A locator for locating a buried electromagnetic marker, the locator comprising: a transmission antenna for generating a first oscillatory magnetic field to couple with an electromagnetic marker; a first reception antenna for receiving an oscillatory magnetic field emitted by the electromagnetic marker; and a first analogue to digital converter, characterised in that, the locator is operable in a marker locate mode and a validation mode in which operation of the transmission antenna is validated against calibration data, wherein: in the marker locate mode: during a first time period, the transmission antenna is configured to generate the first oscillatory magnetic field and the reception antenna is configured to not output a signal; and during a second time period, after the first time period, the transmission antenna is configured to stop generating an oscillatory magnetic field and the first reception antenna is configured to receive an oscillatory magnetic field emitted by the electromagnetic marker; and in the validation mode: the transmission antenna is configured to generate a test oscillatory magnetic field; the first reception antenna is configured to directly receive the test oscillatory magnetic field transmitted by the transmission antenna and thereby generate a first detected test signal; the first analogue to digital converter is configured to generate a first digitised test signal from the first detected test signal, the first digitised test signal being indicative of the test oscillatory magnetic field received by the first reception antenna. a transmission antenna for generating a first oscillatory magnetic field to couple with an electromagnetic marker; a first reception antenna for receiving an oscillatory magnetic field emitted by the electromagnetic marker; and a first analogue to digital converter, characterised in that, the locator is operable in a marker locate mode and a validation mode in which operation of the transmission antenna is validated against calibration data, wherein: in the marker locate mode: during a first time period, the transmission antenna is configured to generate the first oscillatory magnetic field and the reception antenna is configured to not output a signal; and during a second time period, after the first time period, the transmission antenna is configured to stop generating an oscillatory magnetic field and the first reception antenna is configured to receive an oscillatory magnetic field emitted by the electromagnetic marker; and in the validation mode: the transmission antenna is configured to generate a test oscillatory magnetic field; the first reception antenna is configured to directly receive the test oscillatory magnetic field transmitted by the transmission antenna and thereby generate a first detected test signal; the first analogue to digital converter is configured to generate a first digitised test signal from the first detected test signal, the first digitised test signal being indicative of the test oscillatory magnetic field received by the first reception antenna. in the marker locate mode: during a first time period, the transmission antenna is configured to generate the first oscillatory magnetic field and the reception antenna is configured to not output a signal; and during a second time period, after the first time period, the transmission antenna is configured to stop generating an oscillatory magnetic field and the first reception antenna is configured to receive an oscillatory magnetic field emitted by the electromagnetic marker; and during a first time period, the transmission antenna is configured to generate the first oscillatory magnetic field and the reception antenna is configured to not output a signal; and during a second time period, after the first time period, the transmission antenna is configured to stop generating an oscillatory magnetic field and the first reception antenna is configured to receive an oscillatory magnetic field emitted by the electromagnetic marker; and in the validation mode: the transmission antenna is configured to generate a test oscillatory magnetic field; the first reception antenna is configured to directly receive the test oscillatory magnetic field transmitted by the transmission antenna and thereby generate a first detected test signal; the first analogue to digital converter is configured to generate a first digitised test signal from the first detected test signal, the first digitised test signal being indicative of the test oscillatory magnetic field received by the first reception antenna. the transmission antenna is configured to generate a test oscillatory magnetic field; the first reception antenna is configured to directly receive the test oscillatory magnetic field transmitted by the transmission antenna and thereby generate a first detected test signal; the first analogue to digital converter is configured to generate a first digitised test signal from the first detected test signal, the first digitised test signal being indicative of the test oscillatory magnetic field received by the first reception antenna.