A filtering method and apparatus for a hand-held computing device such as a palmtop computer. In one method, an overall code space in which the pilot may be found is partitioned into a number of groups of code sets, with each code set representative of all possible chip offsets of a specific PN sequence. In the first position, the cam member engages a lock operator to prevent the lock operator from moving out of the unlocked position. If the total refrigeration supply exceeds the demand, or consumption, excess refrigerant is directed to cryogenic refrigerators which can utilize the excess helium to complete a current cooling function more quickly.