A system for determining the level of liquid in a shock absorber, the system comprising: a telescopic shock absorber, comprising: a housing; a cavity located within the housing and containing a liquid and a gas; a sensor for measuring the level of the liquid in the cavity, the sensor comprising: a first waveguide having a first end and a second end, wherein the first waveguide is arranged such that when the shock absorber is in normal use the first end is surrounded by the gas and the second end is immersed in the liquid; and a communications interface operable to transfer electrical signals between the first waveguide to the exterior of the housing; and a transceiver arranged to be connected to the first end or the second end of the first waveguide and operable to couple electromagnetic waves into the first waveguide and receive reflected EM waves from the first waveguide; transmit an electromagnetic signal over a range of frequencies into a first end or a second end of the first waveguide; receive a reflected EM signal from the first waveguide; analyse the reflected EM signal to detect one or more amplitude peaks in the reflected EM signal; characterised in that the transceiver is arranged to determine the level of the liquid in the cavity as a function of the frequency of the amplitude peaks and the dielectric constants of the liquid and the gas. a telescopic shock absorber, comprising: a housing; a cavity located within the housing and containing a liquid and a gas; a sensor for measuring the level of the liquid in the cavity, the sensor comprising: a first waveguide having a first end and a second end, wherein the first waveguide is arranged such that when the shock absorber is in normal use the first end is surrounded by the gas and the second end is immersed in the liquid; and a communications interface operable to transfer electrical signals between the first waveguide to the exterior of the housing; and a housing; a cavity located within the housing and containing a liquid and a gas; a sensor for measuring the level of the liquid in the cavity, the sensor comprising: a first waveguide having a first end and a second end, a first waveguide having a first end and a second end, wherein the first waveguide is arranged such that when the shock absorber is in normal use the first end is surrounded by the gas and the second end is immersed in the liquid; and a communications interface operable to transfer electrical signals between the first waveguide to the exterior of the housing; and a communications interface operable to transfer electrical signals between the first waveguide to the exterior of the housing; and a transceiver arranged to be connected to the first end or the second end of the first waveguide and operable to couple electromagnetic waves into the first waveguide and receive reflected EM waves from the first waveguide; transmit an electromagnetic signal over a range of frequencies into a first end or a second end of the first waveguide; receive a reflected EM signal from the first waveguide; analyse the reflected EM signal to detect one or more amplitude peaks in the reflected EM signal; characterised in that the transceiver is arranged to determine the level of the liquid in the cavity as a function of the frequency of the amplitude peaks and the dielectric constants of the liquid and the gas.