ShipPlotter FAQ FileShipPlotter displays complete information about ships that are within VHF range of your position using the Universal Automatic Identification System (AIS).
AIS is the acronym for the “UNIVERSAL SHIPBORNE AUTOMATIC IDENTIFICATION SYSTEM USING TIME DIVISION MULTIPLE ACCESS IN THE VHF MARITIME MOBILE BAND”.
It is a system in which all ships over 200 tonnes operating in international waters automatically broadcast their position, course and speed, together with other information about their name, call sign, dimensions, ship type, destination and eta, on a pair of radio channels in the Maritime VHF band. The system is part of the Safety of Life at Sea system and is intended to assist in reducing collisions between ships.
With suitable equipment, all the participating ships obtain a real-time radar-like display of the ships operating in their area. Better than radar, the AIS system provides name and call sign details of the target ships on the display which greatly facilitates voice communication between them. With ShipPlotter, you can monitor these transmissions and keep track of shipping movements within VHF range or beyond.
AIS uses two frequencies in the Marine VHF band: 161.975 and 162.025 MHz. All ships use both frequencies and successive message transmissions are radiated on alternate channels. Professional AIS equipment monitors both channels simultaneously. This doubles the channel capacity and also combats interference. Monitoring a single channel with ShipPlotter means that you only hear half of the transmissions. Since the transmissions are often made at intervals of only a few seconds, this does not significantly prejudice the topicality of the view obtained. Messages are transmitted at 9600 bits per second using GMSK modulation. Each message is typically only 30ms (1/30 second) long – scarcely more than a click.
You need a suitable antenna, a receiver with a discriminator output, capable of tuning to one or other of the AIS channels, and a PC with a sound card. You also need ShipPlotter software. Some monitoring/scanner type receivers (Katech, WinRadio, PCR1000 and others) are fitted with discriminator outputs. If your does not, take a look at www.discriminator.nl where Rene has catalogued a huge array of simple modifications to many models of receiver to provide the discriminator output that you will need to receive AIS transmissions. The AIS transmissions use a somewhat wider bandwidth than normal VHF FM communications. You may find, if your receiver has a narrow IF bandwidth, that you will only decode a proportion of the messages. A few receivers have a choice of IF bandwidths; in such a case, choose the bandwidth cognate with 25 kHz channel spacing.
There are an increasing number of low-cost dedicated AIS receivers that provided a serial data stream out in NMEA AIS format which can also be used with ShipPlotter. These are simpler to connect and use less computing power to handle than the audio processing but may not deliver all the message types. Note that they are not general purpose receivers and only function for this purpose.
The regulatory framework regarding radio monitoring varies considerably from country to country. In some countries the regulations are very liberal, in others they are very strict. In many, the regulations are strict but are not enforced strictly except in cases of abuse or criminal intent. In deciding whether such monitoring is legal in your own regime, remember that the transmissions from ships are in the nature of a broadcast (there is no destination address in the messages) and are not in any sense private messages. If your interest stems from a boating connection, then you would have a strong safety case for monitoring this traffic.
AIS transmissions are subject to VHF line-of-sight propagation. It all depends on the terrain between you and the shipping. Using the sharing feature of ShipPlotter,you can display ship position reports that are being received by other ShipPlotter users and shared via the Internet, even if you cannot receive signals yourself. This feature is only available to registered users.
Tune to one of the two channels and select AM mode (normally you would use FM mode for this traffic). If there are AIS messages around, every few seconds you will hear a characteristic ‘pop’ sound. It sounds quite different from static clicks. You should hear some stronger and some weaker signals and the frequency of these pops will give you some idea of the AIS traffic intensity in your area.
Assuming that you have set your receiver to the correct frequency and to the correct mode (FM) there are many reasons why you may not decode the messages.
AIS uses baseband signalling (NRZI) at 9600 bits per second; it does not use an audio subcarrier. The audio bandwidth occupied is therefore from dc to something like 9.6Khz. Accordingly, it places great demands on the receiver, the sound card and the demodulating software. Any distortions in, say, the phase or amplitude response of the receiver will be highly prejudicial to the fidelity of the signal. This is why a discriminator output is essential; it bypasses the audio stages of the receiver which typically would reduce the bandwidth to telephonic standards. It should be noted that although baseband signalling can involve frequencies down to dc, the fact that the messages are only 30ms in length means that the bandwidth need only be flat down to 30Hz or so. Even so, this is a much lower frequency than voice communications require.
There are two specifications for the bandwidth of the AIS signal, one is based on 12.5kHz channel spacing and the other on 25kHz spacing. The FM deviation of the latter specification is wide compared with the IF bandwidth in a typical scanner/monitoring receiver. Accordingly, the peaks of deviation of the incoming signal may be falling out of the receiver IF passband and degrading the signal-to-noise ratio and also distorting the shape of the signal recovered from the frequency modulated rf carrier. Despite the adversity, it is still possible to receive the AIS signals with such a receiver but the tuning becomes very critical indeed. Even the slightest mistuning will push one or other side of the deviation peaks into the noise. Note that just because a receiver has a digital frequency display showing the precise nominal frequency, it does not mean that the tuning is accurate. The reference frequencies controlling scanner/monitor receivers vary considerably in their precision and stability. If you have tuning steps that are small enough, try monitoring frequencies on either side of the nominal frequency in steps of, say 0.5kHz.
Assuming that the ships have not all gone away, almost certainly, as the receiver warms up, your receiver tuning is changing and so the signal drifts out of the receiver passband. If you have tuning steps that are small enough, try monitoring frequencies on either side of the nominal frequency in steps of, say 0.5kHz.
Versions of ShipPlotter prior to 4.4 assumed a preamble pattern in accordance with one interpretation of the specification. Some ships used another pattern and this was accommodated by version 4.4 onward.
If you are using the latest version, there must be some subtle difference in the transmissions from the different ships.
Perhaps some are using the wider deviation (25kHz channel) and others are using the narrow one and your system is optimised for one and not the other.
Perhaps there is a small transmit frequency error in some cases which is pushing the signal out of your receiver IF bandwidth. Remember that particularly in situations where the IF bandwidth is too narrow, the tuning can be very critical indeed.
Perhaps one ship has its transmitter set to a slightly narrower or wider deviation. A narrower deviation would probably benefit you if your passband was too narrow. A wider deviation would probably benefit you if your audio levels were too low.
The static information is sent less frequently than the dynamic information. Under some circumstances, such as channel congestion, the static information may be sent even less often. If you never ever see any static information, you might suspect your sound card. The demodulation algorithm assumes perfect clocking of the soundcard which is operated at 48000 samples per second. If there was a small error in this clock rate, the short messages (which include the dynamic information) are more likely to be received without error than the long messages (which include the static information). Sadly, there is not much you can do about that except try another computer/soundcard.
ShipPlotter can generate serial NMEA AIS Messages from an available serial port. These messages can be fed into the serial input of commercial AIS plotting devices which will display the data in their own way. ShipPlotter has also provision for COM Automation which means that you can extract data from the program when it is running using, for example, simple VB scripts. This allows you, very easily to feed the ship data to other applications and processes.
ShipPlotter includes an interface with OziExplorer moving map display program. Ship positions that are decoded by ShipPlotter can be superimposed on the moving map display presented by OziExplorer running on the same computer.
Other plotting programs that run on the same machine may be able to use the data from ShipPlotter if they can accept the serial NMEA AIS message format. If you have Win2k or WinXP, a utility driver called ComEmulDrv, which is to be found on in the ShipPlotter files area, creates a pair of back-to-back virtual comms ports. If you direct ShipPLotter to output the NMEA data on one of these virtual ports and you direct the other program (eg SeaClear) to take its input from the other of these virtual ports, then the messages from ShipPlotter will be displayed on the SeaClear charts. To use this feature, you do not need to have a physical comms port but you do need to be using Windows 2000 or XP. If you are not using either of those, you can achieve the same result less elegantly by using two real comms ports and a null modem cable between them.
There are a few adjustments that you might like to investigate in optimising your monitoring set up.
As noted above, the receiver tuning is very critical, especially in case where the receiver passband is narrower than ideal for the transmission. If you only have 5kHZ tuning steps or larger, there is not much that can be done but if you have 0.5kHz tuning steps, it may be beneficial to experiment with setting the receiver to slightly below or above the nominal frequency. The View..Signal window has a blue trace that shows the dc offset of any successful HDLC header. If, after a number of messgaes, that trace is consistently above or below the centre line, try a very small tuning adjustment to make the trace closer to the centre line. Note that it requires a large number of messages at the new setting to determine if the graph has shifted in the right direction. Beware of being too hasty.
The audio level into the PC can be important. If the input level is very high, the soundcard might be distorting the signal even before the mixer gain control. Unfortunately, most discriminator outputs are not adjustable in amplitude. It may be worth making an in-line volume control for the lead from the receiver to the PC especially if you have to use the microphone input on the sound card. You can also experiment with small changes to the mixer volume setting to see if more or less signal level gives improved performance.
The Options..Decoder dialog in ShipPlotter provides a couple of adjustments that you might also explore. The Rx Bias deals with offsets that might come out of the receiver resulting from dc in the signal or slight tuning errors. Try a range of values around the nominal zero. Values larger than +/- 20 or 30 are unlikely to work. The Auto Bias control addresses a similar but not identical problem. Try experimenting with and without that option selected. The adjustments in this paragraph (but not the previous paragraphs) can be tested objectively by recording a few minutes of signal to a wave file (using the ShipPlotter Process..Record option) and then repeatedly processing the same recording (Process..Wave file) with the different settings and seeing which settings give the largest number of decoded messages by inspecting the View..Messages window. In this way, using a predictable input to the test, you avoid drawing spurious conclusions from chance changes in the level of traffic.
It may be worth trying the effect of inserting the audio into the microphone socket, rather than the line-in socket. You may need to adjust the level, perhaps with an inline volume control, but there is some anecdotal evidence that the filter characteristics of the two inputs may be sufficiently different for one or other to give better results.
There is a very important difference between ACARS and AIS. ACARS uses FSK at 1200 bps and so the occupied bandwidth is nicely centred in the pass band of the audio stages of a typical scanner receiver. AIS uses 9600 bps baseband signalling. There is no audio subcarrier conveying the bits, they are sent as NRZI - the raw bits modulate the transmitter, which means that the bandwidth is DC to 9600 Hz or more. In fact, it is doubtful if a soundcard, much less a receiver audio stage, will render DC but since the data packets are only 30 ms long, the requirement is more correctly described as from much less than 30 Hz to more than 9600bps. Because the bits are much closer together (1/9600 sec instead of 1/1200 sec), the tolerance on phase distortion is eight times more stringent than ACARS and the bandwidth over which the phase response must be flat, is several times greater than that required by ACARS. You will see why comparisons with ACARS are not really relevant. If you want to receive AIS transmissions you really need a discriminator output on your receiver or you must use a dedicated AIS receiver. A few radios already have a discriminator output (I use a WinRadio which has a suitable output labelled 'data' and the PCR1000 receiver has a suitable output labelled '9600'), but most receivers do not have one. The good news is that adding a discriminator output to an existing receiver is generally very simple indeed. www.discriminator.nl is the place to find about discriminator taps. There is an English version and a Nederlands version - click on the appropriate flag.