Abstract: A communications device, such as a GNSS receiver comprises an oscillator, having a temperature-dependent frequency characteristic, for generating signals at a nominal frequency; receiver circuitry, for receiving transmitted wireless signals using the signals generated by the oscillator; at least one temperature sensor, having a known positional relationship to the oscillator; an estimation device, for estimating a frequency of the signals generated by the oscillator, based on a measurement from the temperature sensor, and based on the temperature-dependent frequency characteristic of the oscillator; and at least one heat source. A change in the temperature of the oscillator is predicted, based on a state of the heat source, and further based on a model of the thermal properties of the communications device, and hence a change in the frequency of the signals generated by the oscillator is predicted, based on the temperature-dependent frequency characteristic of the oscillator.