to a change in temperature or pressure or to handling by the user in such cases the detection means 107 may comprise the appropriate signal processing to determine the signal to noise ratio an accelerometer located in the mobile telephone a temperature sensor a pressure sensor and a movement sensor or actuator respectively indeed many other alternatives to these components would suggest themselves to a person skilled in the art also at present gps is most notably associated with the navigation system with time and ranging navstar gps an all weather spaced based navigation system developed and operated by the us department of defense however the general principles underlying gps are universal and not merely limited to navstar accordingly gps refers to any positioning system comprising a plurality of radio transmitters at different locations and a receiver which determines its location based on the time of arrival of the transmissions of the radio transmitters from a reading of the present disclosure other modifications will be apparent to the skilled person skilled and may involve other features which are already known in the design manufacture and use of gps receivers and component parts thereof and which may be used instead of or in addition to features already described herein although claims have been formulated in this application to particular combinations of features it.