Surveillance is the monitoring of behavior. Systems surveillance is the process of monitoring the behavior of people, objects or processes within systems for conformity to expected or desired norms in trusted systems for security or social control. Military surveillance refers to the monitoring of objects such as aircraft, ships, motor vehicles people etc.. Radar is a system that uses electromagnetic waves to identify the location, direction, and/or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. A transmitter emits radio waves, which are reflected by the target and detected by a receiver, typically in the same location as the transmitter. Although the radio signal returned is usually very weak, radio signals can easily be amplified. This enables radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar is used in many contexts, including meteorological detection of precipitation, air traffic control, police detection of speeding traffic, and by the military. Doppler Radar In surveillance, Doppler radars especially require low-noise oscillators. Usually Crystal Oscillators are used, but in some cases where high stability is required one uses Rubidium. The velocity of the target and the radar frequency are primary determinants of the phase noise requirements. Slow-moving targets produce small Doppler shifts, therefore, low phase-noise close to the carrier is required. To detect fast-moving targets, low noise far from the carrier is required. For example, when using an X-band radar to detect a 4 km/hour target (e.g., a slowly moving vehicle), the noise 70 Hz from the carrier is the important parameter, whereas to detect supersonic aircraft, the noise beyond 10 kHz is important. Effect of Noise in Doppler Radar System Echo = Doppler-shifted echo from moving target + large "clutter" signal (Echo signal) - (reference signal) --› Doppler shifted signal from target Phase noise of the local oscillator modulates the clutter signal, generates higher frequency clutter components, and thereby degrades the radar's ability to separate the target signal from the clutter signal. When a radar is on a stationary platform, the phase noise requirements can usually be met with commercially available oscillators. A good quartz crystal (bulk acoustic wave, BAW) oscillator can provide sufficiently low noise close to the carrier, and a good surface acoustic wave (SAW) oscillator can provide sufficiently low noise far from the carrier. Very far from the carrier, dielectric resonator oscillators (DRO) can provide lower noise than either BAW or SAW oscillators. A combination of oscillators can be used to achieve good performance in multiple spectral regions, e.g., a DRO phase locked to a frequency-multiplied BAW oscillator can provide low noise both close to the carrier and far from the carrier. The problem with achieving sufficiently low phase noise occurs when the radar platform vibrates, as is the case when the platform is an aircraft or a missile. The vibration applies time-dependent stresses to the resonator in the oscillator which results in modulation of the output frequency. The aircraft's random vibration, thereby, degrades the phase noise, and discrete frequency vibrations (e.g., due to helicopter blade rotation) produce spectral lines which can result in false target indications. The degradation in noise spectrum occurs in all types of oscillators (BAW, SAW, DRO, atomic frequency standards, etc.). Large phase-noise degradation can have catastrophic effects on radar performance. In coherent radar, the platform-vibration-induced phase noise can reduce the probability of detection to zero. Bi-Static Radar In bi-static radars the illuminator and receiver are widely separated with two reference high stability oscillators such as Rubidium. Mono-static radar is vulnerable to a variety of countermeasures. Bi-static radar can greatly reduce the vulnerability to countermeasures such as jamming and anti radiation weapons, Bi-static radar can also increase slow moving target detection and identification capability via "clutter tuning”. The transmitter can remain far from the battle area, in a "sanctuary." The receiver can remain "quiet.” The timing and phase coherence problems can be orders of magnitude more severe in bi-static than in mono-static radar, especially when the platforms are moving. The reference oscillators must remain synchronized and syntonized during a mission so that the receiver knows when the transmitter emits each pulse and the phase variations will be small enough to allow a satisfactory image to be formed. Low noise crystal oscillators are required for short term stability; atomic frequency standards are often required for long term stability. Identification Friend-Or-Foe Systems (IFF) In a modern battle, when the sky is filled with friendly and enemy aircraft and a variety of advanced weapons are ready to fire from both ground and airborne platforms, positive identification of friend and foe is critically important. For example fratricide due to identification errors has been a major problem in all 20th century wars. Current IFF systems use an interrogation/response method which employs cryptographically encoded spread spectrum signals. The interrogation signal received by a friend is supposed to result in the "correct" code being automatically sent back via a transponder on the friendly platform. The "correct" code must change frequently to prevent a foe from recording and transmitting that code ("repeat jamming"), thereby appearing as a friend. The code is changed at the end of what is called the code validity interval (CVI). The better the clock accuracy, the shorter can be the CVI, the more resistant the system can be to repeat jamming, and the longer can be the autonomy period for users who cannot resynchronize their clocks during a mission.

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