selector switch is moved to the BT position. The recorder chart drive circuits automatically position the chart paper to provide correct chart registration. Recorded scale marks on the chart paper denote the temperature scale being used for each temperature recording. The recorder plots temperature on the vertical axis and depth on the horizontal axis of the moving chart. Recorder Range Mode The recorder RANGE mode is used to obtain continuous strip-chart displays of target echo ranges. Range scale control signals from the receiver RANGE SCALE-KYDS switch are accepted by the recorder sweep circuits to correlate the range sweeps. As the chart paper moves, range scale marks are recorded on the chart paper to denote the range scale being used for each range recording. Target echo video signals are applied to the styluses when they appear in time, as related to the range sweep. The video signals are recorded each time a stylus passes over the range position of a target. The chart advances a small increment for each stylus sweep. Recorder Aspect Mode The recorder ASPECT mode is used to obtain continuous strip-chart displays of target echo signals. Timing and control signals, generated within the recorder, slave the receiver timing circuits to alternate sweep ramps between transmit and receive cycles. During each transmit sweep ramp, a train of short keying pulses is generated, and pulsewidth is regulated in the recorder. This pulse train is applied to the receiver. During each receive sweep ramp, the train of received target echo video pulses is applied to the recorder styluses. Target echo signal level is neither limited nor affected in the system. This permits varying intensity recordings (highlights) of target structural characteristics for optimum target classification. Recorder Test Mode The sonar operator uses the recorder TEST mode to check the operational status of the recorder. The TEST mode effectively checks the operation of the recorder stylus drive, stylus write, and chart drive operations. In addition, all front panel controls on the recorder can be checked by the operator for operational compliance and accuracy. MAGNETIC ANOMALY DETECTION Learning Objective: Recognize components and operating principles of magnetic anomaly detection (MAD). By the beginning of World War II, it had become apparent that the aircraft was a deadly antisubmarine weapon. This was true even though the ability to search and detect submarines was solely dependent on visual sightings. The development of radar extended the usefulness of airborne antisubmarine measures, making detection of submarines possible at night or under conditions of poor visibility. However, visual or radar detection was possible only when the submarine was surfaced. Thus, some method of detecting submerged subs from an aircraft was needed. The use of sonar wasn’t feasible because there was no direct contact between the fast-moving aircraft and the surface of the water. The most feasible way of detecting a submerged submarine was to detect its disturbance of the local magnetic field of the earth. PRINCIPLES OF MAGNETIC DETECTION Light, radar, and sound energy cannot pass from air into water and return to the air in any degree that is usable for airborne detection. On the other hand, lines of force in a magnetic field are able to make this transition almost undisturbed because the magnetic permeability of water and air are practically the same. Specifically, the lines of force in the earth’s magnetic field pass through the surface of the ocean essentially undeviated by the change of medium, and undiminished in strength. Consequently, an object under the water can be detected from a position in the air above if the object has magnetic properties that distort the earth’s magnetic field. A submarine has sufficient ferrous mass and electrical equipment to cause a detectable distortion (anomaly) in the earth’s field. The function of the MAD equipment is to detect this anomaly. Magnetic Anomaly The lines comprising the earth’s natural magnetic field do not always run straight north and south. If traced along atypical 100-mile path, the field twists at places to east and west, and assumes different angles with the horizontal. Angles of change in the east-west direction are known as angles of variation, while angles between the lines of force and the horizontal 4-15