12 



TRANSMISSION EXPERIMENTS OVER MASSACHUSETTS BAY 



markedly with direction, as happens occasionally when Figure 7 shows such a diagram relating the signal 

 the air trajectory is S or SW or at times of skip dis- strength of the target at Eastern Point as observed 

 tance. on the X-band system with the signal strength of tlie 



Maximum Eaxges 



Large variations in the maximnm ranges have also 

 been oliserved at both frequencies, with the upper 

 limit apparently being set only by the lengtli of the 

 sweep: 280 miles on S band and 200 miles on X band. 

 (Xote that these radar sets were far from the high- 

 power class.) Lack of fixed targets at ranges between 

 10 and 2.5 miles made it impossible to follow in detail 

 the way in which substandard conditions reduced de- 

 tection range, but there was no question as to the 

 general trend toward reduction of range. The maxi- 

 mum range of the liigli sited K-band receiver [HTtK] 

 from its location at the Eace Point Light was 46 miles 

 on a land target and about 30 miles on shipping. 



It should be borne in mind that our project deals 

 with propagation near and roughly parallel to the 

 coast line. Thus these results are not necessarily ap- 

 plicable to operations perpendicular to the coast with 

 off-shore winds, where the surface M inversions be- 

 come "washed out." 



Statistics 



The radar observations include about 1,200 hours 

 of operation. Of these, overall, the X-band ranges were 

 'better than "normal" 59 per cent of the time (normal 

 = 29 miles") and the S-band ranges 48 per cent of 

 the time. At both frequencies ranges were below nor- 

 mal 20 per cent of the time. The variations from week 

 to week were great, the maximum values being 95 per 

 cent above normal on X and 75 per cent above normal 

 on S, with about 45 per cent below normal as the 

 lowest value at both frequencies. 



CORRELATIOXS WITH OxE-WaT EeSULTS 



A visual comparison of the radar and one-way data 

 suggests fairly good agreement in general between 

 the two. To get a more quantitative evaluation of this 

 agreement, however, correlation diagrams have been 

 drawn. 



"Unfortunately, in the radar ease it is impossible to estab- 

 lish a precise definition of a "standard" range analogous to 

 standard signal in one-way transmission unless detailed in- 

 formation is available on the radar target. In this case we 

 have attempted to determine the detection range on the low 

 hills available as coast line targets, at times when the M 

 curve is standard or very nearly so. 



100 



60 40 



DB BELOW 1 WATT 

 SIGNAL STRENGTH 



Figure 7. Relation between one-way and radar trans- 

 mission, Race Point to Eastern Point. 



high X-band receiver on the one-way path. As in the 

 previous diagrams, a week has been chosen as the time 

 interval and hourly values are plotted. In this we 

 neglect the difference between the single observation 

 of the radar and the average of an hour's continuous 

 record in the other ca.se. Note also that all radar 

 measurements which give values equal to or below 

 the minimum detectable level are plotted at the min- 

 imum detectable level ; thus if a more sensitive re- 

 ceiver had been used, many of these points would have 

 fallen lower in the diagram. The diagram reveals the 

 nature of the relation: the one-way signal strength 

 must rise considerably above the standard value before 

 the target becomes visible. Thereafter, small changes 

 in the one-way signal correspond to much larger 

 changes in the radar echo. As a matter of interest, 

 which may or may not be significant, the values at 

 times fall close to the square law, as they should if the 

 target-reflecting properties remain constant as the 

 atmospheric conditions change. 



Figure 8 shows the relation between maximum radar 

 ranges on surface targets and the one-way transmis- 

 sion results. In this case the effects of both substandard 

 and better than standard conditions are noticeable. 

 When the one-way signal strength is below standard, 

 the radar ranges are mainly less than normal ; excep- 

 tions occur in cases of strong directional effects and 

 S-shaped J\[ curves. As the one-way signal strength 



