nge of cryptographic tication and access control applications. Traditional keys. Increased accuracy and lowered jitter may be handsets may be modified to capture the necessary inputs accomplished through more sophisticated circuitry. for redundant biometric authentication. For example, The local oscillator in a radio system may be used to cell phones recently have been equipped with digital keep track of time of day. The stability of these clocks is cameras. This sensor, coupled with facial recognition measured at approximately 10-6. These clocks tend to software, may be used to authenticate a user. An iris scan drift over time, and in the course of a single day may or retina scan is also possible. Figure 2.4b-3 shows some accumulate up to 90 ns of error. Atomic clocks have of the potential sensors and their relative strengths and much greater stability (10-11), but have traditionally weakness in terms of reliability and acceptability [33]. been large and power hungry. Chip-scale atomic clocks have been demonstrated and are expected to make pre- 2.4b.5.4 Time cision timing practical. This will enable geolocation ap- plications with lower CEPs. Included in many contracts is the phrase "time is of the essence," testament to the criticality of prompt perfor- 2.4b.5.5 Spatial awareness or situational mance in most aspects of human interaction. Even awareness a desktop computer has some idea about what time it is, what day it is, and even knows how to utilize this in- A very significant role for a CR may be viewed as a per- formation in a useful manner (date and time stamping sonal assistant. One of its key missions is facilitating information). A radio that is ignorant of time has a seri- communication over wireless links. The opposite mission ous handicap in terms of learning how to interact and is impeding communications when appropriate. As an behave. Therefore, it is important for the CR to know example, most people do not want to be disturbed while about time, dates, schedules, and deadlines. in church, in an im