Deep Space Network

The Voyager spacecraft, currently the farthest human-made objects from Earth, are roughly 20 billion kilometers away yet remain in contact with us. When people hear this, they often wonder how such a feat is possible.

Both Voyager 1 and Voyager 2 were launched in 1977, just a few days apart, and embarked on a mission to explore the planets of the solar system, successfully completing their tasks by passing close to various planets and capturing remarkable images. Voyager 1 exited the solar system in 2012 (meaning it crossed the boundary of the Sun’s intense solar wind influence), followed by Voyager 2 in 2018, crossing what is known as the Heliopause, the border of the Heliosphere.

Now, let’s discuss how we maintain communication with these spacecraft from Earth. The technology used is the same as that for communicating with satellites orbiting Earth, i.e., radio wave communication systems. NASA uses a system of ground-based antennas to communicate with any spacecraft in deep space, available in all directions beyond Earth. This network is called the Deep Space Network (DSN).

The DSN consists of three large radio dish antenna installations on Earth, located in California (USA), Madrid (Spain), and Canberra (Australia). At each of these locations, there are antennas with diameters of 70 meters, 34 meters, and 26 meters. These three stations are positioned 120 degrees apart, ensuring that at least one antenna is oriented toward any mission beyond Earth at all times (only the Australian station can communicate with Voyager 2). Signals sent from these stations travel through space at the speed of light and are received by the antenna on the spacecraft. The Voyagers are equipped with a 14-foot dish antenna (High Gain Antenna), which receives these signals and transmits them to the spacecraft’s computer or control unit, instructing it to, for example, send recorded data from its memory back to Earth in small packets.

At this distance, it currently takes about 19 hours for a one-way signal to reach the spacecraft. The communication system on the spacecraft uses the X and S band spectrum to transmit and receive data.

A common question that arises is whether the signals weaken over such a great distance or get lost after colliding with something.

That’s a valid question.

The signal sent from Earth is quite powerful, typically transmitted using a 34-meter diameter antenna with a power of up to 20 kilowatts. The high-power signal is necessary because the Voyager’s antenna cannot receive a weak signal. The uplink frequency is usually near 2 GHz, while the return signal from Voyager is only about 20 watts, which is a thousand times weaker than the signal sent from Earth. The downlink frequency uses the X band at 8.4 GHz, which allows for a higher data rate signal transmission from Voyager to Earth. However, by the time this signal reaches Earth, its power has diminished to about:

9 x 10^-23 kW.

This extremely low-power signal is then amplified, and the data is extracted. Indeed, as the signal travels, it gradually weakens, and some errors occur. These errors (bit flips) are corrected using error correction codes accompanying the data, ensuring the data is accurately interpreted. If we talk about the data rate, the commands sent from Earth to Voyager travel at 16 bits per second (2 bytes/sec), as these are small commands requiring minimal data. The data sent from Voyager via the X band using the High Gain Antenna is transmitted at a normal rate of around 160 bps, with an option of 1.7 kbps for critical data.

It should be noted that during such communication, the signal is always separated from the carrier frequency, amplified, errors are corrected, and then the data is analyzed.

Similarly, communication with Mars rovers and orbiters is also carried out through the same Deep Space Network. Mars is an average of 220 million kilometers away from Earth (ranging from a minimum of 50 million to a maximum of 250 million kilometers). Sending and receiving data from the Martian vehicles and orbiters is easier than communicating with the Voyagers, which are 20 billion kilometers away and utilize older technology.

Currently, the 70-meter antennas in the Deep Space Network are nearing the end of their operational life, and plans are underway to replace them with new 34-meter antennas to maintain seamless communication with future space missions.