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Tuesday, July 14, 2015

Receiving from Pluto

Three years ago I blogged about deep space communications. Now I can elaborate on the New Horizons mission to Pluto.

The primary radio antenna on NH is a seven-foot dish. If you remember the earliest home receivers for satellite TV —  the so-called C-band receivers — they used dishes of about the same size. Antennas on NH can get only so large because the spacecraft has severe constraints on its size and mass at launch. Also, limiting the size of the dish to only seven feet means that the "aim" of the spacecraft toward Earth is a rather broad 0.3 degrees of arc. Every spacecraft drifts and wobbles in flight. One-third of a degree of arc is not too small a target to hit, and therefore the finite amount of hydrazine fuel on board NH is not depleted to keep the antenna pointed precisely.

Another constraint on the communications system is that NH receives only 200 watts of electric power from its plutonium-powered generator. (NH is too far from the Sun for photovoltaic panels to be useful.) This 200 watts must run the entire spacecraft; the radio transmitter cannot use all of it, and furthermore no transmitter is 100% efficient when converting DC power to radio signals. Consequently the radio signal from NH is quite faint by the time it reaches Earth.

In contrast NASA's Deep Space Network uses dish antennas at its sites in California, Spain, and Australia as large as 230 feet in diameter. The antenna in use must be aimed at NH with extreme precision, and the dish must move constantly to continue pointing directly at NH as Earth revolves.

Overall the system supports a data rate of about 700 bits/second in the downlink from NH to Earth — the speed of dialup modems in the mid-1970s. The very slow speed is essential to overcome the billions of miles of signal dispersion, thermal noise, the small size of the NH dish, and the low power of its transmitter. Actually the signals are being transmitted quite a bit faster than 700 b/s, but NH must add intentional redundancy to the signals so that they are recoverable on Earth. The net data rate after the redundancy is processed and removed on Earth is 700 b/s. Consequently it will take nine months for NH to transmit the multiple gigabytes of data and images that it obtains from Pluto and its moons. The 9-hour round-trip delay for radio waves (or light or any other kind of electromagnetic wave) between Pluto and Earth means that a flight controller at the Jet Propulsion Laboratory cannot simply browse the images on NH and prioritize the ones of most interest. Be patient.