How are the Voyager spacecraft able to transmit radio messages so far?

The two Voyage spacecraft certainly have had an amazing track record. They were sent to photograph planets like Jupiter, Saturn and Neptune and have just kept on going past the outer edge of the solar system. Voyager 1 is currently over 7 billion miles (about 11 billion kilometers) away from Earth and is still transmitting -- it takes about 10 hours for the signal to travel from the spacecraft to Earth! The Voyager spacecraft use 23-watt radios. This is higher than the 3 watts a typical cell phone uses, but in the grand scheme of things it is still a low-power transmitter. Big radio stations on Earth transmit at tens of thousands of watts and they still fade out fairly quickly. The key to receiving the signals is therefore not the power of the radio, but a combination of three other things:

• Very large antennas

• Directional antennas that point right at each other

• Radio frequencies without a lot of man-made interference on them

The antennas that the Voyager spacecraft use are big. You may have seen people who have large satellite dish antennas in their yards. These are typically 2 or 3 meters (6 to 10 feet) in diameter. The Voyager spacecraft has an antenna that is 3.7 meters (14 feet) in diameter, and it transmits to a 34 meter (100 feet or so) antenna on Earth. The Voyager antenna and the Earth antenna are pointed right at each other. When you compare your phone's stubby, little omni-directional antenna to a 34 meter directional antenna, you can see the main thing that makes a difference! The Voyager satellites are also transmitting in the 8 GHz range, and there is not a lot of interference at this frequency. Therefore the antenna on Earth can use an extremely sensitive amplifier and still make sense of the faint signals it receives. Then when the earth antenna transmits back to the spacecraft, it uses extremely high power (tens of thousands of watts) to make sure the spacecraft gets the message.

Communicating Over Billions of Miles: Long Distance Comms in the Voyager Spacecraft

The Voyager spacecraft are on a long journey out of our solar system into interstellar space. Despite their great distance from Earth, we are still able to communicate with the spacecraft on a regular basis. This article looks at the basic communication infrastructure that allows us to communicate with the spacecraft. 

The Deep Space Network

After the Voyager spacecraft left Earth and completed their grand tour of the solar system, they began their journey into the regions of space that are beyond the influence of our sun—answering questions about what lies in the great cosmic void between stars.

Thirty-eight hours ago, a 20 kW signal was transmitted from Earth towards the Voyager 1 spacecraft. Nineteen hours ago, the signal was received by Voyager 1 and returned by a 20 Watt transponder. And, as I write this article, a station in Madrid, Spain is receiving that return signal at a power level of  9×10^−23kW=9×10^−8pW (-160.48 dBm.) For reference, a very good FM radio receiver can pick up signals at 9×10^−5pW, the signal received from Voyager is 1000 times weaker. 

The Deep Space Network consists of three antenna complexes that are stationed around the globe approximately 120-longitudinal degrees apart. The global separation of stations allows most spacecraft to have an uninterrupted line-of-sight with at least one station regardless of the time of day. A listening station will rise before the last visible one sets. Voyager 1 is still visible from all three stations, but Voyager 2 is only visible from the Canberra, Australia site. 

Watch which spacecraft the Deep Space Network antennas are communicating with below. 

Visit the full website: https://eyes.nasa.gov/dsn/dsn.html

As the spacecraft travel further from Earth, the signal strength decreases due to free space path loss. Data rates typically fall as a consequence. Improvements in the Deep Space Network receiver sensitivity over the past 40 years have mitigated reductions in data rate. 

Due to the incredible weakness of the spacecraft's downlink by the time it reaches Earth, large parabolic reflectors, and hyperbolic sub-reflectors collect the microwave radiation and focus it on a cryogenically cooled receiver at the base of the antenna. 

Each Deep Space Network location has multiple 34-meter antennas and a single 70-meter antenna. While any one of the antennas is more than powerful enough to transmit to Voyager, a single 34-meter antenna does not collect enough electromagnetic radiation to detect Voyagers downlink. Antennas at each site can be linked to simultaneously receive the signal from the spacecraft, providing increased gain through radio interferometry. 

Accurately locating the spacecraft on its journey was accomplished with Doppler rangefinding, and later with Very Long Baseline Interferometry (VLBI) along two baselines that extend from Goldstone, California to Madrid, Spain, and from Goldstone, California to Canberra, Australia.

The Goldstone-Madrid baseline is used to determine right-ascension of a spacecraft and the Goldstone-Canberra baseline provides a mix between right-ascension and declination. When combined, the data can locate the spacecraft extremely accurately in the celestial sphere with angular measurement error measured in nano-radians (one nano-radian of error at 1 million kilometers is 100 cm).

Technical Details

The following technical details of Voyager communication are provided in Deep Space Communication and Navigation Series (Chapter 3), and JPL DESCANSO Volume 4—Voyager Telecommunications.

Each Deep Space Network site has a highly accurate frequency source that can be tuned to compensate for the Doppler frequency shift caused by relative movement between the transmitting and receiving antennas. Compensation takes into account the movement of the spacecraft, the rotation of the Earth around the sun, and the revolution of the Earth around its axis. Receivers are able to detect frequency shifts that are a fraction of a hertz.

The uplink carrier frequency of Voyager 1 is 2114.676697 MHz and 2113.312500 MHz for Voyager 2. The uplink carrier can be modulated with command and/or ranging data. Commands are 16-bps, Manchester-encoded, biphase-modulated onto a 512 HZ square wave subcarrier.

Voyager's receivers phase lock to the uplink carrier to provide a two-way coherent downlink carrier signal or can use an internal frequency source to produce a non-coherent downlink carrier. The spacecraft can return information to Earth with X-band or S-band transmitters 

Conclusion

The Voyager spacecraft will continue their journey for unknown millennia, but we will only be able to communicate with them for another three years (2022)—by that time, the Radionucleotide Thermoelectric Generators will have depleted to the point that they can no longer power Voyager's remaining scientific instruments and transmitters. The spacecraft will fall silent.  

Scientists at the Deep Space Network will track the downlink signal from the spacecraft as it sputters into silence and becomes part of the background noise of the solar system—never to be heard from by humans again.

Additional resources and references used to write this article can be found at:

• https://descanso.jpl.nasa.gov/

• https://deepspace.jpl.nasa.gov/

• https://www.sti.nasa.gov/

*Even forget a software error due to a cosmic ray is a possible explanation for many issues or possible errors.