Table I lists the site-specific data and the experi- ment duration, uptime, and measurement parameters. The radiometers had 2m diameter, horizontally polarized,.
COMPARISON
OFFOUR INTELSAT 12 GHZ RADIOMETRIC EXPERIMENTS IN RAINY CLIMATES
J.E. Allnutt, B. Arbesser-Rastburg, and D.K. McCarthy INTELSAT 3400 International Drive, N.W. Washington, D.C. 20008-3098, U.S.A.
-
INTRODUCTION
1.
Propagation impairments can be severe 10 above a, especially in rainy climates. Prediction models do not provide consistent results in these regions due atolack of meteorological and measured slant-path data. To increase the data base available INTELSAT instituted a 12 GHz radiometric measurement campaign in high rainfall rate regions. The interim results of two experiments [l] are compared below to two earlier experiments which have been subjected to additional analysis. DESCRIPTION OF EXPER1"TS
2.
Table I lists the site-specific data and the experiment duration, uptime, and measurement parameters. The radiometers had2m diameter, horizontally polarized, front-fed antennas with lo beamwidths and were similar to others deployed in earlier INTELSAT experiments [2]. Rain gauges were located close to each site. Data from the Singapore and Hong Kong experiments were recorded o strip-chart recorders and later digitized for computer analysis. The Peru and Australia experiments also utilized strip-chart recorders but only as back-up a digital for data acquisition system. 3.
MEASUREMENT
AND ANALYSIS PROCEDURES
Automatic calibration was performed hourly. Periodically, in addition, cold-load and hot-load calibrations were performed to ensure the correct operation the equipment[l].
Details of the
Four
Tropical
Radiometric
Experiments
Duration and Percentage rFrea.IElev.1 i (G&)jAnglei Uptime of Rad./R ]Singapore1 Z0 N1104O Er11.6 141.0°126/1/76 - 2/2/77 I I I I I I
]Country
I
I Lat.ILong.
I
I
-
IHong Kong122" N1114O E I 12.0 1~7.9~i1/10/80 30/9/81
I lPeru I
I L I I I I 4 O S1286O E I 11.6 127.7O115/11/83 -15/5/85 I I I I I lAu~tralial17~ S1146O EPresent I 11.6 145.0°11/7/84
-
CH2325-9/86/0000-0229$01.00 @ 1986 IEEE
229
I
of
The sky temperature, Ts, is related to the antenna temperature, Ta, by:
Ts = [Ta
-
(1
- h)Tg]
/ hk
(1)
where h is a factor to take account of the antenna pattern intercepting the ground Tg andis the temperature of the ground. The path attenuation, A, can then be found using: A = 10 log [Tm
- Tcos) /
(Tm
- Ts) dB
(21
where Tm is the effective temperature of the absorbing medium and Tcos is the cosmic background temperature (4 K). Tm can take values between 250 K and 290 K [3] in Europe. Variations in the true physical medium temperature, the degree the rain event totally encloses the radiometer beam, and scattering effects can all be significant. For the Singapore and Hong Kong data, values of Tm of 260 K, 280 K, and 285 K were adopted for 1%, 0.1%, and O.Ol%, respectively and approximately linear interpolation used between the relevant percentage time points[4]. The precise value of T ~ used I will not significantly affect diversity gain results [l and a fixed valueof Tm = 275 K was assumed for the Australia and Peru data. Later analysis will probably change this value. 4.
RESULTS
4.1
Single-Site Data
Cumulative statistics of rainfall rate, sky-noise temperature, and total path attenuation are given in Figs. 1, 2 , and 3, respectively, for the Singapore and Hong Kong experiments. The total rainfall in Hong Kong (1200 mm) was well below that measured in Singapore (2060 mm). The rainfall rate statistics reflect this. The sky-noise data in Fig. 2 , however, do not reflect the rain statistics. The availability of the Singapore radiometer, was only 97% and so possibly some large storms were missed. The unexpected 'ripple'in the Ts data for Singapore around 0.1% supports t h i s idea. The total path attenuatioqdata presented in Fig. 3 use the data 2 and equation2. directly from Fig. 4.2
Diversity Data
The preliminary diversity gain [5] of the Peru and Australia experiments is presented in Fig. 4. The total raining-time analyzed to date 25isand 150 hours in Peru and Australia, respectively.
230
DISCUSSION 5.
Assuming a uniform rainrate and standard scaling criteria [ 6 ] , the Hong Kong : Singapore path attenuation ratios for 0.01, 0.1, and 1% should1.17, be 089, and 1.13. In practice (see Fig.3) they are 2.2and 1.12 for 0.1 and l%, respectively. The ratio at 0.01% is undefined. It would appear that the missing data in Singapore have biased the cumulative statistics downwards in the mi range.
The site diversity data indicate that the performance to be achieved by such a system in the tropics is inferior to similar systems in temperate climates [3] which isa surprising result. It has been speculated [l] that the size of the tropical stormsand their apparent slowness has causedthe degradationin anticipated performance. The flatness of the terrain in both site diversity experiments is also a likely contributor;a major geographical feature between two diversity sites has been shown to gi enhanced diversity performance [2]. 6.
REFERENCES
1.
ARBESSER-RASTBURG, B., McCAR"Y, D.K., and ALLNUTT, J.E.: 'Radiometric propagation experimentsin tropical regions' ESA Int.Symp. on Sat. Trans., Graz, 1985.
2.
ROGERS, D.V.: 'Diversity- and single-site radiometric measurements of 12 (;Hz rain attenuation in different climates, IEE Conf. Publ. 195, 1981, pp. 118 123.
-
3.
BRUSSAARD, G.:
4.
ALLNUTP, J.E. and UPMN, S.A.J.: 'The resultsof a 12 GHz radiometric experiment in Hong Kong', submitted to Electronics Letters.
5.
HODGE, D.B.:
6.
CCIR VolumeV: Propagation in non-ionised media 564: 'Propagation data (Study Group 5) Report required for space telecommunications systems'
'Radiometry: a useful prediction tool?', ESA SP-107, 1985.
'An empirical relationship for path diversity gain', IEEE Trans. AP-24, 1976, pp. 250-251.
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