Military needs have always been the prime driver of frequency control technology. Modern military systems require oscillators/clocks that are stable over a wide range of parameters such as temperature, vibration, acceleration, radiation, etc. Rubidium-Atomic-Standard Technology meets the needs of numerous military applications. It provides highly-stable and low-noise signals, as well as low power-consumption, compact size and weight, very fast warm-up stabilization and cost effectiveness. Rubidium-Atomic-Standard Technology are in used in several military applications such as communication, secure communication, electronic warfare, command and control, telemetry, navigation, surveillance and more.. The impacts of atomic clocks on military applications are highlighted in the following list: Higher jamming resistance & improved ability to hide signals Improved ability to deny use of systems to unauthorized users Longer autonomy period (radio silence interval) Fast signal acquisition (net entry) Lower power for reduced battery consumption Improved spectrum utilization Improved surveillance capability (e.g., slow-moving target detection, bistatic radar) Improved missile guidance (e.g., on-board radar vs. ground radar) Improved identification-friend-or-foe (IFF) capability Improved electronic warfare capability (e.g., emitter location via TOA) Lower error rates in digital communications Improved navigation capability Improved survivability and performance in radiation environment Improved survivability and performance in high shock applications Longer life and smaller size, weight, and cost Longer recalibration interval (lower logistics costs Under the following heading you will find more information about the major applications and will understand the source of the needs for rubidium atomic standards. Communication Systems, Radio & Wire Efficient Spectrum Utilization As has been observed in the past, as the number of users of commercial two-way radios has grown, channel spacinghas been narrowed, and higher-frequency spectra have had to be allocated to accommodate the requirements. Narrower channel spacing and higher operating frequencies necessitate tighter frequency tolerance for both the transmitters and the receivers. The need to accommodate more users will continue to require higher and higher frequency accuracies. Radio silence interval In telecommunications radio silence is a status in which all fixed or mobile radio stations in an area stop transmitting. The radio stations include anything capable of transmitting a radio signal. Radio silence generally applies to the military, where any radio transmission may reveal troop positions, either audibly from the sound of talking, or by its use as a homing signal. Radio silence can also be maintained for other purposes, such as for highly sensitive radio astronomy Keeping the radio station off for long time requires a long autonomy period of the local oscillator. The more stable theoscillator is the longer the autonomy period. Secure Communication Secure communication describes means by which people can share information with varying degrees of certainty and which that third parties cannot know what was said. Other than communication spoken face to face out of possibility of listening, for example, it is probably safe to say that no communication is guaranteed secure in this sense, although practical limitations such as legislation, resources and the sheer volume of communication are limiting factors for surveillance. Nothing is ever truly secure. In a spread spectrum system, the transmitted signal is spread over a bandwidth that is much wider than the bandwidth required transmitting the information being sent (e.g., a voice channel of a few kHz bandwidths is spread over many MHz). This is accomplished by modulating a carrier signal with the information being sent, using a wideband pseudo noise (PN) encoding signal. A spread spectrum receiver with the appropriate PN code can demodulate and extract the information being sent. Those without the PN code may completely miss the signal, or if they detect the signal, it appears to them as noise. Two of the spread spectrum modulation types are: Direct sequence, in which the carrier is modulated by a digital code sequence. Frequency hopping, in which the carrier frequency jumps from frequency to frequency, within some predetermined set, the order of frequencies being determined by a code sequence. (see special heading on this subject) Advantages of spread spectrum systems include the following capabilities: Rejection of intentional and unintentional jamming Low probability of intercept (LPI) Selective addressing Multiple access High accuracy navigation and ranging Frequency Hopping It is possible to jam frequency hopping systems with the availability of fast spectrum analyzers and synthesizers. If a jammer is fast enough, it can detect the frequency of transmission and tune the jammer to that frequency well before the radio hops to the next frequency. However, with a good enough clock, it is possible to defeat such “follower” jamming. As illustrated below, even a "perfect" follower jammer can be defeated if a good enough clock is available. (A perfect jammer is defined here as one that can identify the frequency of a received signal, tune a synthesizer to that frequency, and transmit the jamming signal in zero time.) Because radio waves travel at the speed of light, the radio-to-jammer-to-radio (R1 to J to R2) and radio-to-radio (R1 to R2) propagation delays are 3.3 µs per km. Therefore, if the hopping rate is fast enough for the propagation delay difference to be greater than 1/hop-rate,(i.e., if the radios can hop to the next frequency before the jamming signal reaches the receiver), then the radios are jamming-proof (for follower jammers). In the example above, the propagation delays t1, t2, and tR imply that the message duration tm be less than 30 µs. Since the clock accuracies required by frequency hopping systems are usually 10% to 20% of tm, the allowed clock error is about 6 µs. In a military environment, such accuracies can be maintained for periods of hours and longer only with atomic clocks. To summarize: Transmitter s and receivers contain clocks which must be synchronized; e.g., in a frequency hopping system, the transmitter and receiver must hop to the same frequency at the same time. The faster the hopping rate, the higher the jamming resistance, and the more accurate the clocks must be (see special heading on this subject) Electronic Warfare Electronic warfare (EW) indicates the use of the electromagnetic spectrum in order deny its effective use by an adversary while optimizing its use by friendly forces. Electronic warfare has three main components: electronic support, electronic attack, and electronic protection. Electronic support (ES) is the passive use of the electromagnetic spectrum in order to gain intelligence regarding other parties on the battlefield in order to find, identify, locate and intercept potential threats or targets. Electronic attack (EA) is the active or passive use of the electromagnetic spectrum to deny its use by an adversary. Active EA includes such activities as jamming, deception, active cancellation, and EMP use. Passive EA includes such activities as the use of chaff, towed decoys, balloons, radar reflectors, winged decoys, and stealth. Electronic protection (EP) includes all activities related to making enemy EA activities less successful by means of protecting friendly personnel, facilities, equipment or objectives. EP can also be implemented to prevent friendly forces from being affected by their own EA. Active EP includes such activities as technical modifications to radio equipment (such as frequency-hopping spread spectrum). Passive EP includes such activities as the education of operators (enforcing strict discipline) and modified battlefield tactics or operations. ELINT Receivers One of the Electronic supports (ES) application is ELINT. It stands for ELectronic Signals INTelligence, and refers to intelligence-gathering by use of electronic sensors. ELINT focuses primarily on non-communications signals intelligence. Signal identification is performed by analyzing the collected parameters of a specific signal, and either matching it to known criteria, or recording it as a possible new emitter. The data gathered is typically pertinent to a rival's defence network, especially the electronic parts such as radars, surface-to-air missile systems, aircraft, etc. Gathering can be performed from ground stations near the opponent's territory, ships off coast, aircraft near or in their airspace, or by satellite. The ELINT receivers are used to search a broad range of frequencies for signals that may be emitted by a potential adversary. The frequency source in an ELINT receiver must be as noise-free as possible so as not to obscure weak incoming signals. The frequency source must also be extremely stable and accurate in order to allow accurate measurement of the incoming signal's characteristics. Emitters location The ability to locate radio and radar emitters is important in modern warfare. One method of locating emitters is to measure the time difference of arrival of the same signal at widely separated locations. Emitter location by means of this method depends on the availability of highly accurate clocks, and on highly accurate methods of synchronizing clocks that are widely separated. Triangulation is the process of finding coordinates and distance to a point by calculating the length of one side of a triangle, given measurements of angles and sides of the triangle formed by that point and two other known reference points, using the low of sinus. Since electromagnetic waves travel at the speed of light, 30 cm per nanosecond, the clocks of emitter locating systems must be kept synchronized to within nanoseconds in order to locate emitters with high accuracy. (Multi path and the geometrical arrangement of emitter locators usually results in a dilution of precision.) Without resynchronization, even the best available militarized atomic clocks can maintain such accuracies for periods of only a few hours. With the availability of GPS and using the "GPS common view" method of time transfer, widely separated clocks can be synchronized to better than 10 ns (assuming that GPS is not jammed). An even more accurate method of synchronization is "two-way time transfer via communication satellites," which, by means of very small aperture terminals (VSAT’s) and pseudo noise modems, can attain sub-nanosecond time transfer accuracies. Command, Control, Communication Computer and Intelligence (C4I) In the military the exercise of authority and direction by a properly designated commander over assigned and attached forces in the accomplishment of the mission is crucial. Command and control functions are performed through an arrangement of personnel, equipment, communications, facilities, and procedures employed by a commander in planning, directing, coordinating, and controlling forces and operations in the accomplishment of the mission. Military forces require extensive long-range communications systems that can maintain contact with all of those forces at all times. To enable national command authorities to exercise effective command and control over their widely dispersed forces, a communications system was established to enable those authorities to disseminate their decisions to all subordinate units, under any conditions, within minutes. Such a command and control system, WWMCCS, was created by US DOD titled "Concept of Operations of the Worldwide Military Command and Control System," 27 command centers were equipped with more than 6000 computers build extensive network at geographically separate locations, interconnected by a dedicated wide-band, packet-switched communications subsystem and wideband, encrypted, dedicated, data communications circuits. Command, Control, Communication Computer and Intelligence. requires precise timing and frequency that can be provided by a rubidium standard. Telemetry, Test Fields Telemetry is a technology that allows remote measurement and reporting of information of interest to the system designer or operator. Systems that need instructions and data sent to them in order to operate require the counterpart of telemetry, tele-command. Telemetry is vital in the development phase of missiles, satellites and aircraft because the system might be destroyed after/during the test. Engineers need critical system parameters in order to analyze (and improve) the performance of the system. Without telemetry, this data would often be unavailable. Accurate and stable time tagging of events for to different causes and results is therefore essential in telemetry systems. Navigation Precise time is essential for precise navigation. In the past, navigation has been a principal motivator in man's search for better clocks Even in ancient times, one could measure latitude by observing the stars' positions. However, to determine longitude, the problem became one of timing Since the earth makes one revolution in 24 hours, one can determine longitude from the time difference between local time (which was determined from the sun's position) and the time at the Greenwich meridian (which was determined by a clock): Longitude in degrees = (360 degrees/24 hours) x t in hours. Today's electronic navigation systems still require ever greater accuracy. As electromagnetic waves travel 300 meters per microsecond, e.g., if a vessel's timing was in error by one millisecond, a navigational error of 300 kilometers would result. In the Global Positioning System (GPS), atomic clocks in the satellites and quartz oscillators in the receivers provide nanosecond-level accuracies. The resulting (worldwide) navigational accuracies are about ten meters Global positing systems are composed from two main systems: the satellite and the satellite base stations. These include atomic clocks on board as THE key element. Stable and accurate frequency is also required in the satellite's base stations. Surveillance 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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