Aerospace and Electronic Systems - June 2019 - 19

Giannetti et al.
Table 1.

^
4. Derive an estimate of the rain rate RðtÞ
from
Lrain ðtÞ.

Summary of NEFOCAST System Main Parameters
Feature/Item

Name/Value

Satellite name, orbital
slot

Eutelsat 10 A, 10o East

FL satellite EIRP

48 dBW

FL frequency,
polarization

11.345 GHz, LVP

FL protocol,
modulation, FEC rate

DVB-S2, QPSK, 4/5

FL noise figure of
SmartLNB

0.2 dB @ 290 K

RL SmartLNB EIRP

From 15 to 35 dBW

RL frequency,
polarization

14.216 GHz, LHP

RL protocol

SSA F-SIM

RL figure of merit G/T

4 dB/K

Unfortunately, as we will see in "Impairments
Affecting the Level of a Signal Received from a GEO Satellite," in a real system there are several impairments that
affect this procedure, which, if not taken care of,
compromise the accuracy of the resulting estimate. In the
following sections, we first address in detail all the aspects
related to ideal rain rate retrieval; then, we analyze the
impairments, and finally propose a solution based on a
double-KF architecture.

SNR FOR A SATELLITE LINK AND ITS DEPENDENCE ON
RAIN ATTENUATION
SNR in Dry Conditions: In case of dry conditions, i.e., in
the absence of rain, the (dimensionless) SNR can be
expressed as (notice that the SNR is actually a time-varying link quality metric due to the many impairments
outlined in "Impairments Affecting the Level of a Signal
Received from a GEO Satellite") [18]
!ðdryÞ
Es
ðtÞ
N0

the F-SIM (Fixed Interactive Multi-media Services)
protocol [17].

D

h

¼
Latm Lcloud

PROCEDURE FOR RAIN RATE RETRIEVAL
A key parameter for estimating the rain rate is the ratio
Es =N0 (signal-to-noise ratio, or SNR, for short) measured at the ground terminal, wherein the numerator Es
represents the average radiofrequency received energy
(in J) within the time interval of one information-bearing symbol, and the denominator N0 is the one-sided
power spectral density of the additive white Gaussian
noise (in W/Hz) affecting the received signal. In the
presence of a rain event, the procedure for the estimation of the rainfall rate based upon the measured SNR
can be summarized in four steps:
1. Obtain the reference value of the SNR in dry
conditions ½Es =N0 ðtÞðdryÞ ;
2. Measure the actual SNR, ½Es =N0 ðtÞðwetÞ , whose
value depends on the intensity of rain;
3. Extrapolate the attenuation of the SNR due to the
rain Lrain ðtÞ;

Es
ðtÞ
N0

!ðwetÞ

JUNE 2019

¼

Latm Lcloud Lrain ðtÞ

h

Tc
Latm Lcloud


þ Tm 1 À Latm L1

cloud



þ Tg þ Trx

i

(1)
where F is the signal power flux density (in W/m2) at the
receiving antenna input, GR is the receiving antenna gain
(dimensionless),  is the carrier wavelength (in m), Rs is
the symbol rate (in sÀ1), Latm is the atmospheric attenuation due to water vapor absorption, and other gaseous
effects (dimensionless), Lcloud is the attenuation due to
clouds (dimensionless), k is the Boltzmann constant (in J/
Hz), and Tc , Tm , Tg , Trx are the noise temperatures of
cosmos, meteorological formations (clouds, etc.), and
environment surrounding the antenna and receiver
hardware, respectively (all in K).
SNR in Wet Conditions: In case of wet conditions, i.e., in
the presence of rain, the signal experiences an additional fast
time-varying attenuation Lrain ðtÞ (dimensionless) and
accordingly the SNR in (1) becomes (due to its role in the
evaluation of the rain rate, the time-dependence of the rain
attenuation is here highlighted)

FGR 2
4pRs k
Tc
Latm Lcloud Lrain ðtÞ

FGR 2
4pRs k


þ Tm 1 À Latm L

1



cloud Lrain ðtÞ

IEEE A&E SYSTEMS MAGAZINE

þ Tg þ Trx

i:

(2)

19
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