The Friis Transmission equation is the holy grail of a link analysis. It defines the received power strength in terms of losses, gains, and source output power.
Why is it important to get an understanding of the received power? There are limits to the sensitivity of a receiver. Below a certain level, the bit error rate on a signal becomes too great to extract useful information. Getting an idea of the received power at different points in our orbit allows us to gain insight into how long we'll be able to receive useful data at our ground station when the CubeSat passes over. But more on that later (Eb/N0 and SNR).
In essence, the Friis transmission equation is the product of the transmit power and all the losses and gains in an RF system:
$$
P_R=P_TGL $$
More often, the Friis Transmission Equation is stated as the product of the transmit power, receiver & transmitter antenna gains, and the free space path loss of the system.
$$ P_R=\frac{P_TG_TG_R\lambda^2}{(4\pi R)^2} $$
The geometry of an omnidirectional transmit antenna gives the equation:
$$ p=\frac{P_T}{4\pi R^2} $$
where $p$ is the power density of the signal at distance $R$. Here, $R$ is usually given by:
$$ R=\sqrt{(R_E+h)^2-R_E^2cos^2\phi}-R_Esin\phi $$
where $R_E$ is the average radius of the Earth, $h$ is the altitude of the satellite, and $\phi$ is the minimum elevation angle (see Orbital Mechanics).
Knowing that the definition of antenna gain is the amount by which the power density of a signal is boosted, we can rewrite this as:
$$ p=\frac{P_TG_T}{4\pi R^2} $$
where $G_T$ is the transmit antenna's maximum gain. Given $p$, we know that we can find $P_R$ using our effective aperture equation:
$$ P_R=pG_TA_{ER}=\frac{A_{ER}G_TP_T}{4\pi R^2} = \frac{P_TG_TG_R\lambda^2}{(4\pi R)^2} $$
Notice here that we substituted the equation for effective aperture (Effective Aperture ). In this equation, the factor $\frac{\lambda^2}{(4\pi R)^2}$ is also known as the free space path loss, which we can abbreviate as $L_{path}$.
Of course, $L_{path}$ is not the only loss in our cubesat-ground station system. In previous pages, we also talked about polarization loss, which we can denote as $L_{pol}$, the pointing loss or "antenna misalignment loss" ($L_{aml}$), the atmospheric loss, $L_{atm}$, and others including rain loss, ionospheric loss, and transmission line losses (internal to the receiver or transmitter board, such as impedance mismatch). Therefore, a more complete version of the Friis equation may be written as:
$$ P_R=P_TG_TG_RL_{path}L_{atm}L_{pol}L_{aml}... $$