NASA AIS Engine 2, as screwy as #1

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now excited to have Ben Stein as very able publisher, webmaster, and editing colleague. Panbo is going to the next level in 2019 and beyond.

26 Responses

  1. Edam says:

    Hey Ben,
    On this Dual Channel issue, I’m confused???
    For example, is there really a performance or update improvement with a dual-channel Parallel receiver instead of a dual-channel multiplexing or switching receiver as you mention above?
    I was shopping at the Miami Boat Show and the Furuno Reps showed me a bold statement about their new FA30 AIS Receiver in the brochure that said it had a Dual Parallel Receiver that allows it to receive updates twice as often as Dual Channel Multiplexing AIS Receivers.
    They also said it really matters more with smaller, faster boats that will probably install CLASS B AIS.
    Is this true and does it really matter?
    Edam

  2. The Furuno pitch is true, Edam.
    Multiplexing receivers perform OK with Class A AIS transmissions because they are broadcast at a fairly fast rate. But Class B is much slower—the max transmission rate is 30 seconds per position, on alternating channels—which results in “jumping bean” target plotting. Example:
    http://tinyurl.com/yqasgu
    AIS channel multiplexing is a bit of a black art, but it’s my understanding that one-channel-at-a-time receivers can only plot Class B targets at one minute intervals, which is going to be quite disconcerting with fast boats in close quarters. Parallel receivers may eventually be the norm.

  3. Jim says:

    AIS Class B tranceivers legal and now available in Canada, Recentlty purchased ($900 CDN) a Simrad A150. Nice little piece of kit.
    Was surprised to find it came with own GPS antenna, as it wants signal from own built-in GPS receiver as well as it;s very own VHF antenna.
    According to Link 10 monitoer on my 35 foot sailboat, it draws about 1/2 amp with screen intensity turned up full.
    Lots of interesting little features I had not expected.
    Hope your FCC gets their act together
    Jim

  4. Edam says:

    Ben,
    Thanks for the confirmation.
    The FA30 Brochure says “Dual Parallel Channel, FULL DUPLEX receiver design receives AIS contact messages up to 100% more often than dual channel Multiplexing receivers”
    I found some of your previous entries and on the web that said to look for “Full Duplex” receive capability, which are the better AIS Receiver designs that will update Class B AIS messages twice as fast. One minute between receiving updates seems dangerous to me as a lot can happen in that time.
    Edam

  5. The SeaCAS website gives a very understandable display of the timing involved on their FAQ page under “Why should I buy a Dual-Band AIS Receiver”.
    http://www.seacas.com/oscomm/zz_faq.php
    Terry

  6. norse says:

    The problem I have with the SeaCAS FAQ webpage is item 5. This seems to be the justification for buying a receiver instead of a transponder. These points have been discussed on Panbo before.
    What I would like to point out here is that Fred Pot is president and head developer of SeaCAS according to their web page, and it also says “He is a member of two United Nations IEC workgroups. These workgroups are responsible for developing worldwide standards for AIS transponders, and for developing international standards for navigation displays that integrate AIS with radar, ECDIS, GPS and other sensors.”
    This is true; if you explore http://www.iec.ch/ you will see he is a member of TC 80 / WG 8A “Shipborne (SOLAS & non-SOLAS) automatic identification systems (AIS)” tasked “To prepare a standard 62287-2 for Class B AIS using SOTDMA and review and maintain standard 61993-2 for Class A AIS” TC 80 / WG 13 “Displays for the presentation of navigation related information” tasked “To prepare a standard 62288 to incorporate the requirements of the IMO performance standards for display of navigation related information published in 2004”
    So if Fred is on the committee which designed Class B, why does he dislike it? And if Fred is on the committee which says how AIS should be displayed, why does he say it won’t be displayed?
    Did something go wrong with those committees so that the they produced an inferior result? Is there a story here that isn’t being told?

  7. Jim HebertJim Hebert says:

    Regarding AIS transponders, I would appreciate a concise explanation of how they determine which channel to use for transmission. Does a Class-A transponder automatically alternate its transmissions between the two AIS channels? Does it use one channel in preference to another?
    It seems like an understanding of how transponders decide which channel to use for transmitting would be important to understanding how a single-channel-at-a-time receiver would be affected by its inability to be able to simultaneously monitor both channels.

  8. Yes, Jim, Class A and Class B transponders automatically alternate transmissions between the AIS 1/2 or A/B frequencies (VHF 161.975 Mhz and 162.025 Mhz). That doubles the overall bandwidth of the system, plus provides some hardware redundancy in the radios.
    The data itself is broken up into several types of messages, and the message transmission rate is regulated according to the speed of the vessel, plus static information messages are sent much less frequently than dynamic messages. Class B static and dynamic data is more limited than A’s, and transmission rates slower.
    So any given specific boat message may go out every 2 seconds or every 6 minutes, first on Channel A, then 2 seconds or 6 minutes later on Channel B. The exact milli second they’re transmitted is determined by the underlying organizing protocol, but the goal is to hit those regulated time intervals.
    Both Class A and Class B transponders contain true dual parallel receivers and thus hear every message sent. A one-channel-at-a-time receiver simply can not hear every message. But how they perform depends to some degree on how they switch back and forth between the channels. A Raymarine product manager once told me that its AIS250 can somehow anticipate transmissions and thus catch more than half of them, but I have not been able to test that or find the claim in writing.
    There’s lots more about this scattered around Panbo’s AIS category, like here:
    http://tinyurl.com/yqpsgf

  9. Anonymous says:

    I wonder if the question of who programs the AIS B fixed messages is in some part responsible for the FCC’s paralysis.

  10. Good day,
    The word “full duplex” is in correct in the AIS context. AIS is a simplex (radio term) system . It can either transmit or receive at a given time, not both. Duplex obviously means a device can transmit and receive at the same time.
    AIS receivers are either single receiver with channel multiplexing capability or dual (2) receiver devices.
    Additional to multiplexing, the overall processing capability of a receiver is of great importance. If 2 or more AIS messages are transmitted in a very short period of time, some AIS receivers will be unable to process all these messages. Class A type approval includes a 90% on-air load test; the unit must be able to receive, process and port all these messages via the pilot/presentation interface.
    Class B/CS requires two receivers. These receivers are used for AIS. However, one of these receivers may be used for DSC (channel 70) operation when required.
    There are a number of other considerations that need to be taken into account, i.e receiver design parameters other than sensitivity such as adjacent channel selectivity, blocking / desensitize, inter-modulation, co-channel rejection.
    With some receives, only the receive sensitivity is specified, viz. -116dBm without qualification, i.e. -116dBm @ PER (Packet Error Rate) less than 20%.
    If other radio units are transmitting in the near vicinity of an AIS receiver, you could find that the AIS receiver performance is impaired (desensitized / blocked, etc.).
    Bottom line is, reception of an AIS receiver in close proximity of other transmitting radio equipment or in a congested area could be impaired considering the above.

  11. del says:

    Spot-on Andre!
    Desnsitization can come from many sources, in many parts of the world the Marine channels are interspersed with Land Mobile equipment, which is fine on the high seas, but when you get closer to shore, the problems surface, indeed, in the USA, NOAA have some stunningly powerful transmitters only a few hundred kHz away from the AIS channels.
    It is a shame that NASA haven’t taken this opportunity to correct the obvious errors in their documentation.

  12. Dan (b393capt) says:

    Andre wrote “If 2 or more AIS messages are transmitted in a very short period of time, some AIS receivers will be unable to process all these messages. ”
    … could you elaborate Andre ? My understanding is that there are timeslots, and although there can be a collision within a slot forcing both transmitters to choose another slot to re-transmit and the recievers to end up with an unintelligable message, it dosn’t otherwise take a whole lot of CPU to decode the AIS messages. Why not get 100% of the messages that don’t collide ?

  13. Dan wrote … could you elaborate Andre ? My understanding is that there are timeslots, and although ……..
    Hi Dan,
    Your understanding re. slot use is in principle correct. However, if 2 or more AIS units transmit in the same time slot, meaning full reception thereof is subject to the distances between the transmitting AIS units and the receiving unit which includes the receiver’s co-channel rejection ability (signal discrimination). The transmitting AIS units are unaware of the slot contention, being simplex devices. Slot contention is resolved automatically by the AIS protocol algorithm, i.e. slot time out (between 3 and 7 minutes) and new slot selection.
    I have performed load performance tests on a few AIS receivers (not using a screen chamber – faraday cage). The test bed was the same for all, which is environmentally ideal (no other transmitters close by, 5m distance between transmitter and receiver antennas; calculated free space loss receive signal strength -37dBm).
    An AITS-R (AIS Installation Test Set) is used for generating AIS VDL (VHF Data Link) messages. AIS type 1 message (single slot) is transmitted every 5th slot, alternating between AIS channel 1 and channel 2 (450 transmissions per minute ~ 1/10th of VDL theoretical maximum). The lowest result was 52.6% throughput, the control unit being 99.5% constantly.
    VDL receiver processing of all units without an internal GPS is asynchronous, meaning that the start of a slot boundary is not coupled to GPS 1pps/UTC. AIS Data transfer is a bit-oriented protocol which is based on the High-Level Data Link Control (HDLC). Both the start and stop flags are 0x7E symbols (characters). HDCL is not actually suitable for a RF / wireless environment; it was initially developed as a hardwired computer communication protocol. The AIS adoption thereof is the result of business politics at the time. The 0x7E flag has very poor correlation properties in a RF environment. In order to overcome this deficiency, enhanced correlation techniques need to be implemented by the manufacturer. The receiver requires a continuous detection process; once a valid start of message is detected, the receiver will process the message accordingly. If a receiver does not have the required processing capability it will not recognize all or most of the transmitted messages.
    We have observed that some Class A units’ receive performance deteriorates when the internal GPS is disabled. This is a direct result of the unit’s receiver slot synchronization and message detection capabilities.
    Another consideration is the reception of multi-slot messages. Message 5, AIS Class A static and voyage data is a 2 slot message. The error curve increases exponentially as the length of the message increases.
    The poor results of some of the receivers can be contributed to poor large signal handling and processing capabilities. It can be expected that these receiver’s performance will further deteriorate in a less ideal environment, i.e in the real world out there.

  14. Dan (b393capt) says:

    Very interesting …
    “The transmitting AIS units are unaware of the slot contention, being simplex devices.” … I wasn’t aware of this, this limitation alone must have made the creation of AIS protocol an exception challange.
    “If a receiver does not have the required processing capability ” … your not talking so much about how fast the CPU is then, you mean the A/D processor or something else? Some additional discrimination capability ? … is there a hardware reference architecture everyone follows that provides for a minimum capability ? Has anyone done exceptional job of exceeding this ?

  15. del says:

    Dan:
    Andre is perhaps simplifying the system / protocol – while it is true that the system is simplex – it can only Tx or Rx, not both at the same time, the use of Tx slots is carefully managed by each unit building up a “slot map” of used slots that it detects. This starts off with a period at power-on when all it does is monitor for occupied slots before trying to Tx itself, then every time it receives an AIS message, that slot too is marked as occupied in future. In addition, the AIS messages themselves can “reserve” slots in the future to mitigate against collisions.
    There is no “hardware reference” as such, however the IEC standard specifies the minimum performance standards for all AIS units. However, this is the MINIMUM standard – it can always be bettered, and the more powerful your processing capability (both RF, hardware and software), the more scope you have for exceeding it.

  16. Andre says:

    Dan, Del,
    Class A units are subject to “slot map” processing. Positional reports (msg 1, 2 & 3) are repetitive. An AIS unit selects available slots from within its own “slot map”. The number of slots required depends on the units update rate, which is subject to the units speed and change of course. A Communication State field is included in messages 1, 2 & 3. The ComStat includes a time-out parameter, values randomly selected between 3 and 7 minutes. After each respective transmission, the time-out value is reduced by 1. If the slot time-out value is 0 then the slot offset should indicate the offset to the slot in which transmission will occur during the next frame. If the slot offset is zero, the slot should be de-allocated after transmission. Receiving units record this information in their own “slot maps” accordingly. SOTDMA (self organized) access method is applied for these transmissions. RATDMA (random) is generally applied for non-repetitive (ad hoc) messages, binary and safety related messages, acknowledgments, interrogation. Msg 5, ships static & voyage data, transmitted nominally every 6 minutes applies RATDMA. There are obviously a host of other rules within this protocol.
    Class B/CS does not require a slot map. Ten (10) candidate slots are selected randomly. On candidate slot, the carrier-sense (CS) detection method requires the unit to determine within a time window of 1146us after the start of the time period intended for transmission if there is another carrier on-air. I carrier sensed, the unit must abort its transmission attempt, otherwise it must transmit.
    AIS base stations and Aids to navigation units’ primary access method is FATDMA, fixed slot allocations, reoccurring for a period of time. RATDMA may be used subject to conditions.
    It should be kept in mind that AIS in principle is a cellular broadcast system, operating in a time (TDMA) and distance domain. It is paramount that the units close by receive one another. A Class A ship in excess of 23 knots transmits every 2 seconds. The probability is very high that the majority of its transmissions will be received by the units in close proximity.
    The possibility that a base or repeater stations’ reception can be impaired is much higher, specifically when positioned on a high site covering a large AIS area (multiple cells), as a result of slot contention.
    AIS hardware and software implementation is subject to the respective manufacturer’s discretion. Some manufacturers make use of a discriminator (analogue), others DSP, etc. There are IEC performance standards which specify the minimum requirements for Class A (IEC61993-2), Class B/CS (IEC 62287-1), Base Station (IEC62320-1) and AtoN (IEC62320-2). No standard for receive-only units exist. It is expected that good practice will be applied by these manufactures, or is it only the bottom line $$$$$ that counts.

  17. del says:

    … and not forgetting the AIS SART standard too….

  18. Dan (b393capt) says:

    Has any of the products you tested demonstrated they had more processing capabilities than others .. in a way that provided a measurable difference in performance ?

  19. Andre says:

    Dan wrote “Has any of the products you tested demonstrated …”
    Yes, qualifying that it is a result of both large signal handling and processing capability.

  20. del says:

    Interesting – I played a stereo .wav file of recorded repeated AIS messages (using Audacity to replicate the burst and invert the odd one or two and mess with the timing a bit for over 300 bursts) into the discriminator inputs of my receiver, so simulating a 100% loading of the VDL on both channels – it got 93% of them – sounds like its better than the ones Andre tested?

  21. FWIW I’m still using the AIS Engine I installed three years ago and recently found I could not receive Class B targets. A new chip delivered in two weeks time has fixed that problem … for $24.24.

  22. Don Maylotte says:

    I was recently told that it was a “waste of money to buy a Class B AIS transeiver because Class B transmissions are not received by Class A equipment” Reading the emails on this website makes me believe this is not technically correct. Am I right? However, is the currect perception that Class B tansmissions so overload the Class A receiver that it is common for Class A operators to use whatever tools they have to filter out Class B messages from their screens?

  23. Anonymous says:

    Never had an overload problem on any ais reciever before.
    Class B can be shown on AIS systems but some require a software upgrade to display the vessel name and lengths correctly.
    Andy Murray
    Service Manager

  24. norse says:

    The worry about too many Class B AIS transceivers is way ahead of its time (or FUD). Right now, Class B is only a few percent of all AIS, anywhere in the world as far as I can tell via the internet viewers. It only shows one Class B running on the west coast of North America at this time (Vancouver BC SAR) and none on the east coast. So why would anybody bother to filter out Class B? The leader in Class B adoption might be Turkey. They require pretty well all boats to have Class B AIS now, an estimated 18,000 boats.

  25. Bob says:

    I have a NASA AIS #1 & have considered upgrading to #2. Is it worthwhile doing so?

  26. Bob, I’m glad to see that NASA has stopped claiming that its receivers output NMEA 2000, but why would you want a receiver that only relays AIS messages “1, 2, 3, 4, 5, 11, 18, 21 and 24”? I believe that all other receivers and all transponders pass along all AIS messages.

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