units, which is typical of most coastal areas. Using this value for the index, 

 one can make ranging measurements in most coastal areas with good line- of - 

 sight operation to an accuracy of ± 40 ppm . It is claimed that by making atmos- 

 pheric measurements at all three stations and properly averaging the computed 

 index of refraction, one can reduce the propagation velocity error to approxi- 

 mately ±10 ppm. Thus the tjrpical system accuracy achieved is ± 50 cm + 10 

 ppm times the measured range . 



Errors can be caused by ground reflections and are governed by the 

 modulation frequency, the antenna beam width, and the ground characteristics. 

 Broad beam antennas increase the severity of ground reflections. The errors 

 which may be introduced at the lower modulation frequencies will tend to be 

 greater under certain conditions of high reflection. By using the more directive 

 antennas at the stationary sites and by using data averaging techniques, one can 

 reduce the errors due to reflection. For moving targets, computer data proc- 

 essing can provide accuracies approaching ± 10 cm + 10 ppm of range. 



Data averaging should be particularly useful where a mobile station 

 is hovering or fixed over one position for a period of time. With the Autotape 

 equipment locked in the Fine channel, a hover of 10 seconds will provide 30 

 two-range Fine readings for averaging. This is said to be sufficient to approach 

 a ±10 cm system accuracy prior to addition of the propagation error. 



b. Random Errors 



Because of the digital measurement techniques used, 'an increase in 

 measurement accuracy and long-term stability can be expected in comparison 

 with analog continuous display techniques. Some random errors such as oper- 

 ator error and instrument drift, may be minimized. 



Ground reflection effects can cause readings to deviate by an amount 

 which is a function of the excess length of the indirect ray, the strength of the 

 ground reflection, and, most important, the relative phase of the microwave 

 carrier over the two paths. The effect is a swing or variation of readings about 

 the true reading, the amplitude of which is dependent upon the reflecting prop- 

 erties of the surface. Antenna heights of 200 ft or less should help to reduce 

 the swing error by reducing the grazing angle of propagated energy with the 

 surface of the terrain or water. 



147 



Arthur ai.l.ittlcjnr. 



