This instrument is one of a number of examples of attempts to produce a cheap and easy to use instrument for the determination of latitude, longitude and time at sea, principally for the leisure market.

The instrument is made of plastic and comprises a base plate, a protractor and a pointer with a crosspiece. It can be packed flat and held together with a brass clamping screw, but in use the crosspiece is slotted into the pointer. The base plate has a date scale for aligning the protractor, and a table for applying a time correction to the observed time from the sun according to the time of year. The protractor has a degree scale on its outer edge and a time/arc conversion scale. Instructions for using the instrument are printed on the back of the base plate.

To find the latitude at noon, the protractor is set against the date scale and then pointer is aligned with the sun. This indicates the latitude on the protractor. For the determination of longitude, the instrument is used as a sundial to determine the local mean time (by applying a correction from the table on the base plate to the observed local time) and comparing this to Greenwich Mean Time. The difference in time can then be turned into a difference in longitude by using the time/arc conversion scale on the protractor.