To be useful satellites and spacecraft must communicate, sometimes to relay communications between two points, sometimes to transmit data they have collected. Although there have been some experiments in optical communications using lasers, most satellite communication is accomplished by radio, one part of the electromagnetic spectrum. Radio frequencies must be shared with terrestrial radio services, and international frequency assignment is essential to avoid interference between all the different uses made of the radio spectrum.
The International Telecommunications Union (ITU) is the global body that assigns radio frequency allocations. In doing this they divide the world into three regions, regions I, II and III. Australian lies in region III. The general frequency assignments may be found at the ITU web site, and Australian allocations may be seen at the ACMA web site. The ITU unfortunately, charges horrendous prices for any of its publications. However, the Australian Communications and Media Authority (ACMA) makes all spectral allocation information available for free download. This includes a book, an attractive wall poster and a simplified spectrum graphic.
This note discusses the frequencies that are used for space communications.
THE ELECTROMAGNETIC SPECTRUM
There are four, and only four known forces in the universe (although the so-called dark energy hints at another). These are, in order of strength, the nuclear strong force, the electromagnetic force, the nuclear weak force and the gravitational force. The two nuclear forces exert their influence over only very very short (nuclear) distances, and apart from holding all matter together do not directly influence us in everyday life. It is gravity and particularly electromagnetism that are of direct concern to us in our daily interactions.
Gravity springs from the property of matter we call mass, while electromagnetic effects derive from the property we call charge. When a charge is stationary, it has around it an electrostatic field. If it moves with a constant velocity it produces a magnetic field, and when it accelerates or declerates it generates electromagnetic radiation.
Electromagnetic radiation is a coupled oscillation of electric and magnetic fields that propagates through space with a velocity of about 3 x 108 metres per second. The properties of this electromagnetic radiation vary markedly depending on the frequency of the oscillation. This gives rise to what we know as the electromagnetic spectrum.
The chart below shows the major divisions of the electromagnetic spectrum. An electromagnetic wave may be characterised by its frequency f (the number of times per second the signal undergoes a complete oscillation at a specified point in space) or its wavelength λ (the distance between successive extremal values of the wave at a specified time).
1 For clarity the bands are not shown with uniform frequency or wavelength spacing
2 The visible spectrum occupies only a very small part of the total EM spectrum
3 Bands also have subdivisions (this is particularly true of the radio spectrum)
4 The band divisions are not as sharp as shown, but rather fuzzy, merging into one another
5 In the frequency scale T=1012, P=1015, E=1018
6 In the wavelength scale μ=10-6, n=10-9, p=10-12
THE RADIO SPECTRUM
The radio spectrum is a subset of the electromagnetic spectrum. It extends from frequencies below 1 Hz up to around 3000 GHz or 3 THz, where it gives way to the infrared spectrum. Different frequencies have different uses because of different propagation, generation and general properties. The radio spectrum is divided into many different bands.
This table shows the usually accepted division of the radio spectrum. The left hand column lists the frequency (f), the centre column the band designator and the right column the wavelength (λ). The relationship between frequency and wavelength is given by the expression:
In the above relation, frequency is given in Hertz (Hz) when wavelength is specified in metres (m).
Note that the designated band qualifiers are not in the same order going toward lower frequencies as they are going toward higher frequencies. Also note that the division between ELF and ULF is not universally agreed. It can be placed anywhere from 1 Hz to 100 Hz. The one shown (1 Hz) is that employed by geophysicists.
There is no lower limit to the ULF band and magnetic signals with periods of years can be identified.
Microwave is a term that was historically applied to signals with wavelengths less than one foot (30 cm), and this region has been subdivided into letter bands. However, there are several schemes of designation for microwave bands. Two of these, which we shall call traditional and new, are given below. Despite the efforts of many engineers to have the 'new' division adopted, the 'traditional' scheme seems to be firmly entrenched among space communicators.
WINDOWS TO SPACE
Not all of the electromagnetic spectrum can pass through the Earth's atmosphere. Obviously, visible light can - we can see the stars at night, at least when there is no cloud. However, ultraviolet and higher frequencies are mostly absorbed by different components of the atmosphere.
There are in fact only two main windows of the EM spectrum that are open to space. One is the visible spectrum, as mentioned above, and the other is the radio spectrum. However, not all of the radio spectrum is useable for space communication. The available window spans from about 30 MHz to 30 GHz, although these are not absolute end frequencies.
Below 30 MHz, the ionosphere, at altitudes from around 100 to 500 km, absorbs and reflects signals. Above 30 GHz, the lower atmosphere or troposphere, below 10 km, absorbs radio signals due to oxygen and water vapour. Even between 20 and 30 GHz, there are some absorption bands that must be avoided.
HISTORICAL SPACE FREQUENCIES
The first satellite to orbit the Earth was Sputnik 1, launched by the Soviets in October 1957. It carried two radio beacons on frequencies of 20.005 and 40.01 MHz.
The Soviets continued to use frequencies around 20 MHz and even some around 15 MHz for many subsequent missions.
The first satellite launched by the USA (Explorer 1) carried beacons on 108.00 and 108.03 MHz. This lay just above the terrestrial FM broadcast band (from 88 to 108 MHz) and just inside the civil aviation band which extends from 108 to 136 MHz. This frequency had been specified by an international committee for the International Geophysical Year (IGY - 1957/8) as the one to be used for all scientific satellites launched in pursuit of IGY objectives. The Soviets had chosen to ignore this recommendation and use the much lower frequencies previously mentioned.
SPACE COMMUNICATION BANDS
The following is a list of some of the more heavily used frequency bands for space communication. Specific frequencies may be found in the links provided at the end of this note.
SPECIFIC SPACE COMMUNICATION FREQUENCIES
|Band||Uplink Frequency (MHz)||Downlink Frequency (MHz)|
|S||2110 - 2120||2290 - 2300|
|X||7145 - 7190||8400 -8450|
|Ka||34200 - 34700||31800 - 32300|
The earliest DSN spacecraft used S-band (1960s), then in the 1990s moved to X-band, and Ka-band started to be used in the 21st century. Many spacecraft have dual frequency capability (S/X and lately X/Ka).
The use of higher frequencies allows larger bandwidths, better tracking capability and minimises ionospheric effects. It also requires greater pointing accuracy.
The links here provide detailed information about specific frequencies of various types of satellites.
COMMERCIAL SATELLITE COMMUNICATIONS (BROADCAST)
Australian Space Academy