Abstract

Methods for secure communication of location and other information by two spaced apart receivers in a location determination (LD) system, such as GPS, GLONASS and LORAN-C, that use pseudorange corrections to enhance the accuracy of the computed present location of an LD receiver. A pseudorange correction signal PRC(t;i;j), presenting a correction of a pseudorange measurement made at time t at an LD receiver number i from an LD signal issued by an LD signal source number j, is transmitted at a consecutive sequence of times t=t0, t1, t2, . . . , tn. The pseudorange correction signal PRC(tn ;i;j), or a message sent in a time interval tn-1 <tࣘtn, is encrypted, using an encryption key that is a function of and depends non-trivially upon one or more of the preceding pseudorange correction values PRC(tk ;i;j) (kࣘn-1). This encryption key varies from one time interval tn-1 <tࣘtn, to another as the collection of preceding pseudorange correction values changes. Several suitable encryption algorithms that depend upon one or more of the preceding pseudorange correction values are discussed.