Radar scattering from ionized meteor trails has been used for many years as a way to determine mesopause-level winds. Scattering occurs perpendicular to the trails, and since the ionizing efficiency of the incoming meteoroids depends on the cosine of the zenith angle of the radiant, echoes directly overhead are rare. Stratosphere-troposphere (ST) radars normally sample within 15 deg of the vertical, and thus receive few meteor echoes. Even the higher powdered mesosphere-stratosphere-troposphere (MST) radars are not good meteor radars, although they were used to successfully retrieved meteor winds from the Poker Flat, Alaska MST radar by averaging long data intervals. It has been suggested that a receiving station some distance from an ST radar could receive pulses being scattered from meteor trails, determine the particular ST beam in which the scattering occurred, measure the radial Doppler velocity, and thus determine the wind field. This concept has been named MENTOR (Meteor Echoes; No Transmitter, Only Receivers)

Roper, R. G.

English

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9.5A NENTOR -- ADDING AN OUTLYING RECEIVER TO AN ST RADAR 
FOR METEOR-WIND MEASURPfENTS 
R,  G. Roper 
School of Geophysical Sciences 
Georgia I n s t i t u t e  of Technology 
At lanta ,  GA 30332 
Radar s c a t t e r i n g  from ionized meteor t r a i l s  has been used f o r  many years  
a s  a way t o  determine mesopause-level winds. Sca t t e r ing  occurs perpendicular 
t o  the t r a i l s ,  and s ince  the ioniz ing e f f i c i ency  of the  incoming meteoroids de- 
pends on the  cosine,of the  zen i th  angle of the  r ad ian t ,  echoes d i r e c t l y  o v e r  
head a r e  rare .  ST ,hadars normally sample wi th in  15" of the  v e r t i c a l ,  and thus 
r ece ive  few meteor echoes. Even the higher powered MST radars  a r e  not good 
meteor radars ,  although Avery and coworkers have successfully r e t r i eved  meteor 
winds from the  Poker F l a t ,  Alaska MST rada r  by averaging long data  in t e rva l s .  
It has been suggested t h a t  a receiving s t a t i o n  some'distance from an ST 
. 
radar  could receive  pulses  being sca t t e red  from meteor t r a i l s ,  determine the 
p a r t i c u l a r  ST bean i n  which the  s c a t t e r i n g  occurred, measure the r a d i a l  Doppler 
ve loc i ty ,  and thus determine the  wind f i e l d .  This concept has been named 
MENTOR (Meteor Echoes; No Transmitter ,  Only Receivers). This paper i s  a pre- 
l iminary look a t  system requirements and p o s s i b i l i t i e s .  
There a r e  a number o f  immediate ques t ions  t o  be answered, such a s ;  I f  we 
rece ive  a pulse sca t t e red  from an ST bean, how can we t e l l  which beam i t ' s  from? 
Now can we measure the Doppler ve loc i ty  without access t o  the  ST'S o s c i l l a t o r ?  
Can we a l so  measure the , a l t i t u d e  of s c a t t e r i o g ?  Can we use t h i s  information t o  
determine the wind f i e l d ?  I n  t h i s  paper we sketch one poss ib le  approach to  
these questions.  
SITE LOCATION 
Consider a typ ica l  ST radar  with 3 beans; one v e r t i c a l ,  one s t ee red  15 
degrees due North, and the  t h i r d  poir.ted 15 degrees East ,  a s  shown i n  Figure 1. 
Many STs forego the  v e r t i c a l  betm, and with it the a b i l i t y  t o  measure . 
-IC _ __ 
-. -- . _ __- v e r t i c a l  winds. The I.IENTOR approach probably can' t  measure v e r t i c a l  winds any- 
how, but viewing the  t h i r d  beam of an ST rada r  w i l l  increase the MENTOR r a t e  by 
50%. I f  a l l  three  ST beams use a common frequency, no add i t iona l  hartiware i s  
required. 
For the geometry shown i n  Figure 1, the  bes t  locat ion f o r  a KENTOR s i t e  i s  
probably along a NE-SW l i n e ,  so t h a t  the th ree  ST beams can be seen from the  
MENTOR s i t e  a s  three  separa te  beans with maximal separation.  We w i l l  consider 
two MENTOR s i t e s ;  one 100 kn from the  ST s i t e  and one 300 km from the  ST s i t e .  
ST BEAM DETERMINATION 
Consider a p a i r  of MENTOR antennas def in ing a l i n e  normal t o  the  MENTOR - 
ST axis .  Let D be the spacing between the two antennas. Then the hor izonta l  
projec t ion of the geometry i s  a s  shown i n  Figure 2. (This f igu re  ac tual ly  
shows the  hor izonta l  projec t ion f o r  a MENTOR s i t e  located t o  the SW, ra the r  
than NE, of the ST s i t e . )  We want t o  choose D so tha t  the spacing of the ST 
beams maps in to ,  say, 90 degrees phase d i f f e rence  between t h i s  pa i r  of MENTOR 
antenna s . 
https://ntrs.nasa.gov/search.jsp?R=19850024225 2020-03-20T18:22:07+00:00Z
NORTH 
MENTOR SITE 
\ 
Figure 1. ST con£ igu ra t ion  
and Mentor s i t i n g .  
- 
Figure 2. Beam-determination geometry. 
Take the  a l t i t u d e  r eg ion  of i n t e r e s t  t o  be  95 f 15 bn, and ignore  e f f e c t s  
due t o  the  E a r t h ' s  curvature.  Assume an ST frequency of 50 MHz, so t h a t  = 6 
meters. Let the ST-to-MENTOR d i s t ance  be G (= e i t h e r  100 o r  300 km f o r  the 
two strawman loca t ions  we have chosen.) Then we g e t  
2TD s i n  a h 
Thus we f i n d  the  des i r ed  spacing of the  t ransverse  pa i r  of antennas t o  be 
10.1 meters f o r  the near s i t e  and 26.5 meters fo r  the f a r  s i t e .  With t h i s  
spacing, t he  phase d i f f e rence  ac ross  the antennas w i l l  be 0 degrees i f  the 
received pulse o r ig ina t ed  i n  the STs v e r t i c a l  beam, +90 degrees i f  ir came 
from the ST'S North bean, and -90 degrees i f  it came from the  ST'S East beam. 
The range of a l t i t u d e s  (+/- 15 km) maps t o  a spread i n  the phase-difference 
values  of +/- 25 degrees. 
Some STs opera te  using d i f f e r e n t  frequencies on d i f f e r e n t  beams t o  im- 
prove beam separation.  This makes bean i d e n t i f i c a t i o n  easy,  but means t h a t  two 
MENTOR channels a r e  needed a t  each ST frequency fo r  a l t i t u d e  determinations.  
ALTITUDE DETERMINATION 
For a f t i t u d e  determinations,  we need an antenna (and r ece ive r  channel)  t o  
form a long i tud ina l  pa i r  wi th  one of the f i r s t  two antennas. Now consider the 
phase d i f f e rence  across  t h i s  p a i r  of antennas f o r  an echo from t h r  v e r t i c a l  beam 
a t  an a l t i t u d e  z ,  a s  shown i n  Figure 3. 
-1 2 2rrD s i n  ( t an  A0 ' 
.ORIGIN& PAQ, ,E 
OF POOR Q3/;L!7\1 
Figure 3. Altitude-determination geometry. 
Thus we can choose the separation of the  sltitude-determining p a i r  of an- I 
tennas. so t h a t  the expected range of a l t i t u d e s  (80-110 lan) i s  mapped i n t o  phase 
differences.from, say, 0 to  90 degrees. This w i l l  g ive  an a l t i t u d e  reso lu t ion  
of 0.5 ha with the an t i c ipa ted  MENTOR phase resolut ion of 1.5 degrees ( s l i g h t l y  
b e t t e r  or worse f o r  the t ipped beans, depending on which way they're tipped). 
Notice t h a t  no time-of-flight information about the pulse i s  needed for the 
a l t i t u d e  determination. 
1 I f  it i s  necessary t o  operate the MENTOR receiver from a s i t e  such t h a t  
two of the ST' s beans are  too closely coplanar ( a  projected separat ion of l e s s  
than, my,  3 degrees), then the  transverse pa i r  of IlENTOR antennas cannot 
! I discr iminate  between the overlapped pa i r  of ST beans. However, i f  the MENTOR 
a i s i t e  i s  close enough t o  the ST s i t e  so t h a t  the regions ly ing between 80 and 
110 km don't overlap i n  projection,  then the  longitudinal pa i r  of EENTOR 
antennas can resolve  the overlapped pa i r  of ST beans. This i s  sketched i n  
I Figure 4. From the geometry shown, 
x = 110 tan  (15') = 29.5 km 
-1 110-80) , 45 -50 eL = tan (- 
X 
The l i m i t i n g  ground dis tance i s  78.6 b; beyond t h a t ,  it would be neces- 
sary  to  add logic  t o  MENTOR t o  determine the time-of-flight of the pulse from 
the. ST, and use tha t  information t o  determine which bemn the pulse came from. 
We w i l l  not consider t h i s  case fu r the r .  
Figure 4 .  Limiting geometry fo r  overlapped beams. 
BADIAL DOPPLER FREQUENCY DETERMINATION - 
MENTOR w i l l  use, as  do the  ST radars,  coherent detectors ,  so t h a t  both 
quadrature components of the s ignal  (and thus amplitude and phase) a r e  measured. 
This gives both the magnitude and the sign of the Doppler veloci ty .  The 
problem of determining the r a d i a l  Doppler ve loc i ty  becomes the problem of de- 
termining d$/dt,  with 4 measured every pulse over a typical  meteor-trail  l i f e -  
time of 113 second. At the ST'S pulse r a t e  of 217 pulses/second, t h i s  w i l l  
y i e l d  72 measurements of the phase over the l i f e t ime  of the s ignal .  I f  we take 
150 mfsec t o  be a l ike ly  upper l i m i t  t o  the r a d i a l  component of the mesospheric 
wind speed, then the mirximum d$/dt w i l l  be 83 degreesfpulse, which i s  extreme, 
but measureable. 
BEFEBEN CE 0s GILLATOR STABILITY 
The remote measurement of meteor echoes a s  envisaged here requires  good 
o s c i l l a t o r  s t a b i l i t y  fu r  both the ST t ransmit ter  and the MENTOR receivers.  
Even good commercial o s c i l l a t o r s  have enough aging d r i f t  t h a t  frequent r eca l i -  
b ra t ion  would be needed. Earwer ,  commercial WWVB phase-lock receivers  a r e  
ava i l ab le  a t  moderate cost. Re t ro f i t t ing  ex i s t ing  STs appears t o  be no 
problem. The ve loc i ty  e r ro r  due t o  frequency i n s t a b i l i t y  w i l l  thus be < 1 mf 
WIND VELOCITY DETERMINATION 
. . . . . . . .  
Operation of the MENTOR system w i l l  be s imi la r  t o  other meteor-wind ra- 
da r s ;  meteor echoes and t h e i r  character iza t ions  w i l l  be accumulated u n t i l  a l l  
t h ree  beans a r e  wel l  sampled i n  a s  many a l t i t u d e  bins  a s  possible,  commensurate 
with the  wave periods of i n t e r e s t .  A three-dimensional wind vector can then 
be f i t t e d  t o  the data. An ST radar  uses one bean tha t  i s  careful ly  v e r t i c a l ,  
s ince  a typ ica l  v e r t i c a l  ve loc i ty  i s  overwhelmed by even a small component of 
the (usually) much-larger hor izonta l  veloci ty .  The geometry of the MENTOR 
system appears t o  preclude v e r t i c a l  veloci ty  measurements, or a t  l e a s t  t o  + 
render them d i f f i c u l t .  I t  i s  a l s o  c l e a r  tha t  the error  of the hor izonta l  wind 
determination w i l l  increase a s  the MENTOR - ST spacing increases.  The e r ro r  
goes a s  
z 
so t h a t  the e r ro r  w i l l  be 40% la rge r  than the  "ST equivalent1' a t  the near s i t e  
(G = 100 km) and YL 3.2 l a rge r  a t  the f a r  s i t e  (G = 300 km). 
NETWORK POSSIBILITIES 
C 
* There a r e  two areas,  Colorado and Pennsylvania, where networks of ST ra- 
F : dars  a r e  operating (or soon w i l l  be). It i s  f e a s i b l e  t o  operate a s ing le  
i MENTOR receiving s i t e  t o  monitor meteor winds above a number of ST radars ,  pro- 
& vided they ' re  a l l  within, say, 500 km of a coumon s i t e .  (Received s ignal  
\ s t rength  i s  the  l imi t ing  parameter.) The Colorado network i s  shown i n  Figure 
5. The ST radar s i t e s  now operating a r e  shown a s  l a rge  so l id  c i r c l e s ;  a l ike-  
ly MENTOR s i t e  i s  indicated.  This s i t e  would give a maximum ground dis tance 
of 310 km t o  the fu r thes t  ST s i t e .  The d i f f e r e n t i a l  cost f o r  monitoring an 
add i t iona l  ST radar,  once the i n i t i a l  MENTOR system i s  i n s t a l l e d ,  i s  expected 
t o  be small. 
SUMMARY AND CONCLUSIONS 
Meteor-wind measurements can be made by s i t i n g  a three-channel receiver  
COLORADO S T  NETWORK 
sTElLtNo 0 
CRAlO 
UATlCV1LLE 
SUNSET 
. O e N v r m  
X MENTOR 
CORTLZ 
Figure 5 .  
some dis tance from an ST radar ,  then measuring Doppler v e l o c i t i e s  f &  meteor- 
t r a i l  s ca t t e r ing  of the ST'S pulses. Much of the  gravity-wave a c t i v i t y  i n  the 
mesosphere i s  thought t o  occur with hor izonta l  wavelengths of 10 to  1000 km. 
A s i n g l e  MENTOR s i t e  i n  Colorado or Pennsylvania could add useful information 
over much of t h i s  range. We a r e  working on data-rate ca lcula t ions  and system 
design. 


MENTOR: Adding an outlying receiver to an ST radar for meteor-wind measurement

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