A few attempts have been made to use MST radar interferometers in scatter probing of the ionosphere. A brief description of radar interferometer construction is given. These interferometer observations do not appear to have been successful

Farley, D. T.

English

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237 
3.78 RADAR INTEBFEROMETER MEASUREMENTS 
D. T. Farley 
School of E lec t r i ca l  Engineering 
Cornel1 University 
Ithaca,  t?Y 14853 
INTRODUCTION 
WOODMAN (1971) appears t o  have been the f i r s t  t o  use the interferometer 
technique i n  s c a t t e r  probing of the ionosphere. H i s  observations allowed him t o  
determine the posi t ion a t  which the radar  beam was normal t o  the magnetic f i e l d  
(and hence the d i p  angle) above the Jicamarca Observatory with very high 
accuracy. More recent ly  the technique has been extended, with the inclusion of 
Doppler information, and applied t o  studies of equator ia l  E-region e l e c t r o j e t  
plasma turbulence (FARLEY et  al., 1981; KUDEKI e t  al.,  19821, equator ia l  
F-region i r r e g u l a r i t i e s  (KUDEKI e t  al. , 19811, and auroral  e l e c t r o j e t  
i r r e g u l a r i t i e s  (PROVIDAKES e t  a l . ,  1983). There have been a few references t o  
interferometer observations i n  the MST radar l i t e r a t u r e  also,  but the observa- 
t i o n s  have been of various types, usually d i f f e ren t  from those i n  the 
ionosphere. One purpose of t h i s  paper i s  t o  t r y  t o  c l ea r  up any possible 
confusion, A s  y e t  there  do not appear t o  have been any successful 'true' ( i n  
the sense tha t  w e  describe below) MST radar interferometer observations, but 
only a few brief  attempts have been made, 
TEE BASIC IDEA 
In  i t s  simplest form, a radar interferometer consis ts  of two separated 
receiving antennas, each with i t s  own receiver,  and a s ingle  transmitt ing 
antenna, which could be e i t h e r  a t h i rd  antenna or one of the two receiving 
antennas. Suppose a l l  the antennas are pointed v e r t i c a l l y  (extending the 
r e s u l t s  t o  oblique observations is t r i v i a l )  and t h a t  there is  a s ingle  small 
(-point)  t a r g e t  located a t  some small angle 0 from the zeni th  i n  the plane 
defined by the v e r t i c a l  and the line joining the  phase centers  of the two 
receiving antennas. 
separation, La between the receiving antennas (as i s  always the case i n  
pract ice) ,  then the re  w i l l  be a small t i m e  delay, given by Lsinelc,  between 
s igna l  reception a t  the two antennas. 
difference of 
I f  the range t o  the t a rge t  i s  much greater  than the 
This delay t r ans l a t e s  i n t o  a phase 
A+ = UsinB = Ue (1) 
where k=Za/X i s  the radar wave number. 
difference can be measured eas i ly  by forming the complex cross  product of the 
two s ignal  voltages;  i.e.,  
I n  the absence of noise,  t h i s  phase 
where <*> represents  an ensemble or t i m e  average. 
determines the angular posi t ion of the t a rge t ;  changes i n  time of t h i s  angle 
determine q angular veloci ty ,  t o  which range information can be added t o  give a 
l i n e a r  ve loc i ty  component i n  the d i r ec t ion  of the interferometer baseline. 
order t o  avoid ambiguities of multiples of 2r i n  the phase measurement, it i s  
desirable  t o  have the beamwidth of one or more of the  antennas narrower than the 
interferometer lobe spacing. A s  a rough r u l e  of thumb, t h i s  i s  accomplished i f  
the two receiving antennas, fo r  example, are 'touching' ( t h e i r  l i n e a r  dimension 
i n  the baseline d i r ec t ion  i s  equal t o  L). 
This phase measurenent 
I n  
https://ntrs.nasa.gov/search.jsp?R=19840019046 2020-03-22T09:44:11+00:00Z
238 
COMPLICATING FACTORS 
There are a number of e f f e c t s  encountered i n  pract ice  which a l so  must be 
kept i n  mind. For example: 
1. Receiver noise. Since the noise i n  the two receivers  i s  uncorrelated, 
it w i l l  not contribute t o  the cross produce i n  (2). 
normalization constant, but t h i s  i s  not important. 
It w i l l  change the 
2. Comic noise and/or other  co r re l a t ed  interference.  These a r e  c m o n  t o  
both interferometer channels and so w i l l  cause problems. 
separately (with the t ransmit ter  o f f ,  say) and subtracted out. I f  the s ignal  i s  
weak and the correlated interference changes with t i m e  and/or i s  somewhat 
changed by the presence or absence of the t ransmit ter ,  the subtract ion i s  some- 
what changed by the presence or absence of the t ransmit ter ,  t he  subtraction may 
be d i f f i c u l t  t o  do accurately. 
They must be measured 
3. Multiple targets .  I f  there are several  t a rge t s  i n  the beam a t  once. 
t he  cross product i n  (2) w i l l  be a vector sum of phasors whose magnitude and 
d i r ec t ion  represent the strength and angular posi t ion of the separate targets ,  
and the magnitude of F w i l l  be l e s s  than unity even i f  the signal/noise r a t i o  i s  
high. I f  a l l  the t a rge t s  move with the same veloci ty  and maintain the  same 
r e l a t i v e  strengths,  the veloci ty  can s t i l l  be determined from the r a t e  of change 
of the mean phase angle, but i f  the s i t ua t ion  i s  more complicated it may become 
impossible t o  in t e rp re t  the phase changes. 
4. Broad target .  Much the a t m e  arguments apply t o  a t a rge t  with an 
appreciable angular width. 
i.e., i f  w e  can describe the tar e t  as having an angular variance 02, the 
The magnitude of F gives a measure of the width; 
magnitude of F i s  roughly exp(-k 'i L2u2/2). 
RADIAL MOTION AND CROSS-SPECTRAL ANALYSIS 
So f a r  w e  have neglected t a r g e t  motion i n  the d i r ec t ion  of the radar beam. 
The Doppler Such motion w i l l  produce Doppler s h i f t s  i n  the received signals. 
s h i f t s  thus give addi t ional  information about the sca t t e r ing  medium and a l s o  
o f t en  can be used t o  separate the contributions t o  the s ignal  from multiple 
targets ,  i f  the individual t a rge t s  have d i f f e ren t  r ad ia l  ve loc i t i e s  as w e l l  as 
d i f f e ren t  angular positions. The analysis  of the s ignals  i n  t h i s  case i s  
similar t o  tha t  given above, but w e  introduce the addi t ional  step of f i r s t  
Fourier transforming t h e  two signals.  Next, we replace the simple vol tage cross  
product with the cross spectrum of the two individual spectra;  i.e., we compute 
where V 1 ( w )  and V2(W) a r e  the (voltage) Fourier transforms of Vl ( t )  and V 2 ( t ) .  
By analogy with (2) and from the discussion above of broad targets ,  i t  i s  f a i r l y  
easy t o  see that ,  i n  terms of the t a rge t  parameters, the cross  spectrum i s  
- 
where and are the mean angular posi t ion and spread of the t a rge t  giving 
a Doppler s h i f t  of w .  This r e s u l t  neglects noise contributions,  e tc .  For a 
more complete discussion see FARLEY e t  a l .  (1981). 
This measurement and analysis  procedure has proved tb be a very powerful 
239 
t oo l  for invest igat ing plasma turbulence i n  the ionosphere, as mentioned i n  the 
introduction, but f u l l  cross spectral analysis  does not appear t o  be useful for  
MST applications.  To see why, le t  us assume a typical  v e r t i c a l  veloci ty  t o  be 
“.5 m / s  (e.g., WOODMAN and GUILLPN, 19741, which would give a Doppler s h i f t  of 
0.1-0.2 Hz a t  a radar frequency of 50 Mz. 
resolut ion of *.Ol Hz, say, would require  an observation t i m e  of a t  least 1 min 
f o r  Fourier transform, and several  computations of the cross  spectrum 
must be averaged t o  obtain a meaningful resul t .  I n  the required in t eg ra t ion  
time of several  minutes, a sca t t e r ing  center t ravel ing with a horizontal  velo- 
c i t y  of say 10 m f s  w i l l  move several  kilometers, probably enough t o  move the 
scatterer out of t he  radar  beam and cer ta inly enough t o  give a phase change of 
many radians. Operating a t  higher radar  frequencies w i l l  improve the s i t ua t ion ,  
but probably not by enough. 
To achieve a reasonable frequency 
In  pract ice ,  then, one must rwert t o  the ana lys i s  of (21, which i s  a l s o  
much simpler computationally, and ignore the v e r t i c a l  veloci ty  information i n  
the  interferometer calculations.  
estimates of the v e r t i c a l  veloci ty  by calculat ing the power spectrum of e i the r  
of the received signals,  or w e n  the magnitude of the crosg spectrum, which i s  
e s sen t i a l ly  the same thing. It i s  only the phase information i n  the  cross 
spectrum which i s  of no use i n  MST applications.  
MST INTERFEROMETER OBSERVATIONS 
It i s  s t i l l  of course possible t o  obtain mean 
Some attempts to  observe horizontal  ve loc i t i e s  a t  Jicamarca i n  the way j u s t  
described have been mentioned by RUSTER and WOODMAN (1976) and RUSTER e t  a l .  
(1978). The a t t anp t s  were unsuccessful, however. The successive phase angles 
were more or l e s s  randomly d i s t r ibu ted ,  indicat ing t h a t  s ca t t e r ing  centers  were 
d i s t r ibu ted  throughout the radar beam and t h a t  the sca t t e r ing  could not be 
modeled a s  coming from one or two d i s c r e t e  moving centers.  The horizontal  velo- 
c i t i e s  shown i n  RUSTER e t  a l .  (1978) were obtained from a vers ion of Spaced 
Antenna D r i f t  (SAD) analysis.  
obtained from Fourier analysis  of the s ignals  received on the two separated 
antennas were cross correlated and the horizontal  veloci ty  was determined from 
the delay a t  which the cross  correlat ion maximized. 
interferometer analysis  were not very exhaustive, howwer, and the re  is some 
reason t o  hope t h a t  they might be successful a t  times i n  the mesosphere a t  least 
(WOODMAN, pr iva t e  communication, 1983). Some more recent observations a t  
Jicamarca have led t o  s imilar  conclusions (COENISH, pr ivate  communcation, 1983). 
(Added note:  One successful mesospheric observation was described a t  t h i s  work- 
shop by M. I e r k i c  and J. Rottger.) 
The two t i m e  s e r i e s  of the v e r t i c a l  ve loc i t i e s  
The attempts a t  the ‘ t rue ’  
Some work using the SOUSY radar with spaced antennas and phase coherent 
receivers  has been described by ROTTGER and VINCENT (1978) and VINCENT and 
ROTTGEB (1980). The voltages were cross  correlated,  and the l a g  a t  which the 
magnitude of the cross  correlat ion w a s  greatest  w a s  used t o  determine the velo- 
c i t y  ( the  SAD technique). I n  contrast ,  the technique discussed here u t i l i z e s  
t h e  phase of the cross correlat ion a t  zero l a g  t o  determine position. The 
SOUSY work did involve using phase information t o  determine the character of the 
sca t t e r ing  medium. The ‘ r ad ia t ion  pattern‘,  so t o  speak, of the medium w a s  
measured by s t ee r ing  the lobe pat tern of the interferometer array numerically i n  
the data processing. 
a l t i t u d e  of 2.44 km corresponded t o  i so t rop ic  scatterers but those from 3.79 km 
closely approximated a specular p a r t i a l  ref lect ion.  This sort of analysis  is 
r e l a t ed  to,  but not q u i t e  the s a m e  as, measuring t h e  magnitude of F i n  (2) t o  
determine i n  some sense the angular width of the d i s t r ibu t ion  of s ca t t e r e r s .  
This width would be very small for  a quasi-specular ref lect ion,  but would 
roughly equal the antenna bem width fo r  i so t rop ic  and uniformly d i s t r ibu ted  
scatterer 8. 
I n  one example it was found t h a t  the echoes from an 
240 
REFERENCES 
Farley,  D. T., E. 1. I e r k i c  and BP: G. Fejer  (19811, Radar interPerometrp: A new 
technique for  studying plasma turbulence i n  the ionosphere, J. Geophys. 
Res.. 86, 1467-1472. 
Kudeki, E,, D. T. Farley, and B. G. Fejer  (19821, Long wavelength i r r e g u l a r i t i e s  
i n  t he  equator ia l  e l e c t r o j e t ,  Geophvs. Res. Le t t . ,  9, 684-687. 
Kudeki, E., B. G. Fejer,  D. T. Farley and H. M. I e r k i c  (1981), Interferometer 
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Lett.,  8 ,  377-380. 
Providakes, J. F., W. E. Swartz, D. T. Farley and B. G. Fejer  (19831, F i r s t  VHF 
auroral  radar interferometer observations, Geophys. Res.  L e t t . ,  10, 401-404 
Rottger, J. and R. A. Vincent (19781, WF radar  s tudies  of tropospheric velo- 
c i t i e s  and i r r e g u l a r i t i e s  using spaced antenna techniques, Geoohvs. Res .  
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Ruster, R., J. Rottger and R. F. Woodman (19781, Radar measuranents of waves i n  
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Ruster, R. and R. F. Woodman (19761, Radar measurements i n  the tropo-, s t ra to- ,  
and mesosphere, Preprints,  17th Radar Meteor. Conf ., 354-358. 
Vincent, R. A. and J. Rottger (19801, Spaced antenna VIIF radar observations of 
tropospheric v e l o c i t i e s  and i r r e g u l a r i t i e s ,  Radio Sci.. 15, 319-335. 
Woodman, R. F. (19721, Inc l ina t ion  of the geomagnetic f i e l d  measured by an 
incoherent scatter technique, J. Geophvs. Res., 76, 178-184. 
Woodman, R. F. and A. Guillen (1974), Radar observations of winds and turbulence 
i n  the  s t ra tosphere and mesosphere, J. Atmos. Sci.. 31, 493-505. 


Radar interferometer measurements

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