For a peg sensory portable this was a well made machine aimed more at stronger hobby players, than the toys market. It was designed by Ian Sinclair (also the designer of the Mark V). The program was written by Dane, Kaare Danielsen who achieved remarkable results in just 4K. It is still thought to be one of the strongest 4K programs in the world. Danielsen had already achieved a number of good results at tournaments with his program Logichess, including 3rd place at the European Microcomputer Chess Championship in London 1981. In the Explorer the program achieves an Elo rating of about 1220 (Wiki) which is a good test for anyone up to weak club standard. It also has the same processor (0609V171 bespoke SciSys singlechip) as the SciSys Chess Companion II, Olympiade, Concord, Mark 8 and Tandy/Radio Shack 1650s. The Explorer sold in good numbers for around £70 (219 DM) back in 1983 / 84. The Travel Sensor model is similar, but with a weaker program.