Communicating with the Voyager Spacecraft
- 25 Apr 2023
- Start time
- 7:00 PM
- The Yorkshire Museum
- Professor Andy Marvin, University of York
Communicating with the Voyager Spacecraft
Professor Emeritus Andy Marvin, University of York
The two Voyager spacecraft were designed and launched back in the seventies when I was an engineering student. They use the hardware technology that I was taught about at the time. That they are still operational nearly fifty years later is nothing short of amazing. In the early days we saw spectacular colour images of the outer planets. More recently the cameras have been turned off partly to conserve power and partly because even the outer planets are now so far away from the spacecraft that they just show up as dots. Each of the spacecraft is a vast distance from earth, yet we can still communicate with them, receiving data about the local space environment at the edge of the solar system.
How on earth is this possible?
In this lecture I will show how it is possible. There will be some very very big numbers and there will be some very very small numbers as I explain how it is done and run through the calculations to show how it all works.
I’ll do the sums, all you have to do is wonder!
Lecture to be held in the Tempest Anderson Lecture Theatre, Yorkshire Museum,
YO1 7DR at 7pm
Joint lecture with the “Institute of Physics”.
The two Voyager spacecraft were launched in 1977, with the primary objective of studying the outer solar system including Jupiter, Saturn, Uranus and Neptune. This was then extended to include the outer limits of the Sun’s sphere of influence, and possibly beyond. The mission has been incredibly successful, with both craft entering interstellar space (in 2012 and 2018 respectively), and now transmitting data to earth at distances greater than 20 billion kilometres.
Professor Marvin made creative use of relatively simple mathematical and scientific formulae to illustrate and quantify the technical challenges of communicating between the spacecraft and earth. Perhaps the greatest constraint is that the power available on the spacecraft for each transmitter is only 18W. A parabolic high gain antenna focusses the microwave radio signal into a narrow beam pointed at the earth, and the largest realistic diameter (3.7 metres) of the antenna used to limit the beam divergence to 0.75 degrees. Directional orientation of the antenna is achieved by on-board gyroscopes or reaction wheels, with fine tuning by signals from earth. However, even when Voyager was near Jupiter, the beam divergence led to a beam 11 million kilometres across by the time it reached earth. There are three earth stations, located in California, Spain, and Australia, to ensure contact around the clock. Each station has a 70 meter diameter parabolic reflector antennae that can both receive and send signals. Clearly, the signal intensity on earth is extremely low, with the antennae collecting only 2.84 x 10 ‑16 Watts from the Jupiter location – and 1000 times less from the current location.
The data from the many scientific instruments on the spacecraft is first recorded on magnetic tape, and later transmitted in digital form as a string of ones and zeros, or bits. For example, a colour photograph from the 1975 era primitive cameras requires about 15 million bits. The bit stream is transmitted by a radio wave using a rather complex phase shift keying system. At the earth station, the original bit stream is reconstituted from the radio waves and then decoded to give the original data.
A further major challenge is that the rate at which data can be transmitted is limited by the ability to extract the data from the background ‘noise’. Noise is caused by the random agitation of charge carriers associated with thermal energy in the system. It is present in all physical systems – an everyday example is the background hiss in a radio – and is proportional to the absolute temperature. The most important equation of the lecture was the Shannon/ Hartley Law, which demonstrates the need to minimise noise levels-
Channel Capacity=Bandwidth x log2 (1+Signal Power/Noise Power) bits/second
The effect of various atmospheric and extra-terrestrial noise sources including noise from the microwave background radiation (a remnant of the Big Bang and at a temperature of 2.7 deg K) is minimised by transmitting data at the optimum frequency of 8 to 9 GHz. The larger effect of noise from the receiving equipment is minimised by cooling it with liquid helium at 4.2 deg K. In practice, the effective overall temperature is 20 deg K, and the current maximum possible transmission rate at the spacecraft’s present distance is about 1090 bits per second. However, since Voyager stopped transmitting images and in order to save power from the nuclear batteries, the actual transmission rate has been dropped to 160 bits/second or less.