Aerospace and Electronic Systems - June 2019 - 16

Feature Article:

DOI. No. 10.1109/MAES.2019.2916292

The NEFOCAST System for Detection and Estimation
of Rainfall Fields by the Opportunistic Use of
Broadcast Satellite Signals
Filippo Giannetti, Marco Moretti, Ruggero Reggiannini,
University of Pisa, Pisa, Italy
Attilio Vaccaro, M.B.I., S.r.l., Pisa, Italy

INTRODUCTION
Monitoring of precipitations over a regional territory is an
objective of major concern for public administrators in
order to guarantee an adequate level of security for people
living and operating on the territory [1], [2]. In particular,
great importance is given to the availability of rain maps
featuring i) good accuracy in the measurement of rainfall
rate, ii) space-time completeness and continuity, and iii)
negligible delays in the estimates of rain, so as to be able
to predict from the above precipitation maps the arrival of
severe meteorological events and adopt timely measures
to prevent/reduce the related risk. The methods used to
obtain a rain map are traditionally based, alternatively or
jointly, on: a network of rain gauges [3], deployment of
weather radars [4], and satellite remote sensing in the optical or infrared band [5]. Each of the cited methods entails
a more or less satisfactory degree of compliance with the
requirements listed above. In Giannetti et al. [6]. the
advantages and limitations inherent in the use of these
techniques are discussed. Independent of the approach
taken, a terrestrial operational center is also needed for
processing, control, and alert functions.
More recently, in addition to the techniques mentioned
above, it has been demonstrated [7]-[12] that rain fields

Authors' current addresses: F. Giannetti, M. Moretti,
R.
Reggiannini,
Dipartimento
di
Ingegneria
dell'Informazione, University of Pisa, Pisa 56126, Italy,
E-mail: (filippo.giannetti@unipi.it, marco.moretti@unipi.
it, ruggero.reggiannini@unipi.it). Attilio Vaccaro, M.B.I.,
S.r.l., Pisa 56121, Italy, E-mail: (avaccaro@mbigroup.it).
Manuscript received September 29, 2018, revised
December 20, 2018, and ready for publication May 10,
2019.
Review handled by S. Marcuccio.
0885-8985/19/$26.00 ß 2019 IEEE
16

can also be estimated by exploiting the presence of
"signals of opportunity" generated by communication systems. In particular, for satellite systems, these signals are
transmitted in the microwave region, usually in the Ku
range or higher, and suffer from a significant excess attenuation in the presence of rain. In other terms, they exhibit a
measurable reduction in the signal-to-noise ratio (SNR)
with respect to dry operation, and this reduction is uniquely
related to the intensity of rain. Moreover, they can be
received by a potentially huge number of domestic devices,
widely spread over the whole territory (at least within its
anthropized part). 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
is already available [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.
The problem of estimating the rainfall rate from a
measure of excess signal attenuation or SNR reduction on
a satellite link has already received considerable attention
in the literature (for a representative selection of references see [9]-[12]), and its points of force and weakness
have been investigated. One of the most critical problems,
at the moment not yet satisfactorily resolved, is how to
choose the SNR reference level corresponding to the
absence of rain (a condition referred to as dry in the
sequel), against which the rain-induced SNR variations
should be measured. As a matter of fact, the downlink dry
signal level is far from being exactly constant, due to
various phenomena, notably the apparent movement of
the satellite under the attraction of the sun and the moon.
In spite of the above drawbacks, the method has,
however, some undoubted advantages compared to
conventional techniques; notably, it is not subject to
significant measurement delays, unlike tipping bucket rain

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JUNE 2019

Aerospace and Electronic Systems - June 2019

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