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Chapter 4 Ionospheric Radio Propagations

Chapter 4 Ionospheric Radio Propagations. 4.1-Speaker 徐明慶 4.2-Speaker 文允晟 4.3-Speaker 黃俊雁 4.4 & Conclusion-Speaker 洪 佩綺. Outline. 4.1 Ionospheric Observations and HF Radio Propagation 4.2 Transionospheric Propagation and Ionospheric Total Electron Content (TEC)

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Chapter 4 Ionospheric Radio Propagations

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  1. Chapter 4Ionospheric Radio Propagations 4.1-Speaker 徐明慶 4.2-Speaker 文允晟 4.3-Speaker 黃俊雁 4.4 & Conclusion-Speaker 洪佩綺

  2. Outline • 4.1 Ionospheric Observations and HF Radio Propagation • 4.2 Transionospheric Propagation and Ionospheric Total Electron Content (TEC) • 4.3 Ionospheric Irregularities and Radio Wave Propagation • 4.4 Ionospheric Storms • 4.5 Conclusion

  3. 4.1Ionospheric Observations and HF Radio Propagation-Speaker 徐明慶 • 4.1.1 Radio Wave Propagation and Ionosonde Techniques • The Ionogram • n(h) Reduction • Top side Sounder • 4.1.2 Shortwave Communications

  4. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • Ionosonde：One of the traditional techniques to monitor the ionosphere. It continually sweeps 1~20MHz band frequencies from ionosphere. Radar receive

  5. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • The half value of the echo delay time () multiplied by the speed of light () will get a ideal reflection height, referred as “Virtual height ()”. • Virtual height will be higher than the actual altitude of reflection . • The relationship between and is given by：

  6. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • is smaller than c cause by group delay. • The relationship between and refractive index is given by： • The electromagnetic wave propagation in a cold magnetized plasma has the refractive index .

  7. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • is given by the Appleton-Hartree equation [Maeda, 1970]：

  8. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • In eq.(4.3), the double sign corresponds to the right-hand and left-hand circularly polarized radio waves. • The gyrofrequencies of ions are to times smaller than electrons, thus, ions can be considered as stationary, and their effects are omitted from eq.(4.3). • Assuming that the ionosonde transmits a pulse vertically, we have:

  9. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • in vacuum is 1 and becomes smaller with increases in electron density. • When , the radio wave is reflected back. • To simplifymatters, we omit B, collision of particles, and assume that the sounding radio wave is perpendicular to the ground. • If the double sign in Eq.(4.3) is positive (O-mode, ordinary mode), eq.(4.3) becomes: , for and , Hz (using the values from Table 3.4) • It means that the wave, which is right-hand circularly polarized, is reflected back at the altitude where sounding frequency and electron plasma frequency matches.

  10. If the wave is left-hand circularly polarized (the double sign in eq.(4.3) is negative, X-mode, extraordinary mode), eq.(4.3) becomes: Assuming no collision of particles and the wave propagation vector is perpendicular to the magnetic field line ( • ,

  11. 4.1.1 Radio Wave Propagation and Ionosonde Techniques • For X=1-Y, using eq.(4.4) and eq.(4.5), we can get: • , #

  12. 4.1.1 Radio Wave Propagation and Ionosonde Techniques - The Ionogram • The maximum frequency of an echo corresponds to the maximum electron density and it is referred as critical frequency. • Any signal with a frequency exceeding critical frequency will penetrate the ionosphere. • The electron density of D-region is too low that the ionosonde couldn’t receive echo.

  13. fxF2 foF2 fxF1 foF1 h’F2 h’F1 E-region

  14. 4.1.1 Radio Wave Propagation and Ionosonde Techniques - n(h) Reduction • In order to determine the real altitude of maximum electron density and intermediate heights of densities, a method of iterative calculations will be required. • To simplify matters, there are some conditions of ionosphere that must be complied with. • Ionosphere is layered structure. • The electron density function in each layer is simple. • Considering only the ordinary wave (O-mode). • The boundary condition of each layer is plasma frequency.

  15. 4.1.1 Radio Wave Propagation and Ionosonde Techniques - n(h) Reduction • If the actual thickness of the layers below the layer we are considering is known, then for a sounding pulse reflected at the upper boundary of the layer, the delay time of the radio wave propagated through the underlying layers can be calculated. • The propagation delay time through another added layer whose thickness is unknown can be determined. Thus, layer thicknesses can be determined by assuming the functional form of the density variation within the last layer. • Carrying this calculation through all subdivided layers yields the true ionospheric height profile for the full observation range of the ionogram.

  16. 4.1.1 Radio Wave Propagation and Ionosonde Techniques - Topside Sounder • Cause ionosonde has two major drawbacks: • Information is only obtained below the height of maximum electron density. • The information is restricted to the regions above the observation facility.

  17. 4.1.1 Radio Wave Propagation and Ionosonde Techniques - Topside Sounder When the sounding pulse is transmitted from within the plasma, in addition to the electromagnetic waves, we also have a condition of electrostatic waves in an excited state. These waves travel at a slower group velocity, when the group velocity and the velocity of satellite are nearly equal, the satellite will detect a standing wave, which shows up as the sharp wedges seen in Fig.4.2.

  18. 4.1.2 Shortwave Communications Reflection point

  19. 4.1.2 Shortwave Communications • With respect to shortwave, the change in refractive index within the ionosphere is very gradual as compared to the wavelength of radio wave, and this allows the radio propagation path to be described as a geometrical optics approximation. • Snell’s law gives: for , • To simplify matters, we ignore the effects of the magnetic field.

  20. eq.(4.3) become: for for #

  21. 4.2 Transionospheric Propagation and Ionospheric Total Electron Content (TEC)-Speaker 文允晟 • 4.2.1 Group Delay • MeansuringTotal Electron Content(TEC) • 4.2.2 Doppler Shift • Differential Doppler Technique • Ionospheric Tomography • 4.2.3 Faraday Rotation • TEC by Faraday Rotation Technique

  22. In cases where the radio waves propagated in the ionosphere have a frequency much higher than oF2, we can simplify the refractive index equation (eq.(4.3)). In that equation, if we assume = 100MHz,N= 10MHz,and = 1MHz, then X = 0.01 and Y = 0.01, so X in the denominator under the radical sign can be omitted. (4.14)

  23. In practice, generality is preserved if we except the case in which propagation direction is strictly perpendicular to the geomagnetic field,and assume progapation is quasi-parallel to the magnetic lines of force, or Y2YT4/4. Equation (4.3) thus can be written as (4.15)

  24. Since X 1 and YL1, we can approximate as 1 - X(1) (4.16)

  25. When we can ignore the effects of the geomagnetic field, eq.(4.16) can be further simplified to 1 - X = 1 - (4.17)

  26. 4.2.1 Group delay • When the radio signal transmitted by the altimeter passes through the ionosphere, ionospheric effects cause a propagation delay that results in an error in the measurement. Using the approximation for refractive index (eq.(4.17)), group refractive index from eq.(4.2) is given by ’ = (4.2) ’ = 1 + (4.18)

  27. Consequently, the amount of group delay imposed on a radio signal of frequency is calculated from: • NT • b = (4.20) • NT = (4.21)

  28. Measuring Total Electron Content(TEC) • 1.Group delay values can only be measured using beacon signals with two or more frequencies. • 2.The ATS-6 satellite transmit 40-,140,and360-MHz beacon signals modulated by a 1-MHz or a 0.1-MHz signal for ionospheric observations, and group delay is determined from the phase differences detected in the modulating signal of two the carrier combinations. • 3.The GPS satellites generate a 10.23-MHz pseudorandom noise (PRN) code to modulate the carriers of the L1 signal (f1 = 1575.42MHz) and the L2 signal (f2 = 1227.60MHz) (called the P code). • 4.The difference in the group delay betweem the two frequencies,f1 and f2,is related to TEC as (4.22)

  29. 4.2.2 Doppler shift • 1.Doppler shifts in the signal transmitted by satellites typically occur due to the motion of the satellite. • 2.The effects of Doppler shift on radio waves are far greater than the effects of rotation of the polarization plane caused by the geomagnetic field, we can therefore the approximation in eq.(4.17) to determine the phase of a radio signal (frequency ) as it passes through the ionosphere and reaches ground-level. = t - = t - + (4.24)

  30. It is difficult to determine phase of the signal immediarely following transmission by the satellite, but at ground-level it is possible to continually monitor the amount of phase change.Differentiating eq.(4.24) with respect to time,we have = - + [Hz] (4.25) R = (4.26) (satellite-to-receiver distance)

  31. (a)

  32. (b)

  33. Differential Doppler Technique • The radio signals transmitted by the NNSS satellites have such a relationship.Aussuming150 = 150MHz and 400MHz, we have the relationship of 150 = (3/8). Utilizing eq.(4.25) and differentiating the phase change of each of the frequencies, only the change in TEC will remain. This value is referred to as the differential Doppler shift. - = 7.70-16[Hz] (4.27)

  34. Ionospheric Tomography receiving stations

  35. 4.2.3 Faraday Rotation • When a satellite is transmitting linearly polarized radio wave, they are considered to be a combination of the two circularly polarized (right-hand and left-hand rotation) wave, and eq.(4.16) describes how the waves pass through the ionosphere at different phase velocities, depending on polarization. At the ground-level, combining the right-hand and left-hand polarized waves, we have the polarization phane rotated by the amount of the difference in phase velocity between the two circularly polarized waves. + = 1 - - = 1 -

  36. The phase difference between two circularly polarized waves reaching ground-level is calculated by = ds = = (4.28)

  37. Rotation in the plane of linear polarization is one-half the phase difference. Expressed in radians, the rotation angle becomes = = 2b [rad] (4.29)

  38. TEC by Faraday Rotation Technique • It is also important to continuously monitor temporal variations in ionospheric condition at a fixed point. When a geostationary satellite is available, it is possible to constantly monitor TEC in the radio propagation path between the satellite and ground-level receiver using the above methods. N = (4.30)

  39. Figure 4.6 shows an example of daily TEC variations in the skies over Tokyo, as observed by the ETS 2 satellite, which transmits a 136MHz signal.

  40. 4.3Ionospheric Irregularities and Radio Wave Propagation-Speaker 黃俊雁 • 4.3.1 Spread F • 4.3.2 Scintillations • 4.3.3 Other Radio Phenomena Radio wave scattering by Meteor Trail Sporadic E Layer and Broadcast Signals The Absorption of Shortwave Radio Signals

  41. 4.3.1 Spread F

  42. Range spread F, RSF [彭秉正,2002]

  43. Frequency Spread F, FSF

  44. 4.3.1 Spread F – Global Distribution From this map we can see the characteristic differences in latitude and time.

  45. We know that equatorial spread F occurs seasonally at certain longitudes. In the Atlantic Ocean longitudes ,it occurs predominantly in the northern winter months. Over the Pacific region ,it occurs in the northern summer ,and in India and the areas south of Japan, spread F appears as a spring and autumn phenomenon. Although it is not fully clear what causes these seasonal and longitudinal characteristics ,they appear to be related to factors such as changes in thermospheric wind and/or seasonal changes in the electric fields.

  46. 4.3.2 Scintillations When irregular structures which result in spread F are observed ,we sometimes also observe variations in signal strength of the radio waves which pass through the ionosphere.

  47. Notice that immediately following passage of the radio wave through the irregular density structure, only phase of the signal has changed, and amplitude still remains constant. However, by the time the signal reaches ground-level, it will have been interfered with by adjacent signals having different phases, which leads to spatial variations in electric field strength.

  48. The phases of the signals passing through an area of high electron density are advanced over the phases of those signal passing through the less dense parts of the structure. In other words, as eq 4.17 demonstrates, phase velocity (Vp =c/μ)increases with the presence of electrons, and phase-path length varies by Thus, where wavelength of the radio wave is λ, phase advance is given by

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