6442 HYDROGRAPHIC MANUAL PaGE 600 



reflected from the ionospliere, which is a layer of ionized gas from 50 to 150 miles above 

 the earth. The ionosphere is slightly conductive and its characteristics vary during 

 the 24 hours and with seasons and sim spots. The propagation of radiation from low- 

 frequency transmitters is principally by the ground wave and less by the sky wave, 

 but the ground wave of the higher frequencies does not travel far and reception of such 

 frequencies is by a sky wave through an optical path, or by way of one or more reflec- 

 tions from the ionosphere at distances of thousands of miles. Since there is almost no 

 change in the ground wave and many complicated changes in the sky wave, it is appar- 

 ent that propagation by the ground wave is much more uniform than by the sky wave. 

 For this reason, it is desirable to select a frequency whose ground wave extends as far 

 as required for R.A.R. operations and which is still high enough to require little power. 

 Frequencies between 2000 kc and 5000 kc, transmitted with a power of 5 watts, may be 

 received from 150 to 250 miles via the ground wave. However, the radio signal must 

 not only be heard but it must also be recorded on a chronograph tape. 



It is fairly easy to read radio signals through radio interference on nearly the same 

 frequency, but the chronograph is unable to make this distinction. For this reason, the 

 desired radio signal should be stronger than the others and there should be as little in- 

 terference as possible. Interference can be divided into two broad classes: static and 

 communication. Static interference can be subdivided into atmospheric, caused by 

 electric disturbances in the air such as lightning, and man-made static caused by elec- 

 tric apparatus, sparking commutators, loose coimections, etc. At frequencies above 

 2000 kc, atmospheric interference decreases, but man-made interference increases. 

 Man-made interference is controllable to a considerable degree and can be eliminated 

 on a survey vessel by the use of proper filters in connection with the offending appa- 

 ratus. It might be inferred that the lesser atmospheric interference at higher frequen- 

 cies would make the use of these frequencies desirable for R.A.R., but the advantage 

 to be gained in this respect would be more than offset by the unreliable propagation at 

 the higher frequencies, although it must be stated that higher frequencies have not yet 

 actually been tried in R.A.R. 



Communication interference can be avoided only by using frequencies for R.A.R, 

 which are reasonably free from such interference. Since the frequencies that can be 

 used for R.A.R. are assigned by the Federal Government, and quite often are shared 

 with other radio stations, it is difficult to avoid this type of interference completely, 



6442. Authorized Radio Frequencies 



The frequencies (in kilocycles) authorized by the Federal Communications Com- 

 mission for use by the Coast and Geodetic Survey for R.A.R. and communications are: 



1,738 2,492 4,135 8,270 12,405 16,540 



1,742 2,496 4,160 8,320 12,480 16,640 



1,746 2,500 



Those frequencies that are available to all vessels for communication have not been 

 tabulated. 



Of the above frequencies, 4,135 and 4,160 kc are most frequently used for R.A.R.; 

 2,492, 2,496, and 2,500 kc are also used, but less frequently. All these frequencies are 

 shared with other radio stations. The frequencies available for use in R.A.R. should 

 be monitored for a few days to select the one with the least communication interference 

 in the area at the time its use is desired. 



