The current state of the telecommunication market is driven by the increasing demand of the end users for multimedia services, which require high data rates. Within the Fixed Satellite Service, frequency bandwidths wide enough to carry such high data rates are less and less available in conventional C-band to Ku-band but instead at higher frequency bands such as Ka-band or Q/V-band.
For broadband applications, this trend is reinforced by the saturation of C-band and Ku-band especially in North America or Europe, so deploying new satellite system in these frequency bands involves a complex co-ordination procedure. In tropical and sub-tropical areas, the terrestrial infrastructure is less developed and there is a strong incentive to develop Satcom systems in the next few years, to serve wide regions and to reach a large market size for economic profitability.
Regarding Ku-band in the tropics, no spectrum congestion is expected in the near future, so the use of this frequency band will be generalised for the next few years, with an anticipated evolution towards Ka-band for high data rate multimedia applications.
However, propagation impairments are expected to be quite severe in tropical climates, so standard performances may be difficult to achieve in these regions. Accurate prediction of impairment statistics is thus very important for the design and deployment of satellite systems in tropical regions.
The objective of this study, carried out by ONERA (France) in collaboration with Politecnico di Milano (Italy), CRC (Canada), and CETUC (Brazil), is to improve the knowledge concerning propagation effects, climatological influences, and propagation modelling for SatCom and Satnav systems providing services in tropical and sub-tropical regions.
Tropical areas are characterised by high ambient temperatures, which result in a higher evaporation of water, and then larger cloud structures and more intense precipitation. Therefore, it is the water cycle which is the most specific for tropical and sub-tropical latitudes. On the other hand, specific phenomena can be encountered such as monsoons in South Asia, hurricanes in America, or squall lines in Africa. All of these effects may lead to different behaviour of radio propagation in the troposphere, and therefore may require different modelling approaches.
Therefore, key issues addressed in this study concerned the modelling of precipitating systems and in particular the upgrade of prediction methods of rainfall rate statistics. In a first step, new rain maps have been generated by calibrating ERA 40 precipitation data with independent meteorological data. The superior accuracy of these new rain maps has been verified with respect to TRMM products.
The rainfall rate prediction method recommended by ITU-R (Rec. ITU-R P.837-4) was upgraded and its performance assessed with respect to a large database of experimental statistics (which included substantial augmentation from data collected in Brasil). The accuracy of the model was improved from a RMS value of 40% for the in-force Recommendation ITU-R P.837-4, to a RMS value of 29% for the new model.
Current prediction methods recommended by ITU-R rely on radio-climatological maps of input parameters that are representative of yearly statistical distributions. Further, these prediction methods have been developed and tested essentially with respect to experimental data collected in temperate areas. Consequently, climatological maps and prediction methods are both expected to be less accurate in tropical and equatorial areas, where specific behaviour and seasonal variations are of primary importance.
The main achievements and the main outputs foreseen in this study are the following:
- Constitution of new databases more representative of tropical and equatorial areas for deriving new maps of radio-climatic parameters;
- New or improved propagation prediction methods for SatCom systems, especially for propagation impairment models reliant upon accurate rain modelling.
- New software procedures for analysing propagation-related data and Earth Observation and Numerical Weather Prediction products.
In this study, propagation tools for the prediction of the impact of radiowave propagation for satellite communication and satellite navigation systems will be developed. These propagation tools will rely on a database of radiometeorological parameters, allowing prediction methods for tropospheric delay, cloud attenuation, rainfall rate, rain attenuation and fade duration to be applied worldwide.
The study was divided into three phases.
Phase1: the objective of this phase was to review tropospheric and ionospheric propagation effects, models and experimental data acquisition for both SatCom and GNSS systems. First, a review of propagation effects, propagation models and required radio-meteorological input parameters was carried out. Afterwards, a review of experimental data collected in the tropics and useful for modelling improvement was performed concerning: propagation measurements, satellite imagery from Earth observation, and numerical weather products. Then a databank of experimental data was constituted for model testing and improvement.
Phase2: This phase dealt with development of processing tools to allow tropical and sub-tropical data collected in Phase1 to be analysed. Three kind of processing tools were developed: for tropospheric propagation data analysis (beacon, radiometer, rain gauge data), for Earth observation data analysis (TRMM products); and for numerical weather products analysis (ECMWF ERA40 database). At the end of Phase2, all processing tools were available and tested for producing the required maps of meteorological parameters and statistical distributions to be used in Phase3.
Phase3: The objective of this phase was to provide the best models of the tropospheric effects for tropical and sub-topical regions. According to the results obtained in Phase1, the activity was concentrated on the modelling of the effects that have the major impact on satellite communication systems, that is, effects due to precipitation systems, and more particularly, on the characterisation and modelling of statistics of cloud attenuation, rainfall rate and fade duration; sky noise temperature was also analysed.
The project was divided into three main phases.
In the first phase, a state-of-the-art review of propagation effects, relevant data, and propagation models applicable to tropical and equatorial areas, available at the beginning of this study, was performed.
The characterisation of the propagation channel for tropical and equatorial areas was carried out in the second phase of the project. Meteorological and Earth observation data, numerical weather forecast products, as well as meteorological and propagation measurements collected during propagation campaigns, were analysed. Then a comparative analysis among similar products was carried out.
The modelling activity performed in this study was completed in the third phase of the project. In a preliminary step, new databases were generated. Then, the modelling activities themselves were carried out in terms of : first, rainfall rate; cloud attenuation; sky noise temperature; and fade duration. Associated testing analyses were conducted to assess the accuracy of the models.