Handheld sat phonesJan 1, 2003
The age of the handheld satellite phone is almost here. A new system called Iridium, the major partner of which is Motorola Corp., is set to provide worldwide satphone coverage. Iridium will provide the ocean sailor with another offshore communications choice.
Iridium is scheduled to begin commercial operation at 1300 UTC, September 23, 1998. Initially, a handheld phone will cost about $3,000. The per-minute cost of a call, in North America and in close-by ocean waters, will be about $2.00. Costs farther offshore and in other parts of the world are not yet determined but will be competitive with existing satellite services. The price/value history of cellular services and equipment and other satellite systems is likely to apply to Iridium, with costs going down with increased use.
Geostationary satellites were not a likely option for use with handheld phones. The minimum 19,000-nm distance from the earth's surface to the satellite would require too much transmitter power, both at the satellite and in the handheld phone, for which using anything other than a simple rod-shaped antenna would be out of the question.
A system using satellites orbiting at a relatively low altitude, similar to weather and reconnaissance satellites, is the obvious alternative to a geostationary system. However, these satellites would move past points on the earth at a great speed. For satellites intended to gather weather data and intelligence information, movement over the earth's surface is desirable. For communication purposes such movement is a major technical hurdle. Few people would be content to use a system in which calls could occur only at certain times and continue only until the satellite disappeared over the horizon. (You may recall waiting for a satellite pass when using the Transit navigation system.) Of course, if you put enough satellites in orbit, you might be able to ensure that at least one would be in sight of any point on the earth at all times.
Motorola initially determined that 77 satellites, orbiting the earth in seven polar orbit constellations of 11 satellites each and crossing the equator at an angle of 86.4°, at an altitude of 780 km (421.5 nm), could provide a system in which any point on the earth's surface would always be in view of at least one satellite. The planned satellite complement of 77 birds gave the system its name. Iridium is the 77th element in the periodic table of the elements.
The Iridium system differs from conventional practice in areas other than that of satellite orbital altitude. In geostationary systems, the user and a ground station must be simultaneously in view of the satellite. Communication is between the mobile station, the satellite, and a ground station that immediately transfers the signal to the Public Switched Telephone Network (PSTN) to complete a call. Clearly, Iridium's low-earth-orbit (LEO) satellites could not operate in this manner. Too many ground stations would be required.
Satellites talk to each other
In the Iridium system, satellites function as space-borne relay stations, forwarding calls from one satellite to another in a constellation, and from constellation to constellation, around the globe, until the signal can be directed to a gateway (an earth station) near the call's final destination.
This approach offers a number of advantages. The total communication distance is much shorter than that required with geostationary systems. The minimum communication distance for a geostationary call is more than 38,000 nm, plus the distance traversed in the PSTN. Although the signals travel at near the speed of light, annoying delay times in two-way communication occur with these systems. In the Iridium system, the communication distance can be as short as 1,000 nm, while a conversation that travels halfway around the earth will traverse "only" about 16,000 miles.
The practicality of building, launching and operating a system containing 77 active satellites was seriously doubted by many people skilled in the space industry. Even after Motorola refined the system so that 66 satellites (still 11 per constellation, but reduced from seven to six constellations) much doubt existed. Over time, 17 risk-sharing partners have joined in a consortium, now known as Iridium LLC.
One of the basic design features critical to the success of the Iridium system is its ability to function as a conventional cellular telephone when the handheld phone is in range of a cellular system, and to automatically switch to satellite mode when no cellular service is available.
The users of this type of communication system will be world travelers. There are 28 different and often incompatible cellular systems in use around the world. A workable Iridium phone would have to be able to function with most, if not all, of the various cellular standards. Motorola responded to this challenge by building a phone containing a removable cassette that defines the phones' operating mode. Before traveling to a place that uses a cellular system different from that of the home area, the user will obtain the cassette that matches the needs of the system in the foreign area.
Making a call with an Iridium phone is simple. Turn it on and punch in the number. If in range of a compatible cellular system the call will be completed via that cellular network. If the phone is unable to make contact via a cellular system it will switch to satellite mode. Iridium keeps track of phone location using a system of area codes similar to those used for terrestrial phone service. Each gateway can manage up to 2,047 area codes. Over oceans, area codes may be assigned to differing geographic areas, depending on communication traffic density. Areas of intensive fishing or oil exploration, for example, will probably generate heavy traffic.
With 66 active satellites in orbit, at least three satellites will normally be in view of any point on the ground. (One spare satellite is in orbit for each constellation, to back-up the constellation's 11 active satellites). The system uses the three satellite groups to triangulate on the calling phone, thereby determining its position. This information is used to assign the caller to a particular satellite and subsequently to one of the 48 spot-beam antennas with which each satellite scans the earth below. Each of the spot-beam antennas covers a 26 km radius on the earth's surface, an area of 2,122 square kilometers (820 square miles). One satellite can therefore provide service to an area of almost 40,000 square miles.
The spot-beam antennas on each satellite can handle 780 simultaneous two-way calls, plus 180 paging channels. The calls may be analog voice or digital data at 2,400 baud. The maximum message handling capacity of a single satellite is therefore 37,440 voice channels, plus paging services for an additional 8,640 users.
Calls handed off
As the satellite proceeds on its orbital path, the calling phone is handed off from spot beam to spot beam until the system software determines the need to switch to the next satellite in the constellation. All of this activity occurs without the notice of the user.
When a call is initiated, the system first verifies the caller's eligibility for service. In other words, has the bill been paid? Assuming a positive account verification, which requires only a second or two, the call from the phone proceeds from the serving satellite overhead, through as many other satellites as may be required by the relative positions of the caller and the station being called, until it arrives at a satellite that can communicate with the gateway serving the called telephone.
To complete a call to any mobile station, a cellular phone or another Iridium phone, the system must be able to locate the called phone. Iridium handles this task in a manner similar to that used for cellular services. Whenever the phone is on, but not in use, it silently communicates with the system, continuously updating the record of its location. Calls can proceed to and from an Iridium phone regardless of its position on the earth, as long as it can either communicate with a cellular system or contact a satellite.
Communication between the handheld phone's 0.6-watt transmitter and the satellite occurs at L Band, 1,616 to 1,626.5 MHz. Cross-link communication, from satellite to satellite, is handled at Ka Band, 23.18 to 23.38 GHz, using separate antennas. Satellite communication with ground stations, called gateways, is at Ka Band, 19.4 to 19.6 GHz downlink and 29.1 to 29.3 GHz uplink.
Since signals at this frequency may be seriously attenuated by heavy rain, spatial diversity is used at the ground stations. Antennas are located about 25 miles apart, assuring, on the basis of the reasonable horizontal extent of any extreme rainstorm, that at least one antenna will be able to maintain communication integrity. The gateway stations are highly automated, requiring only minimal staffing.
Building more than 70 satellites is not an everyday task. The world's first satellite mass-production line had to be established to produce all the needed hardware. Where many satellite production operations take upward of a year to produce a finished bird, Motorola's Chandler, Ariz., Satellite Communications Group worked on a 35-day cycle, producing a satellite about every five days. Anyone who has worked in the satellite field can only wonder at this production rate and volume.
Placing an Iridium satellite in operation is not a simple process of shooting it into space with enough energy to reach the desired orbital altitude. Initially, the satellites are placed in a parking orbit while numerous checks are carried out on all systems. With checks complete, thruster firings over a period of several days boost the satellites to the required orbital altitude and place them in their assigned slots in the 66 satellite constellation.
Sophisticated control facilities
The task of managing a flotilla of 66 active, plus six spare, in-orbit satellites is immense. Control communication with the satellites is carried out near the North Pole. All satellites must pass over the poles during each orbit. There are two control relay stations located in North America, in northern Canada. An additional station is located in Iceland. These relay stations are in turn managed from two master control stations, one in Landsdowne, Va., and the other in Rome, Italy.
The Landsdowne facility resembles mission control for a manned spacecraft program. The daily workload in managing the total Iridium satellite fleet is greater than for many manned missions. The extent of the station's uninterruptable power supply provides an indication of the amount of computer and control equipment required to manage the satellites. The uninterruptable power supply is based on a 1.2-megawatt, lead-acid battery bank, which occupies a large air-conditioned room. The system can support the total energy demand of the center, including lighting, air-conditioning, and all of the electronic equipment for between two and six hours. A V-16 Caterpillar diesel engine, turning a massive alternator, backs up the battery bank, cutting in automatically within seconds of any interruption in the main power supply.
The Iridium satellite weighs approximately 1,560 pounds (on earth), consumes up to 590 watts of electrical power, supplied from a battery bank supported by a solar cell array that can produce 1,200 watts in full sunlight. The satellite is approximately 15 feet long and, with its solar panel extended, 27.5 feet in diameter. The satellites are launched from three countries: the U.S., using a Delta II rocket, carrying five satellites at a time; Russia, with the Proton rocket carrying seven satellites; and China, where the Long March 2C/SD rocket carries two satellites into orbit.
Each satellite is loaded with a small amount of in-orbit maneuvering fuel (approximately 12 pounds). Since variable drag effects from the solar wind exist at the satellites' orbital altitude, corrections to the orbital path must be made to compensate. The useful life of a satellite may be determined by its fuel supply, since once the tank runs dry the spacecraft can't maintain station. Anything that can maintain desired orbital path, while conserving fuel, is a valuable resource.
Each satellite is equipped with large, sail-like solar panel arrays. Since the in-orbit drag of a satellite is in part determined by the relative position of its solar panels to the solar wind, an opportunity exists to "sail" the satellite, in effect tacking the satellite relative to the solar wind to correct for orbital deviations, without using scarce thruster fuel. The system controllers may therefore be the world's most traveled sailorsafter all, their sailing craft travel tens of thousands of miles each day. They don't have to wear foul-weather gear or try to cook when heeled at 25°, however.
Aiding search and rescue
It is reasonable to expect that the availability of this worldwide telephone/data system will have an impact on maritime search-and-rescue (SAR) activities. A sailor in trouble might be reluctant to turn on the 406 EPIRB until certain that outside assistance is really needed. With an Iridium phone at hand, a call to alert the outside world to the problem will be an attractive option. Furthermore, if the situation degrades to the point where activating the EPIRB is called for, additional communication, via Iridium, will be a great help. The vessel in trouble will be able to provide information beyond that sent by the EPIRB; that an emergency exists and with new generation EPIRBs, vessel position from the GPS or loran. In an emergency, nothing beats talking with someone who might be able to provide assistance. In the event the vessel's navigation system is out of commission, the Iridium system's ability to resolve the caller's position to within about 26 kilometers can also aid in locating the position of the vessel.
Should an EPIRB be triggered accidentally, especially if it is lost overboard while operating, a call on the phone can eliminate anxiety about the state of the vessel and the cost of activating a SAR mission. The Iridium system may also play an active role in making assistance available to a vessel in distress in the minimum possible time.
The SAR authority responsible for the ocean area involved could call other vessels known to be in the area to alert them to the need for assistance. At present, many commercial vessels are usually part of the AMVER system, whereby their positions are known to SAR authorities. The Iridium system may expand the availability of rescue assets by including other vessels, such as yachts, in the resource pool.
The 2,400-baud digital capability of the system will make it possible to obtain weather charts and data from a cooperating commercial service provider. The cost, for the approximate two minutes required for each page of moderate-resolution information, will probably be acceptable to many sailors.
Many of the uses to which the Iridium system will be put by mariners cannot be foreseen today. Following the history of land-line and cellular telephony, the sky is the limit and, in this instance, the medium for communication.