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Keeping Track

Jan 1, 2003

How does one track a vessel sailing around the world? Keeping tabs on the boats involved the BOC singlehanded around-the-world race is perhaps the ultimate tracking challenge. For the 1994/95 race, the tracking will be handled using a combination of Inmarsat C and GPS.

It's important to track the location of each boat in such a race, but also required is a fast method for the racers to communicate any life-threatening emergencies during their transit of the planet's most desolate waters.

One way to track these racers would be to have them call in, using HF SSB, with a regular position report. This method does have some drawbacks: the position report is wholly dependent on each racer faithfully making a regular call; propagation conditions have to be right; and if a racer doesn't call in, does the race committee immediately assume an emergency exists?

Obviously, an automated system would be better. Instead of having to make a daily call, each boat would "call in" without the racer's involvement. And rather than using long-range HF radio to call race headquarters, a more reliable approach would be a worldwide satellite-based system. (Since HF propagation is always changing, an HF system normally requires a human operator for selecting the best frequency. Recently, however, automated control systems have been developed for HF radio. One of these is called automatic link establishment, or ALE. Radios with ALE capability are able to establish and maintain links automatically. See "Direct dial HF" Issue No. 53.)

In the past four BOC races, the tracking function was performed by a French/American system called Argos. Each racer carried an Argos module on his boat that provided tracking capability as well as a built-in emergency signaling capability.

The Argos system, which became operational in 1979, is a joint project of NOAA and the French space agency CNES. Designed as a data collection and location system, Argos uses instrument packages on board NOAA television and infrared observational satellites (TIROS) to pick up transmissions from individual platform transmitter terminals (PTT). The PTTs broadcast a message at 401.65 MHz, every 90 seconds. Included in the transmission is a PTT identifier and up to 32 bytes of data.

The position of a PTT is determined using the Doppler shift principle in much the same way as the SARSAT system uses EPIRB signals to locate those in distress. As the satellite pops up over the horizon, it picks up the 401.65 MHz signal from a broadcasting PTT. The satellite is, of course, traveling at high speed relative to the PTT. Due to the Doppler effect, the signal coming from the PTT is shifted to a slightly higher frequency. As the satellite makes its closest approach to the PTT, the signal is briefly received at the oscillator frequency of 401.65. Leaving the PTT behind, the signal received by the satellite is shifted to a lower frequency.

The change in frequency during a satellite pass, along with the known coordinates of the satellite's orbit, can be used to calculate the location of the PTT. Since PTTs broadcast continuously, either of the two Argos-equipped TIROS spacecraft can be used to determine the location of a PTT.

The second part of Argos is data collection. Each PTT is sending data during its 90-second broadcast loop. The type of data depends on the application. Argos is extensively used for collecting data from scientific instruments such as drift buoys or animal tracking collars. In fact, all a PTT broadcasts is an identifier code and data; the Doppler shift imparted on the signal is "free" purely a result of the satellite moving relative to the PTT.

In previous BOC races, sailors carried an Argos transmitter in a sealed package that included solar panels for charging the unit's batteries. These modules, usually secured on a vessel's stern, were autonomous, the racer never needed to touch them for race headquarters to get their position reports.

There was one way for a racer to influence his or her Argos module: should an emergency strike, a lanyard on the side of the module could be pulled and the PTT would begin transmitting a distress code. Then, if the situation required, a racer could take the Argos module with him into his life raft.

In the upcoming 1994/95 race, the race coordinators have decided on a different approach. In this race, a combined Inmarsat C/GPS system will be used to track the race boats, to communicate with the racers (although there will also be a regular SSB schedule as well), and for signaling emergencies.

Inmarsat is an international consortium that operates four geosynchronous satellites for worldwide communications by mariners. They offer several services, including Inmarsat A (voice, data, and fax capability), Inmarsat M (voice, data, fax - but at a slower transfer rate than A), and Inmarsat C. The Inmarsat C service uses a small, non-stabilized omnidirectional antenna and is restricted to text and data, with no voice capability. With Inmarsat C, a user types out his or her message on a keyboard before hitting the "send" key. The data file is broadcast to a satellite, which then rebroadcasts the file down to an earth station. At the earth station, depending on how the file has been tagged by the sender, the file is reconstituted as a telex message, or as a data file in ASCII or binary format and then sent over land lines to the end user. Similarly, telex messages or data files can be sent to a racer during the race. Received messages are held in the memory of the Inmarsat C transceiver until the racer decides to look at it on screen, print it, or save it to disk.

The tracking function will be provided using GPS: each of the C-link terminals to be used in the BOC will have a built-in multichannel GPS receiver. (Both the Inmarsat C antenna and the GPS antenna are combined in a single unit.) This radionavigation receiver (usually all the components are mounted on a computer-type expansion card) will supply the Inmarsat C terminal with continuous GPS positions. Unlike Argos, which is limited to a handful of positions a day, the Inmarsat C/GPS combination can track the vessels on a more accurate basis, with the provision for continuous position-fixing, if desired.

One of the ways this positioning bonanza can be realized is via the use of polling: Race headquarters will be able to call each vessel (probably on a four-hour schedule) and "request" a position from its Inmarsat C terminal. The sailor on board can continue about his business, though, the terminal will handle the task automatically, sending a data message back to race headquarters announcing the vessel's current location.

An advantage the Inmarsat C terminals have over Argos is the capability for two-way communications (a capability the racers will also possess via HF SSB). Racers can communicate with race headquarters, and with support teams on shore, without having to sit at their SSB radios selecting frequencies. And to prevent anyone with an Inmarsat C terminal from badgering the racers with a flood of messages or unauthorized polling, special access codes will be required to send messages or to poll the competitors.

Should there be an emergency, a racer can send a message to race headquarters detailing the exact situation, allowing race officials to take the most appropriate action. If the situation is especially dire - a boat has hit an iceberg and is sinking fast - the racer need only press a button and an emergency message will be sent immediately.

Since they are self-contained units, Argos modules are independent of ship's power and can be taken into a raft. The Inmarsat C terminal, on the other hand, is wired into the vessel's power. As such, and given the fact that the terminal is not designed to be used out in the elements, or without its separate antenna, the Inmarsat unit is not something to be taken into a life raft. Of course, that's a task EPIRBs can perform. All competitors will be required to carry two 406 MHz beacons.

There are several companies that offer Inmarsat C terminals. Among these, Trimble Navigation has arranged with the BOC race committee to provide the racers with terminals. This unit, called the Galaxy, will use Trimble GPS receiver circuitry and Trimble's tracking software. Since each competitor will be equipped with the same hardware and software, BOC race director Mark Schrader is hoping to avoid any problems with incompatibility. "It's extremely important to us that this stuff works the way it should,"says Schrader. "We want to run a safe race."

Some racers will also carry a second Inmarsat C terminal. American racer Michael Carr, for example, will be using a terminal from Mobile Telesystems Inc.

Inmarsat will be providing the race committee with two of their Inmarsat M units for use during the BOC. The M service uses a larger, gyro-stabilized antenna, and can provide voice communications. Race director Schrader plans to rotate these units through the fleet, selecting two boats to be M-equipped for each leg of the race.