Автор неизвестен - Сборник научных трудов 3-го международного радиоэлектронного форума прикладная радиоэлектроника - страница 48

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Литература

1. Шарыгин Г.С. Статистическая структура поля УКВ за горизонтом. М.: Радио и связь, 1983. 140с

2. Петров В.А., Баранова Л.В. Структура источников и направленность вторичного излучения при дальнем тропосферном распространении радиоволн11 Радиотехника. 2005. Вып. 143. С. 83-88.

3. Петров В. А. Векторное поле радиоволн, рассеянных в тропосфере на флуктуаци-ях диэлектрической проницаемости11 Радиотехника.2006. Вып. 145. С. 126-129.

4. Шур А.А. Характеристики сигнала на тропосферных радиолиниях. М.: Связь, 1972. 105с

5. Дальнее тропосферное распространение ультракоротких волн.1 Под ред. Б.А. Введенского и др. М.: Сов. радио, 1965. 418с.

MIXED PATH LOSS MODELING FOR HIGH FREQUENCY SURFACE WAVE RADAR A.L. Dzvonkovskaya, L.I. Dzvonkovsky JSC SPC Scientific Research Institute of Long-Range Communications 1st Bukhvostova Str. 12/11, 107258 Мoscow, Russia E-mail: dzvonkov@niidar.ru

The mixed propagation paths are considered for high frequency surface wave radar (HFSWR) and radar path losses are estimated for homogeneous patches with different electrical properties of sea and land as well as disturbed sea state influence.

1. Introduction. Long distance surveillance of 200 nautical miles Exclusive Economical Zone (EEZ) using HFSWR is a developing technology [1], which has already shown its capa­bilities to detect ships and aircrafts, to collect data about the ocean environment remotely. The working principle of HFSWR is based on the diffraction of HF radio waves with vertical polari­zation along the earth surface [2]. There are different scenarios of HF surface wave propagation depending on the region and HFSWR coverage, e.g. the propagation along smooth sea surface is a homogeneous path, the propagation along disturbed sea surface is a inhomogeneous path, the propagation along the sea surface with many islands of different size is a mixed path. In the lat­termost case the electric properties of the land can be inhomogeneous.

The radar equation for HFSWR can be expressed via the signal-to-noise ratio q for the working frequency f after a coherent integration of reflected target signals:

where P is the energy potential, a is the radar cross section of a target, Tcit is the coherent integration time, W is the two-path radar attenuation, Lsys is the system losses, N is the in­terference level.

Following (1) the main component of radar propagation losses is the radar attenuation W because there are several hardware and software techniques to minimize these losses for the ac­tive and passive interference level N that influences the HFSWR operation. It is necessary to propose an adequate propagation model in order to estimate the signal attenuation along the path for different working frequencies. The final accurate solution hasn't been found yet. The formulas in [2] are complicated to realize therefore the Millington's empirical technique [3] seems to be simple and gives satisfactory results compared to the theory in [2]. This technique is used in CCIR Rec. 368-8 [4] and includes the problems of homogeneous and mixed HF sur­face propagation paths. Nevertheless the use of graphical data from [4] is not convenient for the HFSWR permanently choosing the operating frequencies with minimal active interference lev­els to adapt to complicated electromagnetic environment. Such estimation is necessary in real time especially during the HFSWR operation along the mixed paths.

To overcome this problem there is a necessity to develop an interface-friendly software tool to estimate radar path losses. The program is based on the program GRWAVE [4] to calcu­late the field strength according to [5] and uses the mathematical model of exponential atmos­phere CCIR. Actually this program is used to calculate the filed strength of surface waves along the smooth homogeneous earth with the conductivity a and the relative permittivity s. Mil-lington's technique is utilized to analyze the radar losses estimation under the mentioned above environment.

2. Millington's technique for mixed paths. The surface wave propagation attenuation between the transmitter and the receiver at the distance d can be determined as [4]

PaTc

(1)

W(d) = 10log10 (Pr /Pt),

(2)

where Pr is the signal power at the receiver input, Pt is the transmitted power.

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The received power from the short electrical dipole with the transmitting power

Pt =1kWatt can be defined using the field strength E :

(3)

Pr (d ) = E(d )222/(4nZ 0) where X is the wave length, Z0 = 120n is the impedance in free space.

Substituting (3) in (2) the one-way propagation attenuation for the path consisted of several ho­mogeneous patches is expressed as

W(d ) = 142+20logf + 20logE(d), (4)

where the working frequency f is in MHz and the filed strength E is in uV/m.

The recursive equations of Millington's technique correspond to the plots in [4]

N     ,   . N

ED = ZEk(sk)- ZEk(sk-1) sk = Zdn = d1 + d2 + d3 +... + dk

k=1 k=2

N N

ER = Z Ek (rk ) - Z Ek-1 (rk ),

k=1 k=2

where ED and ER are the filed strengths for direct path (transmitter-receiver) and return path (receiver-transmitter) respectively, dn is the length of the n-th path patch. The path patches sk and rk are shown in Fig. 1.

k

Zdn

n=1

k

= Z dN-n+1 = dN + dN-1 + dN-2 +... + dN-k n=1

S4

Si

S2

S3

r5

Fig. 1. Different patches of the mixed propagation path.

The total field strength of the mixed propagation with several patches is equal to

Et =(Ed + Er )/2.

The recursive equations of Millington's technique in case of two-patch propagation path can be written as

ed = e1(d1)-e2(d2)+e2(d1 + d2) , er = E2(d2)-e1(d2) + e1(d1 + d2) ,

where the filed value E1(d1), E1(d2), E2 (d1), E2 (d2), E1(d1 + d2) and E2(d1 + d2) are defined as following: E1(d1) is the field strength at distance       for the homogeneous medium 1;

) is the field strength at distance d2 for the homogeneous medium 1; E2 (d^) - is the field strength at distance d-1 for the homogeneous medium 2; E2(d-2) is the field strength at distance d2 for the homogeneous medium 2; + d2) is the field strength at distance d-1 + d2 for the homogeneous medium 1; E2(d1 + d2) is the field strength at distance d-1 + d2 for the homogene­ous medium 2.

If the path consists of three patches then the equation are

Ed = E^)-E2 (d-2)+E2(d1 + d2)-E3(d1 + d2)+E3(d1 + d2 + d3), ER = E3(d3 )-E2 (d3 ) + E2(d3 + d2 )-E1(d3 + d2 ) + E1(d3 + d2 + d1)

The similar procedure continues in case of n patches of the propagation path. 3. Radar losses modelling. Using the developed software the research is done to estimate radar losses for the HFSWR in case of mixed propagation paths, i.e. there are islands inside the

ri

r

r

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coverage zone, as well as in case of additional losses due to disturbed sea surface. To estimate HFSWR losses in case of propagation along the mixed paths with homogeneous properties, sev­eral path scenarios are considered. Initially the propagation scenario of the path "sea-land-sea-land-sea" (see Fig. 2, left) is considered at patch lengths equal to 100 km, electric properties of sea surface with a =5 S/m and s = 70 (average sea water salinity), electric properties of dry soil with a =0.01 S/m and s = 15. The case of homogeneous propagation path along the smooth sea surface is shown in the right part of Fig.2 by dashed lines. The case of mixed propa­gation path including two islands is shown in Fig.2 by solid lines. The electric properties of sea surface are a =5 S/m and s = 80, the electric properties of land are a =0.003 S/m and s = 15. The islands of 20 km and 50 km sizes are at the distances 50 km and 170 km respectively. In Fig. 2 the homogeneous path (thick horizontal line) and EEZ's border (vertical line) are shown.

Fig. 2. Two-path surface wave attenuation while propagating along the mixed path. Left part: using frequencies 5-25 MHz stepped by 5 MHz (the upper curve is at 5 MHz, the lower curve is at 25 MHz). Right part: using frequencies 3.5 MHz, 5.5 MHz and 10-25 MHz stepped by 5 MHz (the upper curve is at 3.5 MHz, the lower curve is at 25 MHz).

The Millington's effect is seen clearly in these figures. The rapid increase of attenuation at the border "sea-land" and the reconstruction at the border "land-sea" show a capability to de­tect surface targets behind the island. This effect has been already confirmed experimentally but it is still necessary to investigate it further. Hereby it is important to notice that islands increase the surface wave propagation attenuation; additional one-way attenuation can be 15 dB due to mixed paths; the islands surrounding to the HFSWR induce more attenuation compared to out­lying ones; the critical parameters of mixed path attenuation are radial lengths of patches and distances between them; additional attenuation can be minimized by HFSWR operating at lower frequencies.

4. Disturbed sea surface influence on radar losses. D. Barrick has expanded his surface wave propagation theory by the case of rough sea surface consideration [6], which is accepted to estimate the influence of disturbed sea surface on the radar losses. In Fig. 3 the curves of addi­tional one-way attenuation are shown at the frequencies 5 MHz (solid line), 10 MHz (dashed line) and 20 MHz (dotted line) in case of different sea states, i.e. wind speeds, and antenna loca­tions at sea level [6]. The additional attenuation A W increases according to increasing wind speed (or wave height), working frequency and distance. For example, the wind influence at the frequency 10 MHz makes additional attenuation about 20 dB at the distance 500 km and the wind speed 15 m/s. It should be noticed that at lower band 3-5 MHz the additional losses have negative sign. Practically it means the increasing receiving signal level. Such an effect takes place when an increasing impedance is reactive, i.e. the sea wave lengths are small while com­paring to radio wave length [6]. Taking into account the disturbed sea surface losses, the model­ing of potential HFSWR losses is done both for homogeneous paths (see Fig.4, left) and mixed paths (see Fig.4, right) using radar frequencies 5, 10 and 25 MHz marked by dashed-dotted, dashed and solid lines respectively. The utilized data about HF radar attenuation along dis­turbed sea surface are limited to wind speed 15 m/s, i.e. sea state code is 7. The influence of sea

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surface under strong storms and hurricanes is necessary to estimate theoretically and experimen­tally for distances equal at least the distance to the EEZ's border.

22---------------------!

24 |-1-1-1-1-1-1-1—|-1-1-1-1—|-1-1-1-1-1-1-1_

10 20 30 50 100 200 300        400 500

Range, km

Fig. 3. Additional wave attenuation according to Phillip's sea spectrum model

at the frequencies 5, 10 and 20 MHz vs. distances. 1 - wind speed is 2.5 m/s; 2 - wind speed is 5 m/s; 3 - wind speed is 7.5 m/s; 4 - wind speed is 10 m/s; 5 - wind speed is 12.5 m/s; 6 - wind speed is 15 m/s.

Fig. 4. Two-path surface wave attenuation while propagating along the disturbed sea surfce, the homogeneous path (left) and the mixed path (right). The upper curve is at 5 MHz, the lower curve is at 25 MHz. Wind speed is 15 m/s.

Conclusions. The mixed propagation paths bring additional radar losses for the HFSWR in case of homogeneous patches with different electric sea and soil properties as well as dis­turbed sea state influence. The developed software allows predicting and estimating the radar losses for the mixed propagation paths in real-time operation.

References

1. A.L. Dzvonkovskaya, L.I. Dzvonkovsky, F.F. Evstratov, V.A. Sobchuk. Modern state and prospects of HFSWR development. Naukoemkie Tekhnologii, Vol.8, No. 10, 2007, pp. 3­16 (in Russian).

2. Feinberg E.L. Wave propagation along the earth surface. M.: Nauka, 1999. 496 p. (in Russian).

3. Millington G. Ground Wave propagation over an Inhomogeneous Smooth Earth. Proc. IRE, Part III, Vol.96, 39, January, 1949, pp. 53-64.

4. CCIR. Ground wave Propagation Curves for Frequencies Between 10 kHz and 30 MHz. CCIR Rec. 368-8, Genf, 2005.

5. Rotheram S. Ground-wave propagation. Part 2: Theory for medium and long distances and reference propagation curves. IEE Proc. F. Commun., Radar & Signal Process., 1981, 128,

n.5, pp. 285-295.

6. Barrick D. E. Theory of HF and VHF propagation across the rough sea. Part 2. Application to HF and VHF propagation above sea. Radio Science, Vol.6, No. 5, May 1971, pp. 527-533.

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ЭКСПЕРИМЕНТАЛЬНЫЕ ИССЛЕДОВАНИЯ ОТРАЖАТЕЛЬНОЙ СПОСОБНОСТИ ЗЕМНЫХ ПОВЕРХНОСТЕЙ С ПОМОЩЬЮ БОРТОВЫХ РАДИОЛОКАТОРОВ ЛЕТАТЕЛЬНЫХ АППАРАТОВ.

В.А. Зелинский Научно-технический центр АН ПРЭ 61166, г. Харьков, просп. Ленина, 14, тел 702-18-09 E-mail: akad@kture.kharkov.ua The goal of the given work is review of modern current state and estimation of future trends in development of air traffic control radars.

1. Введение. Для обеспечения безопасности полетов авиации используются средст­ва навигации и наблюдения. В настоящее время определяющее значение в развитии ука­занных средств играет внедрение систем космического базирования GPS, Глонас и дру­гих. Однако, абсолютизация возможностей спутниковых систем явно неправомерна ( 1 ). От качества функционирования спутникового сегмента, возможных нарушений в его ра­боте зависит очень многое.

В силу отмеченного традиционные радиотехнические средства навигации и наблю­дения предполагается использовать еще достаточно длительное время. Гармоническое же развитие различных видов средств, базирующихся на различных физических или техни­ческих принципах, может действительно создать гарантии безопасности полетов лета­тельных аппаратов, особенно на малых высотах. Дублирование различных полей в одном и том же воздушном пространстве, в частности со слабо развитой радиолокационной ин­фраструктурой, обеспечит выполнение задач авиацией в чрезвычайных ситуациях.

Целью проведенных исследований является создание перспективных бортовых ра­диовысотомеров для обеспечения полетов на малых и предельно малых высотах, в том числе и над пересеченной местностью. При этом по-прежнему является актуальной зада­ча учета специфики эхо-сигналов, отраженных от различных участков земной поверхно­сти при полетах ЛА на разных высотах.

2. Основными тактическими характеристиками радиовысотомеров (РВ) являются пределы и точность измерения высоты и вертикальной скорости. В зависимости от на­значения измерителя требования к его параметрам изменяются в широком диапазоне. Одним из них является сохранение заданной точности измерения при различных эволю-циях полета ЛА. Для уменьшения возникающих из-за кренов ЛА погрешностей приме­няют широкие ДНА. Построение и характеристики РВ определяются видом зондирую­щих сигналов.

В проведенных экспериментальных исследованиях реализован частотный метод измерения дальности с использованием непрерывного симметричного линейного частот­ного модулированного сигнала в сантиметровом диапазоне длин волн. Антенная система состояла из двух рупорных антенн: передающей и приемной. Электромеханический при­вод обеспечивал изменение ориентации обеих антенн в вертикальной плоскости от 0° до 50°.

В приемном трактате сигнал с выхода частотомера подавался на вычислительное устройство (СВУ) и индикатор. В СВУ реализован алгоритм обработки эхо-сигналов, учитывающий изменение профиля земной поверхности по всей трассе полета ЛА. В нем учтена специфика отражения радиосигналов СВЧ - диапазона от протяженной поверхно­сти со случайным рельефом, а также наличие высоковольтных ЛЭП, автотрасс с различ­ным числом автотранспорта, высотных зданий, состояние растительного покрова по вре­мени года, наличие объемных рассеивателей (дождь, туман). При этом определялся про­странственный радиус корреляции для эхо - сигналов от земной поверхности.

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