Automatic Frequency Control system compensates for Doppler shift in two-way communication systems where one or both stations are moving. This automatic correction can be applied to the reply link to elimate frequency search for the reply or an excessive bandwidth to accommodate the Doppler

Muller, R. M.

English

NASA Technical Reports Server

December ,1967	 Brief 67-10643 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Concept for Automatic Doppler Compensation in Two-Way 
Communication Systems 
'In two-way communication systems where the 
relative velocity between the two ends of the com-
munication link is not zero, there will be a frequency 
shift proportional to the relative velocity and to the 
frequency used for the communication link (Doppler). 
In many cases, this frequency shift may be large com-
pared to the bandwidth of the data to be transmitted 
over the links. In conventional communication, 
either the receiver bandwidth is widened to accommo-
date this shift plus the data bandwidth, or a tech-
nique is used to tune out this frequency shift. This last 
might be done manually by a receiver operator, or it 
may be accomplished automatically, called Auto-
matic Frequency Control (AFC). 
The AFC technique is very useful for automatic 
communication systems, but it requires that both ends 
of the communication system "lockup" before any 
data may be transferred. The time required for this 
lockup varies with system design. For many systems 
this is of no significance, but for the case where one 
station (master) rapidly interrogates many other 
stations (remote stations), the lockup time is usually 
intolerable on the return link from the remote to the 
master station. This results from the fact that the 
master station usually is always transmitting and all 
remote stations within range will have time to acquire 
and lockup before they are interrogated by the master 
station. However in * the general case, each remote 
station may be moving at a different relative velocity 
to the master and thus the master must either search 
for each reply or else it must have a wide enough 
bandwidth to accommodate each reply plus the maxi-
mum Doppler.
The operation of the two-way system is as follows. 
The master station radiates a carrier which is slowly 
swept in frequency over a suitable range. The remote 
stations are tuned to a nominal frequency and when 
the swept transmitted frequency plus the Doppler 
shift equals this nominal frequency, the remote station 
activates an Automatic Frequency Control (AFC) 
circuit. This circuit then follows the frequency sweep 
of the master plus the Doppler due to the relative 
motion between the master and remote station. AFC 
is achieved by varying a local oscillator in the remote 
receiver such that the received signal falls at the center 
of a frequency discriminator. 
An automatic Doppler correction is derived from 
the AFC system and applied to the reply link such 
that the master receiver may be fixed tuned and 
requires neither a time consuming frequency search 
for the reply nor an excessive bandwidth to accom-
modate the Doppler. 
The master station might be an airplane control 
tower communicating to automatic readout devices 
located on airplanes flying in the area. It could also be 
located in a satellite and be interrogating remote sta-
tions located on the ground, floating in the sea, sus-
pended from balloons, flying in airplanes or in 
another satellite. The automatic Doppler compensa-
tion is applicable to any other system where one or 
both stations are moving, which gives rise to a Dop-
pler frequency shift. 
Notes: 
I. This development is in conceptual stage only, and 
as of date of publication of this Tech Brief, neither 
a model nor prototype has been constructed. 
(continued overleal) 
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=19670000642 2020-03-11T21:32:55+00:00Z
Inquiries concerning this invention may be di-
	 Patent status: 
rected to:
	 Inquiries about obtaining rights for the commercial 
Technology Utilization Officer
	 use of this invention may be made to NASA, Code 
Goddard Space Flight Center
	 GP, Washington, D.C. 20546. 
Greenbelt, Maryland 20771
	 Source: Ronald M. Muller 
Reference: B67-10643
	 (GSC- 10213) 
Brief 67-10643	 Category 01


Concept for automatic Doppler compensation in two-way communication systems

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