Do Iridium developments suggest the satcom future?


As satellite-based communications grow smaller and more effective they become increasingly attractive to voyagers. One activity that terrestrial HF SSB radio still does best, however, is provide "cruisers' nets." These radio chat groups are a way for voyagers, in say, the Bahamas, to have a communal discussion and exchange information particular to their location. A recently developed satellite communications technique used by the U.S. military on the Iridium satellite system may, however, point the way to a similar "cruiser net" capability using satellite communications.

In addition to developing these small networks, Iridium is doing further work for the Defense Department on devising a way to assist GPS receivers using Iridium satellite signals. The company is also selling space on future Iridium satellites for government agencies, or private companies, for remote earth observation. 

A prime mover for all these new Iridium applications is the upcoming generation of Iridium satellites now in development. Called Iridium Next, these spacecraft are scheduled to start launching in 2014 with the full constellation of Iridium Next satellites in place by 2016.

Iridium, of course, is the low earth orbit (LEO) global coverage satellite communications system originally developed by the Motorola Corporation in the 1990s as a worldwide cell phone system. Motorola greatly underestimated the growth of terrestrial cell phone infrastructure and when customers didn't sign up, the company was forced to sell its stake in Iridium.

The Iridium system has 66 LEO satellites in 500-mile-high orbits in six orbital planes. These sophisticated spacecraft not only communicate with users on the earth's surface, they also can relay communications between satellites; a capability that allows a user in the Southern Ocean, for instance, to speak with his office in Miami even though there is no earth station in the Southern Ocean to carry the traffic.

It works like this: When our hypothetical Southern Ocean user uplinks from his phone to the satellite, if there is no earth station in view, the satellite that received the call from the surface relays the call to another orbiting Iridium satellite. If that satellite does not have an earth station in view, it will relay the call to yet another Iridium bird. Eventually, one of the satellites will have an earth station in view and that spacecraft will downlink the call data. Then the data is sent over the telephone network to the Iridium user's office in Miami.

The ability of Iridium to relay data between satellites makes the system truly global, allowing it to carry communications traffic in areas that have no earth station infrastructure. This capability makes Iridium phones an attractive option for ocean voyagers who want to stay in touch. It also makes Iridium useful to the U.S. military, which is often called on to operate in remote areas. The Defense Department even operates its own Iridium earth station in Hawaii.

One of the intriguing new ways that the military is using Iridium is called Netted Iridium, or more formally, the Distributed Tactical Communications System (DTCS). According to a recent article in Signals magazine, the DTCS setup allows groups of soldiers equipped with special "push to talk" satellite phones to "talk with many listeners on a common channel within designated user "nets." With this system, a satellite phone attains some of the "broadcast" capability of SSB, allowing users to talk to an entire group of listeners. Much like current marine HF SSB cruiser's nets. The advantage over a HF SSB, of course, is ease of use. Turn on your Iridium satellite phone, select the proper channel and join the net. When you have something to add to the group, you can hit the talk button and chime in. In this current military system, users can even set their transceivers to receive from multiple nets.

Part of the DTCS system is something called the troop position locator information. This takes GPS information and adds it to the communications message allowing net users to see the position of all members of the net. The DTCS system is also set up for users to send text files and small data files.

While Iridium DTCS is currently only a military system, a similar civilian system could have attractive features for voyagers. The ability to participate in cruiser nets or listen to a group weather briefing as provided by Herb Hilgenberg of Southbound II, while at the same time automatically providing position information is a nice package for voyagers.

Another Defense Department initiative that uses Iridium's readily accessible constellation of LEO satellites is called iGPS. According to Iridium, iGPS "enables second-generation GPS-aiding signals to be broadcast through the entire Iridium constellation. These broadcasts will enable rapid, more accurate GPS position fixes than are available with current technology." In other words, data broadcast by Iridium satellites can aid GPS receivers in developing faster, more accurate fixes. The idea behind this program is to allow military GPS units to continue to acquire and determine position even in challenging environments such as heavy foliage, in built-up urban areas, in deep canyons and in cases when the GPS signal is being jammed by enemy forces.

Voyagers, of course, don't often find themselves or their boats in conditions of heavy foliage, city streets or in canyons. Yet they do often have to navigate in poor weather conditions, which are similar to GPS jamming. Presumably, this iGPS Iridium augmentation approach could aid voyagers in cases of heavy rain or lighting interference.

Both the new capabilities of Iridium and the upcoming Iridium Next satellites suggest that Iridium satellite phones will remain a voyaging communications tool for some time to come. That's quite a turnaround for a system that, only a few years ago, was threatened with "de-orbiting" its satellites and burning them up in the earth's atmosphere due to a lack of customers.

By Ocean Navigator