The Chung-Li Very High Frequency (VHF) radar is used in the dual-mode operations, applying Doppler beam-swinging as well as the spaced-antenna-drift method. The design of the VHF radar is examined. Results of performance tests are discussed

Chao, J. K.

Chu, Y. S.

Fu, I. J.

Kuo, F. S.

Liu, C. H.

Rottger, J.

English

NASA Technical Reports Server

° •
N87-10495
6.15 THE FIRST OPERATION AND RESULTS OF THE CHL_G-LI VHF RADAR
J. K. Chao, F. S. Kuo, Y.
National Central University
Chung-Li, Republic of China
J. Rot tger
Arecibo Observatory
Box 995
Arecibo, Puerto Rico
C. H. Liu
Department of Electrical and Computer Engineering
University of Illinois
Urbane, IL
S. Chu, I. J. Fu
/77c; s
I C_ , i"
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It was decided that the Chung-Li VHF radar would be used in dual-mode
operation, applying Doppler beam-swinging as well as the spaced-antenna-drift
method. The original plans of this radar were discussed by BROSNAHAN et el.
(1983). The total system was developed and constructed by Tycho Technology Inc.
in Boulder, CO, under supervision of the Department of Atmospheric Science of
the National Central University in Chung-Li, and the National Science Council
of the Republic of China, advised by an international advisory committee. In
May 1985, the radar was set up at the campus of the National Central University
in Chung-Li and the first echoes were recorded on May 29, 1985.
The design of the radar was based on a proposal by ROTTGER (1981) to use
three phase-coherent transmitters feeding three separate antenna modules with
vertical beams; each can be used as the three receiving antennas of the spaced-
antenna-drift and interferometer method (see Table I). In the Doppler bess-
swinging mode, either of the three antennas can be steered independently to
five directions (vertical, 12 ° off-vertical to north, south, east or west) or
all three antenna modules can be steered into the same direction. Three
antenna modules of 64 four-element Yagis are used (see Figure i), which are set
up in a T-shape. The total aperture is about 2500 m2. Three transmitters
(Tycho MST-50-TX-3), each with a peak power of 50 kW, can be operated at a
duty cycle of 2% and shortest pulse length of 1 _s. The final power-aperture
product should be close to 107 Wm 2. Each transmitter has its own transmit/
receive switch, from which the received signals are fed to three independent
receiver ADC and preintegrator channels. The ADCs can be set optionally to 8,
I0 or 12 bits accuracy. The preintegration is done in a new high speed signal
processor (BROSNAHAN and WOODARD, 1984) and it allows coherent additions up to
32 bits. Through a multichannel bus, the coherently integrated raw data for up
to 400 range gates are transferred to a Codata 3300 computer from where they
are dumped on tape (Cipher F880 tape drive). The Codata computer with the
preintegration signal processor can be used later for on-line decoding and/or
further data processing. The radar control is performed by a system synchro-
nizer (Tycho MST-50-1APC) which generates also complementary codes (up to 16
bits) and controls the antenna beam steering. The raw data on tape are analysed
with the CYBER-720 and the VAX 11/750 computers at the University campus. The
latter computer is used also for image processing of power and velocity plots
(e.g., ROTTGER et al., 1986). More technical details of the Chung-Li VHF radar
will be presented elsewhere by BROSNAHAN et al.
During the first system tests, the radar was operated with one vertically
beaming antenna module, one transmitter and one receiver channel. Single pulse
operation with shortest pulse length of 1 _s was tested at a duty cycle up to
https://ntrs.nasa.gov/search.jsp?R=19870001062 2020-03-23T14:37:10+00:00Z
360
Table 1.
Chung-Li VHF Radar
National Central University, Chung-Li, R.O.C.
Dual-Mode: Doppler beam swinging and spaced-antenna
at 52.0 MHz
transmitters, each 50 kW (peak),
1 ps shortest pulse (compl. code),
2% duty cycle,
phase coherent.
antenna modules
of 64 four-element Yagis each,
2500 m 2 total effective aperture,
vertical and four off-vertical
directions.
independent receiver -- and
preprocessor channels
(max 400 range gates each)
system synchronizer,
Codata computer
Cipher tape drive
(transmitters, receivers, antennas, system
synchronizer and preprocessor made by
Tycho Technology, Inc.)
IZ. It was shown during the acceptance tests that this system configuration,
comprising less than 1/10 of the final system sensitivity, is suitable for
quite a variety of experiments, although improvements of the system performance
are necessary, such as elimination of some digital noise, receiver saturation
at short ranges and occasional data transfer failures, besides the addition of
antenna beam steering. Using the vertical beam, ground clutter was not a
problem, although the facility is built on a flat area sloping into a large
valley with many buildings and power lines. Also, high mountains at about 20
km distance did not show up as strong clutter. Part of the time, interference
was encountered and echoes from aircraft spoiled some range gates for a small
fraction of time.
Figure 2 shows some first spectra of tropospheric signals detected with
the vertically beaning Chung-Li VHF radar. The ground clutter was obtained
from a longer time series and was subtracted before the spectra were computed.
Range gate KH=6 corresponds to 3-km altitude, the range gate separation is 300
m, thus, range gate 25 is at 8.7 km. The spectra are normalized which can be
seen by the noise level varying from gate to gate. They show the well known
features of narrow and broad spectrum width due to dominance of scattering or
partial reflection. Figure 3a shows the distribution of signal plus noise
power as a function of height. Due to receiver transition effects (causing an
attenuation out to delays of 40 ps (= 6 km range)), the absolute power at the
lower heights is incorrect. However, a layered structure is clearly noticed.
A first physical result, which we regard as a new finding, is the lack
of an enhanced echo from the tropopause (the enhanced power in the upper range
m P 
361 
Figure 1. P a r t  of the antenna f i e l d  of the Chung-Li VHF radar, 
cons i s t ing  of 192 four-element Yagi antennas. 
ga t e  40 i s  due t o  preprocessor or  da t a  t r a n s f e r  problems. 
height w a s  between 12 km and 13 km. but no echo was seen a t  these a l t i t udes .  
This evidence w a s  checked a l s o  with o ther  pulse schemes. 
communication, 1985) reported an ana lys i s  of MU-radar observations where he 
apparently d id  not  f i nd  such c l e a r  i nd ica t ions  of the  tropopause as seen i n  
mid- and high-lati tude observations (e.g., ROTTGER. 1981). The l a t i t u d e  of 
the  MU-radar (35ON) and of the  Chung-Li radar  (25ON) a r e  i n  the  region of t h e  
tropopause break separa t ing  the  t rop ica l  and midlati tude tropopause. Since 
the  tropopause i s  d i s s imi l a r  here  as compared t o  higher l a t i t u d e s ,  w e  would 
expect i ts  c h a r a c t e r i s t i c s  with respect  t o  VHF radar reflectivit ies t o  be q u i t e  
d i f f e ren t .  
tropopause echo with the  Chung-Li VHF radar .  
The tropopause 
rmKA0 (personal 
This may be a poss ib le  reason why ue did not  see the  enhanced 
In  Figure 3b. t h e  d i s t r i b u t i o n  of r ad ia l  velocity i s  displayed. Single 
ve loc i ty  da t a  were deduced ( a f t e r  ground c l u t t e r  and instrumental  
dc-elimination) f r an  4 sec averages of the  f i r s t  l ag  of t he  complex auto- 
c o r r e l a t i o n  function. The d i s t r i b u t i o n s  i n  Figure 3b c l u s t e r  around zero  
ve loc i ty  wi th  rms v a r i a t i o n s  of about 10 cm s-1. The mean r a d i a l  (ve r t i ca l )  
ve loc i ty  i s  close t o  zero,  as  expected, and it increases continuously i n  t h e  
upper troposphere up t o  10 cm s-1 a t  11 km al t i tude.  
ve loc i ty  t i m e  series ind ica t e s  that part of t h e  velocity v a r i a t i o n  is due t o  
short-period gravi ty  waves. Above range ga te  34. the  ve loc i ty  estimates are 
random. which i s  expected because the s igna l  had disappeared i n  t h e  noise. 
Analysis of the  
We regard  our f i r s t  tests and observations as w e l l  a s  the  results of the  
preliminary analyses t o  prove the relevance and app l i cab i l i t y  of t he  Chung-Li 
VHF radar .  
atmosphere. 
It w i l l  comprise another e s s e n t i a l  tool t o  study t h e  subt ropica l  
362
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KH=I9 KH=20
t KH=21
FR EQUENCY
Figure 2. Normalized spectra of tropospheric returns with vertical beam.
KH = 6 corresponds to 3 km altitude and the range gate separation is
300 m.
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VERTICAL VELOCITY (m/s)
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Figure 3. Distributions of a) signal plus noise power, b) radial (vertical)
velocity.
W
ACKNOW LEDG_ENT
Two of us, C. H. Liu and J. Rottger, appreciated very much the outstanding
hospitality and cooperation experienced during their stay at the National
Central University.
REFER_ CES
Brosnahan, J. W., J. K. Chao, and J. Rottger (1983), Chung-Li Taiwan Dual-Mode
VHF Radar, Handbook for MAP, 9, 383-386.
Brosnahan, J. W., and D. M. Woodard (1984), A high-speed digital signal
processor for atmospheric data, Handbook for MAP, 14, 280-284.
Rottger, J. (1981), The capabilities of VHF radars for meteorological
observations, ESA-SP-165, (Proc. Nowcasting Symposium, Hamburg 1981),
143-148.
Rottger, J., I. J. Fu, F. S. Kuo, C. H. Liu, and J. K. Chao (1986), On the use
of colour reflectivity plots to monitor the structure of the troposphere and
stratosphere, this volume.


The first operation and results of the Chung-Li VHF radar

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