Speed of sound

The speed of spound varies according to your weather conditions which include temperature and barometric pressure.

Speed of sound

Speed in mach

1 Mach equals the speed of sound at the given time and location.

Mach

The models mach speed is calculated by dividing the model's speed in km/h through the speed of sound in km/h.
The reason for measuring the model's speed in mach is to have a constant measurement relating to weather conditions.
That constant is the speed of sound.
Theortically with more investigation pending, depending on model characteric and flying behaviours, your km/h reading might vary under different weather condition but the model's mach reading should be the same.

Pergrine weather Station

The weather station is used to read the weather conditions to determine the speed of sound.
It should always be used and connected to the Peregrine base station when speed readings is done.

Model that do not have mach reading is due to no weather station being used while speed measurements was done.