From the outside looking in, the task of designing a handset for direct communication to a satellite verges on the impossible. The handset has to be small, it can't glow in the dark while transmitting, it can't depend on a shopping cart full of batteries, and it has to reach a receiver that may be 40,000 kilometers away. Not a promising set of requirements.
But designers have found that handset design, while demanding, is not as close to science fiction as the specifications suggest. They report that it is possible to stay fairly close to the basic formula used in GSM cellular handsets, with some small but significant enhancements. "The necessary gain to noise temperature (G/T) requirements are typically in the area of -24 dBK," explained Hassan Zamat of Hughes Network Systems. "The required effective isotropic radiated power (EIRP) is about 5 to 7 dBW. The G/T figure can usually be met with the low-noise amplifiers already widely used in the industry. The EIRP figure requires a transmitter power of about 2 to 4 W-a lot more than GSM but well within the capabilities of a handheld device, if the power drain is discontinuous."
The key to the handset design, Zamat said, is the antenna. "The most typical antenna for satellite services is a quadrifilar helix. This type of antenna provides a pattern optimized for hemispherical coverage, directing the radiation upward."
With the trick antenna, a standard LNA, a more powerful transmitter and circuitry for forward error correction, the whole electronics package for the handset is manageable. Iridium is selling handsets for something like $3,000 each. But they are a production item, not a military miracle.
