A ground station of a radionavigation system, responding to interrogation pulses from aircraft passing within range, emits message pulses whose timing enables the on-board equipment of the interrogating aircraft to determine its distance from the ground station and/or its azimuthal position relative thereto. To prevent such information from being utilized by aircraft not entitled thereto, the emission of the message pulses is subjected to an artificial delay whose randomly varying magnitude is conveyed by a multibit code word subjected in a distorter to a predetermined series of transformations based upon selected bits thereof. The on-board equipment of privileged aircraft includes a decipherer complementary to the distorter which restores the original code word and derives from it an indication of the artificial delay to be deducted from the measured time interval.

The instant invention permits a communication function to be performed by utilizing the transmitter/receiver portion of existing navigational equipment such as, for example, DME equipment, TACAN/DME equipment, and VOR/DME equipment. Rather than utilizing the same channel as the navigation function, the communication function utilizes another of the equipment's channels specifically assigned for communication purposes. Specifically, when it is desired to communicate with a communication or master station, the airborne or remote navigational equipment is momentarily turned from the navigational frequency channel to the frequency channel specifically assigned for the communication function. While momentarily tuned to the communication channel, it sends data to, and receives data from, the communication station. Having completed the communication process, the airborne or remote unit tunes back to the navigational channel and resumes its navigational function. Tuning from the navigational channel and the time spent on the communication channel is sufficiently short so that the navigational function is not impaired.

A method and apparatus for encoding and decoding data on navigation signal pulse pairs utilizes pulse position modulation (PPM). The pulse interval between a first pulse and a second pulse of the pulse pair is varied depending on whether the logic value of the data to be encoded is a logic "0" or a logic "1". A tri-graph encoding process converts the raw input data into encoded input data. Each input data bit is encoded into three bits with each bit having the same logical value as the input data bit. The encoded input data is then encoded on the navigation signal using a pulse position modulation (PPM) technique. A decoder recovers the encoded data and provides an error correction process for high data integrity that corrects certain bit errors that may occur during the transmission and reception of the data on the navigation signal. The error correction process detects and corrects errors such as a missing bit, an extra bit or a bit error.

A method and apparatus for encoding and decoding data on navigation signal pulse pairs utilizes pulse position modulation (PPM). The pulse interval between a first pulse and a second pulse of the pulse pair is varied depending on whether the logic value of the data to be encoded is a logic "0" or a logic "1". A tri-graph encoding process converts the raw input data into encoded input data. Each input data bit is encoded into three bits with each bit having the same logical value as the input data bit. The encoded input data is then encoded on the navigation signal using a pulse position modulation (PPM) technique. A decoder recovers the encoded data and provides an error correction process for high data integrity that corrects certain bit errors that may occur during the transmission and reception of the data on the navigation signal. The error correction process detects and corrects errors such as a missing bit, an extra bit or a bit error.