### Abstract

- Top of page
- Abstract
- 1. Introduction
- 2. Measurements and Data Processing
- 3. Attenuation and Rainfall Statistics
- 4. Conclusions
- Appendix A:: Fourier Series Fitting
- Appendix B:: Statistical Combination of Individual Tropospheric Effects to Obtain the Total Attenuation Statistics
- Appendix C:: Rain Attenuation Statistics Using the Full Rainfall Distribution
- References
- Supporting Information

[1] Long-term statistics of tropospheric attenuation were derived from almost 4 years of measurements made in the south of England using the ITALSAT F1 beacon signals at 49.5, 39.6, and 18.7 GHz; coincident rainfall rate measurements were made at the site of the receiving ground station. A method to remove the nonatmospheric changes of the beacon signals and to establish the reference levels from which to measure the excess and total attenuation has been presented in detail. The accuracy of fade level retrieval is estimated to be ∼±0.5 dB. A new method for predicting the annual total attenuation statistics has been proposed and validated against our data and data collected in Italy at 18.7, 39.6, and 49.5 GHz. For both locations, the new proposed method gives much better predictions compared with the established International Telecommunication Union recommendation method. A significant monthly and seasonal variation was observed in the attenuation and rainfall statistics and should be taken into consideration when planning the design and use of future slant path systems. We have seen that the attenuation statistics are subject to diurnal variations; however, for the period analyzed, this variation does not seem to follow a particular pattern.

### 1. Introduction

- Top of page
- Abstract
- 1. Introduction
- 2. Measurements and Data Processing
- 3. Attenuation and Rainfall Statistics
- 4. Conclusions
- Appendix A:: Fourier Series Fitting
- Appendix B:: Statistical Combination of Individual Tropospheric Effects to Obtain the Total Attenuation Statistics
- Appendix C:: Rain Attenuation Statistics Using the Full Rainfall Distribution
- References
- Supporting Information

[2] The increasing demand for more bandwidth for the numerous new services using Earth-space links, and the congestion of the lower-frequency bands, C (4–8 GHz) and Ku (12–18 GHz), is leading to the use of higher-frequency bands K (18–26 GHz), Ka (26–40 GHz) and U (40–60 GHz) or V (50–70 GHz). Services using systems operating at higher frequencies can benefit not only from the high data rates available at those frequencies but also from the smaller component sizes. The latter is an important factor for the expansion of satellite services in small business and direct to home applications.

[3] However, link quality and availability is likely to be severely degraded by the troposphere. In particular, rain attenuation, which increases rapidly with increasing frequency (at least for frequencies up to 100 GHz), is not the only propagation factor likely to degrade system performance as it is at the lower frequencies (Ku band). Light rain, clouds and gaseous attenuation, which have been neglected at the lower frequencies, can significantly limit the performance of Ka, U and V band Earth-space systems. Propagation experiments have been performed to measure, characterize and model the propagation effects on Earth-space paths. The first propagation experiment using satellite beacons occurred after NASA launched the ATS 5 and ATS 6 satellites in the 1970s [*Ippolito*, 1981]. Then a number of satellite experiments followed, mainly, in North America, Japan and Europe such as COMSTAR [*Cox and Arnold*, 1982], ETS-II [*Yamada and Yokoi*, 1974], SIRIO [*Mauri*, 1981], CS [*Fukuchi et al.*, 1983], and the most recent OLYMPUS [*Arbesser-Rastburg and Paraboni*, 1997], ITALSAT [*Paraboni et al.*, 2002], and ACTS [*Davarian*, 1996].

[4] The Radio Communications Research Unit (RCRU) of the Rutherford Appleton Laboratory (RAL) has made measurements of tropospheric induced fading by monitoring the 18.7, 39.6 and 49.5 GHz beacon signals carried on the geostationary Italian satellite, ITALSAT F1, for nearly four consecutive years. In addition, a 3 GHz multiparameter radar “CAMRa,” a microwave radiometer, a video camera, a ceilometer and a variety of other meteorological equipment provided information on the structure of rain and clouds. Measurements from this propagation experiment extended those already made using the beacons carried on the ESA satellite, Olympus, at frequencies near 12, 20 and 30 GHz [*Ventouras et al.*, 1995]. Figure 1 shows an example of the comparative levels of attenuation experienced at Ku, K, Ka, and U band as measured at the receiving station of RCRU during the ITALSAT and Olympus propagation campaigns. The annual total attenuation statistics shown in Figure 1 were measured from April 1997 to March 1998 using the ITALSAT 49.5, 39.6 and 18.7 GHz beacons whereas the statistics for the Olympus 19.8 and 12.5 GHz frequencies were measured in 1993, the satellite's final operational year.

[5] The severity of propagation impairments raises fundamental questions regarding the way in which Ka band and above services would be used and what system availability could be achieved. The rapid growth in Earth-space services using very small aperture terminals (VSAT), and ultra small aperture terminals (USAT), which necessarily have low fade margins, means that the impact of light rain, clouds and gaseous attenuation on these systems becomes a crucial consideration. Also, as the frequency increases, in a practical system subject to technological and economical limitations, the available fade margin alone is unlikely to compensate for the atmospheric attenuation. Therefore fade mitigation techniques might have to be employed. It is therefore very important to identify, measure and study all sources of loss along Earth-space paths for use in system design and the development of accurate propagation prediction methods.

[6] Section 2 of this paper describes the experimental characteristics and the processing of the recorded raw data from the ITALSAT propagation experiment. The algorithm used for the extraction of slant path attenuation from the received beacon signals is given in detail, as this is critical for the final outcome and conclusions. The results relating to long-term propagation measurements are discussed in section 3. These include attenuation and point rainfall statistics presented in annual, monthly, seasonal and diurnal format. The long-term attenuation statistics are compared with the current International Telecommunication Union recommendation (ITU-R) predictions and a new proposed prediction of total attenuation.

[7] Throughout this paper the attenuation is referred to as total or excess attenuation. The total attenuation is considered as the sum of two components: the gaseous attenuation due to oxygen and water vapor, which is always present, and the excess attenuation, which is sometimes present, due to clouds and rain.

### 4. Conclusions

- Top of page
- Abstract
- 1. Introduction
- 2. Measurements and Data Processing
- 3. Attenuation and Rainfall Statistics
- 4. Conclusions
- Appendix A:: Fourier Series Fitting
- Appendix B:: Statistical Combination of Individual Tropospheric Effects to Obtain the Total Attenuation Statistics
- Appendix C:: Rain Attenuation Statistics Using the Full Rainfall Distribution
- References
- Supporting Information

[65] We have presented the long-term total attenuation statistics from the analysis of almost 4 years beacon signal measurements at 49.5, 39.6 and 18.7 GHz and coincident rainfall rate data; these data have been collected in the south of England from the geostationary satellite ITALSAT F1. A comparison of the attenuation statistics at 39.6 GHz (Ka band) and 49.5 GHz (U band) with those from the 18.7 GHz beacon (K band) highlights the significantly increased attenuation levels that are experienced when using high-frequency slant path communication systems.

[66] In any propagation experiment using satellite beacons, the received signals undergo changes originating from behavior of the satellite and/or receiving station. These changes occur simultaneously with the atmospheric phenomena and can bias the measured attenuation if they are not removed from the received signals. A method to remove the nonatmospheric changes of the beacon signals, and to establish the reference levels from which to measure the excess and total attenuation, has been presented in detail. The accuracy of fade level retrieval is estimated to be ∼±0.5 dB.

[67] A new method for predicting the annual total attenuation statistics has been proposed and validated against our data and data collected in Italy at 18.7, 39.6 and 49.5 GHz. For both locations the new proposed method gives much better predictions compared with the established ITU-R method Recommendation P.618-8.

[68] The significant monthly variation that was observed in the attenuation and rain rate statistics should be taken into consideration when planning the design and use of future slant path systems. A design based only on annual and worst month statistics would give uneconomical or impractical solutions, and would not give any insight into potential advantages to be obtained by operating services during specific time slots. Summer and autumn were found to be the seasons with the most rain and attenuation. We have seen that the attenuation statistics are subject to diurnal variations; however, for the period analyzed, this variation does not seem to follow a particular pattern.