The results of data obtained at The University of Texas at Austin during a total of 551 days of righthand circularly polarized transmission at 11.7 GHz from the CTS satellite are presented. Measured were attenuation, cross-polarization and rain rate. Results indicate that ice depolarization can be of significant importance

Vogel, W. J.

English

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• 
CTS ATTENUATION AND CROSS POLARIZATION MEASUREMENTS 
AT 11 . 7 GHz 
(NASA-CR-159918) CTS AT TE NUAtION AND CROSS N19-16113 
POLARI ZATION ~EASURE~ENTS AT 11.1 G~z Final 
Report, 18 Oct. 1916 - 31 JaD. 1918 (Texas 
Univ. at Austii: .) 23 p HC A02/ IH A01 Unclas 
W . J . Vogel 
Electrical Engineering Research Laboratory 
The University of Texas at Austin 
10100 Burnet Road 
Austin. TX 78758 
CS CL 20N G3/32 43610 
Final Report Covering the Period 18 October 1976 to 31 January 1978 
Under Contract NAS5-22576 
September 1978 
Prepared for 
GODDARD SPACE FLIGHT CENTER 
Greenbelt . MD 20771 
I. 
11. 
111. 
TABLE OF CONTENTS 
Introduction 
Receiver Description 
Meteorology of Austin 
Page 
1 
1 
Z 
A. Thunderstorm Climatology for Central Texas Z 
B. Meteorology for 18 October 1976 to 31 January 
1978 
IV. Results of the Attenuation and Cross-polarization 
Isolation Measurements 
V. Conclusions 
iii 
4 
6 
15 
LIST OF FIGURES 
No. Page 
1 Rain rate exceedence plot for the period 
2 
3 
4 
5 
18 Oc t 76 - 3 1 Jan 78 
Attenuation and cross-polar ization exceedence plot 
for the period 18 Oct 76 - 31 Jan 78 
Attenuat ion and cross-polarization exceedence plot 
for the period 12 June 76 - 31 Jan 78 
Equal probability fit for all CPI- A pairs 
The percentage with which ice depolarization con-
tributed to the total depolarization events 
i v 
8 
12 
13 
14 
17 
1. INTRODUCTION 
The nominally right hand eire uh.rly polarized 11 . 7 GHz beacon 
transmitter on the CTS satellite has been monitored at The University 
of Texas at Austin for the periods of its peration . A two channel re-
ceiver . using a turnstile junction as feed and polar ization separator 
was built . Its polar ization is a djusted to give better than 45 dB isolation 
between the c hannel m easuring the co-polarized signal and the one mea-
suring the cross-polarized signal during clea r air propagat ion. 
A previous r eport (Vogel and Stra iton. 1977) described the receiver 
and summar ized the data collected from 12 June 1976 to 30 August 19',6 . 
The b eacon transmitter aboard CTS was d~activated to conserve power 
during the first eclipse period . starting 31 August 1976 until 17 October 
1976 . This report presents the data collected from 18 October 1976 to 
31 January 1978 . 
The signal levels in both receiver bands were reco rded fo r 471 days. 
Thus . the initial 80 days added. data have been obtained for a total of 
551 days. The data were used to determine attenuation (to 25 dB) and 
cross-polarization isolation (bel ow 35 dB). 
II. RECEIVER DESCRIPTION 
Only the pertinent characteristics of the receiver are g;ven in 
Table I. 
Vogel. W. J. and Straiton, A . W . . 1977, "CTS Attenuation and Cross-
polar ization Measurements at 11.7 GHz," Final Report under Contract 
NAS5-22576 for 12 June 1976 - 30 August 1976. 
1 
TABLE I 
Dua.l Polarization Receiver Characteristics 
Antenna: 
Feed: 
Polar ization: 
Isolation: 
Fade Margin: 
3m parabola . prim e focus feed. f/D = . 375. 
program po inting to .020. elevation = 49 0 . 
turnstile polariz er 
RHC. LHC (nominally). matched by adjusting for 
minimum power in cross-polar channel during 
clear- air propagation. 
better than 45 dB with optimum po inting 
30 dB for co- polarized channel 
45 dB for cross-polarized channel 
z 
Calibration: Prec ision attenuated 11 . 7 GHz signal injected into 
front end. 
Output: logarithrnic amplitudes. recorded on strip chart. 
Ill. METEOROLOGY OF AUSTIN 
A. Thunderstorm Climatology for Central Texas 
Information on thunderstorms in Central Texas is summarized 
in Table II . The frequency of occurrence is based on a thirty year mean 
as published by the weather bureau . ~1ay and August have the hi,ghest ob-
serve d mean frequencies (though the standard deviation is very large). 
An average total of 41 thunderstorms occur annually . The height of the 
tops of the thunderstorm clouds shows a slight bimodCi.l tendency . Also 
given are the translational speed of the individual rain cells and their 
3 
Table II 
Mean Values of Thunderstorm Parameters 
For Central Texas by Month of Year 
Frequency of Height of Cloud Tops Cell Speed Direction of Movement 
Month Occurrence km (km/hr) (degrees) 
Jan 1 8.5 46 Z40 
Feb Z 9 . 1 S6 Z50 
Mar 3 11. 6 65 Z70 
Apr 5 12.Z 56 250 
May 7 12.8 46 Z40 
Jun 4 10.7 37 180 
July 4 9·8 28 170 
Aug 5 9.8 28 150 
Sept 4 lZ.Z 46 230 
Oct 3 11. 6 46 230 
Nov 2 9. 1 46 240 
Dec 1 8.5 46 240 
.. 
direction of movement (clockwise direction from true north from which 
the cell is moving). It should be pointed out that when the showers are 
arranged in a line (a squall line) as they frequently are in the spring 
(March through May) the line usually moves from 300 0 to 310· (i. e . from 
the northwest) . The individuaJ. s hower cells within the line. however. 
move in the direction as indicated in the Table. The direction of move-
ment of individual cells has a strong southwesterly to westerly component 
most of the year. except in the summer time when cells move from south 
(June) to southeast (August) . 
B . Meteorology for 18 October 1976 to 31 January 1978 
Prec ipitation occurred in the form of liquid water only. Table 
III summarize& the obtained rain data . Given are the 40 year average 
quantities and the amounts recorded during this period . Under A . are 
listed the official weather bureau data based on m easuring the accumu-
lated liquid captured by a standard 20 em gage. Column B was obtained 
by a 7 cm gage with 2.5 mm tip interval. located 70 m from the receiver 
and the data in column C are from a standard 20 cm tipping bucket gage. 
derived by counting its tips. (.25 mm liquid per tip). This gage is 
located at the receiver site . 
Some discrepancy exists between the amounts measured with the 
three gages. The weathe r bureau station is located several miles from 
the receiver and one would expect only a rough agreement between the rain 
measured there and at the receiver. Monthly variations of column A with 
5 
Table III 
Rain Measured in Austin During the Reporting Period. 
All quantities in mrn. 
40 year 1976 1977 1978 
Month Mean A B C A B C A B C 
Jan 52 57 51 42 22 30 19 
Feb 62 66 64 53 
Mar 56 55 51 40 
Apr 90 154 183 144 
May 108 31 41 37 
Jun 72 31 25 25 
Jul 56 5 5 5 
Aug 57 2 0 0 
Sep 92 79 69 61 
Oct 80 94 84 75 30 64 53 
Nov 60 45 28 37 43 64 62 
Dec 63 63 56 49 9 8 6 
Annual 848 562 625 528 
• 
6 
respect to column B are of the order of 10 to ZO percent. The 1977 
annual total at B exceeds A by 110/0 . Gage C . even though located very 
close to B totalled 60/0 less rain:iall than A in 1977 . 
By dripping water into gage C it was found that at a rate of ZO 
mm/hr it underestimates the precipitation by 30/0 and at a rate of 100 mm/ 
hr by IZ%. In addition to this error introduced by the dynamic behavi or 
of the gage. its placement on top of a building near the roof edge may have 
resulted in a reduced capture efficiency due to air turbulence. (The gage 
subsequently was moved further away from the roof edge . ) 
During the reporting period a total of 28 thundershowers were 
recorded by the Austin Weather Bureau. as compared to a long term 
average of 46 for the 15.4 months covered. These data are shown in 
Table IV. The number of thunderstorms and the total rain were reduced 
by about the same ratio as one can see if the reporting period numbers 
are compared to their long term averages. 
The fraction of time the rainrate exceeded values between 0 and 
so mm/hr is shown in Fig . I for this time period. 
IV. RESULTS OF THE ATTENUATION AND CROSS-POLARIZATION 
ISOLATION MEASUREMENTS 
The chart records were digitized at one minute intervals for all periods 
of data events. Thes e are defined by either the attenuation exceeding 1 dB 
or the cross-polarization isolation being reduced below 35 dB. Since the 
values recorded are the power levels in the co- and cross-polarized 
7 
Table IV 
Number of Thunderstorms in Austin During Reporting Period 
Month 1976 1977 1978 
Jan 1 0 
Feb 1 
Mar 3 
Apr 5 
May 3 
Jun 1 
Jul 0 
Aug 1 
Sep 7 
Oct 0 4 
Nov 1 1 
Dec 0 0 
l&J 
~ 
~ 
~ 
0 
Z 
0 
~ 
U 
<l 
a: 
~ 
10- 2 
10- 5 
RAIN RATE EXCEEDENCE PLOT 
18 OCT 76 - 31 JAN 78 
10-4 ~--------~----------;--------------4--------------+------------~~ 
o 10 
Figure 1 
20 30 40 50 
RAIN RATE - mm/hr 
Rail' Rate Exceedence Plot for the Period 
I 8 Oc t 76 - 31 J an 78 
8 
9 
channels. the desired quantities. attenuation and isolation have to be 
derived. The relations are 
Attenuation (dB) = clear air powe . (dBm) in co-polarized channel 
power (dBm) in c'>-polarized channel 
and 
cross-polarized isolation (dB) = power (dBm) in co-polarized channel -
power (dBm) in cross-polarized channel. 
Logarithmic curve fits were made between the cross- polarization isolation 
(CPI) in dB and the attenuation (A) in dB. They are of the general form 
cpr = a + b log A 
The coefficient of determination r2 expresses the scatter of the pOints 
about the curve and has also been calculated. 2 An r of 1 means that all 
points fall on the curve and an r2 = 0 means that the points were randomly 
distributed in the CPI - A plane. The fits were made for each month for 
all periods for which the attenuation was equal to or exc eeded 5 dB. This 
value was chosen to eliminate ice depolarization events. Table V summarizes 
the results of the fitting. 
The result is quite amoiguous. There are months when the logarithmic 
relationship holds reasonably true, but not with the same coefficients from 
month to month. There are also months for which the fit is not good at all. 
No clear seasonal dependence is indicated. The reason for this seems to 
be the variability of the parameters of rain which produce polarization " 
effects, i. e. drop shape and canting angle distributions. Their statistic s 
10 
Table V 
Logarithmic Fit for cpr vs . A: cpr = a + b log A while A ~ 5 dB 
# of points 2 Month (minutes) a b r 
Oct 76 3 53 . 6 - 31. 6 .89 
Nov 0 
D ec 4 20 . 5 18.2 . 42 
Jan 77 40 42 . 4 -18.5 .67 
F eb 0 
Mar "7 21 7.5 .05 
Apr 37 44.6 -22.7 .89 
M ay 21 45.8 -22.0 . 75 
Jun 9 50. 1 -29.3 . 66 
Jul 0 
Aug 12 41. 6 -20.0 . 89 
Sep 71 ):< 
Oct 48 42 . 1 -20 . 0 . 62 
Nov 12 61. 9 -50 . 6 . 75 
D ec 0 
Jan 78 1 1 50.6 -28 . 5 . 80 
* The cross polarization receiver failed during the two storms that pro-
duced essentially all of the rainfall of September 1977. consequently 
no isolation data were obtained for that month. 
.j 
11 
-
are different for different rain events, making it impos sible to formulate 
a universal deterministic relationship between cpr and A . Fortunately 
such a relationship is not really required by designers of satellite 
communication systems. For their purpose a furmula which relates cpr 
and A on a statistical basis is much more useful. That type of relationship 
can be derived from the " exceedence plots" for attenuation and isolati on. 
Figure 2 gives the fraction of time for the reporting period (18 Oct 
76 - 31 Jan 78 ) and Fig. 3 for the 551 observation days following 12 June 
1976 until 31 January 1978 during which the attenuation exceeded and the 
isulation was reduced below the abscissa. Since the cross polarization 
receiver had a failure during the storms that pr o duced esentially the entire 
September 1977 data . a second curve for the attenuation exceedence . with 
the month of September 1977 d ele t e d , has been drawn in Fig . 3 . This 
curve and the one labelled cpr are used to der ive th e qual probability 
relationship between CPE and A. They r e present all simultaneous atten-
uation and isolation data gathered. The equal probability cpr- A pairs have 
been drawn up in Fig. 4. They lie very closely to the line 
cpr = 41 - 20 . 6 log A 
with a coefficient of determination of .99. The relationship between cpr 
and A recommended by the CCrR is 
cpr = 30 - 20 log A 
and has been added to Fig. 4. The slope is in very close agreement but the 
CCIR curve is more conservative from a design viewpoint. It overestimates 
A 
1&.1 10-4 
~ 
~ 
~ 
0 
Z 
0 
~ 
U 
<l 
a: 
~ 
10-5 
Figure 2 
CROSS-POLARIZATION ISOLATION- dB 
30 
10 
25 20 
EXCEEOENCE PLOT 
18 OCT 76 - 31 JAN 78 
15 20 
ATTENUATION - dB 
15 
Attenuation and Cro ss-po l a rizati on Exceedc:mce Plot 
for the Period 18 O c t 76 - 31 Jan 78 
12 
&&J 
2 
~ 
lL. 
0 
Z 
0 
i= 
u 
C( 
a: 
lL. 
13 
CROSS-POLARIZATION ISOLATION- dB 
35 30 25 20 15 
10-2,-------__ ~--__ --__ ~--------~-----------~------------~-------~ 
10-" 
10-5 
A, 
SEPT. 77 
DELETED 
EXCEEDENCE PLOT 
12 J ~ .'NE76-31 JAN 78 
10~ ~--------+---------+----------~---------+---------~------~ 
5 
Figure 3 
10 15 20 25 
ATTENUATION -dB 
Attenuatiun and Cro ss-polar ization .sx c et!dence Plot 
for th~ Pt!riod 12 June 76 - 31 Jan 78 
CD 
" I 
Z 
o 
~ 
10 
15 
20 
25 
ct 30 5 
en 
35 
EOUAL PROBABILITY 
CPI- A PAIRS 
FIT: 
CPl. 41-20.' lot A 
(rl •.• 9' 
40~----~----~--~--~--4--+----------~-------+----~----~ 
4 5 6 7 8 9 10 15 20 25 
ATTENUATION - d B 
Figu rt! 4 e: qu a l Probability Fit for a ll CPI··A P ai rs 
14 
15 
the isolation degradation by 11 dB. 
A portion of all depolarization events was distinguished by t.he 
absence of simultaneous fading. This type of depolarization has been 
linked to the presence of ice crystals in the propagation path. While 
such ice crystals absorb very little electromagnetic energy, they do 
scatter it (mostly in the forward direction) and because of their asymetri-
cal shapes introduce a phase shift which depends on the polarization of the 
wave. In general. the output polarization of such a medium will be different 
from the transmitted input polarization . i . e. cross polarization isolation 
between orthogonally polarized transmitted waves will have been reduced . 
T~) assess the importance of phase events from a system design view-
point, the percentage of time during whic h the isolation was reduced below 
these values was determined . Thes e results are given in Table V. Ice 
depolarization events observed in Austin were assoc iated either with thunder-
storms during the summer months or with clouds in the presence of polar 
airmass e s during the winter. The averages given in Table V have also 
been plotted in Fig. 5. 
V. CONCLUSIONS 
Attenuation and c ross polarization isolation have been measured for 
circular polarization at 11 . 7 GHz . While a one to one relationship between 
the two quantities cannot be supported by the data. their cumulative pro-
bability equates lead to a dependence of 
CPI = 41 - ZO.6 log A. 
16 
Table V 
Percentage of Time the Loss of Isolation was not Coincident with 
Fades greater than 1 dB . 
Isolation (dB) below) 
Month 35 30 25 
Nov 76 78 56 62 
Dec 51 0 0 
Jan 77 60 3 0 
Feb 84 100 0 
Mar 63 11 0 
Apr 59 43 10 
May 6 0 0 
June 31 0 0 
Jul 0 0 0 
Aug 0 0 0 
Oct 11 21 8 
Nov 41 35 22 
Dec 0 0 0 
Jan 78 37 29 0 
Average (Nov 76 - 51 27 15 
Jan '78) 
Average (12 June 76- 47 24 11 
31 Jan 78) 
L&.I 
2 
~ 
~ 
0 
~ 
Z 
L&.I 
0 
a: 
L&.I 
Q. 
17 
60 ~------------------------------------------------------~ 
so 
40 
30 
20 
10 
o 
10 
TIME(ISOLATION S ABSCISSA/A S IdB) X 100 TIM E (JSOLATlON~ ABSCI SSA) 
15 20 25 
ISOLATION-dB 
12 JUN 76- 31 JAN 78 
30 
F :gure 5 The P~rcentag~ with which Ice Depolarization 
Contributed to the T o tal Depolarization Events 
40 
18 
Whether ice depolarization is of a major system design important de-
pends very much on the level of isolation required. At 35 dB isolation 
500/0 and at 25 dB isolation 15% of the depolarization events were due to 
this effect. 


CTS attenuation and cross polarization measurements at 11.7 GHz

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