Marine Band stations in the 156-163 MHz range useful to monitor for band openings. The duct from Ascension Island to Cape Town was first identified by the reception of Cape Town Harbor Radio on Ascension. However, transmissions can be fairly sporadic, and ships usually only communicate with the port facilities when they are fairly close to the harbor. With commercial vessels ranging the full extent of the oceans, it would be very helpful if they would be on transmitting frequently as they crossed the oceans, our area of interst. Essentially, that would amount to a beacon network with thousands of stations spread across the oceans. Fortunately, that beacon network is exactly what is available!
The maritime Automatic Identification System (AIS) is a systems used by ships to communicate their positions to each other as part of the global maritime safety system. AIS provides identification, position, course, and speed, with other nearby ships and VTS stations. This information can be displayed on a PC using simple receivers and PC software, or dedicated receivers with internal modems can also be used. While intended for exchanging data between vessels directly, and between vessels and shore-based Vessel Tracking Services, it is also monitored by many private individuals, operating on two channels at 161.975 and 162.025 MHz, Marine channels 87B and 88B. Several of the software packages also link the data onto Internet position servers. It is reminiscent of a maritime APRS system, except that ships to do not relay each others positions.
Detailed information on the AIS network can be found on the web sites below An Internet search will yield many more information sites.
WikiPedia - Automatic Identification System
Automatic Identification System Overview - USCG Navigation Center
On-Line Tracking
Live Ships Map - AIS - Vessel Traffic and Positions
A useful feature on this service is to investigate what ranges the individual reception sites are receiving. This information is retained for teh previous two weeks. To view this data. on any screen showing the world or the region of interest, under "Notation and Display Options", select the "Stations" button . The green stations ill then be shown on the map. Left-click on the station of interest. The "Coverage Map" shows the regions from which ship signals have recently been heard. The "Station Details and Statistics" link provides useful historical data over the previous two weeks, especially the maximum ranges received.
MarineTraffic vessels on Google Earth, in Real-Time
The Google Earth display provides a familiar user interface, but lacks some of the data and reception station data referred to above.
This is another on-line AIS display service, and can display ship data in both Virtual Earth and Google Earth formats.
Receivers
AIS receivers have two components; the receiver and the demodulator (Modem). Receiving stations can either use a commercially available receiver/modem that provides data output to a PC, or a VHF FM receiver and PC with a software modem. Higher quality receivers can monitor and decode both AIS channels simultaneously.
Commercial receiver/modems are available with quite a wide price range from $150 to over $600. They provide RS-232 data output to a chart plotter on a vessel, or in our case, a PC with mapping software. That PC can feed the received data to AIS servers on the web for Internet viewing by others. Links to two commercial equipment providers are listed below.
AIS Receivers from Milltech Marine
Millitech Marine, a US distributor, offer a wide-range of AIS receivers that have received good reviews. Some of note are:
SmartRadio SR-162 dual channel receiver with RS-232 data output
Comar AIS-2-USB dual channel receiver with USB interface to PC
Comar SLR 200N dual channel receiver with Ethernet interface
Comar Systems main web page has more detailed information on their AIS receivers
The Icom dual-channel AIS receiver provided data output in RS-232 format.
Any receiver capable of covering the two channels on 161.975 and 162.025 MHz with a bandwidth of 10-12 kHz for the 9600 baud signals can be used to receive AIS signals as well. Quite often a direct discriminator output is used. The receiver output is fed into a PC sound card. Software in the PC provides a software modem. Information on building your own receiver is provided on the links below. Many amateur radio 144 MHz transceivers can receive the AIS channels, and have 9600 baud capability already for packet radio operation. These older radios perform very well. The N7BHC receiver is an old Icom IC-281H radio.
Build your own AIS monitor.
Cover your Area on the Live Navigation Map
This site does a very good job describing how to build your own AIS receiving site, what software to use, and how to feed the data to Internet servers. MarineTraffic will even provide free receivers in certain circumstances for unique or needed locations. MarineTraffic - Free AIS receiver
Discriminator.nl - How to build inexpensive AIS Receiver
This excellent site describes how to build an AIS receing site using commercial AIS receivers.
AISMon is a high-performance freeware AIS demodulator/decoder which outputs AIS data in NMEA format. Input may be from any installed sound card (radio discriminator output required) or 44.1/48K .wav file. Output may be used to drive charting applications such as SeaClear or ShipPlotter.
Antennas
Several different antennas are suitable for AIS reception. Most typical monitoring sites will use omnidirectional antennas such as the collinear antenna described below. Stations interested investigating long-range transoceanic ducting may prefer to use directional vertically polarized antennas like a yagi or a large vertical array stack of short yagis.
Shiplotter has excellent pages that compare different receiver and antenna performance, and detailed instructions on building high performance collinear AIS antennas.
AIS Aerial Performance Comparisons
High Gain Collinear AIS Receiving Aerial
Software
ShipPlotter decodes the AIS digital signals from each ship using the sound card in your PC. You need a suitable VHF band radio receiver tuned to one of the two AIS channels. The program decodes the received digital data and displays it in a variety of formats.
AISMon is a high-performance freeware AIS demodulator/decoder which outputs AIS data in NMEA format. Input may be from any installed sound card (radio discriminator output required) or 44.1/48K .wav file. Output may be used to drive charting applications such as SeaClear or ShipPlotter.
SeaClear GPS Navigation Software
SeaClear navigatoin software has an AIS display mode.