house originally built in the 14th century is certain to need some careful refurbishment. And that is exactly what is happening at Ightham Mote, one of the loveliest and most interesting of the medieval manor houses that survives in England.

Although the early history of the Kent mansion is unclear, it appears that the main part of the house was constructed between 1340 and 1530. Now, an extensive programme of renovation and refurbishment being carried out is aided by the use of advanced technology - radar.

GE Geotechnics, a specialist British company based in Cambridge, eastern England, used impulse radar equipment to determine the condition of external stonework and to plot the extent of voids or cavities before remedial work.

The pulse radar equipment, working in a similar way to bat sonar, sends out pulses of high frequency radio energy which pass through the stone and bounce back from the far side. The time it takes for the signal to pass through each stone is measured and the information analysed from a paper printout. This shows the thickness ofthe stone, and, by interpretation, cracks, voids or moisture within the wall.

Repairs to the Gate Tower were dominated by concerns for the quality, texture and stability of the ragstone elevations and brick crenellated parapet. Photogrammetry as well as pulsed radar were used to record the masonry in great detail and to analyse its structure.

Following GE Geotechnics' investigations, the project team was able to specify conservative repairs rather than wholesale replacement of stone.

Ightham Mote is built around a courtyard and surrounded by a moat, or mote, hence its name. Conservation of this ancient and fragile house is one of the largest projects undertaken by the National Trust, a charitable organisation that helps to preserve some of the UK's oldest and most interesting buildings.

The repairs and renovation are being carried out in phases with traditional techniques and materials being used wherever possible. The house has remained open to visitors during each stage and an exhibition about the project is an added attraction.