|MadSci Network: Physics|
Greetings: Reference: 1 W. L. Morgan, G. D. Gordon, “Communications Satellite Handbook”
John Wiley & Sons, 1989
Jet Propulsion Laboratory (JPL) Mission and Spacecraft Library
Space communications systems are highly dependent on the frequency (and wavelength) used for communication. The lower the frequency (the longer the wavelength) the greater will be the impact of man-made interference and solar and galactic noise on the system. At lower frequencies the ionosphere also reflects more transmitter power back toward the earth. Also, antennas will become larger the as the wavelength increases.
The higher the frequency (the shorter the wavelength), the greater will be
impact of rain, snow and hale, on the system and the greater will be the
absorbed by atmospheric gasses. However, the size of the antennas will
smaller as the frequency increases.
It turns out that the microwave frequencies between 1 GHz and about 15 GHz are the optimum region in the electromagnetic spectrum for earth to space communications for they are the least affected by the opposing factors of interfering noise and ionospheric reflections on the lower frequency end and weather and absorption by atmospheric gasses atmospheric on the higher frequency end. These microwave frequencies are in great demand for communications, entertainment, education, radar, navigation and defense related applications.
Let me begin to answer your questions by presenting some background information on frequency bands and orbits. For simplicity I will use general numbers not exact values in this discussion.
Four frequency regions of the electromagnetic wave spectrum are currently being used for space to space and ground to space communications. These are :
VHF (Very High frequencies) have metric waves ranging from 30 Megahertz ( 10 meter wavelength) to 300 Megahertz (1 meter wavelength) Note one Megahertz is one million cycles (waves) per second.
UHF (Ultra high frequencies) have decimetric waves ranging from 300 Megahertz (1 meter wavelength) to 3 Gigahertz (10 centimeter (cm) wavelength). Note: One Gigahertz is one thousand million or one billion cycles per second.
SHF (Super high frequencies) have centimetric waves ranging from 3 Gigahertz (10cm wavelength) to 30 Gigahertz (1 cm wavelength).
EHF (Extremely high frequencies) have millimetric waves ranging from 30 Gigahertz (1 cm or 10 millimeter (mm) wavelength) to 300 Gigahertz (1 mm wavelength).
Microwaves are considered to begin at about 1 Gigahertz (30 cm wavelength) in the UHF band and extend through the SHF band to 30 Gigahertz (1 cm wavelength).
Since World War II the microwave spectrum has also been divided into lettered bands and many satellite dish antenna manufacturers use these band designations for their products. Different groups use slightly different definitions for the bands; however, the following are some of the more common definitions:
L-band --390 MHz to1.05 GHz
S-band --1.05 GHz to 3.9 GHz
C-band--3.9 GHz to 6.2 GHz
X-band--6.2 GHz to 10.9 GHz
Ku-band-- 12.4 to 18 GHz
K-band--18 GHz to 26.5 GHz
Ka-band--26.5 to 40 GHz
The majority of space communications systems are in the upper UHF band which is called the L-band and the lower SHF band (1 to 14 GHz) using S, C, X and Ku bands. Ku and Ka bands have problems with heavy rain and K- band is not used because of power absorption by a resonance in water vapor molecules in the atmosphere. Because the electromagnetic spectrum is becoming over crowded with traffic in the microwave bands, new space systems are moving to millimeter waves in the EHF region using Ka band and higher frequency bands requiring excess transmitter power to overcome weather related attenuation of the signals. Systems designers are also considering laser communications links, particularly for space to space and deep space communications where weather and atmospheric absorption are not a problem. However, there have been some laser communications experiments using ground stations in the desert and on high mountain peaks
Distance to Orbits
Low earth orbits (LEO) range in altitude above the earth’s surface from about 160 km (100 miles) to 800km (500 miles) and the time to orbit the earth is about 90 minutes. LEO orbits are generally used for weather and photographic satellites and were also used for the bankrupt Iridium satellite communication system, the Space Shuttle, the Hubble Space Telescope (HST) and many more.
Medium earth orbits (MEO) are at an altitude of about 20,000 km (12,500 miles) and orbit the earth in 12 hours. MEO orbits are being used by the Global Positioning System (GPS) navigational satellites and are being used for the new ICO communications system. The Van Allen radiation belt extends from 6400 km (4000 miles) to 11,000 km (6800 miles). The Van Allen belts do not directly effect communication signals; however, MEO orbiting spacecraft encounter higher rates of degradation of their solar cell arrays from this radiation. Also, extra metal shielding must be used to protect the electronic devices from radiation damage. .
Geostationary earth orbits (GEO) are at an altitude of about 36,000 km (22,400 miles) and orbit the earth in 24 hours. If the orbit is in the plane of the equator and the velocity of the spacecraft is in the direction of rotation of the earth, the spacecraft stays above a specific longitude on the equator. There are about 150 operational commercial communications satellites in GEO orbits surrounding the earth and many military spacecraft. These satellites are the largest and most expensive being manufactured and they are the backbone for commercial and military space communications systems today. GEO spacecraft are used for international television, telephone, Internet traffic, business systems, direct broadcast television, defense early warning satellites, NASA’s Tracking and Data Relay Satellite System (TDRSS), MILSTAR, pagers, credit card verification and many more applications.
Antennas, Transmitters and Receivers
The key parameters for space communications are the transmitted Equivalent Isotropically Radiated Power (EIRP) and for receivers the antenna area divided by the system noise temperature in degrees Kelvin (Ts).
Transmitters and Transmitting Antenna For the first 20 years of space communication the prime power developed by silicon solar cell arrays, which are about 11 percent efficient in converting solar power to electrical power, limited spacecraft transmitters to about 10 watts of microwave power and the size of the booster rocket fairing limited antennas to dimensions between one and two meters in diameter. In the 1990s, with the development of large GEO spacecraft using large arrays of gallium arsenide solar cells, which are 22 to 32 percent efficient, and unfurling parabolic antennas up to 10 meters (30 feet) in diameter, transmitter powers have increased to over 4000 watts in direct to the home television systems.
An isotropic antenna transmits and receives energy equally in all directions. A frosted lightbulb is a close approximation of an isotropic light source. All spacecraft have isotropic (low gain) antennas for use when the spacecraft is tumbling before the high gain antennas are deployed or when some unexpected system failure causes the spacecraft to loose communication with the high gain antennas. Most spacecraft have one or more high gain antennas that concentrate the transmitted energy into beams, just as an automobile headlamp concentrates the energy from an isotropic radiating bulb into a beam of light. The larger the area of the antenna, the smaller will be the angle of the beamwidth and the greater will be the gain of the antenna. Also the greater the area of an antenna the greater will be the energy captured for the receiver. Depending on the type of antenna, the gain, beamwidth and area are interrelated.
Any spherical surface has an area of 41253 square degrees. We can obtain a close estimation of antenna gain by dividing the area of the beam that we want into the area of the sphere. For example, from GEO orbit the earth has a width of 17.4 degrees or 238 square degrees ( Area = pi times diameter squared divided by 4). Thus to focus our transmitter power to cover only the earth need an antenna gain (G) of 41253/238 = 173.From GEO orbit the continental USA is covered by 17 square degrees so that the antenna gain to only cover the USA would be G = 41253/17 =2427.
The EIRP is equal to the transmitter power times the transmitter antenna gain. Thus if we use a 10 watt transmitter and an antenna gain of 2427 the EIRP is 24,270 watts. What this means is that an isotropic antenna would need to transmit 24,270 watts to deliver the same power density (watts per square meter) to the surface of the USA as my 10 watts produce when focused with an antenna gain of 2427. The only drawback is that I must keep the beam of my high gain antenna precisely pointed at the USA at all times. I can use even higher gain antennas to focus on a single city of state which will require even greater antenna pointing accuracy.
The gain and beamwidth of an antenna are determined by the area of the antenna in square wavelengths. Thus for the same beamwidth and gain, an antenna operating at 1 GHz (30 cm wavelength) will have dimensions 10 times greater than an antenna operating at 10 GHz (3 cm wavelength). The higher the operating frequency (the smaller the wavelength) the smaller will be the antennas for a specific application. The EIRP for many different satellites is given in the referenced Spacecraft Library.
A dish antenna with a circular parabolic reflector having a one degree circular beam width will need to be 70 wavelengths in diameter and will have a gain of 31600. A 10 degree beam antenna will be 7 wavelengths in diameter and have a gain of 178 while and antenna with a 0.1 degree beam will be 700 wavelengths in diameter and have a gain of 998 million!
Receiving Antennas and Microwave Receivers
For receiving energy we want as big an area antenna as possible to gather as much of the extremely small amount of power passing by the spacecraft and focus the power into the receiver electronics. From the transmitter antenna discussion above we no that the larger the area of an antenna, the greater will be the gain and the smaller will be the beamwidth, so that we must also keep the receiving antenna precisely pointed toward the transmitter to gather the available power. From the beamwidth we can calculate the receiver antenna gain as we did for the transmitter antenna. Sometimes the same antenna will be used for both transmitting and receiving. Deep space probes often use single antennas.
All mater at temperatures above absolute zero generate radio frequency noise which is caused by the thermal agitation of the atoms in the material. Temperature is caused by the agitation of rubbing atoms. The greater the temperature the greater will be the noise power. This includes the atoms in the receiver electronics, the atoms in the atmosphere, the atoms in the antenna and even the atoms in the sun which is a huge noise source. We can hear this thermal noise as the hissing of our radio speakers at full volume when we are not tuned to a station or view it as the snow on a television picture tube when the is no TV signal present. Usually the majority of this noise is caused by the receiver electronics. This noise power can be measured and specified as a system noise temperature (Ts).
Noise can also enter the receiver from external sources such as the sun or lightning strikes or man-made interference. These noise powers all add in the receiver and they must be overcome by the received signal. We call this the Signal to Noise ratio (S/N) of the receiver. Typical values of signal power needed to over come receiver nose are 10 to 100 times the noise power( S/N = 10 to S/N=100). The greater S/N the higher will be the quality of the signal and the lower will be the errors in received digital data. An S/N=100 will have a bit error rate (BER) of better than one error bit in a million. Error correction coding can reduce this to as low as one error in a billion at the expense of lower data rates. Receivers can be cooled to improve their noise temperature. The receivers in ground stations of NASA’s Deep Space Tracking System are cooled with liquid helium to 2 degrees above absolute zero and the antennas are the size of football fields. Even with this huge area and very small Ts, the S/N ratio for signals from deep space are much less than one.
The record distance for Earth to space communications has been carried out to well beyond the planet Pluto as the Voyager spacecraft leave the solar system. The Voyagers have enough electrical power and thruster fuel to operate at least until 2020. By that time, Voyager 1 will be 12.4 billion miles from the Sun and Voyager 2 will be 10.5 billion miles away. ( http://www.jpl.nasa.gov /voyager/inter.html).
Elecromagnetic energy from all antenna spreads in a spherical wavefront and thus the power density in watts per square meter decreases as the square of the distance traveled. This is true for all antenna including isotropic and high gain antennas. For example assume we have a one watt isotropic source. At a distance of 100 meters a sphere has an area of 4 times Pi Times 100 squared = 125663 square meters. So the power density is 1 watt divided by 125663= 8 microwatts per square meter. The the distance from the equator to GEO is 35786 km. Calculating the power density for our one watt source at GEO orbit gives about one tenth of one billionth (1x 10 to the power -10) of a watt per square meter! To overcome this huge loss in power the direct broadcast satellites (DBS) in the U.S.A. transmit with an EIRP of 250,000 watts so that we can receive digital TV signals in our homes with a 45 cm (18 inch) diameter dish antenna using a room temperature receiver with a G/Ts of about 13
Atmospheric and weather attenuation of microwave signals The power in radio waves below 300 MHz (1 meter) are mostly reflected back toward earth from the ionosphere. The longer the wavelength the greater the reflection. These waves can also reflect back and forth several times between the ground and the ionosphere. This is why I can listen to Radio Australia each evening here in California on my Sony portable receiver operating at 6MHz and 10 MHz! However, not much power from Radio Australia would reach a spacecraft traveling above the atmosphere. Above 300 MHz radiowaves and microwaves pass through the ionosphere. We call these line-of-sight frequencies because on earth they can only be used as far as the horizon and then they pass on into space.
The gasses in the atmosphere and weather attenuate (reduce) the radio frequency power. The greater the frequency, the greater the power lost to absorption and scattering. Resonances in water vapor molecules greatly absorb microwave power between 20GHz and 28 GHz and and around 180 GHz. Resonances in the oxygen molecule greatly absorb millimeter waves around 60 GHz and 120 GHz. These frequencies would not be used for communications between earth and space.
Rainfall attenuates microwave signals at frequencies above 10 GHz. The heavier the rain fall the greater the signal loss and the greater the frequency above 10 GHz the greater the signal loss for a given rain fall rate.
Percentage of power lost passing through one kilometer of atmosphere
Column 1 Frequency
Column 2 Clear Atmosphere
Column 3 Light Rain
Column 4 Moderate Rain
Column 5 Heavy Rain
20 GHz---2----0.5 --69---99
Solar storms bombard the earth’s atmosphere increasing the density of ions in the ionosphere. This increases the absorption and reflection of electromagnetic signals entering the ionosphere and the effects often extend into the microwave region causing signal fades. Depending on the BER required for the link these fades could be a loss of a TV picture for a few minutes or a loss of data for a few minutes which might be alarming. Of coarse if the fade was during a championship football game the fans would be very unhappy. Even during calm solar activity when a spacecraft can eclipse the sun so that solar radiation directly enters the antenna beam, solar noise can overwhelm the microwave signal. GEO spacecraft encounter this twice a year and many of the network TV signals experience brief fades for several minutes each day as the spacecraft passes the sun
Best regards, Your Mad Scientist
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