the distribution function of the frequency fluctuations has s t r o n g e r tails than a G a u s s i a n function, and in c o r r e s p o n d e n c e with (9) the value of a is d e t e r m i n e d not only by the p a r a m e t e r s of the m e d i u m , but also by the time constant of the r e c e i v e r . LITERATURE 1.

2. 3. 4.

5. 6.

7. 8.

CITED

J. V. HoIlweg and J. V. H a r r i n g t o n , J. Geophys. R e s . , 7.33, 7221 (1968). R. Goldstein, Science, 166, 598 (1969). O. I. Yakovlev, ]3. P. T r u s o v , V. A. Viktorov, A. I. Efimov, Yu. M. Kruglov, S. S. Matyugov, and V. M. R a z m a n o v , Kosmich. I s s l e d . , 12, 600 (1974}. V. G. G a v r i l e n k o and N. S. Stepanov, Radiote, kh. E l e k t r o n . , 1.88, 1105 ('1973). O. I. Yakoviev, Radio-Wave P r o p a g a t i o n in the Solar System [in Russian], Izd, Soy. Radio, Moscow(1974). M. H. Cohen and E. J. G u n d e r m a n n , A s t r o p h y s . J., 155, 645 (1969). A. M. P r o k h o r o v , V. F. ]3unkin, K. S. Gochediashvili, and V. I. Shishov, Usp. Fiz. Nauk, 114, 415 (1974). A. V. Pynzar,, V. I. Shishov, and T. D. Shishova, Astron. Zh., 5_~2, 1187 (1975).

QUASIOPTICA

L RADIO

l~. I . G e l ' f e r , Yu. S. E . F i n k e l , s h t e i n ,

VISION V.

IN T H E

Lebskii, a n d N. A.

PASSIVE

REGIME

UDC 535:621.378 Yakun'

The possibility of d i r e c t quasioptieal m i l l i m e t e r radio vision in the p a s s i v e r e g i m e is t h e o r e t i cally and e x p e r i m e n t a l l y investigated. E x p e r i m e n t s have been r e p o r t e d on studies of radio vision [1-3] f o r the active r e g i m e , in which an object e m i t s a c o h e r e n t wave f r o m the t r a n s m i t t e r . However. radio vision in the p a s s i v e r e g i m e , i.e., the c o n s t r u e tion of r a d i o images of objects derived f r o m t h e i r s e l f - r a d i a t i o n , is c l e a r l y the m o s t interesting case. In the c u r r e n t a r t i c l e the possibility of p a s s i v e o b s e r v a t i o n of distant objects with m i l l i m e t e r waves is evaluated as a function of the b o d y - m e d i u m t e m p e r a t u r e c o n t r a s t , the sensitivity of the r e c e i v e r , and observation conditions, and a l a b o r a t o r y e x p e r i m e n t on p a s s i v e radio vision is d e s c r i b e d and the e x p e r i m e n t a l r e s u l t s presented. The i m a g e s of the objects w e r e constructed using a standard m i l l i m e t e r antenna [3]. In c o n t r a s t to r a d a r s e t s , in r a d i o vision we d e t e r m i n e not the total e n e r g y received by the antenna, but r a t h e r the field intensity distribution in a plane conjugate to the plane of the object. In selecting the dimensions of the fixed element (for e x a m p l e , a horn), both e n e r g y relations, as well as the demands f o r m a x i m a l resolution of the s y s t e m , m u s t be taken into account. C l e a r l y , the optimal a r e a of the horn is d e t e r m i n e d by the size of the resolving e l e m e n t in the image plane. In this c a s e , the emitted power f r o m an object with s i m p l e configuration z units f r o m the antenna is given by [4] Peck_. kTa A f ,l e-~z ,

(1)

w h e r e k is B o l t z m a n n ' s constant, T a is the a p p a r e n t t e m p e r a t u r e of the body, Af is the r e c e i v e r frequency band, u = 0.4-0.7 is the antenna a r e a utilization f a c t o r [5], and ~ is the a b s o r p t i o n coefficient in f r e e space. The power r e c e i v e d by the antenna m u s t exceed the fluctuation sensitivity threshold of the r e c e i v e r ( r a d i o m e t e r ) APmin: Prec> n a Pm,n.

(2)

It is usually a s s u m e d that the w e a k e s t signal that can be detected by a r a d i o m e t e r is equal to the standard deviation a of the output fluctuations. It can be verified in the c o u r s e of an e x p e r i m e n t (ef. Fig. 6) that in radio G o r ' k i i State University. T r a n s l a t e d f r o m I z v e s t i y a Vysshikh Uchebnykh Zavedenii, Radiofizika. VoL 19, No. 10, pp. 1512-1517, October, 1976. Original a r t i c l e submitted July 16, 1975. This material is protected by copyright registered in the name of Plenum Publishing Corporation, 227 West 17th Street, New York, N.Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechaiffcal, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $7.50.

1059

1.6 ram

; 1

a

16mrn

b Fig. 1. P r o b e p a t t e r n of image of a point source, a) O n e - d i m e n s i o n a l sweeping by b a c k r e f l e c t o r ; b) twod i m e n s i o n a l sweeping. vision reliable o b s e r v a t i o n of objects r e q u i r e s a received signal two to three t i m e s g r e a t e r than the width of the noise channel (about 4 - 6 a [6]); i.e., n ~ 10. It is known [7] that the p o w e r threshold sensitivity of a s u p e r m o d e r e c e i v e r is given by Pro,. = k A Tin,. A f,

(3)

w h e r e ATtain is the t e m p e r a t u r e threshold sensitivity. Substituting(i) and (3) in (2)yields the m i n i m a l t e m p e r a t u r e c o n t r a s t which can be detected by a r e c e i v e r with t e m p e r a t u r e sensitivity ATmin: Ta ~ n--e'~ ~ r.,..

(4)

V

Since, in the l a b o r a t o r y e x p e r i m e n t e z << i , setting v = 0.5, we obtain

T~ ~ 20 a T~..

(5)

E x p e r i m e n t s on d i r e c t quasioptieai r a d i o vision in the p a s s i v e r e g i m e w e r e conducted in the range X ~ 3 m m under l a b o r a t o r y conditions at a distance z = 5 m between the object and the r e c e i v e r s y s t e m . C e r a m i c e l e m e n t s of an o r d i n a r y hot r e f l e c t o r and e l e c t r i c hot plates w e r e used as the object. The r e c e i v i n g antenna, which c o n s i s t s of a p a r a b o l i c m i r r o r D = 600 m m in d i a m e t e r andelliptical counter r e f l e c t o r , c r e a t e s an i m a g e of the object in the plane behind an a p e r t u r e at the c e n t e r of the antenna; here a fixed r e c e i v i n g horn is set up. The i m a g e is swept m e c h a n i c a l l y by shifting the c o u n t e r r e f l e c t o r in the horizontal and v e r t i c a l d i r e c t i o n s in a plane p e r p e n d i c u l a r to the antenna axis. The t h e r m a l radiation detected by the horn along the ultra high-frequency channel e n t e r s the 3 - m m range r a d i o m e t e r [8].

Fig. 2

Fig. 3

Fig. 2. H a r s h image of point source. Fig. 3. H a r s h image of two s o u r c e s spaced apart.

1060

a

b

c

Fig. 4. H a r s h i m a g e s of c e r a m i c tablets f o r the following d i f f e r e n c e s between the m e a n t e m p e r a t u r e s of the object and e n v i r o n m e n t : a) AT = 20~ b) ~T = 28~ c) AT = 42~

a

Fig. 5. P h o t o g r a p h s of tablet: a) AT = 28~

b

b ) A T = 54"K,

The r a d i o m e t e r is a two--channel s u p e r h e t e r o d y n e modulated r e c e i v e r intended for reception of signals with a continuous s p e c t r u m ; it exhibits a fluctuating sensitivity threshold of 0.25~ w i t h a c o n s t a n t i n t e g r a t i o n * of 1 sec. The high sensitivity and stability of the r a d i o m e t e r w e r e achieved through the use of an u l t r a h i g h frequency channel with an o v e r s i z e d waveguide [9], a Z e n d e r - M a c h i n t e r f e r o m e t e r to s u p p r e s s heterodyne noise and " p a r a s i t i c " modulation [10, 11], a gallium a r s e n i d e m i x e r diode with Schottky b a r r i e r with ultrahigh i n t e r m e d i a t e - f r e q u e n c y yield and c o n v e r s i o n loss of 5-6 dB, and a low-noise i n t e r m e d i a t e - f r e q u e n c y , d e c i m e t r i c t r a n s i s t o r i z e d a m p l i f i e r with b a n d - p a s s 250 MHz and noise f a c t o r of about 4 dt3. A type PDS-021M :~-Y r e c o r d e r whose sweeps w e r e synchronized with the d i s p l a c e m e n t s of the antenna c o u n t e r r e f l e c t o r was used as the p r i m a r y detecting device. B o t h b r i g h t n e s s r e c o r d i n g on incandescent lamp film and h a r s h (two-level) r e c o r d i n g with the r e c o r d e r pen a r e possible. The r e c e i v i n g s y s t e m (antenna and c o u n t e r r e f l e c t o r ) a r e adjusted f o r a point s o u r c e (type P R K - 4 m e r c u r y tamp with d i a p h r a g m ) , f i r s t in the optical band, and then, finally, relative to the uhf signal. F i g u r e s 1 and 2 depict probe p a t t e r n s and the image of the lamp in the m i l l i m e t e r band, while Fig. 3 shows the image of two c e r a m i c e l e m e n t s with d i a m e t e r s 55 and 50 m m whose c e n t e r s a r e 130 m m apart. The a r r o w indicates the computed value of the width of the diffraction c u r v e at the 0.5 power level. A sheet of foam plastic I 5 - m m thick was placed between the source and r e c e i v e r during the e x p e r i m e n t , which eliminated effects on the r e c e i v e r due to infrared and visible light. Our e x p e r i m q n t was not only intended f o r obtaining i m a g e s of objects f r o m their t h e r m a l e m i s s i o n , but a l s o for d e t e r m i n i n g e x p e r i m e n t a l l y the minimally detectable t e m p e r a t u r e c o n t r a s t between a body and its environment. The r e s u l t i n g i m a g e s and probe p a t t e r n s a r e presented in Figs. 4-6 for different d e g r e e s of heating of the object (ceramic r a d i a t o r 16 c m in d i a m e t e r ) . It is c l e a r f r o m the figures that the signal inc r e a s e s and image quality i m p r o v e s with i n c r e a s i n g body t e m p e r a t u r e . The t e m p e r a t u r e of the c e r a m i c was regulated by c o p p e r - C o n s t a n t a n t h e r m o c o u p i e s simultaneously at 8 points on its surface at a depth of about 3 mm~ the standard deviation of the t e m p e r a t u r e s amounts to 15-20~ of the a v e r a g e value. Nonuniform heating of the c e r a m i c leads to a nonuniform distribution of the intensities in the image of the object, which is p a r ticularly noticeable at low t e m p e r a t u r e c o n t r a s t s (Figs. 4a, 5a). *The e x p e r i m e n t was p e r f o r m e d for a constant integration of 0.5 s e c , so that the sensitivity threshold amounted to 0.35~ A black body i m m e r s e d in boiling nitrogen was used to c a l i b r a t e the r a d i o m e t e r .

1061

b Fig. 6. P r o b e p a t t e r n s of the image of the tablet with two--dimensional sweeping, a) AT = 28~ 54~

b) AT =

We may conclude f r o m Figs. 4-6 that the least c o n t r a s t at which it is still possible to obtain an image of the object in o u r setup AT ~ 20-25~ The t h e o r e t i c a l value of the least detectable c o n t r a s t can be obtained by substituting in (5) the fluctuating t e m p e r a t u r e sensitivity threshold of the r a d i o m e t e r ATmi n = 0.35~ Ta~=7 "K. The difference between this value and the m e a s u r e d value m a y be attributed both to the fact that we have d i s r e g a r d e d e x p e r i m e n t a l e r r o r s and to the fact that the a p p a r e n t and physical t e m p e r a t u r e s of the c e r a m i c a r e not the s a m e . The authors wish to take this opportunity to e x p r e s s their thanks to V. N. Katinichenko and A. M. Shtanyuk f o r active p a r t i c i p a t i o n in the construction of the r a d i o m e t e r , and to V. P. Mezentsev for p r e p a r i n g the m i x e r diodes used in the e x p e r i m e n t . LITERATURE 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11.

1062

CITED

V. !. Vainberg, V. A. P a v e i ' e v , L. P. Stenina, N. A. Kharitonova, and V. N. Shuyukova, Radiotekh. l~lektron., 1.~9, No. 3, 602 (1974). I . I . Shchukin, Radiotekh. E l e k t r o n . , 2._00, No. 2, 405 (1975). l~. I. G e l , f e r , V. B. K r a v t s o v , S. E. F i n k e i ' s h t e i n , and A. V. Shisharin, Izv. Vyssh. Uchebn. Zaved., Radiofiz., 1_88, No. 5, 731 (1975). A . G . NikoIaev and S. V. P e r t s o v , Radio T h e r m a l Sounding [in Russian], Voenizdat MO SSSR, Moscow (1970). r M . S . Zhuk and Yu. B. Molochkov, Designing Antenna F e e d e r Devices [in Russian], E n e r g i y a , M o s c o w Leningrad (1966). N . A . E s e p k i n a , D. V. K o r o I ' k o v , and Yu. N. P a r i i s k i i , Radio T e l e s c o p e s and R a d i o m e t e r s [in R u s sian], Nauka, Moscow 0--973). Yu. A. Dryagin and L. I. F e d o s e e v , Izv. Vyssh. Uchebn. Zaved., Radiofiz., 1_.~2, No. 6, 813 (1969). V . N . Kalinichenko, Yu. V. "Lebskii, V. P. Mezentsev, V. M. T'evelevich, L. I. F e d o s e e v , and A. M. Shtanyuk, in: A b s t r a c t s of R e p o r t s Read to the Second A [l-Union S e m i n a r - S c h o o l on UHF Radio Receiving Devices [in Russian], E r e v a n (1974), p. 125. J. Bled, A. B r e s s o n , R. P a p a u l a r , and J. J. Wegrowe, L'Onde J E l e c t r i q u e , 4_~4, No. 442, 80 (1964). Yu. A. Dryagin, L. M. Kukin, and L. V. Lubyako, Radiotekh. E i e k t r o n . , 1__99,No. 8, 1779 (1974). L . I . F e d o s e e v and Yu. Yu. KuIikov, Radiotekh. E l e k t r o n . , 1._.66, No. 4, 554 (1971}.