Frequency of carrier received by aircraft is measured and compared with reference to indicate magnitude of Doppler shift. One Doppler frequency range is selected and indicated by digital signal. Difference between frequency is offset of apparent carrier frequency transmitted by aircraft

Allen, W. K.

Hollenbaugh, R. C.

Laughlin, C. R.

English

NASA Technical Reports Server

April 1974	 B74-10024 
NASA TECH BRIEF 
Goddard Space Flight Center 	
A 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Traffic Control System and Method 
. 
S
The problem: 
Supersonic transports (SST's), in addition to their 
objectionable sonic boom, impose a communication 
problem. Because of their speed of approximately 
2000 knots, they require constant monitoring by the 
ground control stations to prevent possible collisions 
with other aircraft. Supersonic speeds, however, intro-
duce large Doppler shifts in the communication fre-
quencies. At 2000 knots, a two-way Doppler shift is 
about ±10 kHz on a I .5-GHz carrier. This shift requires 
some means of compensation. One method of over-
coming this problem is to allocate a sufficiently broad 
bandwidth for each SST to include the maximum. 
Doppler shift. However, the bandwidth associated with 
each SST would then be too great for efficient com-
munication. Another approach involves the exact com-
pensation for the Doppler shift, whereby each SST 
would be equipped with an ultrastable oscillator to 
maintain a constant transmitted frequency regardless 
of the shift. This approach is impractical because 
oscillators with the required stability cannot be flown 
on each aircraft. 
The solution: 
An air traffic control system and method were 
developed which use a compromise between exact 
Doppler frequency compensation and a system requiring 
a large bandwidth. 
How it's done: 
The frequency of the carrier received by an aircraft 
is measured and compared with a reference to indicate 
the magnitude of the Doppler shift. In response to the 
difference between the received and reference fre-
quencies, one of the Doppler frequency ranges is 
selected and indicated by a digital signal. This signal 
modulates the carrier transmitted from the aircraft. 
The difference between the boundary value of the 
selected range and the difference frequency is an offset
of the apparent carrier frequency, or frequency without 
Doppler shift, transmitted by aircraft. The ground 
station responds to the digital signal from the aircraft 
and feeds it to a computer for aircraft position 
indication, simultaneously monitoring the actual carrier. 
Phase coherence between the aircraft and ground station 
is thus preserved without the requirement for a wide 
bandwidth. 
To prevent the carrier frequency from oscillating 
between two boundary values as the Doppler frequency 
shifts slightly about the boundary, hysteresis is provided 
in the aircraft Doppler compensation circuit, indicating 
the frequency range. Hysteresis delays the shifting from 
first frequency range to the second adjacent, overlapping 
range, as long as the shifting persists. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Code 207.1 
Greenbelt, Maryland 20771 
Reference: B74-10024 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,715,663). Inquiries concerning nonexclu-
sive or exclusive license for its commercial development 
should be addressed to: 
Patent Counsel 
Goddard Space Flight Center 
Code 204 
Greenbelt, Maryland 20771 
Source: Charles R. Laughlin, 
Roger C. Hollenbaugh, and
Walter K. Allen

Goddard Space Flight Center 
(GSC.10087) 
Category 02 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19740000024 2020-03-17T14:02:00+00:00Z


Traffic control system and method

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server