Motorola STU-III Secure Telephones (SECTEL) are secure voice/data telephone terminals. The STU-III/A is a version of the STU-III family designed for use within the NATO community. It contains all the basic STU-III functions and capabilities, while adding the STU-III modes of operation. The STU-III SECTEL models are black in color and incorporate the capability to store 17 net keying variables compared to the storage of one net keying variable with the Limit Rate Initial Production model. The STU-III models have two and four wire adapters built into the terminal.

The STU-III SECTELs operate at 2400, 4800, and 9600 bps full duplex secure voice. Secure digital data transmission is possible at 75, 110, 300, 600, and 1200 bps in the asynchronous mode. In the synchronous mode, digital data can be securely transmitted at 2400, 4800 and 9600 bps.

The STU-III is capable of being keyed in three different ways. For STU-III interoperability, the first uses the FIREFLY II key concept where all keys are in electronic form and initial keying can be done locally or by access to the Key Management Central Facility. No per call access is required. For STU-III interoperability, the second and third way uses the Bellfield Key Distribution Center (KDC) concept with per call access or commonly held NET key.

Motorola SECTEL 1500. Both the Motorola and AT&T phones used digitized and scrambled voice at 4800 bps using Code Excited Linear Prediction called CELP. Keying had to be done every three months. (Photo courtesy Motorola)