are static ships / at anchor), it might take upto 12 minutes for a single AIS receiver to get the info, compared to 6 minutes for a dual one.

 

Can you improve a single channel AIS receiver?

Single AIS receivers tend to be cheaper than dual AIS receivers, because of the less complexity of the electronics needed – and if you are sailing in less crowded waters might be ideal to you.

But if you want to use a single channel AIS receiver the key is to select one that uses specialised auto-hopping technology – like that found in the QK-AO21 and QK-A023.

Standard single channel AIS receivers hop between channels every 30 seconds. Quark-elec’s patented auto-hopping technology varies the hopping rate to maximise the number and completeness of messages captured. Crucially it also increased the speed of capturing the same amount of message.

Why is this important ?

We did a simple ‘field test’ in Dover comparing the difference in how many boats were spotted when using auto-hopping technology.

And depending on the time period – auto-hopping would spot 2 to 3 more boats than non-specialised technology. Might not sound a lot but could be a big difference to your safety.

Dover messages received
1min2mins3mins4mins5mins
Auto-hopping1320243134
0.25s919243032
1s1120232833
30s1018212832

 

A Single or Dual Channel AIS receiver?

Yes, using auto-hopping technology you can improve a single AIS receiver. And they can be the ideal & cost effective solution for less busy waterways.

But if you are in busy waters, or want more detailed information you will want a dual channel receiver. why not consider something like the QK-A026

More information

All the AIS receivers & multiplexers mentioned are available from our

Specialist Marine Products On-line Shop

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