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2 JPL D-9774A Recap. When a... current is applied to a wire the current flow builds an field around the wire propagating a wave outward If the applied current were made to alternate with a uniform period of time, a series of waves will be propagated at a discrete the amount of electromagnetic energy passing through a unit area decreases with the of the distance from the source.. electromagnetic 2. frequency 3. square Electromagnetic Spectrum Light is electromagnetic radiation at those frequencies that can be sensed by the human eye. But the electromagnetic spectrum has a much broader range of frequencies than the human eye can detect, including, in order of increasing frequency: radio frequency (RF), infrared (IR, meaning below red ), light, ultraviolet (UV, meaning above violet ), X rays, and gamma rays. These designations describe only different frequencies of the same phenomenon: electromagnetic radiation. The speed of light in a vacuum, 299,792 km per second, is the rate of propagation of all electromagnetic waves. The wavelength of a single oscillation of electro-magnetic radiation is the distance that the wave will propagate during the time required for one oscillation. There is a simple relationship between the frequency and wavelength of electromagnetic energy. Since electromagnetic energy is propagated at the speed of light, the wavelength equals the speed of light divided by the frequency of oscillation. Wavelength = Frequency of Oscillation = Speed of Light Frequency of Oscillation Speed of Light Wavelength 44 THE ENVIRONMENT OF SPACE

3 MOPS The Electromagnetic Spectrum: Wavelength/frequency chart Examples Frequency 3 kilohertz ( 3 Hz) Wavelength 0 kilometers ( 3 m) Long Radio Waves 30 kilohertz ( 3 Hz) Lower Frequencies Football Field Human AM Short Waves FM & TV Radar S-band X-band 300 kilohertz ( 3 Hz) 3 Megahertz ( 6 Hz) 30 Megahertz ( 6 Hz) 300 Megahertz ( 6 Hz) 3 Gigahertz ( 9 Hz) 30 Gigahertz ( 9 Hz) Grains of Sand 300 Gigahertz ( 9 Hz) Infrared Radiation 3 Terahertz ( 2 Hz) Bacterium Radio Frequencies (RF) 0 Meters 0 millimeters ( -3 m) 0 micrometers ( -6 m) Longer Wavelengths Visible Light 30 Terahertz ( 2 Hz) 300 Terahertz ( 2 Hz) Virus Ultraviolet Radiation 3 Petahertz ( 5 Hz) 30 Petahertz ( 5 Hz) 0 nanometers ( -9 m) Higher Frequencies Atoms X-rays Gamma Rays Atomic Nucleus 300 Petahertz ( 5 Hz) 3 Exahertz ( 8 Hz) 30 Exahertz ( 8 Hz) 300 Exahertz ( 8 Hz) 3 X 2 Hz 0 picometers ( -2 m) 0 femtometers ( -5 m) Shorter Wavelengths 30 X 2 Hz 300 X 2 Hz 3 X 24 Hz 0 attometers ( -8 m) 30 X 24 Hz 300 X 24 Hz 45

5 MOPS Range of Wavelengths Frequency Band (cm) (GHz) L S C X K Note: Band definitions vary slightly among different sources. These are ballpark values. Spectroscopy The study of the production, measurement, and interpretation of electromagnetic spectra is known as spectroscopy. This branch of science pertains to space exploration in many different ways. It can provide such diverse information as the chemical composition of an object, the speed of an object s travel, its temperature, and more information that cannot be gleaned from photographs. Today, spectroscopy deals with closely viewed sections of the electromagnetic spectrum. For purposes of introduction, imagine sunlight passing through a glass prism, casting a bright rainbow (spectrum) onto a piece of paper. Imagine bands of color going from red on the left to violet on the right. Frequencies of the light increase from left to right; the wavelengths decrease. Each band of color is actually a wide range of individual frequencies which cannot be discerned by the human eye, but which are detectable by instruments. Now, instead of the green band near the middle, imagine a dark band, like a shadow at that point, as if something had absorbed all the green out of the sunlight. Call this a dark line. What is spoken of as a bright line could be imagined as an excessively bright color band, say for example if the green were several times brighter than it normally would appear, as though the sunlight were somehow augmented at this band. In actuality, the bright and dark lines spoken of in spectroscopy are extremely narrow, representing only a very specific shade of color, one discreet frequency at a time. In principle, a bright line represents the emission of radiation at a particular frequency, and a dark line indicates the absorption of a frequency otherwise expected to be seen at that point. In 859, Gustav Kirchhoff ( ) described spectroscopy in terms of the three laws of spectral analysis: I. A luminous (glowing) solid or liquid emits light of all wavelengths (white light), thus producing a continuous spectrum. II. III. A rarefied luminous gas emits light whose spectrum shows bright lines (indicating light at specific wavelengths), and sometimes a faint superimposed continuous spectrum. If the white light from a luminous source is passed through a gas, the gas may absorb certain wavelengths from the continuous spectrum so that those wavelengths will be missing or diminished in its spectrum, thus producing dark lines. 47

6 JPL D-9774A By studying the bright and dark lines (emission and absorption lines) in the spectra of stars, in the spectra of sunlight reflected off planetary surfaces, or of starlight passing through planetary atmospheres, much can be learned about the composition of these bodies. Historically, spectral observations have taken the form of photographic prints showing spectral bands, in which light and dark lines can be discerned. Modern spectrometers (discussed again under Chapter 2, Typical Science Instruments) have largely done away with the photographic process, producing their high-resolution results in the form of X-Y graphic plots, whose peaks and valleys reveal intensity on the vertical axis versus frequency or wavelength along the horizontal. Peaks of high intensity in this scheme represent bright spectral lines, and troughs of low intensity represent dark lines. Atmospheric Transparency Because of the absorption phenomena, observations are impossible at certain wavelengths from the surface of Earth, since they are absorbed by Earth s atmosphere. There are a few windows in its absorption characteristics which make it possible to see visible light, and receive radio frequencies, for example, but the atmosphere presents an opaque barrier to much of the electromagnetic spectrum. Even though the atmosphere is transparent at X-band frequencies as seen above, there is a problem when liquid or solid water is present. Water exhibits noise at X-band frequencies, so precipitation at a receiving site increases the system noise temperature, and this can drive the SNR too low to permit communications reception. 48 THE ENVIRONMENT OF SPACE

7 MOPS Atmospheric Windows to Electromagnetic Radiation Frequency Wavelength 3 Kilohertz (3 Hz). 0 km 30 Kilohertz (3 ) 300 Kilohertz (3 ) 3 Megahertz (6 ) 30 Megahertz (6 ) 300 Megahertz (6 ) 3 Gigahertz (9 ) 30 Gigahertz (9 ) Radio Frequencies (RF) km km 0 m m m 0 mm mm IONOSPHERE OPAQUE Radio Window 300 Gigahertz (9 ) 3 Terahertz (2 ) 30 Terahertz (2 ) 300 Terahertz (2 ) 3 Petahertz (5 ) 30 Petahertz (5 ) mm 0 µm µm µm 0 nm nm Infrared Windows Visible Light Window 300 Petahertz (5 ) 3 Exahertz (8 ) 30 Exahertz (8 ) 300 Exahertz (8 ) 3 x 2 30 x 2 nm 0 pm pm pm 0 fm fm ATMOSPHERE OPAQUE Absorption by Interstellar Gas 300 x 2 fm 3 x 24 0 am 30 x 24 am 300 x 24 am 49

11 MOPS Incoming Electromagnetic Waves Prime Focus Antenna Receiver feed horn Prime Focus Cassegrain Focus Antenna Incoming Electromagnetic Secondary Waves Reflecting Surface (Hyperbolic Section) Reflecting Surface (Parabolic Section) Cassegrain Focus Receiver feed horn Primary Reflecting Surface (Parabolic Section) thus affecting calibration. A solution is the Cassegrain Focus arrangement. Cassegrain antennas add a secondary reflecting surface to fold the electromagnetic waves back to a prime focus near the primary reflector. The DSN s antennas are of this design because it accommodates large apertures and is structurally strong, allowing bulky equipment to be located nearer the structure s center of gravity. The reflective properties of electromagnetic waves have also been used to investigate the planets using a technique called planetary radar. With this technique, electromagnetic waves are transmitted to the planet, where they reflect off the surface of the planet and are received at one or more Earth receiving stations. Using very sophisticated signal processing techniques, the receiving stations dissect and analyze the signal in terms of time, amplitude, phase, and frequency. JPL s application of this radar technique, called Goldstone Solar System Radar (GSSR), has been used to develop images of the surface features of Venus, eternally covered with clouds, Mercury, difficult to see in the glare of the sun, and some of the satellites of the Jovian planets. 53

12 JPL D-9774A Recap. Radio frequency electromagnetic radiation generally travels through space in a. 2. The angle of reflectance of RF waves equals their angle of. 3. DSN s antennas are of the Cassegrain design because it accommodates large apertures, and is structurally allowing bulky equipment to be located nearer the structure s center of gravity. 4. JPL s application of this radar technique, called (GSSR), has been used to develop images of the surface features of Venus.. straight line 2. incidence 3. strong 4. Goldstone Solar System Radar Refraction Refraction is the deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another. The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the substance of the observed medium. The law of refraction states that electromagnetic waves passing from one medium into another (of a differing index of refraction) will be bent in their direction of travel. Air and glass have different indices of refraction. Therefore, the path of electromagnetic waves moving from air to glass at an angle will be bent toward the perpendicular as they travel into the glass. Likewise, the path will be bent to the same extent away from the perpendicular when they exit the other side of glass. Air-glass-air Refraction Air Path of Electromagnetic Waves Glass Perpendicular to Pane of Glass Perpendicular to Pane of Glass Air Path of Electromagnetic Waves 54 THE ENVIRONMENT OF SPACE

13 MOPS In a similar manner, electromagnetic waves entering Earth s atmosphere from space are slightly bent by refraction. Atmospheric refraction is greatest for signals near the horizon, and cause the apparent altitude of the signal to be on the order of half a degree higher than the true height. As Earth rotates and the object gains altitude, the refraction effect reduces, becoming zero at zenith (directly overhead). Refraction s effect on the sun adds about 5 minutes to the daylight at equatorial latitudes, since it appears higher in the sky than it actually is. Refraction in the Earth's Atmosphere Note: Angles have been greatly exaggerated to emphasize the effect Apparent Direction to Source Observation Point (Receiver) Electromagnetic Waves from Source True Direction tosource Earth If the signal from a spacecraft goes through the atmosphere of another planet, the signals leaving the spacecraft will be slightly bent by the atmosphere of that planet. This bending will cause occultation (spacecraft moving into a planet s RF shadow of Earth) to occur later than otherwise expected, and exit from occultation will occur prior to when otherwise expected. Ground processing of the received signals reveals the extent of atmospheric bending (and also of absorption at specific frequencies and other modifications), and provides a basis for inferring the composition and structure of a planet s atmosphere. Phase As applied to waves of electromagnetic radiation, phase is the relative measure of the alignment of two waveforms of similar frequency. They are said to be in phase if the peaks and troughs of the two waves match up with each other in time. They are said to be out of phase to the extent that they do not match up. Phase is expressed in degrees from 0 to 360. Waves in Phase 80 Out of Phase 20 Out of Phase 55

14 JPL D-9774A Recap. Refraction is the of electromagnetic waves when they pass from one kind of transparent medium into another. 2. The path of electromagnetic waves moving from air to glass at an angle will be bent toward the as they travel through the glass. 3. If the signal from a spacecraft goes through the atmosphere of another planet, the signals leaving the spacecraft will be slightly by the atmosphere of that planet. 4. Waves are said to be in if their peaks and troughs match up.. deflection or bending 2. perpendicular 3. bent 4. phase 56 THE ENVIRONMENT OF SPACE

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