Communications system maintains phase lock of weak telemetry signals with a minimal expenditure of power and bandwidth. An estimate of the frequency variation as a function of time is used to achieve coherent phase demodulation

Gilchriest, C.

Goldstein, R.

Mathison, R.

English

NASA Technical Reports Server

IS-DOC-12t4S 
Rh. 1313 
August 1970
	 KSC HQS.	 Brief 70-10008 
v.1..
NASA TECH BRIEF 
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Telemetry Receiver
Estimate
of Frequency
-1
Telemetry 
Subcarrier 
Modulation 
L ----- -
Block Diagram of Telemetry System 
A communications system has been developed with 
an important feature of maintaining phase lock of 
weak telemetry signals with a minimal expenditure of 
power and bandwidth. Direct potential application 
would be in the commercial satellite communications 
network. The electronics hardware can also be used 
as an aide to commercial navigation (ship-to-shore) 
and in long range, fixed-point earthbound telemetry 
networks. 
The problem: 
Telemetry signals received from a capsule entering 
a planetary atmosphere are weak and subjected to 
large Doppler dynamics. The received signal fre-
quency is a function of time that is only predictable 
to a zero or first order approximation. It would be 
possible, by means of a wide band coherent receiving 
system, to maintain phase lock with the telemetry 
carrier under conditions of planetary entry. However,
the power requirements of the capsule transmitter 
would be prohibitively high for the wide bandwidth 
required. 
The solution: 
An estimate of the frequency variation as a function 
of time can be made by preprocessing a recorded 
version of the received signal in nonreal time, and 
using that estimate to perform an aided tracking 
operation whereby coherent phase demodulation can 
be achieved with a minimal use of the available band-
width. This allows most of the transmitted energy to 
be put in the information-carrying sidebands rather 
than in the carrier. 
How it's done: 
A block diagram of the receiving system is shown 
in the figure. The received signal from the space probe 
is mixed in the detector with a local oscillator signal 
which is programmed according to an ephemeris pro-
(continued overleaf) 
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gram from the storage. This program subtracts out the 
components of Earth's rotation, the assumed trajec-
tory of the capsule, and the Earth/planet relative 
motion. The output of the detector is a signal at an 
intermediate frequency with some of the known Dop-
pler dynamics such as Earth's rotation, Earth/planet 
relative motion and assumed trajectory subtracted 
out. This signal is amplified and filtered in an H. 
amplifier and recorded on a magnetic tape recorder. 
The output of the recorder is converted to digital 
signals by an analog-to-digital converter and applied 
to a spectrum estimator computer. This computer can 
be a specialized digital device to perform the specific 
task or, alternatively, can be a general purpose com-
puter utilizing a spectrum estimation program. The 
remaining function of the receiver, i.e., the phase-
lock demodulation, can also be accomplished in soft-
ware if desired, by employing a phase-lock-loop simu-
lator program. 
The computer estimates the frequency in each of 
the segments of the Doppler frequency vs time curve 
and algebraically adds the carrier frequency estimate 
from the tape recorded telemetry signal. The algebraic 
sum is then recycled and compared with the recorded 
telemetry signal. This process is repeated until the 
error in the estimate is reduced to the degree of 
smoothness where the residual frequency can be 
tracked by the phase-lock-loop. The telemetry modu-
lation output is then available at the output of the 
phase-lock-loop. By this iterative estimation of the 
carrier frequency vs time, the signal dynamics can be 
reduced so that the remaining signal is perturbed only 
by the additive noise and the error in the estimate. 
The remaining signal is narrow band and can be 
tracked by a narrowband phase-lock-loop. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B7040008 
Patent status: 
This invention is owned by NASA, and a patent
application has been filed. Royalty-free, nonexclusive 
licenses for its commercial use will be granted by
NASA. Inquiries concerning license rights should be 
made to NASA, Code GP, Washington, D.C. 20546. 
Source: R. Mathison,C. 'Gilchriest, 
and R. Goldstein of
Caltech /JPL
under contract to
NASA Pasadena Office
(N P0-10746)
r_7 
Brief 70-10008	 Category 02


Telemetry receiver

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