RF signal attenuation due to atmospheric gases
L = gaspl(
L, when signals propagate through
range represents the signal path
freq represents the signal carrier
T represents the ambient temperature,
the atmospheric pressure, and
the atmospheric water vapor density.
gaspl function applies the International
Telecommunication Union (ITU) atmospheric gas attenuation model  to calculate path loss for signals
primarily due to oxygen and water vapor. The model computes attenuation
as a function of ambient temperature, pressure, water vapor density,
and signal frequency. The function requires that the signal path is
contained entirely in a uniform environment. Atmospheric parameters
do not vary along the signal path. The attenuation model applies only
for frequencies at 1–1000 GHz.
Atmospheric Gas Attenuation Spectrum
Compute the attenuation spectrum from 1 to 1000 GHz for an atmospheric pressure of 101.300 kPa and a temperature of C. Plot the spectrum for a water vapor density of 7.5 and then plot the spectrum for dry air (zero water vapor density).
Set the attenuation frequencies.
freq = [1:1000]*1e9;
Assume a 1 km path distance.
R = 1000.0;
Compute the attenuation for air containing water vapor.
T = 15; P = 101300.0; W = 7.5; L = gaspl(R,freq,T,P,W);
Compute the attenuation for dry air.
L0 = gaspl(R,freq,T,P,0.0);
Plot the attenuations.
semilogy(freq/1e9,L) hold on semilogy(freq/1e9,L0) grid xlabel('Frequency (GHz)') ylabel('Specific Attenuation (dB)') hold off
Plot Attenuation Due to Atmospheric Gases and Free Space
First, plot the specific attenuation of atmospheric gases for frequencies from 1 GHz to 1000 GHz. Assume a sea-level dry air pressure of 101.325e5 kPa and a water vapor density of 7.5 . The air temperature is C. Specific attenuation is defined as dB loss per kilometer. Then, plot the actual attenuation at 10 GHz for a span of ranges.
Plot Specific Atmospheric Gas Attenuation
Set the atmosphere temperature, pressure, water vapor density.
T = 20.0; Patm = 101.325e3; rho_wv = 7.5;
Set the propagation distance, speed of light, and frequencies.
km = 1000.0; c = physconst('LightSpeed'); freqs = [1:1000]*1e9;
Compute and plot the atmospheric gas loss.
loss = gaspl(km,freqs,T,Patm,rho_wv); semilogy(freqs/1e9,loss) grid on xlabel('Frequency (GHz)') ylabel('Specific Attenuation (dB/km)')
Plot Actual Atmospheric and Free Space Attenuation
Compute both free space loss and atmospheric gas loss at 10 GHz for ranges from 1 to 100 km. The frequency corresponds to an X-band radar. Then, plot the free space loss and the total (atmospheric + free space) loss.
ranges = [1:100]*1000; freq_xband = 10e9; loss_gas = gaspl(ranges,freq_xband,T,Patm,rho_wv); lambda = c/freq_xband; loss_fsp = fspl(ranges,lambda); semilogx(ranges/1000,loss_gas + loss_fsp.',ranges/1000,loss_fsp) legend('Atmospheric + Free Space Loss','Free Space Loss','Location','SouthEast') xlabel('Range (km)') ylabel('Loss (dB)')
range — Signal path length
nonnegative real-valued scalar | M-by-1 nonnegative real-valued column vector | 1-by-M nonnegative real-valued row vector
Signal path length used to compute attenuation, specified as a nonnegative real-valued scalar or vector. You can specify multiple path lengths simultaneously. Units are in meters.
freq — Signal frequency
positive real-valued scalar | N-by-1 nonnegative real-valued column vector | 1-by-N nonnegative real-valued row vector
Signal frequency, specified as a positive real-valued scalar, or as an N-by-1 nonnegative real-valued vector or 1-by-N nonnegative real-valued vector. You can specify multiple frequencies simultaneously. Frequencies must lie in the range 1–1000 GHz. Units are in hertz.
T — Ambient temperature
Ambient temperature, specified as a real-valued scalar. Units are in degrees Celsius.
P — Dry air pressure
positive real-valued scalar
Dry air pressure, specified as a positive real-valued scalar. Units are in Pa. One standard atmosphere at sea level is 101325 Pa.
den — Water vapor density
nonnegative real-valued scalar
Water vapor density or absolute humidity, specified as a nonnegative real-valued scalar. Units are g/m3. The maximum water vapor density of air at 30° C is approximately 30.0 g/m3. The maximum water vapor density of air at 0°C is approximately 5.0 g/m3.
L — Signal attenuation
real-valued M-by-N matrix
Signal attenuation, returned as a real-valued M-by-N matrix. Each matrix row represents a different path where M is the number of paths. Each column represents a different frequency where N is the number of frequencies. Units are in dB.
Atmospheric Gas Attenuation Model
This model calculates the attenuation of signals that propagate through atmospheric gases.
Electromagnetic signals attenuate when they propagate through the atmosphere. This effect is due primarily to the absorption resonance lines of oxygen and water vapor, with smaller contributions coming from nitrogen gas. The model also includes a continuous absorption spectrum below 10 GHz. The ITU model Recommendation ITU-R P.676-10: Attenuation by atmospheric gases is used. The model computes the specific attenuation (attenuation per kilometer) as a function of temperature, pressure, water vapor density, and signal frequency. The atmospheric gas model is valid for frequencies from 1–1000 GHz and applies to polarized and nonpolarized fields.
The formula for specific attenuation at each frequency is
The quantity N"() is the imaginary part of the complex atmospheric refractivity and consists of a spectral line component and a continuous component:
The spectral component consists of a sum of discrete spectrum terms composed of a localized frequency bandwidth function, F(f)i, multiplied by a spectral line strength, Si. For atmospheric oxygen, each spectral line strength is
For atmospheric water vapor, each spectral line strength is
P is the dry air pressure, W is the water vapor partial pressure, and T is the ambient temperature. Pressure units are in hectoPascals (hPa) and temperature is in degrees Kelvin. The water vapor partial pressure, W, is related to the water vapor density, ρ, by
The total atmospheric pressure is P + W.
For each oxygen line, Si depends on two parameters, a1 and a2. Similarly, each water vapor line depends on two parameters, b1 and b2. The ITU documentation cited at the end of this section contains tabulations of these parameters as functions of frequency.
The localized frequency bandwidth functions Fi(f) are complicated functions of frequency described in the ITU references cited below. The functions depend on empirical model parameters that are also tabulated in the reference.
To compute the total attenuation for narrowband signals along a path, the function multiplies the specific attenuation by the path length, R. Then, the total attenuation is Lg= R(γo + γw).
You can apply the attenuation model to wideband signals. First, divide the wideband signal into frequency subbands, and apply attenuation to each subband. Then, sum all attenuated subband signals into the total attenuated signal.
 Radiocommunication Sector of International Telecommunication Union. Recommendation ITU-R P.676-10: Attenuation by atmospheric gases 2013.
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