Electronic beam steering is described as complex and expensive. The Sunset implementation of electronic steering is described, and it is demonstrated that such systems are cost effective, versatile, and no more complex than fixed beam alternatives, provided three or more beams are needed. The problem of determining accurate meteorological wind components in the presence of spatial variation is considered. A cost comparison of steerable and fixed systems allowing solution of this problem is given. The concepts and relations involved in phase steering are given, followed by the description of the Sunset ST radar steering system. The implications are discussed, references to the competing SAD method are provided, and a recommendation concerning the design of the future Doppler ST/MST systems is made

Clark, W. L.

Green, J. L.

English

NASA Technical Reports Server

6.2A PRACTICALITY OF ELECTRONIC BEAM STEERING FOR MST/ST RADARS 
W. L. Clark and J. L. Green 
Aeronomy Laboratory 
National Oceanic and Atmospheric Administration 
Boulder, Colorado 80303 
Elect ronic  bean s t ee r ing  has been prominently described a s  complex and 
expensive (SKOLNIK, 1962). I n  t h i s  paper we describe the Suaset implementation 
of e l e c t r o n i c  s t ee r ing ,  which has been i n  rou t ine  use since 1981, and demon- 
s t r a t e  t h a t  such systems are ,  i n  f a c t ,  cost  ef fect ive ,  v e r s a t i l e ,  and no more 
complex than f ixed  bean a l t e rna t ives ,  provided three  or more berms a r e  needed. 
F i r s t ,  a s  an example of the  need f o r  multibean systems, the problem of 
determining accurate  meteorological wind components i n  the  presence of s p a t i a l  
v a r i a t i o n  i s  considered. Next, a cost  comparison of s t ee rab le  and f ixed systens 
allowing so lu t ion  of t h i s  problem i s  given. Then the concepts and r e l a t i o n s  
involved i n  phase s t ee r ing  a r e  given, followed by the descr ip t ion of the  Sunset 
ST radar  s t ee r ing  system. F ina l ly ,  the  implications a r e  discussed, references 
t o  t h e  competing SAD method a r e  provided, and a recommendation concerning the  
design of the fu tu re  Doppler ST/MST systems i s  made. 
ESTIMATION OF METF.OROLOGICAL WIND COMPONENTS 
Beam pointing ST/MST radars  measure the component of wind i n  the  d i rec t ion  
of probing. The wind components of meteorological i n t e r e s t  (u, v ,  and w) a r e  
r e l a t e d  t o  these r a d i a l  ve loc i ty  ( v r )  mensurenents by a s e t  of l inea r  vector 
equations. The v e r t i c a l  component, w, can be d i r e c t l y  measured by a v e r t i c a l l y  
pointing bean, but the u and v components must be obtained by solving these , 
equations. I~nfor tunate ly ,  so lu t ion  i s  poss ible  only i f  a model of the loca l  
wind f i e l d  i s  assumed. 
The most often used model assumes t h a t  there  i s  no hor izonta l  change i n  
the  ve loc i ty  components so t h a t  measurenent of v r  a t  only th ree  bean pos i t ions  
i s  s u f f i c i e n t  t o  determine u, v and w. This assumption i s  o f t en  not j u s t i f i e d  
and leads t o  inaccuracies and b ias  i n  determinations of u and v (CLARK e t  - 
al. ,  1983). 
The next simplest model assumes t h a t  only the f i r s t  s p a t i a l  de r iva t ive  of 
the  ve loc i ty  components i s  s ign i f i can t ,  requir ing measurements of v r  fo r  a given 
height a t  seven bean posi t ions  t o  solve f o r  u and v. There i s  a bonus f o r  
dealing with t h i s  added complexity. The necessary estimation of the f i r s t -  
order s p a t i a l  der ivat ives  a 1  lows estimation of the divergence and v o r t i c i t y  a t  
each probed height above the  radar. Thus, a seven-bean system provides not 
only more accurate,  but addi t ional  meteorological information. A de ta i l ed  con- 
s ide ra t ion  of optimum bean posi t ions  fo r  solving t h i s  f  i rs t -order  model has 
been given by KOSCIELNY and DOVIAK, 1983. 
The f i r s t -o rde r  model above may be inadequate i n  some cases. For example, 
under l e e  wave conditions a pseudo-sinusoidal model of the flow would be more 
appropriate i n  t h e  d i rec t ion  of the wind. Under convective conditions,  the 
flow can be too complex t o  model i n  a useful way. These cases w i l l  not be 
considered f u r t h e r  except t o  note t h a t  the f l e x i b i l i t y  of e l ec t ron ic  s t ee r ing  
can allow use of more appropriate wind f i e l d  models than a fixed-bean system. 
https://ntrs.nasa.gov/search.jsp?R=19850024203 2020-03-20T18:07:43+00:00Z
COST COMPARISON 
The biggest  b a r r i e r  t o  implenenting a system capable of seven or more bean 
posi t ions  i s  probably cost .  Fixed bean i n s t a l l a t i o n s  require  seven separate 
a r rays  a t  $25,000 to $50,000 each. Elect ronic  beam s teer ing,  coupled with 
time-multiplexing of berm posi t ions ,  can reduce t h i s  cost  t o  t h a t  of two ortho- 
gonal antenna arrays  and the  cost  of the  s t ee r ing  system. The s teer ing system 
i n  the  Sunset radar  was added t o  two previously ex i s t ing  antenna arrays  a t  a 
cos t  of about $15,000. Thus a seven-bean time-multiplexed system was imple- 
mented for l e s s  than the cost  of a f ixed three-bean system, and f o r  l e s s  than 
317 the  cost  of a f ixed seven beem system. Furthewore,  o ther  configurations 
f o r  o ther  types of experiments can be, and of ten are ,  implemented i n  minutes 
a t  no f u r t h e r  cost. The cost  of t r ansmi t t e r s  and receivers  i s  ignored because 
of the  many p o s s i b i l i t i e s  f o r  both types of systen. 
Referring t o  Figure 1, i t  can be seen t h a t  when the constant phase surface 
of a plane wave j u s t  a r r ives  a t  antenna element n with angle A t o  the zeni th .  
it has the  dis tance d(n) remaining before i t  reaches the l a s t  element. By 
adding compensating lengths of feedl ine  t o  each element equal t o  t h i s  d is tance,  
the  received s igna l s  a r e  made t o  add i n  phase a t  the receiver.  Thus, the  gain  
of the  antenna i s  made maximum for  plane waves a r r iv ing  a t  angle A and the an- 
tenna i s  sa id  t o  point i n  t h i s  direction.  A more de ta i l ed  descr ip t ion of t h i s  
process and approximations fo r  the antenna. pat tern  a re  presented i n  SKOLNIK 
(1962). 
ZENITH 
I 
I 
I 
Figure 1. This i s  a side view of a 
I 
/ constant-phase surf ace fo r  a plane 
wave jus t  reaching element 4 of a 
' . 16-element antenna array.  The + 
quant i ty  of i n t e r e s t ,  d ( 4 ) ,  i s  the 
dis tance from tihe surface t o  the 
CONSTANT 
PHASE l a s t  antenna element. The zeni th  
angle of incidence i s  A and the 
freespace radar wavelength i s  L. 
'The black squares represent the 
r r m m  phaseable antenna elements spaced 
4 ' 1 / 2 L a p a r t .  
1 - - - r  XYZK 
From the geometry shown i n  Figure 1 it i s  easy t o  develop the feedl ine  
compensating length r e l a t i o n  
(1.1)... d(n) = (15-n) (L'/2) sin(A) [m] 
Similar ly ,  fo r  a plane wave a r r iv ing  from the other d i rec t ion  
where 
d(n) i s  the  length of feedl ine  t o  add t o  elenent n. 
L' i s  the radar wavelength i n  the feedl ine  i n  meters. 
A i s  the  zeni th  angle of the radar  probing di rect ion.  
..... n = 0,  1 15 i s  the dipole  number. 
These r e l a t i o n s  w i l l  be used l a t e r  on. 
Tl3E SUNSET ELECTRONIC PHASE STEERING SYSTEM 
An e lec t ron ic  phase s t ee r ing  system, implenented i n  1981, i s  i n  rout ine  
use i n  the  Sunset radar  system. The system has proven t o  be very r e l i a b l e  and 
year ly  checks with a vector voltmeter show v a r i a b i l i t y  within each module t o  be 
l e s s  than a degree of phase. The system se lec t ive ly  s t e e r s  e i t h e r  the north- 
south or east-west probing antenna t o  wi thin  .4 degree of any zenith angle be- 
tween 0 and 45 degrees. The s t ee r ing  i s  under progran control  of the online 
computer control l ing the radar  and processing the re turd  signals.  Before be- , 
ginning each sounding the  program s e t s  the probing d i r e c t i o n  from a sequent ia l  
l i s t  of d i rect ions ,  input by the operator when the system is  i n i t i a l l y  s t a r t ed .  
A s  present ly  implemented, up t o  e ight  posi t ions  may be i n  the  l is t ,  which i s  
repeatedly cycled. The operator may change t h i s  posi t ion list a t  any time. 
The time needed t o  change probing d i rec t ions  i s  about a mill isecond, but 
ground c l u t t e r  removal f i l t e r s  have made the pract icable  switching r a t e  about 
30 seconds. It should be poss ible  t o  remove t h i s  delay should f a s t e r  cycling 
become important. 
Power loss  through the s t ee r ing  systen i s  d i f f i c u l t  t o  measure because i t  
i s  l e s s  than 1 dB, including the harness where the element feedl ines  a r e  joined. 
Each of the  16 phase s h i f t  modules can handle 10 kW of peak power, of 1 kW of 
average power a t  40 MKz. This i s  the  same capaci ty  a s  t h a t  of the RG-8 cable  
used throughout the  antenna system a f t e r  the harness so t h a t  the system can 
handle 160 kW t o t a l  peak power. 
PHASE SHIFT MODULES 
Each phase s h i f t  module i s  an aluminum box which i s  inse r t ed  i n t o  the 
f eed l ine  t o  a s ing le  antenna elaaent.  A mechanical re lay ins ide  the box se- 
l e c t s  connection t o  an element of e i t h e r  the north/south or east/west s t ee r ing  
array.  Eight addi t ional  mechanical r e l ays  a r e  used t o  i n s e r t  up t o  e ight  se- 
l ec ted  lengths of coaxial  cable  i n t o  the feedl ine  as  i t  passes through the box. 
The posit ion,  0 or 1, of each of the r e l ays  is  control led  with nine TTL 
l w e l  control  s igna l s  enter ing the phase module through a 25 pin D connector. 
When i n  pos i t ion  1 the  f i r s t  phasing re lay  adds L1 /2  meters i n t o  the feedlcne. 
The second adds L1/4,  and the  t h i r d  L' 18, and so on geometrically till the 
eighth and l a s t  phasing r e l a y  adds L1/256 meters. Thus, any f r a c t i o n  of a . 
wavelength tha t  can be expressed a s  an eight-digit  binary f r a c t i o n  can be in- 
serted.  The ninth re lay i n  the box i s  used t o  se lec t  the north/south or e a s t /  
west s t ee r ing  a r ray ,  
STEERING CONTROL 
The t a sk  of s e t t i n g  each of the 16 phase delay modules t o  the proper delay 
i s  handled by the main radar  system computer. A t  the s t a r t  of each sounding 
the  des i red zeni th  angle i s  Converted t o  ,the appropriate 16 delays, expressed 
a s  f r a c t i o n s  of a wavelength, Fn, using the r e l a t i o n s  
(3.1)... Fn = (n/2)s in(a)  mod 1, n=O, 1, ... ,15 
3 . 2 .  Fn = [ ( 1 5 - n ) / 2 ] s i n ( ~ )  mod 1, n=O, 1,. . .15 
These are,  of course, equations (1.1) and (1.2) where the length informa- 
t i o n  has been removed; the phase modules themselves contain the length informa- 
t i o n  i n  the  form of cut cables. The mod func t ion  i s  used because lengths  great-  
e r  than a wavelength a re  not necessary; the goal i s  t o  keep the  elements i n  
phase, not  t o  sample the same phase surface. The se lec t ion  of (3.1) o r  (3.2) 
i s  determined by the sign of the zeni th  angle. 
'.j. , 
The con t ro l  s igna l s  f o r  the phasing r e l a y s  a r e  obtained by converting the  
f r ac t ion ,  Fn, t o  an eight-bit  binary in teger ;  Fn i s  mul t ip l ied  by 256 and 
truucated. An addi t ional  b i t  i s  appended t o  se lec t  northfsouth or eastlwest 
steering.  This operation i s  performed f o r  a l l  16 phase s h i f t  modules and the 
r e s u l t i n g  144 b i t s  a r e  sent s e r i a l l y  over coaxial  cable t o  s h i f t  r e g i s t e r s  lo- 
ca ted near the modules. The output of these s h i f t  r e g i s t e r s  controls  the re- 
l ays  i n  the  modules. 
DISCUSSION 
Implenentation of phase s t ee r ing  a t  the  Sunset f a c i l i t y  has proven t o  be 
simple, cos t  e f fec t ive ,  v e r s a t i l e ,  and re l i ab le .  The radar  has a l l  the capa- 
b i l i t i e s  of fixed-bean sys tens  except f o r  simultaneous probing i n  two o r  more 
di rect ions .  This i s  an important lack, but the addi t ion of a second trans- 
ce ive r  could allow simultaneous probing i n  two d i rec t  ions. Furthermore, mutual 
in te r fe rence  makes d i f f i c u l t  the probing of more than two d i rec t ions  simulta- 
neously f o r  any type of system. Final ly ,  the  added f l e x i b i l i t y  i n  experimental 
prograns provided by a s t ee rab le  system more than compensates f o r  t h i s  l imita- 
t ion.  ' 
Only Doppler systems a r e  considered here, but it should be noted t h a t  
spaced antenna d r i f t s  (SAD) systems may be cos t  competitive and provide superior 
r e s u l t s  i n  some instances.  HOCKING (1983) argues strongly f o r  t h i s  technique, 
while ROYRVIK (1983) describes an observed l imi ta t ion.  ROTTGER (1983) compares 
t h e  two techniques from a theore t i ca l  standpoint. 
Beam-swinging f a c i l i t i e s  s t i l l  being designed, especia l ly  a s  prototypes, 
should include expansion t o  e l ec t ron ic  s t ee r ing  capab i l i ty  i n  the design, even 
i f . e l e c t r o n i c  s t ee r ing  i s  not o r ig ina l ly  impleaented. This w i l l  place few con- 
, 
s t r a i n t s  on the design because there  a r e  many ways t o  achieve the necessary 
phasing delays  and w i l l  allow l a t e r  expansion t o  s teer ing a t  minimal cost .  
REF- CES 
- Clark, W. L., J. L. Green and J. I. Uarnock (19831, Estiroating unbiased horizon- 
t a l  v e l o c i t y  components from STfMST radar  measurements, Handbook f o r  MAP, 
--- - --- A Vol. 9, SCOSTEP Secre ta r i a t ,  Dep. Elec. Eng., Univ. 11, Urbana. 
- 
Hocking, W. K. (1983), The spaced antenna d r i f t  method, Handbook f o r  MAP. Vol. 
SCOSTEP Secre ta r i a t ,  Dep. Elec. Eng., Univ. 11, Urbana. 
Koscielny, A. J. and R. J. Doviak (1983), An evaluation of the accuracy of some 
radar  wind p ro f i l ing  techniques, Handbook f o r  MAP, Vol. 9, SCOSTEP 
Secre ta r i a t ,  Dep. Elec. Eng., Univ. 11, Urbana. 
Rottger,  J. (19831, Techniques for measurement of hor izonta l  and v e r t i c a l  ve- 
l o c i t i e s ,  Handbook f o r  MAP. Vol. 9, SCOSTEP Secre ta r i a t ,  Dep. Elec. Eng., 
Univ. 11.. Urbana. 
Royrvik, 0. (19831, Spaced Antenna Dr i f t ,  Handbook f o r  MAP, Vol. 9, SCOSTEP 
Secre ta r i a t ,  Dep. Elec. Eng., Univ. 11, Urbana. 
Skolnik, M. I. (19621, Introduction t o  Radar Systems, McGraw H i l l .  


Practicality of electronic beam steering for MST/ST radars, part 6.2A

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