Some low cost options for data acquisition computers for ST (stratosphere, troposphere) and MST (mesosphere, stratosphere, troposphere) are presented. The particular equipment discussed reflects choices made by the University of Alaska group but of course many other options exist. The low cost microprocessor and array processor approach presented here has several advantages because of its modularity. An inexpensive system may be configured for a minimum performance ST radar, whereas a multiprocessor and/or a multiarray processor system may be used for a higher performance MST radar. This modularity is important for a network of radars because the initial cost is minimized while future upgrades will still be possible at minimal expense. This modularity also aids in lowering the cost of software development because system expansions should rquire little software changes. The functions of the radar computer will be to obtain Doppler spectra in near real time with some minor analysis such as vector wind determination

Watkins, B. J.

English

NASA Technical Reports Server

40 
1.5A THE ECONOMICS OF DATA ACQUISITION COMPUTERS FOR ST AND MST RADARS 
B. J. Watkins 
Geophysical I n s t i t u t e  
University of Alaska 
Fairbanks, AK 99701 
INTRODUCTION 
The goal of t h i s  paper i s  t o  present some low cost options f o r  data 
acquis i t ion computers f o r  ST (stratosphere,  troposphere) and MST (mesosphere, 
s t ra tosphere,  troposphere) radars. The pa r t i cu la r  equipment discussed w i l l  
r e f l e c t  choices made by the University of Alaska group but of course many o the r  
options ex i s t .  
approach presented here has several  advantages because of i t s  modularity. 
inexpensive system may be configured fo r  a minimum performance ST radar,  whereas 
a multiprocessor and/or a mult iarray processor system may be used f o r  a higher 
performance MST radar. 
because the i n i t i a l  cost  can be minimized while fu tu re  upgrades w i l l  s t i l l  be 
possible  a t  minimal expense. 
We bel ieve the low cost microprocessor and array processor 
An 
This modularity i s  important fo r  a network of radars  
This modularity a l so  aids i n  lowering the cost  of software development 
because system expansions should require  l i t t l e  software changes. 
It i s  assumed i n  t h i s  paper t h a t  the functions of the radar computer w i l l  
be t o  obtain Doppler spectra  i n  near real-time with some minor analysis  such as  
vector wind determination. 
SYSTEX REQUIREMENTS 
The costs  f o r  computer and s ignal  processing components depend great ly  on 
the desired radar  performance. The  height coverage, height resolut ion,  t i m e  
' resolut ion,  Doppler resolut ion,  and number of antenna beam posi t ions a l l  a f f e c t  
the quant i ty  of data t o  be processed and hence the equipment cost .  An ST radar  
with coarse height resolut ion (e.g., 1-2 km resolut ion with about 16 range 
ga te s )  and poor time resolut ion (data every few minutes or more) can be 
purchased fo r  a low cost. 
capab i l i t y  of height coverage i n t o  the mesosphere w i l l  require  a g rea t e r  
capacity CPU and/or an a r r ay  processor and have a higher cost .  
A higher s p a t i a l  and temporal resolut ion with the 
To simplify the cost  compkrison, Figure 1 shows possible radar  performance 
spec i f i ca t ions  and w e  w i l l  estimate the cost  for  each configuration. 
s implici ty  i t  i s  assumed t h a t  Doppler spectra  w i l l  be derived from 64 point  
FFTs (Fast Fourier Transforms), and t h a t  the radar antennas w i l l  be directed i n  
th ree  d i r ec t ions  f6 r  vector wind measurenents. 
For 
I f  these radars  a r e  t o  be used only fo r  average wind measurements, then 
t i m e  r e so lu t ion  i s  l i k e l y  t o  be of l i t t l e  significance.  Measurements every 9-10 
minutes may be adequate. 
dist inguished, the sample rate must be f a s t  enough to  prevent a l ias ing.  
Observed wave periods can be as low as 4-5 minutes i n  the lower atmosphere. 
Therefore to  make vector measurements of wave motions, a t o t a l  sampling and 
ana lys i s  time f o r  three antenna d i r ec t ions  should be l e s s  than about 2 1 f 2  
minutes. 
On the other  hand, i f  wave motions a r e  t o  be 
Doppler data  a re  generally obtained i n  three direct ions by changing the 
antenna posi t ion i f  i t  i s  physically s teerable .  o r  phasing an array.  
sequence of data  taking and real-time analysis  i s  assumed a s  follows: 
The  
https://ntrs.nasa.gov/search.jsp?R=19840019016 2020-03-22T09:40:35+00:00Z
4 1  
2-30 L 60-100 
OPFION OPTION OPTION OPTION OPTION OPTION OPTION OPTION 
A B C 0 e P G 11 
Figure 1. Eight possible radar performance specif icat ions (A--€I). For the 
purposes of t h i s  paper, coarse and f ine  height resolut ions a re  defined as 
1.5 km and 150 m, respectively.  The t i m e  resolut ion i s  defined as the 
t o t a l  time t o  determine a vector wind measurement (data from 3 antenna 
direct ions) ;  f i n e  resolut ion is considered <2.5 minutes and coarse reso- 
l u t ion  is  2.5--10 minutes. 
Select  f i r s t  antenna d i r ec t ion  
Transmit Np = Nf Nc pulses 
w h e r e  Nf = number of FPT points 
Sample and s to re  a complex receiver  sample a t  each range, f o r  a l l  Np 
pulses. 
A t  the completion of t ransmit t ing Np pulses, the coherent i n t eg ra t ion  
process i s  performed. 
A t  each range a power spectrum of the returned s ignals  i s  computed using 
an FFT. 
I f  multiple spectra  are t o  be averaged, s t eps  (2) t o  (5) above are 
repeated IAV times. 
The NAV spec t r a l  from each range ga te  are averaged and f i n a l l y  stored on 
tape. 
A new antenna d i r ec t ion  i s  selected and s t eps  (2) t o  (7) are repeated. 
Nc = number of coherent integrat ions.  
(NAV = number of averaged spectra).  
I f  phase-coded pulses a r e  used then an addi t ional  decoding s t ep  is 
necessary a f t e r  the coherent integrat ion i s  performed. 
t i m e w i s e  by comparison with other  computations so w e  have neglected i t  i n  our 
timing estimates. 
CIIOICE OF COMPUTER 
This i s  a minimal task 
The computer should idea l ly  be the lowest cost  un i t  t h a t  w i l l  perform the 
required tasks .  
researchers r a re ly  agree on the  mode of operation for radar experiments and 
frequently place more demands on equipment as t i m e  progresses. 
i s  now recognized t h a t  high resolut ion and hence more range gates  a re  desirable  
f o r  studying turbulence s t ructures .  Several  ST and MST radars a r e  now upgrading 
f o r  t h i s  higher resolution. Thus, an important specif icat ion i s  expandsbility. 
However, the exact needs are d i f f i c u l t  t o  define because 
For example, it 
We have considered many hardware options t h a t  would provide the absolute 
42 
lowest cost  system s u i t a b l e  f o r  a simple ST radar t ha t  may be used i n  a network 
and y e t  be expandable t o  a high performance MST radar. 
The University of Alaska group has chosen a microcomputer t h a t  uses a 
Motorola 68000 microprocessor i n  conjunction with two low cost array processors 
(APs). While there  are many other hardware options avai lable  t h a t  other 
researchers may choose, our choice i l l u s t r a t e s  the modularity concept and the  
subs t an t i a l ly  lower cost  by comparison with the computers a t  established radar  
s i t e s .  
The 68000 microprocessor has the advantage of 32-bit i n t e rna l  archi tecture ,  
and coupled with an a r r a y  processor provides f a s t  ar i thmetic  capab i l i t i e s .  Each 
AP ($6,000) can perform one mi l l i on  32-bit f loating-point operations per second; 
they are made by Sky Computer Corporation. 
but share  a common memory with the  main CPU. This has t h e  advantage of low cost  
memory, the a b i l i t y  for the AP t o  access a very l a rge  amount of memory (up t o  
16 M bytes i n  our case),  and minimizing data  t r ans fe r  times. 
The microcomputer cost  depends great ly  on the amount of memory required but 
The APs have no memory of t h e i r  own 
should be i n  the range $5,000 t o  $15,000. 
l a t e r .  
A more de t a i l ed  costing i s  given 
By comparison, presently es tabl ished radars have computer costs  about a 
f ac to r  of ten larger .  For example a t  Millstone H i l l  and Arecibo, the Harris 
Computers and Floating-Point System APs have costs  f a r  i n  excess of $100,000. 
These APs do indeed provide a speed advantage, but as w e l l  as t h e i r  i n i t i a l  
high cost ,  the addi t ion of ex t r a  memory i s  very costly.  
low p r i ce  APs t h a t  are now avai lable  become very costly when any subs t an t i a l  
amount of menory i s  added t o  them. 
Applications, Inc., sells  an AP f o r  about $24,000 with minimum memory, but costs  
$85,000 with 2 M bytes of memory. 
Even other r e l a t ive ly  
For example Computer Design and 
Although many radar experiments, pa r t i cu la r ly  a simple ST radar  network, 
may i n i t i a l l y  have no need fo r  an AP, it i s  worthwhile planning f o r  t h e i r  use so 
t h a t  upgrade costs  w i l l  be minimized. 
MODULAR APPROACH 
To i l l u s t r a t e  how a modular approach can be used t o  assemble computers of 
d i f f e r e n t  processing capab i l i t i e s ,  we present some possible examples including 
the system now being constructed by the University of Alaska group. 
Figure 2 shows the s ing le  board computer (made by Omnibyte Corporation) and 
array processor used. 
specif icat ions (SNIGIER, 1982; WILSON, 19821. These boards, together with a 
card cage, power supply and case form t h e  bas i s  of a computer system. Providing 
the re  a re  enough spare s l o t s  i n  the card cage, the system may be expanded by 
plugging i n  more manory (up t o  15 M bytes),  multiple CPUs and mult iple  APs. 
The boards a r e  7" x 12" and conform t o  the IEEE Multibus 
Some examples of various computer configurations are shown i n  Figure 3. No 
construction costs  are necessary because these boards are commercially ava i l ab le  
and simply plug together.  The University of Alaska system has one CPU, 64.0 K of 
memory and two APs operating i n  pa ra l l e l .  
DETERMINATION OF RJiQUT.RED PROCESSING POWER 
With a given radar  specif icat ion it i s  necessary t o  determine both the time 
duration t o  gather the da t a  and the processing t i m e .  
should not be excessive; fo r  example 3 measurements must be made i n  l e s s  than 
about 2 112 minutes t o  determine waves i n  the stratosphere.  
This t o t a l  t i m e  duration 
Further, the t i m e  
43 
Figure 2. TOP: Single board computer with 128 k of memory, two 
T J O  por t s ,  and two 16-bit p a r a l l e l  110 por t s  (cost  $2000). 
BOTTOM: Array processor (two board set, cos t  $6000). 
t o  process the data should not be la rge  compared t o  the t i m e  required t o  gather 
the data. Ideal ly  t h i s  processing time should not  exceed about 5 - 10% of the 
t i m e  required t o  take the data. 
instead of sampling, fewer spectra  may be integrated i n  a given time, and hence 
s ignal  de t ec t ab i l i t y  suf fers  a t  the upper heights.  
When too much t i m e  is wasted processing data 
The time required t o  co l lec t  samples for  three antenna direct ions,  and 
averaging RAv spectra a t  each range gate  a f t e r  N, coherent in tegra t ions  i s  
given by 
44 
A NIElIKUM CONPUTEB. 
ONE CPU, NO ARRAY PROCESSOB B 
ONE 80. ONE OR MORE 
ARRAY pBocEssoRs. 
MULTIPLE CPD'S AM) 
MULTIPLE AXRAY PROCESSORS 
Figure 3. Some possible  computer configurations u t i l i z i n g  
one o r  more CPUs and up t o  four array processors. 
University of Alaska has implemented the middle configura- 
t i o n  above with two array processors. 
The 
3 9 N f -  Ne- NAV 
DATA = PRF 
where PRF = t ransmit ter  pulse r e p e t i t i o n  frequency. 
For example i f  N f =  64, N c =  64, NAV = 10, PRF = 1250 Hz then TDATA' 98 8ec. 
This value f o r  the PRF i s  about the maximum possible f o r  an MST radar without 
range a l i a s ing ;  it gives a maximum unambiguous range of 120 km. A higher PRF 
i s  possible  fo r  an ST radar  t h a t  receives no da ta  from the mesosphere. 
values f o r  N f  and N c a r e  somewhat a rb i t r a ry ,  but together with the PRF and 
radar  wavelength A, determine the Doppler resolut ion 6v where 
The 
PRF - A 
6 V  = m l  s 2 * NE * N, 
For a 50 MHz radar ( X  3 6 m),  using the above values yields  6v = 019 mlsec. 
This should be a usable value as the Poker F l a t  MST radar has operated with 
6v = 1.3 m / s  with excel lent  r e su l t s .  
The method w e  have adopted fo r  determining the required computer processing 
power i s  t o  f i r s t  determine the required radar parameters (e.g., PRF, Nc, NAY, 
N f ,  6v) required t o  obtain the data  with su f f i c i en t  resolut ion and signal/noise 
r a t i o .  The parameters a r e  best  estimated from experience and extrapolat ion from 
establ ished radars. The values quoted above a re  typical  for  the 50 MBz Poker 
F l a t  radar although higher values of NAV have been used a t  Poker Flat .  
Next TDATA i s  calculated; t h i s  sets an upper l i m i t  t o  the data processing 
A computer i s  then selected so t h a t  the processing t i m e  plus the data- t i m e .  
a cqu i s i t i on  time i s  not too large.  For example, t o  detect  waves, t h i s  t o t a l  
t i m e  should be less than . 2 112 minutes (data i n  th ree  direct ions) .  
The t i m e  T PROCESS. required t o  process the data (from 3 antenna d i r ec t ions )  
i s  the t o t a l  t i m e  required t o  perform coherent integrat ion plus the FFTs 
where Ng = number of range gates 
TFLOAT = Time fo r  AP t o  change complex integer  array of Ng samples t o  
f l o a t i n g  point. 
TSm = T i m e  fo r  AP to  sum a complex vector. 
T = Time for  AP t o  perform FFT on array of Nf samples. 
FFT 
The minimum amount of memory required by given by: 
M = 4 Nf Ne - N bytes 
g ( 3 )  
The factor  4 comes from the use of 16-bit complex samples. 
We have adopted the  technique of f i r s t  acquiring a l l  data before performing 
any coherent integrat ion.  By contrast  i t  i s  possible t o  use f a r  l e s s  memory by 
performing the  coherent i n t eg ra t ion  pulse by pulse. Bowwer, t h i s  places 
constraints  on the minimum interpulse  period of the t ransmit ter  because without 
a very powerful, and hence cost ly ,  computing system it i s  d i f f i c u l t  t o  perform 
t h i s  i n t eg ra t ion  as we l l  as other  required tasks  during the  interpulse  period. 
It i s  f a r  more Gost e f f ec t ive  t o  use a lesser capacity computer and AP i n  
conjunction with a f a i r l y  large memory. I n  the case where memory requirements 
become excessive (e.g., Case G on Figure 1 and Table 1 )  it i s  then desirable  t o  
use a dedicated preprocessor for  performing the coherent integration. 
preprocessor is hard-wired t o  perform f a s t  additions and may be constructed fo r  
about $6,000 (JOKNSTON, 1983). 
Such a 
It should be noted t h a t  t he re  are many possible compromises avai lable  i n  
es tabl ishing a radar 's  operating parameters. For example, both the data- 
acquis i t ion time, TDATA, and the  processing time are affected by the number of 
averaged spectra.NAv. I f  t h i s  parameter value i s  decreased it may be possible 
t o  use a computer of l e s se r  capabi l i ty  and cost .  Howwer, a lower NAV w i l l  
46 
0 0 ro 
- 
a 
% 
D 0 4
g $42,300/$56,450 
h v1
- 
$18,300 
(R 
- 
8 $19,850 
(R 
- 
0 o, VI $18,300 
- 
$30,500/$30,300 
0 0 
h *
- 
0 
VI 
,0 $12,250 
- 
$18,300 
r. VI 
- 
0 
Y 
1.I 
Tr 
47 
decrease the  s ignal  d e t e c t a b i l i t y  and i t  i s  l i k e l y  t h a t  data from some upper 
heights may be lo s t .  
deciding on the radar operating parameters. 
COST SUMMARY 
Such compromises should be careful ly  evaluated before 
Using the method outlined i n  the previous section, w e  have evaluated the 
cos t s  f o r  d i f f e ren t  radar specif icat ions A - G i n  Figure 1. 
i n  Table 1. 
d a t a  from three antenna d i r ec t ions  are given. These t i m e s  are worst-case values 
because it has been assumed t h a t  during data  acqu i s i t i on  the computer i s  only 
required t o  perform the sampling. This reserves  some t i m e  during the in t e rpu l se  
periods f o r  other tasks such as graphics display,  calculat ion of vector winds, 
s ignal lnoise  r a t i o s ,  e tc .  
A summary i s  given 
I n  addition, the approximate t i m e s  required t o  acquire and process 
For the computer configuration B i n  Table 1 t h a t  has no AP, the  data- 
We estimated the t ime  fo r  floating-point processing time i s  r e l a t i v e l y  slow. 
FFTs using the University of Alaska's microcomputer (8 MHz clock). It could be 
speeded up e i the r  by using an integer  FFT instead of f loa t ing  point,  o r  use of 
an addi t ional  simple hardware ar i thmetic  un i t ,  or a CPU with higher clock 
frequency. The 68000 microprocessor i s  now avai lable  for operation with a 12- 
MBz clock and a 16-MRZ version should be avai lable  i n  future .  
It i s  assumed t h a t  some type of hard copy p r i n t e r  (with graphics),  and a 
9-track tape dr ive a r e  common t o  a l l  configurations. 
mate cost  of analog-to-digital converters i s  included and the Appendix b r i e f ly  
discusses some cost options. 
I n  addition, the approxi- 
A l l  the costs l i s t e d  i n  Table 1 are fo r  a quant i ty  of one. 
(15 - 30%) a re  avai lable  for  larger  quan t i t i e s  t h a t  would be purchased f o r  a 
network of radars. 
Discounts 
It should b e  emphasized t h a t  the costs i n  Table 1 are f o r  hardware only. 
The use of an AP 
There may be subs t an t i a l  i n i t i a l  costs  f o r  software. A competent programmer may 
take several  months to develop the data-aquisit ion program. 
w i l l  reduce the software costs  because much s igna l  processing software i s  
provided by the AP manufacturer. It i s  most e f f i c i e n t  t o  develop software using 
a high-level language, an operating system and disk drives and a t  the University 
of Alaska w e  are doing t h i s  with a real-time operating system, a 20 Mb disk 
d r ive  and the C programming language. The costa f o r  t h i s  ex t r a  hardware, soft-  
ware and labor should be considered, but fo r  a large network the cost  per radar 
would not be large. 
APPENDIX: ANALOG AND DIGITAL CONVERTERS 
The choice of analog-to-digital (AID) converter resolut ion may a f f e c t  the 
cos t  of the radar  computer. I f  an 8 - b i t  AID i s  chosen the computer w i l l  require  
only half  the memory (for  s to r ing  samples) compared t o  a 10- or 12-bit da t a  word 
t h a t  i s  commonly used since da t a  a re  stored i n  8-bit increments. However, the 
ove ra l l  dynamic range of the radar w i l l  generally be l imited by the  AID 
converter not by the receiver.  
reduce ground c l u t t e r  (e.g., placed i n  a val ley with nearby shielding h i l l s ) ,  
the dc o f f s e t  a t  the receiver output from c l u t t e r  echoes w i l l  be large compared 
t o  the noise and signa1 f luctuat ions.  In  pract ice ,  a 10- or 12-bit converter i s  
preferred and the calculat ions i n  Table 1 have assumed th i s .  
Unless the radar i s  s i t e d  t o  subs t an t i a l ly  
T h e  cos t  of AID converters i s  r e l a t i v e l y  small. A s  a cost  example i n  Table 
1 we have used a 12-bit 2 vsec AID converter made by ILC Data Device 
Corporation ($150 each) and a SamplelHold made by Analog Devices ADSHM-SK 
($199). 
48 
.A  faster but lower resolution AID i s  approximately the same cost ,  e .g . ,  
Analog Devices 10 b i t s ,  1 vsec, MAH-1001 ($219). 
REFE~ENCES 
r% 
Johnston, P .  E. (1983), (at  NOAA Aeronomy Laboratory) , private communication. 
Snigier, I?. (1982), Designers guide to the multibus, Digital Design, 52. 
Wilsbn, D:'(l982), Multibus: Evolving to meet new svstem stands, Digital 
Dgsign,. 1 6 .  


The economics of data acquisition computers for ST and MST radars

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server