A celestial link: Long transmission distances and rural locations pose unique power challenges for satellite-based communications systems

Satellites are providing desperately needed telephone access to remote and thinly populated areas through the use of small dish antennas that measure between 1 and 2 meters in diameter. These very small aperture terminals provide economical access to satellites in geostationary orbits 22,300 miles above the equator.

Satellites bridge even the most daunting geography with aplomb, but system engineers still must grapple with the issue of providing a reliable power source. Power gives ground-based VSAT equipment the zip needed to communicate with satellites located 22,300 miles in the sky.

Jim Hartman, marketing director for Scientific-Atlanta's Telecom Products division in Norcross, Ga., warns that more provocative topics, such as ISDN, sometimes lure well-intending engineers from the importance of power. "The fundamentals of power interfaces may be mundane, but they are critical," he says. Scientific-Atlanta manufactures a variety of satellite and telephony network platforms.

Any VSAT installation requires electrical power to operate, and VSATs used for telephony demand a highly reliable power source to avoid service interruptions (Figure 1). For sites in remote areas, that power source might be required to operate with little or no maintenance for years at a time.

VSAT equipment is becoming more cost-effective as newer, more sophisticated satellites allow the ground installations to employ smaller antennas and lower-power transmitters (Table 1). The cost of satellite access for telephony also is decreasing as digital signal processing allows acceptable voice links over thinner slices of bandwidth. As costs shrink, satellite systems become a practical option for bringing telephone service to areas where the power supply from the local utility is spotty, or even to areas where there simply is no socket in the wall to plug in a power cord.

It boils down to backup Where a premium is placed on reliability of the satellite phone service, system designers provide backup power systems for the VSAT installations to supplement local infrastructure and to take over immediately in case of an outage. In remote regions of the globe, where terrestrial infrastructure falls short, network operators also face the added challenge of having no power infrastructure in place.

In these instances, operators turn to the sun for help. High above the earth, the satellites themselves use vast panels of solar cells to generate power for their communications payloads. The sun's reliability record is, of course, exemplary.

Batteries allow operations to continue while a satellite passes through the earth's shadow. Some VSAT earth station installations employ the same technology-they employ solar cells for primary power and a backup battery system for periods of insufficient sunshine. In some cases, cells fall into a sleep mode when they are not in use, reserving power until it is again needed.

Before choosing a network's power source, operators must decide on the overall network design. "Whenever we do a new installation, power is a system design criterion," notes Hartman. "We don't supply boxes of products, we supply turnkey systems. [By doing this] we are involved in power selection. Every country in Latin America, for example, has different power requirements. The customer decides the objective-what they want and what they're interested in."

Scientific-Atlanta's customers, which include international carriers and public network operators, are responsible for specifying power sources and backup options when submitting original network designs, he notes. For international markets, power supplies must be designed for a variety of environmental factors, including variant voltage and power fluctuations.

As the cost of deploying VSAT networks continues to plummet, the attraction of VSATs for rural telephony solutions is rising. The demands-particularly power sources-remain rigid in rural applications. Rural telephony system designers sometimes turn to the local power authority and local engineers when deciding on power requirements and solutions. Sometimes solar power is the only available power source.

"You have to look at what the conditions are," Hartman says. "Voltages vary within a country as well as the voltage and consistency of the power source. Of course, urban areas are more reliable than rural areas where there are more fluctuations in voltage, more brownouts."

Furthermore, voltage requirements around the world are different.

"In Mexico, we had some areas where the voltage was slightly higher than the U.S. standard 120 volts," says Hartman. "It was something more like 127 volts, using uninterruptible power supplies. Within the network, 127 volts read as a surge. This is an example of people [neglecting] the importance of power. A lot of times, customers go for whatever power supply is cheapest and forget about how important [this network element] is."

Four nines Customers are becoming less tolerant of downtime in any communications service, whether the message is transmitted as voice, video or data, says Eric Wentz, director of marketing communications at Bellingham, Wash.-based Alpha Technologies, which specializes in power supplies for broadband communications systems. "Power has definitely come to the forefront of the industry," he says. "The reliability issues associated with the sophistication of delivery networks today has certainly made power move up the list of priorities for system operators and service providers."

The reliability goal for most customers is "four nines," a reference to near-perfect performance. If a power system delivers the goods 99.99% of the time, it can be off-line for only 53 minutes a year.

"Four nines is definitely the target to hit," says Wentz. "There are a number of things that go into reaching that objective."

One technique is to build in redundancy with a "hot" spare unit ready to take over instantly if a unit fails or must be taken out of the loop for service. Some customers also use generators fueled by propane or natural gas to keep equipment running through lengthy power outages, he says.

Like Wentz, Hartman says 99.99% is the target for reliability.

"In Mexico, our Skylinx 8000 [VSAT telephony platform] is in use in an emergency network," he says. "There can be virtually no downtime. So we took the necessary steps to design the system with constant availability. We'll never get to 100%, but we're shooting for 99.99%."

Most of Scientific-Atlanta's customers have a battery backup built into their VSAT networks. "We ask [our customers] how much backup capacity they want. The more batteries you have, the longer your network can operate," Hartman says.

In rural areas, government-imposed economic constraints and restrictions may come into play, and customers may not be willing to put in any type of backup system as an alternative. Not surprisingly, rural VSAT networks and network equipment "generally don't have huge supplies of backup power," according to Hartman.

That makes the primary power source even more important.

"There's no one magic solution-the best choice that a system designer can make is to end up partnering with a power expert," says Wentz.