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RESEARCH OF POSSIBILITIES OF RADAR PROBING SIGNAL DETECTION OF EARTH-VIEWING SPACECRAFT
Valeriy I. Volokhov,
senior researcher, Ph.D, Military Academy of Strategic Missile Forces of Peter the Great (VA Strategic Missile Forces of Peter the Great), Moscow, Russia, val.volohov@yandex.ru
Vladimir I. Philatov,
Ph.D, Bauman MSTU, Moscow, Russia, vfil10@mail.ru
Keywords: wideband signal, linear frequency modulation, synthetic aperture, spatial expansion, wave length.
Aleksandr A. Gukel,
teacher, Ph.D, Military Academy of Strategic Missile Forces of Peter the Great (VA Strategic Missile Forces of Peter the Great), Moscow, Russia
The description of different systems of radar monitoring of the Earth from a board of a spaceship is given in the research, as well as the analysis of their main technical details, which provided the basis for demands to the laboratory setup aimed to locate and register their probing signal in the field. In order to register the fact of irradiation of surface of the Earth in the given point the new physical principle of direct transformation of energy of electromagnetic field probing signal into electric current is offered. Based on the offered principle the device allowing to locate radar probing signal from a board of a spaceship in a broad emission band of UHF (1.0 - 10.00 GHz) and effective band of broad band signals with interpulse (till 500 MHz) is developed. In order to register and monitor probing signals a set of digital oscilloscopes with a storage function, a two-channel attachment to a personal computer with a fast Fourier transformation and a two-channel accumulator of recurring pulse energy on super-capacitor are used.
Для цитирования:
Волохов В.И., Филатов В.И., Жукель А.А. Исследование возможностей обнаружения радиолокационных зондирующих сигналов космических аппаратов обзора Земли // T-Comm: Телекоммуникации и транспорт. 2016. Том 10. №12. С. 67-80.
For citation:
Volokhov V.I., Philatov V.I., Gukel A.A. Research of possibilities of radar probing signal detection of earth-viewing spacecraft. T-Comm. 2016. Vol. 10. No. 12, pр. 67-80.
7Тл
The upcoming century is noted with a wide use of space radar stations to serve the civic interests. Currently foreign countries use the following radar probing spacecrafts and space stations: SC Radars at-1 (Canada), SC ERS-2 (Europe), SC EN VIS AT (Europe), SC ALOS-2 (Japan), SS MIGS (Japan), SC Yogan Weixing-1 (China). Spacecrafts and space stations are aimed to:
-monitornatural disasters;
- study wave processes in oceans, speed and wind directions;
-control coastal areas, monitor ships, detect and monitor oil
pollution;
-evaluate seismic hazard, forecast earthquakes, detect ground movement;
- monitor agriculture and forestry — to update maps, monitor the condition of crops, control
- the use of lands, illegal cutting etc.;
- monitor glacial and snow situation by land and sea;
- create and update maps and plans; etc.
The common features of these spacecrafts and space ships are the use of synthetic-aperture radars with a high resolution (till 0.3 ra) and intention to decrease the cost of the program due to the use of small-sized space platforms. On the basis of analysis of the main technical details of the above mentioned systems and means of radar probing of earth-viewing spacecrafts the following conclusions can be drawn:
- Earth-viewing spacecrafts are ejected into circular heliosynchronous orbits with altitude of 500-800 km, declination of i = 97-98", and with orbital period of 95-100 min.
- Mapping is accomplished at an angle of 10-60° to the local vertical at a distance of 540-1600 km of oblique-visual range.
- Probing is accomplished on an operating wave length within the range of 3-22 cm with help of photosynthetic active radiation having an area of S = 10-20 m or paraboloids with diameter of D = 3-4 m".
- The chosen structure of signal provides spatial expansion within the range of I - J 000 m.
- hi the technical descriptions given on websites the following important characteristics are missing; type of modulation, pulse duration and its repetition frequency, maximum and average power of emission, effective signal spectral band, viewing time of one pixel of terrain [1 ]. Thus, it is necessary to give an assessment of parameters of the characteristics, missing in the stated sources, analytically. The assessment may be given on the basis of the theory of radar measurements, particularly of the theory of use of broad band probing signals with interpulse linear frequency modulation (linear FM), which provide high spatial resolution of targets [2,3,8, 11, 15, 19].
Main tactical parameters of a radar: radio range, resolution and accuracy depend on the type of a probing signal emitted by a radar. Radio range of the radar is the greater, the greater is the energy of a probing signal. Radio range of a radar in free spacc is determined from equation of detection range, one form of which is:
= JMmA,
(1)
Where Px - transmitterpower; G, -amplification coefficient transmitting/receiving antenna of a radar; tj and - efficiency of a feeder path of a transmitter and a receiver;
of 'h
X - wavelength of emitted signal; p - threshold power of a
signal on input of a receiver, at which a reflected signal is detected with a given probability of correct detection and false alarm. From the relation {1) follows:
= *i fl-
it)
Where kt -coefficient including all the parameters, which do not depend on energy of a probing signal. Threshold power is determined from the relation:
(3)
nr. r..
^Ihip y in
Where qnn/} - threshold value of the ratio of impulse signal
power to noise power, depending on the given probability of correct detection and false alarm (pulse packets are considered to be coherent); kiu - receiver noise coefficient; ,\'o - unilateral
spectral density of noise; n - the number of impulses in the pulse packets, received from the target; rm - probing signal pulse; L - coefficient of loss in processing the signal; it, -constant coefficient for the given radar . Using the equation for p in (3), we obtain the desired dependence:
where ¿£ - energy of a probing signal (one impulse). Thus, in order to increase the effective range it is necessary either to increase power of the receiver P| or to apply impulses with greater duration r . Limiting value P] is always limited,
especially if semiconductor devices are used. Therefore, the most praetic means of increase J? is increasing the pulse duration
[4, 5]. In order to achieve greater effective range keeping the given values of resolution and accuracy complex or broad band signals are used whose B = Aft ru »I ■ Bandwidth is achieved by
input of interpulse KM or phase modulation (PM) (manipulation). In optimal processing the compression of received signals is held using optimal filters, which provides high resolution and accuracy of range measurement; high value Tn gives an opportunity of getting the desired effective range of
radar by limiting peak capacity of its receiver. Resolution and potential accuracy with the given type of probing signal may be determined by normalized two-dimensional correlation function: P(t,F) = RM(z,F)/R(0,0) = R„(t,F)/E , (5)
where E - energy of signal. Then the probing signal ambiguity function
Z(r,F) = [p(r,F)|. (6)
At any signal of the probing signal ambiguity function it may be represented in form of ambiguity body above the plane tOF , When analyzing probing signals probing signal ambiguity diagrams are more convenient to use then the probing signal ambiguity functions. In order to get the probing signal ambiguity diagram the cutting of the probing signal ambiguity function by the plane (parallel to the plane rOF and drawn on the level ;f{r.F) = 0,5) should be found. Projection of this Cutting on the
plane rOF is the desired probing signal ambiguity diagram. When changing the parameters of the signal square S of the ambiguity diagram remains constant. Width of the ambiguity diagram /(r,0)„, or j(0, F)5 is the measure of resolution by
T-Comm Vol. 10. #12-2016
VI y
the time Si or with the frequency SF, which may be achieved with optimal processing of the signal:
(7)
-1/2 ' O/ = d2Z{F)
dr- r=0_ ci dF3
S{oj) = Un
— exp{-y[
Mo-(on)r n
4y
"1}
(13)
"u4
Sbmx (o>) = Um — exp{-j(oT,p \
(15)
(16) (17)
with I©—&>0| < • Then the output signal of the filter A', ni; = A/V,, —compression ratio of linear FM impulse.
As range is measured by the time of signal lag, and radial speed is F - at Doppler shift of the frequency f - p , i.e.
R = 0,5c/,, ^=0,5^, (8)
where ?? - speed of propagation of electromagnetic wave, resolution by distance and radial speed relatively
M - 0,5cSr , SVr = Q,5A5F . (9)
In order to solve two or several targets (signals) their ambiguity diagrams shouldn't intersect. Potential accuracy of measurement of signal lag or frequency time is characterized by mean-square deviations aT or a, relatively, obtained during
optimal signal processing. As in sueh processing the value of range R and radial speed Vr are determined according to the position of maximum of two-dimensional correlation function, accuracy of fixation of this maximum depend on acuity of two-dimensional correlation function peak and signal and noise power ratio q-E/N0- If (/»I and independent measurement
r h F
(10)
(18)
At the output of the matched tiller the instantaneous power of the signal increased, the pulse duration on 0.7 level decreased by £ times in comparison with the pulse power and duration at the
input Thus, during the matched processing of the impulse with linear FM the signal compression effect is obtained, having the greater duration the higher resolution and accuracy on range measuring are provided. It should be noted that implementation of high resolution of a probing signal with interpulse linear FM is possible only with use of special receiver, provided that the retransmitted signal didn't undergo any random distortions in amplitude, frequency and phase in time ru- Otherwise, the
resolution worsens and may reach the minimum at the limit, equaling to the pulse duration: tcx = r„ • The last property
allows to register the fact of emission with broad band probing signal with use of the simplest energy receiver of, for instance, detector type. According to the theory of signal detection [7], in case of constant amplitude of a signal and random fluctuations of the phase, probability of the signal detection will be close to 1
10"
at a ratio of
Taking into account (2.3) we find out that potential range and radial speed measurement accuracy is characterized w ith errors
aR = 0,5ccrr, at = 0,5Z&/ (11)
Optimal processing of linear FM. Usually optimal processing of linear FM is performed with use of matched filters. Frequency characteristic of matched Jiher may be presented as
u(t) = Umexp{j(co0t + yt2)} (12)
where ,, _ R- rate of frequency change inside the impulse. If
A/rw»1, signal specter S(eo) may be calculated by an approximate approach — method of stationary phase, at that
if \co-(ot\<2xAf .Then Frequency characteristic of the filter,
matched with linear FM impulse,
k(a) = 4?(0)[exp(-/$||)]. (14)
where 71— signal lag in the filter; A - coefficient, having a
dimension, eountrary to the dimension of spectral density of the signal ¡6]. Using (13) and assuming the coefficient of amplification of the filter equaling to I, (14) may be written in the form of
Signal specter at the output of the matched filter
SBm((o) = Mv)kG>)
On the basis off 14) and (16) we get
with probability of false alarm signal/interference of more than 10 dB.
Estimation of parameters of probing signals with interpulse linear frequency modulation. The circumstance which limits the choice of a probing signal while synthesizing radar station apertures is the impossibility to combine probing and receiving the reflected signal. These conditions may be formulated in the following form:
mT + Ai < rmin, (19)
(/w + UT^Ai + r^, (20)
^2Ar + r_-rnlin, (21)
where T - repetition period of probing pulses; At - pulse duration; r , r ■ - maximal and minimal delav of rellected
• mj\ ' ITIin J
signal; m - integer. Besides, the factors limiting the choice of a probing signal are the limitations to its average and peak power, which may be written in the following form
(22)
P <P
p /jmax
" j"
Finally, the choice of a probing signal while synthesizing radar station apertures should allow to conduct selection to a distance with the desired resolution to realize angular resolution in the direction, perpendicular to a motion pattern of a carrier. At that the bandwidth of the signal should be no less than
A F = —■ (23)
2 SI
Here c — speed of light: si ~ resolution to a distance. Pulse repetition rate should provide extracting of Doppler frequencies of all areas of radar information, i.e. should be no less than
„ i VAx
ÀR
(24)
where V - carrier's speed; Av - radar information frame size along the motion pattern of a carrier; X - wavelength; R -range. The choice of element base and parameters of a receiver is the lirst range of issues, necessary to solve while designing such
T-Comm Tom 10. #1 2-2016
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a synthetic-aperture radar. In certain cases the choice of centimeter range of wavelengths is appropriate to use. At that, in the first place, necessary resolution along the motion pattern of a spacecraft may comparatively simply be provided, secondly, the effect of atmosphere is fairly small in comparison to, for instance, millimetre wavelength range, thirdly, there is a broad element base of both electrovacuum and solid-state devices. As an antenna of synthetic-aperture radar if is appropriate to choose the antenna of reflector type with opening umbrella reflector. We will assume that square of reflector aperture may reach 10...20 nr. This value determines the realizing amplification coefficient of antenna, which determines energetic characteristics of synthetic-aperture radar. The desired value of
synthetic aperture of antenna A may be evaluated from the ratio - <25>
2Sa
where X - wavelength; R - range; Sa - resolution on an
azimuth. For instance, at the orbital altitude of 550 km and viewing angles from 25 to 50° the range is within the range of 607...856 km. In that case, for instance, for X-range of wavelengths with the resolution of 2 m the value A is within the range of 4.6...6.4 km, and for L-range with the resolution of 6m- 11.6... 16.4 km, i.e. accumulation time reaches approximately 0.8 s for X-range of wavelengths, and 2 s for L-range. The desired aperture sizes of real antenna are determined from the condition of provision of necessary power of the received reflected signal, as well as from the sizes and position of the viewed surface area [9]. Using a synthetic aperture radar, when terrain is used as a probing object, value <r, may by presented as
where ¿f / - resolution of synthetic aperture radar across the
trajectory; a" - specific EPR of a surface considering radiation incidence angles on the surface. Amplification coefficient of antenna may be defined as follows
G -
4aS„
X
(27)
A
A/, Si R-
D„ " cos/?
where D , D ~ sizes of the aperture of antenna, it can be
obtained that the square of the probing terrain can be expressed with the formula
x
ASvjR2-
4 nR2
S„ cos fi G cos ß
or,
G«
ATIR-
(29)
(30)
AS cos ft
The given expression determines the necessary amplification coefficient of antenna, and therefore the square of its aperture.
Resolution across the trajectory of the carrier is determined by the probing signal modulation band, namely:
c (31)
S =-' 2AFstn ß
Considering also the face, that ineffective band AF
AF =-
m
(32)
where M - length of modulating consequence (using FC modulation ); N - number of times of probing, during which the accumulation is effected, and beyond that
M = r„AF, A = TrNV, (33)
Where t„ ~ probing pulse duration; Tr - repetition rate of pulses; V - speed of carrier, it may be obtained
S T>MrR m
N 4nASlcos2 fi-LkTiKj?
Here P - average power of a probing signal, which can be defined by the given expression. In detectoin mode, when the axis of the antenna's sight remains immobile, for the azimuthal direction the following expression can be written:
À H
HLAL, + A or D. <
ÀH
(35)
D cos ¡3 " ' cos + A )
where H — orbital altitude; A - size of synthesized aperture; 0 - size of real aperture of antenna in the azimuthal direction;
Al, : - radar information frame size along the spacecraft trajectory. Expression for o can also be presented in a form of 2 S„ (36)
D.<-
+ COSÖ—
' m u
where S„ ~ effective area of antenna's aperture, which, in
comparison with the size of the aperture of antenna in case of short wavelength of the radiation, is close to geometric square of the aperture (while evaluating in many cases it may be assumed that the effective square of the aperture of antenna comprise 0.8 of the geometric square of the aperture). Considering the fact that sizes of a probing area through the length and breadth of the trajectory are different if follows
 ■■ - J_, (28)
where g ~ resolution in the azimuthal direction. In extreme
case, when radar information represents consequently forming narrow bands across the carrier's trajectory, the following can be obtained:
D„ <2S„ ■ (37)
For the direction, perpendicular to the direction of the carrier's travel, the condition for determining the real antenna aperture size, have a form of
11
> A/., cos ß Dc<
All
(38)
Dr cos ft ' ' ' cos'/?AL_ where o. ~ size of real aperture antenna in the elevation direction: AL, ~ size of the observed terrain in the direction, perpendicular to the carrier's travel trajectory. As an example, the requirements to transmitter and to antenna of an Earth-viewing radar spacecraft with certain average parameters will be evaluated. Let space-based synthetic aperture radar be placed on the orbit with altitude //=500 km, performing detectoin mode at an angle (3 = it/4. Resolution makes up 5a = Sn -2 m, radiation wavelength - 3 cm, specific EPR of a surface - 0,001 nr, signal and noise ratio - 10, probing area square - 50 km2, Kv-L = 5- In
that case average power of a probing signal in case of pointing directivity pattern of the antenna accumulating on the probing area should make up
- = 4nAS'Cor/} LkT„K,r S « 6oo Wt, (39)
£<r"dpR N
And square of antenna's aperture is:
T-Comm Vol. 10. #12-2016
\ /
5 «R2——_~12m2. (40)
AS cos (i
The received data allows us to substantiate claims to the transmitter and to probing signal detection on the Earth.
Conclusions
On the basis of the analysis of properties of broad band probing radar stations with interpulse linear KM it is established that the required spatial resolution is defined by the effective spectral band, which depends on the pulse duration and its contraction coefficient at the output of a matched filter. In order to register the fact of irradiation by such signal the use of the simplest receiver is enough, for instance, detector receiver, provided signal and noise ratio at its input no more than 10. The obtained evaluation of requirements to the power of the on-board transmitter and to antenna, which will provide the required resolution at the given observation range and area; a transmitter of 600 Wt power, 3 cm wavelength, and ¡2 m radio dish {parabolic-shaped reflector). Time of the signal accumulation is 2 s.
References
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ИССЛЕДОВАНИЕ ВОЗМОЖНОСТЕЙ ОБНАРУЖЕНИЯ РАДИОЛОКАЦИОННЫХ ЗОНДИРУЮЩИХ СИГНАЛОВ КОСМИЧЕСКИХ АППАРАТОВ ОБЗОРА ЗЕМЛИ
Волохов Валерий Иванович, старший научный сотрудник, к.в.н., ВА РВСН им. Петра Великого, Москва, Россия,
val.volohov@yandex.ru
Филатов Владимир Иванович, преподаватель, к.т.н., МГТУ им.Н.Э. Баумана, Москва, Россия, vfill0@mail.ru
Жукель Александр Александрович, старший преподаватель, к.т.н., ВА РВСН им. Петра Великого,
Москва, Россия
Аннотация
Приводится описание различных систем радиолокационного мониторинга Земли с борта космических аппаратов, дается анализ их основных технических характеристик, на основе которых обосновывается требования к лабораторной установке, предназначенной для обнаружения и регистрации их зондирующих сигналов в полевых условиях. Для регистрации факта облучения земной поверхности в данной точке местности предлагается новый физический принцип непосредственного преобразования энергии электромагнитного поля зондирующих сигналов в электрический ток. На основе предложенного принципа разработано устройство, позволяющее обнаруживать радиолокационные зондирующие сигналы с борта космических аппаратов в широкой полосе излучения СВЧ (1,0-10,0 ГГц) и эффективной полосы спектра широкополосных сигналов с внутриимпульсной модуляцией (до 500 МГц). Для регистрации и наблюдения зондирующих сигналов используется комплект цифровых осциллографов с функцией запоминания, двухканальная приставка к персональному компьютеру с быстрым преобразованием Фурье и двухканаль-ным накопителем энергии повторяющихся импульсов на ионисторе.
Ключевые слова: широкополосный сигнал, линейная частотная модуляция, синтезированная апертура, пространственное разрешение, длина волны.
Литература
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4. Вознесенский Д.И. Антенны с обработкой сигнала. M.: Сайнс-Пресс, 2002. 80 с.
5. Каганов В.И. Радиопередающие устройства. M.: ИРПО, 2002. 288 с.
6. Конструирование и расчет полосковых устройств. Под ред. И.С, Ковалева. M.: Сов. радио, 1974. 295 с.
7. Котт В.M., Гаврилов T.K., Баваров С.Ф. Туннельные диоды в вычислительной технике. M.: Сов. радио, 1967. 216 с.
8. Залогин Н.Н., Кислов В.В. Широкополосные хаотические сигналы в радиотехнических и информационных системах. M.: Радиотехника, 2006. 208 с.
9. Ширман Я.Д. Разрешение и сжатие сигналов. M.: Сов. радио, 1974. 360 с.
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