Real-Time Rain Rate Evaluation via Satellite Downlink Signal Attenuation Measurement

Abstract

We present the NEFOCAST project (named by the contraction of “Nefele”, which is the Italian spelling for the mythological cloud nymph Nephele, and “forecast”), funded by the Tuscany Region, about the feasibility of a system for the detection and monitoring of precipitation fields over the regional territory based on the use of a widespread network of new-generation Eutelsat “SmartLNB” (smart low-noise block converter) domestic terminals. Though primarily intended for interactive satellite services, these devices can also be used as weather sensors, as they have the capability of measuring the rain-induced attenuation incurred by the downlink signal and relaying it on an auxiliary return channel. We illustrate the NEFOCAST system architecture, consisting of the network of ground sensor terminals, the space segment, and the service center, which has the task of processing the information relayed by the terminals for generating rain field maps. We discuss a few methods that allow the conversion of a rain attenuation measurement into an instantaneous rainfall rate. Specifically, we discuss an exponential model relating the specific rain attenuation to the rainfall rate, whose coefficients were obtained from extensive experimental data. The above model permits the inferring of the rainfall rate from the total signal attenuation provided by the SmartLNB and from the link geometry knowledge. Some preliminary results obtained from a SmartLNB installed in Pisa are presented and compared with the output of a conventional tipping bucket rain gauge. It is shown that the NEFOCAST sensor is able to track the fast-varying rainfall rate accurately with no delay, as opposed to a conventional gauge.

The NEFOCAST System for Detection and Estimation of Rainfall Fields by the Opportunistic Use of Broadcast Satellite Signals

“…The results show that in the absence of rain (first and last segments of the plot) the outputs of the two filters are almost coincident, while during the precipitation event they depart significantly from one another. Using (13) and (12) the corresponding rain rate is then estimated, and the result is shown in the lower part of Fig. 9.…”

“…NEFOCAST is a research project, funded by the government of the Tuscany Region (Italy), aimed at assessing the feasibility of a satellite-based system for real-time construction of rain maps over the regional territory [13]- [15]. The main idea is to make opportunistic use of broadcast satellite signals related to domestic services such as 6 DVB-S/S2 to estimate the intensity of rain events by measuring the satellite signal attenuation at the receiver.…”

“…Figure 4 illustrates the geometry of the radio link and represents the various precipitation layers. To simplify the notation in the upcoming equations, the reference system has been chosen to set the zero quota at the level of 0 °C isotherm and the vertical coordinate is oriented to increase in the direction of the ground, so that at the ground an- 13 where ML  (km) is the vertical extension of the melting layer, given by the difference between the 0 °C isotherm level and the full liquid rain height [20], and we assumed ML 0 h . We also assume that the specific rain attenuation ( ; )…”

“…IV). Thus, employing the values of rain () L kT provided from (13), an estimate of the rain rate is obtained according to (12). At the end of the rain event, the FT state returns in the close proximity of the "frozen" reference ST state, and when their difference falls below a further threshold (e.g., 0.1 dB), the rain detector states "end of rain" and resets the flag to false.…”

“…This prospective "opportunistic" approach to the construction of rain maps has spurred the NEFOCAST project, funded by the Tuscany Region from 2016. A significant body of results on the project progress are already available in [6], [13]- [17]. The NEFOCAST project is based on the use of interactive domestic terminals capable of continuously measuring the received SNR and relaying the data to the operational center.…”