System provides rapid command sync acquisition between widely separated transmitter-receivers. It is based on a rapid, automatic range-adjustment approach rather than the time-consuming cycle slipping or stepping techniques of conventional phase-locked loops

Anderson, T. O.

Gallo, A. J.

English

NASA Technical Reports Server

October 1969
	
Brief 69-10538 
NASA TECH BRIEF 
NASA Teèh 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. 
-. 4	 s	 ------- - ----,-
Design for a Rapid Automatic Sync Acquisition System 
	
ft Reg	 Timing Pulse Data From From Telemetry 
Sys Ena 
Telemetry 
System
	
	
System 
Control Unit
able Pulse 
From GND 
	
Clock NGND PN Gen
	
Command System 
Clock 
Selector 
21s
	 Co
e	 L	 Input From 
Advancer r
	
Ranging System 
GROUND SYSTEM 
Interrogate	 PN Generator 
Pulse from 
Telemetry System 
Timing Pulse	
Control Unit 
From Command 
System	 Shift Rteg	 To Telemetry 
Data Bit Stream 
Bit Rate 
Shift Pulse 
From Telemetry 
System	
FLIGHT SYSTEM 
A design has been conceived for a system in-
tended to provide rapid command sync acquisition 
between widely separated transmitter-receivers, 
such as between a spacecraft telemetry transmitter 
and a ground-based receiver. Use of the system in 
commercial satellite communications would facili-
tate rapid sync acquisition between stations and 
regaining of data lock after interruption or equip-
ment failure. The system is based on a rapid, 
automatic range-adjustment approach rather than 
the time-consuming cycle slipping or stepping 
techniques of conventional phase-locked loops.
The basic scheme of the proposed system is 
that of a one-step adjustment of the ground PN 
code generator in order to make it synchronous 
with the flight PN generator. The state or phase 
of the flight PN generator is transmitted to Earth 
via the telemetry channel, and the status of the 
flight PN generator is detected by the ground 
system. This information is then used to set the 
ground PN generator. Assuming there were no 
delays in the telemetry processing and transmis-
sion time, the above process would cause syn-
chronization between the two systems. Inasmuch 
(continued overleaf) 
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as there is processing time and propagation time 
involved, these parameters must be accounted for. 
Once the ground PN generator is set by the telem-
etry information, it would be clocked rapidly by 
the code advancer to account for the delays. The code 
advancer, it should be noted, derives its data from 
the ranging system. Assume the total time elapsed 
between sampling of the flight PN generator and 
transmission of the ground PN code is T seconds. 
By multiplying' T by the total number of symbols 
which appear in the PN sequence, the status of the 
flight command code generator can be predicted. 
Only the fractional second-portion of the delay time 
is used. 
At the proper time in the telemetry bit stream, 
a pulse is initiated in the telemetry system which 
attenipts to activate the control unit. The purpose 
of the control unit is to transfer the information 
from the flight command PN code generator into 
a shift register which will be clocked out in the 
telemetry bit stream at the bit rate. To prevent 
sampling of the flight PN generator during a 
transition time, a timing pulse from the command 
system is required, in addition to the interrogate 
pulse from the telemetry system to activate the 
control unit. 
Lock is obtained by this. sequence of events: 
The information from the telemetry bit stream is 
recovered and shifted into a register in the 
ground system. Upon completion of the input in-
formation, the coincidence of a timing pulse from 
the telemetry system and an enable pulse from 
the command system causes the ground control 
unit to set the ground PN generator. This action 
causes the ground code to represent the condition 
of the flight PN generator at the sample time. 
Since the flight code generator has changed 
status during the transmission and processing 
time, it is necessary to predict where the flight 
PN generator will be at the time of transmission 
to the spacecraft. This prediction must then be 
used to advance the ground PN generator. The 
function of the code advancer is to provide high 
frequency clock pulses to advance the ground
generator to coincide with the flight generator. 
The number of pulses generated is dependent upon 
the total propagation and telemetry process time. 
The code advancer, it is proposed, will receive 
the propagation time information directly from 
the ranging system or through extrapolation. Since 
the code is repetitious at 1 bps multiples of 1 
second are discarded; only the fractional portions of 
a second which represent the phase difference 
between the two codes are utilized. 
The proposed system implementation will cause 
the spacecraft to receive a code which is in phase 
or very close to being in phase, depending upon the 
tolerances involved. To overcome some of the prob-
lems associated with the tolerances, the codes will 
be transmitted a fixed number of PN symbols out of 
phase. The two codes will then drift with respect to 
one another for a short period of time until lock is 
acquired. 
Using a system such as this, it would not be 
unrealistic to achieve lock times in the order of 
a few seconds. In contrast with the present lock 
techniques, this number would be fixed. Present 
systems such as those used on Mariner '69 re-
quire anywhere from 0 second to 9 minutes to 
acquire lock. 
Notes: 
1. This Tech Brief formerly appeared as Brief-68-
10428. Documentation is now available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10538 
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: T. 0. Anderson and A. J. Gallo

Jet Propulsion Laboratory

(N P0-10214)
. 
Brief 69-10538	 Category .01


Design for a Rapid Automatic Sync Acquisition System

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