The New Horizons spacecraft made its historic rendezvous with Pluto this week. While there is no direct Amateur Radio involvement in the Pluto flyby, many amateurs are curious about how NASA communicates with New Horizons at a distance of nearly 3 billion miles.

 At that vast distance, New Horzions’ radio signal is extremely weak — so weak that only the Deep Space Network’s largest 70-meter parabolic dish antennas and receivers are capable of detecting it. New Horizons downlink transmissions take place on an X-Band frequency of approximately 8.4 GHz (the uplink is 7.2 GHz). In terms of raw RF output, the traveling wave tube amplifiers (TWTAs) aboard the spacecraft supply only 12 W to its 2.1-meter high-gain antenna.

There are two TWTAs aboard New Horizons. Each is connected to a separate radiating element at the antenna. One element is configured for left-hand circular polarization and the other for right-hand circular polarization. The original intent for using two TWTA was for redundancy.

As the spacecraft was on its way to Pluto, however, engineers discovered that they could use this cross-polarized configuration to transmit two signals simultaneously. At the Deep Space Network they designed a system to detect the separately polarized signals and combine them for substantially greater gain.

A stronger signal means New Horizons can transmit at a higher data rate — about 1.9 times the rate as with a single TWTA. Unfortunately, New Horizon’s nuclear power generator has decayed during its 10-year flight, and there is no longer enough power to run two TWTAs at the same time, unless the team shuts down another onboard system.

This is why it will take considerable time to download the treasure trove of images and other information that New Horizons carries in its memory. At present, New Horizons is transmitting data at just 1 kbps. A typical image produced by LORRI, the Long Range Reconnaissance Imager, is about 2.5 Megabits, even when compressed. At such a low transmitting data rate, it takes about 42 minutes for New Horizons to transmit a single image to Earth — and then there is the 4.5-hour trip at the speed of light! This is why mission scientists are warning an impatient public that it will be well into 2016 before all of the data arrives at Earth.

For more details about the New Horizons RF communication system, see “The RF Telecommunications System for the New Horizons Mission to Pluto,” from the Johns Hopkins University Applied Physics Laboratory.