NAVIGATION — CANOES TO SPACESHIPS — DRAPER 



307 



USE OF RADAR PROVIDES 

 DIRECT LINE OF SIGHT TO 

 KNOWN LANDMARK AND Rc- 

 SULTS IN BOTH RANGE AND 

 BEARING INFORMATION 



Figure 5. — Navi^ration bv use of raJar. 



An alternative method of using wavelengtlis of the radar spectrum 

 for the purposes of navigation is to transmit continuous waves instead 

 of pulses. This technique is particularly useful when the transmit- 

 ter and reflecting surfaces are moving so rapidly v/ith respect to 

 each other that a measurable shift in frequency between reflected and 

 transmitted waves exists. This frequency sliift is a manifestation of 

 the well-understood Doppler effect and is the basis for a number of 

 radiation-contact guidance equipments that are classed as Doppler 

 systems. 



It is fair to state that toward the middle of the '20th century, radio 

 and radar methods had substantially eliminated the difficulties that 

 attend the use of visual contacts for the purposes of navigation by 

 terrestrial landmarks. A number of other benefits had also ap- 

 peared when visible light was replaced by longer wavelengths for 

 navigational purposes : direct range and velocity measurements were 

 available, working distances were greatly extended, direct-line-of- 

 sight restrictions were removed, and complete automation of equip- 

 ment became feasible. By about 1950, developments of equipment 

 and methods had substantially exhausted the possibilities of improv- 

 ing navigation by terrestrial and celestial references. Improve- 

 ments in details will always occur, but it is unlikely that these fields 

 will see revolutionary changes in the future. 



MODERN PROBLEMS IN NAVIGATION 



All the advances in methods, theory, tables, instruments, and tech- 

 niques that appeared during the course of several thousand years to 

 perfect the art of navigation combined to solve the same problem that 

 confronted the first caveman navigator. This problem was and still 

 is that of finding position of the earth from information provided by 



