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Reconnaissance Flights

The most accurate center fixes are obtained by reconnaissance aircraft. There are few erroneous reports, and over the years, the following standards have been set: dead reckoning fixes are accurate to within 30 miles, and Loran fixes to within 10 to 15 miles, depending on the number of Loran stations that a fix can be based on. Aircraft radar fixes normally fall within the same limits of accuracy.

Dead Reckoning fixes are determined by measuring the direction and distance traveled by

Figure 9-3-9.—Center of tropical storm located by observation of two ships. Short dashed arrows show direction of waves.

Figure 9-3-10.—Examples of errors in locating direction of storm center. (A) When the wind field is elliptical and the wind rule is used; (B) when the wind rule is applied to fast-moving storms.

the reconnaissance aircraft from its point of departure (a known point of latitude and longitude). The navigator uses aircraft speeds and directions to measure the distance traveled to a tropical cyclone and to compute the fix. However, because the direction and speed values are rarely exact, the fixes are rarely exact.

Loran is an electronic system that uses land-based radio transmitters and an air-craft receiver. The transmitter sites include a master station and several secondary stations that are separated by several hundred miles. A receiver onboard the reconnaissance aircraft measures the time difference between the receipt of synchronized pulses transmitted by the master station and one of the secondary stations. This time difference translates to a line of position (LOP) on a Loran chart. The aircraft is then somewhere along this LOP. By repeating this procedure using a different secondary station, a second LOP is obtained. The air-craft’s position is at the intersection of the two LOPs.

Another extremely important aspect of re-connaissance flights is the on-scene wind observations.

WINDS.— Surface wind reports from recon-naissance crews able to observe the sea are quite reliable. The error in the reported direc-tion is usually under 10 degrees. As for wind speed, the error does not exceed 5 knots when the wind is below 60 knots, and it is less than 10 to 20 knots at speeds of 60 to 100 knots. Most authorities disagree as to how accurate the speeds are above 100 knots, but it should be assumed that the error will be much greater. The accuracy of the surface wind must be balanced against the height of the aircraft. The accuracy is less reliable if the aircraft is above 10,000 feet; and if there are many clouds present, the reported wind may not be the highest, but the highest observed.

Flight-level winds are about as accurate as the surface wind in areas where Loran facilities are available. Flight-level winds obtained with Doppler equipment are very accurate and compare closely with rawin reported winds. Other flight-level winds are averaged over a given distance and should be considered judiciously, especially where Loran facilities are poor.

OTHER ELEMENTS.— Surface pressure reports are usually quite accurate, and heights of standard upper levels are good but may be in error at times. A reported height can be compared to the height reported in the latest rawinsonde observation of a nearby station; if there is a difference, the difference is usually applicable to all the aircraft’s reported heights.

Other elements such as weather, state of the sea, turbulence, clouds, temperature, and the like are all important and should be judged in the same manner as other similar data.

Learning Objective: Recognize tropical cyclone advisories and warnings and the conditions of readiness associated with them.

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