Weak signals

One of the problems encountered in the reception of signals from the unmanned space vehicles are their low power transmitters in order of 20 watts, corresponding to the inside bulb domestic refrigerator, that is also attenuated progressively with the square of the distance between transmitter and receiver.

When this signal reaches the Earth from, for example, a vehicle in the vicinity of Saturn, the microwave beam has spread over an area whose diameter is equal to 1,000 Earth diameters. Therefore, the energy collected by the complex communication’s antenna is only a tiny fraction of that emitted by the vehicle, mixed with background noise to be added to that produced by each of the elements of the communications channel.

This space-ground link is formed by the onboard vehicle transmitter and its antenna, the region of space between the vehicle and the ground station and the antenna of the ground station with its receiver.

The Noise concept

To focus the discussion on noise we will look at the thermal type one. A metal conductor element, or a resistor, contains an unknown number of free electrons. Due to the thermal motion the free electrons move untidily causing collisions with the atoms and producing an energy exchange. This is the basis of the conductor resistor, so even if no current flows in the resistor, a voltage noise is caused by the erratic movement of electrons.

Effective noise temperature

Experimental results have confirmed that the power due to noise voltage is proportional to the absolute temperature, being the constant of proportionality which has the name of the Austrian physicist Boltzmann.

It thus appears an alternative concept associated to the power of noise, which is the effective noise temperature. Not to be mixed up with the noise that traditionally measures the quality of the microwave systems, and that when it has a value close to the unit, it loses the desired fineness to characterize the contribution of noise of the system itself.

The noise temperature concept can be applied to any source, not necessarily related to a physical temperature. For example, an antenna that captures a galactic noise power, solar noise, etc., may be associated with an effective noise temperature. Equally, this concept applies to noise generated by active elements such as amplifiers.

In very sensitive radio communication systems, such as those in this complex, the concept of noise temperature provides a very useful tool to precisely know those systems’ capabilities.

Remember that in the centigrade scale, or Celsius, zero degrees are equal to 273 degrees in the absolute or Kelvin scale, and that absolute zero is fixed in -273 degrees Celsius.

Low Noise Receivers

When listening the signals from a spacecraft, it is necessary to reduce the contribution of noise of each one of its elements, starting with the antenna and continuing with the RF receivers, currently achieving effective temperatures systems with noise below 20 degrees Kelvin.

A key element in this chain is the receiver set, which is formed by an amplifier mounted in the focal point of the antenna, and a receiver itself installed in control room together with the other electronic equipment.

The amplifier is designed to increase the weak signal captured by the antenna as much as possible, without adding any noise. The obtained amplification ratio is between 100,000 and 700,000 times, and it is when the signal can be transmitted from the antenna to the receivers.

This amplifier consists of a special very low noise device, of around 2 degrees Kelvin, based on the MASER effect (Microwave Amplification by Stimulated Emission of Radiation).

The maser is made up of a synthetic ruby crystal (aluminum oxide with chromium impurities), located in a strong magnetic field and super cooled with liquid helium to near absolute zero. The process requires advanced cryogenic techniques to achieve the temperature of -269 degrees Celsius.

The Doppler Effect

The problem caused by weak signals is normally sorted out using large antennas and receivers with very low noise amplifiers. When this is not enough, the relation signal/noise can be improved combining the signals received by several stations that are simultaneously in contact with the same spacecraft.

However, when communicating with a moving vehicle, a phenomenon known as Doppler Effect appears, The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of an actual change in the frequency of the source. If as in the case of a terrestrial base, the receiver also moves, must take into account the composition of transmitter and receiver speeds. One of the most common examples is that of the pitch of a siren on an ambulance or a fire engine. You may have noticed that as a fast moving siren passes by you, the pitch of the siren abruptly drops in pitch. At first, the siren is coming towards you, when the pitch is higher. After passing you, the siren is going away from you and the pitch is lower. This is a manifestation of the Doppler Effect.

To compensate this Effect in communications with spacecraft, the Jet Propulsion Laboratory, JPL, designed in the early 60’s a receiver equipped with a device called “Phase Locked Loop”, known internationally as PLL by his initials, having become a standard used by the electronics industry.

Thanks to these devices we manage to maintain communication with the vehicle, continuously adapting in an automatic way the signal of the correspondent recipient. The PLL can also evaluate the speed of the spacecraft, and direction of relative movement between transmitter and receiver.