Non-terrestrial networks (NTN) introduce a range of impairments that must be accurately modeled during testing. One key impairment is Doppler shift.
Doppler shift occurs when a transmitter and receiver move relative to one another, causing a change in the observed signal frequency. This effect is especially significant in low-Earth orbit (LEO) systems, where satellites travel at speeds exceeding 17,000 mph. As a result, both the carrier frequency and the signal channel bandwidth can be affected, requiring the receiver to remain locked onto a signal that is continuously changing over time.
Why Doppler Shift Changes During a Satellite Pass
In simple cases where the relative motion is constant, the Doppler shift appears as a fixed frequency offset, as described by the basic Doppler equation:
$$
F_s = F_a \left(\frac{v}{c}\right)
$$
$$
\begin{array}{rl}
F_s & = \text{Doppler shift (Hz)} \\
F_a & = \text{transmitted frequency (Hz)} \\
v & = \text{relative velocity (km/s)} \\
c & = \text{speed of light } (299{,}792.458\ \text{km/s})
\end{array}
$$
However, a real satellite pass is not static. As the satellite moves across the sky, it is not always moving directly toward or away from the receiver. The portion of its motion in that direction (radial velocity) changes continuously. Because of this, the Doppler shift varies over time rather than remaining a single fixed value. The observed frequency increases as the satellite approaches, crosses zero at closest approach, and decreases as it moves away.
The interactive example below illustrates how Doppler shift evolves during a satellite pass, showing the physical motion, the resulting frequency shift over time, and the corresponding spectrum changes.
Visualizing Doppler Shift During a Satellite Pass
Move the slider to simulate a satellite pass. As the satellite moves towards and away from base station, the signals compress and expand due to relative motion. The plots below show the resulting carrier Doppler shift and corresponding spectrum shift over time.
Time
0 sec
Carrier Doppler Shift
+398 kHz
Satellite State
Approaching
Why Emulating Doppler Shift Matters
The time-varying Doppler behavior shown above must be accurately reproduced during system testing.
The Maury ACE9000 Advanced Channel Emulator enables this by applying a time-varying frequency and corresponding phase shift to reproduce what a receiver would experience during an actual satellite pass.
With advanced channel emulation, engineers can evaluate receiver performance under realistic operating conditions, identify potential issues, and ensure reliable operation before deployment.
