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Vệ tinh thông tin (tiếng Anh: communications satellite, đôi khi viết tắt là SATCOM) là vệ tinh nhân tạo đặt trong không gian dùng cho viễn thông. Vệ tinh thông tin hiện đại có nhiều loại quỹ đạo như quỹ đạo địa tĩnh, quỹ đạo Molniya, quỹ đạo elip, quỹ đạo (cực và phi cực) Trái Đất thấp.Vệ tinh thông tin là kỹ thuật tiếp sức vô tuyến vi ba bên cạnh thông tin cáp quang biển trong truyền dẫn điểm điểm cố định. Nó cũng được dùng trong các ứng dụng di động như thông tin cho tàu xe, máy bay, thiết bị cầm tay và cho cả tivi và quảng bá khi mà các kỹ thuật khác như cáp không thực tế hoặc không thể.
TRƯỜNG ĐẠI HỌC VINH VIỆN KỸ THUẬT VÀ CÔNG NGHỆ TÀI LIỆU HƯỚNG DẪN THÍ NGHIỆM THƠNG TIN VỆ TINH NGHỆ AN, 11/2017 MỤC LỤC BÀI KHẢO SÁT SUY HAO TRONG KHÔNG GIAN TỰ DO BÀI SUY HAO DO MƯA TRONG TUYẾN THÔNG TIN VỆ TINH BÀI SUY HAO DO SƯƠNG MÙ VÀ MÂY TRONG TUYẾN THÔNG TIN VỆ TINH .16 BÀI SUY HAO DO HẤP THỤ PHÂN TỬ TRONG THÔNG TIN VỆ TINH 21 BÀI KHẢO SÁT SUY HAO TRONG KHÔNG GIAN TỰ DO I MỤC ĐÍCH THÍ NGHIỆM Bài thí nghiệm giúp sinh viên nắm rõ yếu tố ảnh hưởng lên suy hao sóng vơ tuyến truyền khơng gian tự Từ đó, sinh viên phân tích tính tốn tác động suy hao không gian tự lên tuyến truyền dẫn thơng tin vệ tinh Ngồi ra, thí nghiệm giúp sinh viên khả mơ hình hóa mơ vấn đề kỹ thuật sử dụng MATLAB II CƠ SỞ LÝ THUYẾT Trong viễn thông, suy hao không gian tự suy giảm lượng sóng vơ tuyến bên thu so với bên phát kích thước anten thu khơng đủ lớn để thu hết tồn sóng điện từ bên phát phát Suy hao đặc biệt lớn các tuyến truyền dẫn với cự ly xa Đối với vệ tinh địa tĩnh độ cao 35.768 km, cự ly thông tin cho tuyến lên hay tuyến xuống gần 35.768 km Do cự ly truyền sóng thơng tin vệ tinh lớn nên suy hao không gian tự suy hao lớn Gọi suy hao xác định theo công thức (1.1) (1.1) =( ) đó: d chiều dài tuyến lên hay xuống tính đơn vị m bước sóng cơng tác tính đơn vị m Tính đơn vị đề-xi-ben, suy hao không gian tự tính theo cơng thức (1.2) = 10 log (( )) = 10 log ( ) = 20 log () (1.2) đó: = × 10 / vận tốc ánh sang Với d tinh m f tính Hz suy hao khơng gian tự tính theo cơng thức (1.3) đơn vị dB = 10 log (( )) = 20 log10 ( ) (1.3) = 20 log10( ) + 20 log10( ) + 20 log10 ( = 20 log10 + 20 log10 ) − 147.55 Nếu f tính GHz tính km suy hao khơng gian tự tính theo cơng thức (1.4) (1.4) = 20 log 10 + 20 log10 + 92.5 Nếu f tính kHz tính m suy hao khơng gian tự tính theo cơng thức (1.5) (1.5) = 20 log10 + 20 log10 − 87.55 Nếu f tính MHz tính m suy hao khơng gian tự tính theo công thức (1.6) (1.6) = 20 log10 + 20 log10 − 27.55 Nếu f tính MHz tính km suy hao khơng gian tự tính theo cơng thức (1.7) (1.7) = 20 log10 + 20 log10 + 32.45 Suy hao không gian tự tuyến lên hay xuống công tác băng C vào khoảng 200 dB, băng Ku, Ka thường lớn 200 dB Để bù vào suy hao này, đảm bảo cho máy thu nhận tín hiệu đủ lớn cỡ -90 dBm đến -60 dBm, người ta sử dụng anten có đường kính đủ lớn hàng chục mét để có hệ số tăng ích lớn khoảng 60 dBi máy phát có cơng suất lớn hàng trăm đến hàng ngàn watt Ngồi suy hao khơng gian tự có suy hao khác khơng lớn tính tốn tuyến thơng tin vệ tinh mà ta không xét hết khả xấu ảnh hưởng mơi trường truyền sóng xảy tượng chất lượng thơng tin xấu làm gián đoạn thơng tin III THIẾT BỊ THÍ NGHIỆM Máy tính có cài sẵn phần mềm Matlab phiên 7.0 trở lên Yêu cầu cấu hình tối thiểu: + Bộ vi xử lý Pentium + Hệ điều hành Windows XP (Service Pack 1, 3) + Card hình tối thiểu 256 màu + Dung lượng ổ cứng trống 1Gb, tới 2,1Gb cài đặt Matlab đầy đủ + Bộ nhớ RAM 512Mb IV CÁC BƯỚC TIẾN HÀNH THÍ NGHIỆM Viết chương trình định nghĩa hàm suy hao không gian tự fspl - Khởi động phần mềm Matlab - Khởi tạo sổ soạn thảo Editor chọn Menu File -> New -> MFile - Gõ theo chương trình mẫu sau vào cửa sổ soạn thảo (Editor) ghi lại theo tên tệp fspl.m ======================================================== function [L] = fspl(f,d) % Neu f tinh bang[GHz] va d tinh bang[km] L = 20*log(f)+20*log(d)+92.5 % Neu f tinh bang[Hz] va d tinh bang[m] % L = 20*log(f)+20*log(d)-147.55 % Neu f tinh bang[kHz] va d tinh bang[m] % L = 20*log(f)+20*log(d)-87.55 % Neu f tinh bang[MHz] va d tinh bang[m] % L = 20*log(f)+20*log(d)-27.55 % Neu f tinh bang[MHz] va d tinh bang[km] % L = 20*log(f)+20*log(d)-32.45 end ======================================================== Viết chương trình tính suy hao không gian tự phụ thuộc vào khoảng cách tần số công tác - Khởi động phần mềm Matlab - Khởi tạo sổ soạn thảo Editor chọn Menu File -> New -> MFile - Gõ theo chương trình mẫu sau vào cửa sổ soạn thảo (Editor) ghi lại theo tên tệp Suy_hao_KGTD.m ======================================================== fc = input(Nhap gia tri tan so cong tac [GHz]: ) lambda = physconst('LightSpeed')/fc; d = input(Nhap cu ly tuyen truyen dan [km]: ) L = fspl(d,lambda) ======================================================== Vẽ đồ thị suy hao không gian tự phụ thuộc vào tần số - Khởi động phần mềm Matlab - Khởi tạo sổ soạn thảo Editor chọn Menu File -> New -> MFile - Gõ theo chương trình mẫu sau vào cửa sổ soạn thảo (Editor) ghi lại theo tên tệp Suy_hao_KGTD_f.m - Nhận xét phân tích đồ thị phụ thuộc thu ======================================================== % Tan so cong tac dai tu den 100 GHz fc = 1:0.01:100; lambda = physconst('LightSpeed')./fc; d = input(Nhap cu ly tuyen truyen dan [km]: ) L = fspl(d,lambda) % Ve thi phu thuoc suy hao KGTD theo tan so plot(f,L) ======================================================== Vẽ đồ thị suy hao không gian tự phụ thuộc vào khoang cach - Khởi động phần mềm Matlab - Khởi tạo sổ soạn thảo Editor chọn Menu File -> New -> MFile - Gõ theo chương trình mẫu sau vào cửa sổ soạn thảo (Editor) ghi lại theo tên tệp Suy_hao_KGTD_d.m - Nhận xét phân tích đồ thị phụ thuộc thu ======================================================== fc = input(Nhap tan so cong tac [GHz]: ) lambda = physconst('LightSpeed')./fc; % Cu ly tuyen truyen dan tu 100 den 40000 km d = 100:1:40000; L = fspl(d,lambda) % Ve thi phu thuoc suy hao KGTD theo tan so plot(d,L) ======================================================== V KẾT LUẬN Sau hoàn thành thí nghiệm sinh viên nắm vững kiến thức hao không gian tự ảnh hưởng lên tuyến truyền dẫn Nội dung báo cáo thể rõ: Mục đích yêu cầu thí nghiệm, tóm tắt lý thuyết, bước thực nhận xét kết thu Trả lời câu hỏi kiểm tra đây: Trình bày nguyên nhân gây suy hao không gian tự do? Để giảm suy hao không gian tự cần thực biện pháp nào? VI TÀI LIỆU THAM KHẢO [1] Proakis, J Digital Communications New York: McGraw-Hill, 2001 [2] Thái Hồng Nhị, Thông tin vệ tinh NXB KHKT, 2008 BÀI SUY HAO DO MƯA TRONG TUYẾN THƠNG TIN VỆ TINH I MỤC ĐÍCH THÍ NGHIỆM Bài thí nghiệm giúp sinh viên nắm rõ yếu tố ảnh hưởng lên suy hao sóng vơ tuyến truyền mưa Từ đó, sinh viên phân tích tính tốn tác động suy hao mưa lên tuyến truyền dẫn thông tin vệ tinh Ngồi ra, thí nghiệm giúp sinh viên khả mơ hình hóa mơ vấn đề kỹ thuật sử dụng MATLAB II CƠ SỞ LÝ THUYẾT Suy hao mưa có tác động mạnh đến chất lượng tín hiệu tuyến thơng tin vệ tinh Suy hao phụ thuộc vào lượng mưa trung bình, tần số góc ngẩng anten trạm mặt đất Để xác định suy hao mưa, phương pháp dự đoán tiến hành theo bước: (1) dự đốn phân bố xác xuất lượng mưa (2) tín tổn hao sóng mưa dựa vào hệ số hấp thụ xác định Hệ số tổn hao mưa, , tính từ lượng mưa trung bình theo biểu thức (2.1) [dB/km] (2.1) = ∙ đó: hệ số phụ thuộc vào tần số phân cực sóng Đối với sóng có phân cực thẳng phân cực tròn biểu thức để tính hệ số k (2 a) (2.2 b) =[ =[ ℎ ℎ + +( ℎ + +( ℎ − ) cos2 − ℎ ℎ (2.2 a) ]/2 ) cos 2 ]/2 (2.2 b) đó: góc ngẩng anten góc nghiêng phân cực so với mặt ngang ( có giá trị 45 phân cực tròn) Các giá trị kh, kv, ℎ, xác định bảng 3.6 Bảng 2.1 Các hệ số hồi quy để xác định tổn hao sóng mưa (ITU-R) Tần số (GHz) ℎ ℎ 0.0000650 0.0000591 1.121 1.075 0.000175 0.00155 1.308 1.265 10 0.101 0.00887 1.308 1.264 12 0.0188 0.0168 1.217 1.200 15 0.0367 0.0335 1.154 1.128 20 0.0751 0.0691 1.099 1.065 30 0.187 0.167 1.021 1.000 Suy hao mưa xác định theo công thức (2.3) (2.3) == [ ] đó: Leff quãng đường sóng truyền vùng mưa (km) L độ dày vùng có mưa (km) III THIẾT BỊ THÍ NGHIỆM Máy tính có cài sẵn phần mềm Matlab phiên 7.0 trở lên Yêu cầu cấu hình tối thiểu: + Bộ vi xử lý Pentium + Hệ điều hành Windows XP (Service Pack 1, 3) + Card hình tối thiểu 256 màu + Dung lượng ổ cứng trống 1Gb, tới 2,1Gb cài đặt Matlab đầy đủ + Bộ nhớ RAM 512Mb IV CÁC BƯỚC TIẾN HÀNH THÍ NGHIỆM Viết chương trình định nghĩa hàm suy hao mưa rainpl - Khởi động phần mềm Matlab - Khởi tạo sổ soạn thảo Editor chọn Menu File -> New -> MFile 10 14 Rec ITU-R P.676-10 For 66 GHz < f ≤ 120 GHz: γo = 3.8 3.02 ×10−4 r3.5 t + 0.283rt (f − 118.75) + 1.6 2.91r r pt + 0.502ξ6 [1− 0.0163ξ7 ( f − 66)] ( f − 66) 1.4346ξ f 2r + 1.15ξ −3 ×10 p (22e) For 120 GHz < f ≤ 350 GHz: with: 0.3 3.02×10−4 0.283r t γ = + f 2r2r3.5 −5 2 1.6 o pt 1 + 1.9×10 f 1.5 ( f −118.75) + 2.91r r pt −3 ×10 +δ (22f) ξ1 = ϕ(rp , rt ,0.0717,−1.8132,0.0156,−1.6515) (22g) ξ2 = ϕ(rp , rt ,0.5146,−4.6368,−0.1921,−5.7416) (22h) ξ3 = ϕ(rp , rt ,0.3414,−6.5851,0.2130,−8.5854) (22i) ξ4 = ϕ(rp , rt ,−0.0112,0.0092,−0.1033,−0.0009) (22j) ξ5 = ϕ(rp , rt ,0.2705,−2.7192,−0.3016,−4.1033) (22k) ξ6 = ϕ(rp , rt ,0.2445,−5.9191,0.0422,−8.0719) (22l) ξ7 = ϕ(rp , rt ,−0.1833,6.5589,−0.2402,6.131) (22m) γ 54 = 2.192ϕ(rp , rt ,1.8286,−1.9487,0.4051,−2.8509) (22n) γ 58 = 12.59ϕ(rp , rt ,1.0045,3.5610,0.1588,1.2834) (22o) γ 60 = 15.0ϕ(rp , rt ,0.9003,4.1335,0.0427,1.6088) (22p) γ 62 = 14.28ϕ(rp , rt ,0.9886,3.4176,0.1827,1.3429) (22q) γ 64 = 6.819ϕ(rp , rt ,1.4320,0.6258,0.3177,−0.5914) (22r) γ 66 = 1.908ϕ(rp , rt ,2.0717,−4.1404,0.4910,−4.8718) δ (22s) = −0.00306ϕ(rp , rt ,3.211,−14.94,1.583,−16.37) ϕ(rp (22t) a b , rt , a,b,c, d) = rp rt exp[c(1− rp ) + d(1− rt )] where: f : frequency (GHz) rp = ptot/1013, where ptot represents total air pressure rt = 288/(273 + t) p : pressure (hPa) (22u) Rec ITU-R P.676-10 15 t : temperature (°C), where mean temperature values can be obtained from maps given in Recommendation ITU-R P.1510, when no adequate temperature data are available For water vapour, the attenuation γw (dB/km) is given by: 3.98η exp[2.23(1 − r )] 11.96η1 exp[0.7(1 − rt )] t g( f ,22) + γw= 2 2 ( f − 183.31) ( f − 22.235) + 9.42η1 + 11.14η1 + 0.081η1 exp[6.44(1 − rt )] + 3.66η1 exp[1.6(1 − rt )] ( f − 321.226) + 6.29η2 ( f − 325.153)2 + 9.22η2 1 (23a) + 25.37η1 exp[1.09(1 − rt )] + 17.4η1 exp[1.46(1 − rt )] ( f − 448)2 ( f − 380) +844.6η1 exp[0.17(1 − rt )] g( f ,557) + 290η1 exp[0.41(1 − rt )] g( f ,752) ( f − 752)2 ( f − 557) 8.3328 × 10 η exp[0.99(1 − r )] + 2.5 t ( f − 780) with: η1 = 0.955rprt η2 = 0.735 rp rt −4 g( f ,1 780) f rt ρ × 10 0.68 0.5 (23b) (23c) + 0.006ρ (23d) + 0.0353 rt ρ f − f 2 i g( f , fi ) = + f + f i where ρ is the water-vapour density (g/m ) Figure shows the specific attenuation from to 350 GHz at sea-level for dry air and water vapour with a density of 7.5 g/m Path attenuation 2.1 Terrestrial paths For a horizontal path, or for slightly inclined paths close to the ground, the path attenuation, A, may be written as: (24) A = γ r0 = (γ o + γ w ) r0 dB where r0 is the path length (km) 16 Rec ITU-R P.676-10 FIGURE Specific attenuation due to atmospheric gases 10 10 Total Specific attenuat ion (dB/km ) 10–1 Total Dry air 10–2 Water vapour 10–3 10 Frequency (GHz) Pressure: 013 hPa Temperature: 15° C Water vapour density: 7.5 g/m 102 3.5 Rec ITU-R P.676-10 2.2 17 Slant paths This section contains simple algorithms for estimating the gaseous attenuation along slant paths through the Earth’s atmosphere, by defining an equivalent height by which the specific attenuation calculated in § may be multiplied to obtain the zenith attenuation The equivalent heights are dependent on pressure, and can hence be employed for determining the zenith attenuation from sea level up to an altitude of about 10 km The resulting zenith attenuations are accurate to within ±10% for dry air and ±5% for water vapour from sea level up to altitudes of about 10 km, using the pressure, temperature and water-vapour density appropriate to the altitude of interest For altitudes higher than 10 km, and particularly for frequencies within 0.5 GHz of the centres of resonance lines at any altitude, the procedure in Annex should be used Note that the Gaussian function in equation (25b) describing the oxygen equivalent height in the 60 GHz band can yield errors higher than 10% at certain frequencies, since this procedure cannot reproduce the structure shown in Fig The expressions below were derived from zenith attenuations calculated with the procedure in Annex 1, integrating the attenuations numerically over a bandwidth of 500 MHz; the resultant attenuations hence effectively represent approximate minimum values in the 50-70 GHz band The path attenuation at elevation angles other than the zenith may then be determined using the procedures described later in this section For dry air, the equivalent height is given by: 6.1 ho = 1+ 0.17 rp −1.1 (1+ t1 + t2 + t3) (25a) where: 4.64 t = + 0.066 r f − 59.7 2.87 + 0.14 exp (2.12 rp ) p t2 = t3 = exp − −2.3 12.4 exp (−7.9 rp 2 ) (25b) (25c) ( f −118.75)2 + 0.031 exp (2.2 r ) p 0.0114 f + 0.14 r −2.6 p − 0.0247 + 0.0001 f + 1.61 × 10 − 0.0169 f + 4.1 × 10 −6 f −5 f + 3.2×10 (25d) −7 f with the constraint that: h 10.7 r0.3 when f 70 GHz o (25e) p and for water vapour, the equivalent height is: h w = 1.66 1 + 1.39σ w ( f − 22.235) + + 2.56σ w σw 3.37σ w ( f − 183.31) + + 4.69σ w for f ≤ 350 GHz 1.013 = 1.58σ w ( f − 325.1) + (26a) 2.89σ w (26b) + exp[−8.6 (r − 0.57)] p The zenith attenuation between 50 to 70 GHz is a complicated function of frequency, as shown in Fig 7, and the above algorithms for equivalent height can provide only an approximate estimate, in general, of the minimum levels of attenuation likely to be encountered in this frequency range For greater accuracy, the procedure in Annex should be used 18 Rec ITU-R P.676-10 The concept of equivalent height is based on the assumption of an exponential atmosphere specified by a scale height to describe the decay in density with altitude Note that scale heights for both dry air and water vapour may vary with latitude, season and/or climate, and that water vapour distributions in the real atmosphere may deviate considerably from the exponential, with corresponding changes in equivalent heights The values given above are applicable up to altitudes of about 10 km The total zenith attenuation is then: (27) A=γ h +γ h dB o o w w Figure shows the total zenith attenuation at sea level, as well as the attenuation due to dry air and water vapour, using the mean annual global reference atmosphere given in Recommendation ITU-R P.835 Between 50 and 70 GHz greater accuracy can be obtained from the km curve in Fig which was derived using the line-by-line calculation as described in Annex 2.2.1 2.2.1.1 Elevation angles between 5 and 90 Earth-space paths For an elevation angle, ϕ, between 5° and 90°, the path attenuation is obtained using the cosecant law, as follows: For path attenuation based on surface meteorological data: dB (28) A= A o + Aw sin ϕ where Ao = ho γ o and Aw = hw γ w and for path attenuation based on integrated water vapour content: A(P) = dB Ao + Aw (P) sin ϕ (29) where Aw (P) is given in § 2.3 2.2.1.2 Inclined paths To determine the attenuation values on an inclined path between a station situated at altitude h1 and another at a higher altitude h2, where both altitudes are less than 10 km above mean sea level, the values ho and hw in equation (28) must be replaced by the following ho' and hw' values: km (30) h o' –h /h –h /h –e hw ' –h /h –h /h = hw e –e = ho e 1 o w 2 o km (31) w it being understood that the value ρ of the water-vapour density used in equation (23) is the hypothetical value at sea level calculated as follows: (32) ρ = ρ1 × exp h1 / 2 where ρ1 is the value corresponding to altitude h1 of the station in question, and the equivalent height of water vapour density is assumed as km (see Recommendation ITU-R P.835) Equations (30), (31) and (32) use different normalizations for the dry air and water-vapour equivalent heights While the mean air pressure referred to sea level can be considered constant around the world (equal to 013 hPa), the water-vapour density not only has a wide range of Rec ITU-R P.676-10 19 climatic variability but is measured at the surface (i.e at the height of the ground station) For values of surface water-vapour density, see Recommendation ITU-R P.836 2.2.2 2.2.2.1 Elevation angles between 0º and 5º Earth-space paths In this case, Annex of this Recommendation should be used The same Annex should also be used for elevations less than zero 2.2.2.2 Inclined paths The attenuation on an inclined path between a station situated at altitude h1 and a higher altitude h2 (where both altitudes are less than 10 km above mean sea level), can be determined from the following: –h / h –h / h R + h ⋅ F(x +h R A = γ o ho +γw w ⋅ F(x ) e cos ϕ 1 e – ' 1 e cos ϕ ' ⋅ F(x ) e –h cos ϕ – o cos ϕ R+h –h R + h ⋅ F(x ) e 1/ hw e )e o h e /h w dB (33) where: Re : effective Earth radius including refraction, given in Recommendation ITU-R P.834, expressed in km (a value of 500 km is generally acceptable for the immediate vicinity of the Earth's surface) ϕ1 : elevation angle at altitude h1 F : function defined by: F(x) = 0.661 x + 0.339 R e + h1 ϕ2 = arccos xi = tan ϕi Re + h i h x' = tan ϕ i +h R e (34) x + 5.51 cos ϕ (35a) for i = 1, (35b) for i = 1, (35c) o Re + hi i hw it being understood that the value ρ of the water vapour density used in equation (23) is the hypothetical value at sea level calculated as follows: (36) ρ = ρ1 ⋅ exp h1 / 2 where ρ1 is the value corresponding to altitude h1 of the station in question, and the equivalent height of water vapour density is assumed as km (see Recommendation ITU-R P.835) Values for ρ1 at the surface can be found in Recommendation ITU-R P.836 The different formulation for dry air and water vapour is explained at the end of § 2.2 20 Rec ITU-R P.676-10 FIGURE Total, dry air and water-vapour zenith attenuation from sea level 10 102 10 Zenith attenuat ion (dB) Total –1 10 Dry air Water vapour 10–2 10–3 102 10 Frequency (GHz) Surface pressure: 013 hPa Surface temperature: 15° C Surface water-vapour density: 7.5 g/m 350 Rec ITU-R P.676-10 21 FIGURE 10 15 20 10 –2 Zenith attenuation (dB) 10 –1 10 2 25 10 10 35 50 km 52 54 56 58 Frequency (GHz) 60 62 64 66 6870 Zenith oxygen attenuation from the altitudes indicated, calculated at intervals of 50 MHz, including line centres (0 km, km, 10 km, 15 km and 20 km) 22 Rec ITU-R P.676-10 2.3 Zenith path water-vapour attenuation The above method for calculating slant path attenuation by water vapour relies on the knowledge of the profile of water-vapour pressure (or density) along the path In cases where the integrated water vapour content along the path, Vt, is known, an alternative method may be used The total water-vapour attenuation can be estimated as: Aw P= 0.0173Vt (P)γ w ( f , pref ,ρv,ref ,tref ) γ ( f , p ,ρ w ref ref dB (37) ,t ) v,ref ref where: f: f : ref p ref ρ v,ref t ref = = = Vt(P): γW(f, p, ρ, t): frequency (GHz) 20.6 (GHz) 780 (hPa) V (P) t (g/m3) 14 ln 0.22 V (P) t + (°C) integrated water vapour content at the required percentage of time (kg/m or mm), which can be obtained either from radiosonde profiles, radiometric measurements, or Recommendation ITU-R P.836 (kg/m2 or mm) specific attenuation as a function of frequency, pressure, water-vapour density, and temperature calculated from equation (23a) (dB/km) Recommendation ITU-R P.840-6 (09/2013) Attenuation due to clouds and fog P Series Radiowave propagation ii Rec ITU-R P.840-6 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio -frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU- R/ISO/IEC referenced in Annex of Resolution ITU-R Forms to be used for the submission of patent statements and licensing declarations by patent holders are available from http://www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found Series of ITU-R Recommendations (Also available online at http://www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR BS BT Recording for production, archival and play-out; film for television Broadcasting service (sound) Broadcasting service (television) F M P RA RS Fixed service Mobile, radiodetermination, amateur and related satellite services Radiowave propagation Radio astronomy Remote sensing systems S SA SF SM SNG TF V Fixed-satellite service Space applications and meteorology Frequency sharing and coordination between fixed-satellite and fixed service systems Spectrum management Satellite news gathering Time signals and frequency standards emissions Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R Electronic Publication Geneva, 2013 ITU 2013 All rights reserved No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU Rec ITU-R P.840-6 RECOMMENDATION ITU-R P.840-6 Attenuation due to clouds and fog (Question ITU-R 201/3) (1992-1994-1997-1999-2009-2012-2013) Scope This Recommendation provides methods to predict the attenuation due to clouds and fog on Earth-space paths The ITU Radiocommunication Assembly, considering a) that there is a need to give guidance to engineers in the design of Earth-space telecommunication systems for frequencies higher than 10 GHz; b) that attenuation due to clouds may be a factor of importance especially for microwave systems well above 10 GHz or low-availability systems; c) that for the calculation of the time series of total attenuation and space-time prediction methods, an analytical expression for the statistics of the total columnar content of cloud liquid water is needed, recommends that the curves, models and maps given in Annex should be used for the calculation of attenuation due to clouds and fog; that the information in Annex should be used for global calculations of propagation effects, required by, inter alia, space-time channel models, that require an analytic expression for the statistics of the total columnar content of cloud liquid water Annex 1 Introduction For clouds or fog consisting entirely of small droplets, generally less than 0.01 cm, the Rayleigh approximation is valid for frequencies below 200 GHz and it is possible to express the attenuation in terms of the total water content per unit volume Thus the specific attenuation within a cloud or fog can be written as: γc = Kl M dB/km where: γc : specific attenuation (dB/km) within the cloud Kl : specific attenuation coefficient ((dB/km)/(g/m 3)) M : liquid water density in the cloud or fog (g/m 3) (1) Rec ITU-R P.840-6 At frequencies of the order of 100 GHz and above, attenuation due to fog may be significant The liquid water density in fog is typically about 0.05 g/m for medium fog (visibility of the order of 300 m) and 0.5 g/m3 for thick fog (visibility of the order of 50 m) Specific attenuation coefficient A mathematical model based on Rayleigh scattering, which uses a double-Debye model for the dielectric permittivity ε ( f ) of water, can be used to calculate the value of Kl for frequencies up to 000 GHz: Kl = 0.819 f ε"(1 + η2 ) (2) (dB/km)/(g/m ) where f is the frequency (GHz), and: η ε' ε" (3) The complex dielectric permittivity of water is given by: f (ε0 – ε1) ε"( f ) = ε'( f ) = f (ε1 – ε2 ) fp 1+(f/fp) ε0 – ε + + (4) + ε2 (5) fs + ( f / fs ) ε1 – ε2 + ( f / f p )2 + ( f / fs )2 where: (6) ε0 = 77.66 + 103.3 (θ – 1) (7) ε1 = 0.0671ε (8) ε2 = 3.52 θ = 300 / T (9) with T the temperature (K) The principal and secondary relaxation frequencies are: fp = 20.20 – 146 (θ – 1) + 316 (θ – 1) fs = 39.8fp GHz GHz (10) (11) Cloud attenuation along slant paths To obtain the attenuation due to clouds along slant paths for a given probability, the statistics of the total columnar content of liquid water reduced to a temperature of 0°C, Lred (kg/m2 or, equivalently, mm) for a given site must be known yielding: A = L K red sin θ l dB for 90° ≥ θ ≥ 5° (12) Rec ITU-R P.840-6 where θ is the elevation angle and Kl is calculated from equations (2) to (11) for a water temperature of 0°C The annual values of total columnar content of reduced cloud liquid water, Lred (kg/m2 ), exceeded for 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% of an average year are an integral part of this Recommendation and are available in the form of digital maps The monthly values of total columnar content of reduced cloud liquid water, Lred (kg/m ), exceeded for 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% of each average month are an integral part of this Recommendation and are available in the form of digital maps The annual and monthly values of total columnar content are provided in the file R-REC-P.840-6-201309-I!!ZIP-E The data is from 0° to 360° in longitude and from +90° to –90° in latitude, with a resolution of 1.125º in both latitude and longitude The total columnar content of reduced cloud liquid water at any desired location on the surface of the Earth can be derived by the following interpolation method: a) determine the two probabilities, pabove and pbelow, above and below the desired probability, p, from the set: 0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% for annual statistics and from the set: 1, 2, 3, 5, 10, 20, 30, 50, 60, 70, 80, 90, 95 and 99% for monthly statistics; b) for the two probabilities, pabove and pbelow, determine the total columnar content of reduced cloud liquid water, Lred1, Lred2, Lred3, and Lred4 at the four closest grid points; c) determine the total columnar content of reduced cloud liquid water, Lredabove and Lredbelow, at the probabilities, pabove and pbelow, by performing a bi-linear interpolation of the four values of total columnar content of reduced cloud liquid water, Lred1, Lred2, Lred3, and Lred4 at the four grid points, as described in Recommendation ITU-R P.1144; d) determine the total columnar content of reduced cloud liquid water, Lred, at the desired probability, p, by interpolating Lredabove and Lredbelow vs pabove and pbelow to p on a linear Lred vs log p scale 3.1 Approximation of Lred by a log-normal distribution The annual statistics of the total columnar content of reduced cloud liquid water content can be approximated by a log-normal distribution The mean, m, standard deviation, σ, and probability of reduced liquid water, Pclw, parameters of the log-normal distribution are an integral part of this Recommendation in the form of digital maps The total columnar content of reduced cloud liquid water at any desired location on the surface of the Earth can be derived by the following interpolation method: a) determine the parameters, m1, m2, m3, m4, σ1, σ2, σ3, σ4, PCLW1, PCLW2, PCLW3 and PCLW4 at the four closest grid points; b) determine the total columnar content of reduced cloud liquid water Lred1, Lred2, Lred3, and Lred4 for the desired probability, p, at the four closest grid points from the parameters m1, m2, m3, m4, σ1, σ2, σ3, σ4, PCLW1, PCLW2, PCLW3 and PCLW4 as follows: −1 mi σiQ L red ,i =e P P CLWi for i = 1, 2, 3, (13) Rec ITU-R P.840-6 where: Q x 2π c) ∞ e − t2 dt (14) x determine the total columnar content of reduced cloud liquid water at the desired location by performing a bi-linear interpolation of the four values of total columnar content of reduced cloud liquid water, Lred1, Lred2, Lred3, and Lred4 at the four grid points as described in Recommendation ITU-R P.1144 ... tan so cong tac [GHz]: ) lambda = physconst('LightSpeed')/fc; d = input(Nhap cu ly tuyen truyen dan [km]: ) L = fspl(d,lambda) ======================================================== Vẽ đồ thị... 100 GHz fc = 1:0.01:100; lambda = physconst('LightSpeed')./fc; d = input(Nhap cu ly tuyen truyen dan [km]: ) L = fspl(d,lambda) % Ve thi phu thuoc suy hao KGTD theo tan so plot(f,L) ========================================================... input(Nhap tan so cong tac [GHz]: ) lambda = physconst('LightSpeed')./fc; % Cu ly tuyen truyen dan tu 100 den 40000 km d = 100:1:40000; L = fspl(d,lambda) % Ve thi phu thuoc suy hao KGTD theo