Test data of the RFS Program in the Production phase and computer automation are presented, as an essential element in the evaluation of the RFS performance in a simulated spacecraft environment. Typical production test data will be discussed for stabilities from 1 to 100,000 seconds averaging time and simulated time error accumulation test. Also, design considerations in developing the RFS test systems for the acceptance test in production are discussed

Dederich, D. J.

Koide, F.

English

NASA Technical Reports Server

RUBIDIUM FREQUENCY STANDARD TEST 
PROGRAM FOR NAVSTAR GTS 
Frank K. Koide and Darrell J. Dederich 
Electronic Systems Group 
Rockwell International, Anaheim, Califoruia 
ABSTRACT 
Space- qualified Rubidium Frequency Standards (RFS) are  
being developed for the NAVSTAR Global Positioning System 
(GPS) program by Rockwell International Electronic Sys tems 
Group. 
This paper will present the test data of the RFS Program in 
the Production Phase and computer automa tion a s  an essential 
element in the evaluation of the RFS perfarmance in a simu- 
la ted spacecraft environment. 
Typical production test data will be discussed for stabilities 
from 1 to 100,000 seconds averaging time and simulated time 
e r ror  accumulation test. Also, design considerations in devel- 
oping the RFS test systems for the acceptance test in production 
wi l l  be discussed in the paper. Finally, as  part of a life-cycle 
test in vacuum, stability data to 1,000,000 seconds averaging 
time will be presented. 
INTRODUCTION 
- --. 
The Electronic Systems Group (ESG) of Rockwell International has been 
involved in the engineering development and production of the Rubidium 
Frequency Standard for the GPS Phase I effort since 1974. In Phase I of the pro- 
gram, ESG will produce a total of 29 RFS; one proto type, three engineering 
models, an6 25 production units. Currently, all but four units have been 
delivt .:d to our sister Group, the Space Systems Group who is the Prime 
Conh-actor for the satellite NAVSTAR-GPS Program. 
r'he first series of four GPS satellites are in orbit, with each carrying three 
redundant RFS. Three of the satellites have been declared operational, and 
initial test results proved that the user equipment is  capable of navigation 
accuracies better than three meters in three dimensions. When the fourth 
NAVSTAR satellite is declared operational, the prime objective of validating 
the Glob1 Positioning System, will be accomplished. 
https://ntrs.nasa.gov/search.jsp?R=19790016574 2020-03-20T18:50:25+00:00Z
The RFS test cycle (Figure 4) covers two major phases, the pre-production 
and the acceptance level testing. The pre-produc tion testing covers the board 
and system assembly, where tests a r e  made of the components, absolute 
frequency, temperature compensation, and repairs, if  required. The 
accephnce-level testing includes the environmental and certification tests. 
X l i  of the certification tests and part of the pre-produc tion and environmental 
tests a r e  performed on three test systems located within the Metrology Lab- 
Labora tory of ESG . 
Testing completed to date indicates that the RFS meets o r  exceeds the GPS 
specifications for Phase I, and is a potential candidate for the forthcoming 
Phase I1,'III Programs. 
TEST SYSTEM DESCRIPTION 
The three automated test systems (Figure 1) a r e  supported by a DEC PDP 
11/35 computer which utilizes a timeshare operating system. A valuable 
feature of this operating sys tem utilized in this application,is the ability to 
access data being stored by executing an independent data analysis program 
at  another remote termina1,withou t interrupting the data collection process. 
The frequency stability certification test (Figure 4) is performed using test 
stations one and threz (Figures 2 and 3). This test requires that data be 
collected for nine days without interruption. Microcomputers a r e  included in 
these test stations to provide redundant data collection and storage if the main 
computer should fail. At the end of the test, these data would then be trans- 
ferred to the PDP 11/35 for analysis. Failsafe devices a r e  incorporated into 
all test stations to protect against RFS supply over-voltage and current, and 
over and under base-plate temperatures. 
The RFS supply current and telemetry-monitored testpoint levels a r e  recorded 
throughout the test cycle. The RFS frequency is measured using a "Time 
Mark System. [2] This system, developed by ESG Metrology, i s  similar in 
principle to the NBS chronograph. The heterodyned beat period i s  measured 
without dead time (the loss of time required to rearm the counter). In this 
system, the beat-frequency zero crossings load the value of a free running 
counter into an output buffer. This sys tern was implemented by simple 
modification of an electronic counter. 
Figure 5 shows the RFS mounted in the vacuum chamber which similates the 
spacecraft pressure. Cooling i s  provided through the R* S baseplate to the 
mounting block which simulates the spacecraft mounting. Coolant from a 
ternperature-stabilized bath is circulated through a maze within the block. 
The RFS is fundamentally an Efratom Rubidium Frequency Standard repackaged 
with extensive modiiica tion to meet space environmental, reliability and space- 
craft interface requirements. The RFS is 8 in. x 5 in. x 4.5 in. in size, 
weights 8-1/2 lb and consurnes less  than 30 watts of power. The nominal 
output frequency is 10.23 MHz. 
RFS PERFORMANCE 
The data collected to obtain the two-sample Allan Variance [3,4] computer 
printout of Section 1, Figure 6, i s  for  a 1-second Fundamental beat period. 
Adjacent data pairs a r e  averaged to obtain the 2-second Allan Variance, and so 
on for the 4- and 8-second results. The second section fundamental beat is 
10 seconds. The third and fourth section results a r e  calculated from stored 
data that is the average of 10 (100-seconds) and 80 (800 seconds) 10-second 
beat periods. This method is  a reasonable alternative to storing large amounts 
of data, and still retaining acceptable codidence limits. The calculated drift 
per  day is based on the last  5 days of testing. The Allan Variance results pre- 
sented in Figure 6 reflect the removal of a firs t-order curve (drift). 
Figure 7 is a graphical representation of the 800-second data used to calculate 
the Allan Variance of Figure 6 .  The graphs a r e  the average of 10, 20, and 50, 
800-second data, resulting in 8000, 10,000 and 40,000 seconds per  bar. The 
graph readily demonstrates the exponential warmup cha~.acteristic present in 
the RFS frequency. 
Figure 8 is a graphical representation of the printout of Figure 6 .  The vertical 
bars e t  each data point represent the range of confidence limits[3r4]. The 
inset in the top-right corner i s  the graph of Figure 7 for the average of 50, 
800-second data points. It is apparent, that the warmup characteristic dis- 
played by the inset is the predominant cause of the Allan Variance f 1 atteni ng 
o r  turning upward for  longer sample times. In Figure 9 where the inset shows 
essentially flat data, o r  in Figure 10 where removal of the drift results in a 
relatively flit plot, the Allan Variance values continue a downward trend. 
Figure 12 is  a plot of the RFS frequency versus mounting base-plate tempera- 
ture data. This test simulates possible RFS base-plate temperature excur- 
sions during a 12-hour space vehicle orbit. The accumulated time e r r o r s  and 
the elapsed times over which they a r e  specified a r e  marked on the plot. The 
computer printout of Figure 13 shows the time e r r o r  accumulations and the 
procurement specifications. Representative profile plots of other units a r e  
shown in Figures 14 and 15. 
Figure 16 is a plot of the Allan Variance calculated from data collected in a 
RFS life cycle test conducted by the Space Systems Group of Rockwell 
International. The calculations reflect removal of the fir: !-order drift char- 
acteris tic. The test term was 100 days (8600 data of approximately 1000- 
second averaging time). A graph of the data revealed two distinct drift char- 
acteris tics separated near the midpoint of the test. Both a r e  very nearly 
linear. It seems liltely that this change in the drift slope was caused by an 
external influence on the RFS. The plot using the . symbol is the Allan 
Variance calculated from data points 1-8600, and the plot using the x symbol is 
calculated from data points 4300 - 8600. 
These data indicate that the RFS, after adequate stabilization, is capable of 
meeting the GPS Phase II/III specification a s  shown in the figure. 
ACKNOWLEDGEMENTS 
The authors wish to express their sincere appreciation to Laurie Baker of 
Metrology, who made significant contributions in the development of the 
vacuum test systems and the tempera ture-control system for the Rubidium 
Frequency Standards. 
REFERENCES 
1. F. K. Koide and E. M. Hicks, llAtomic Clock Test Program for 
NAVSTAR," Fifth Cal-Poly Measurement Science Conference, December 
1975. 
2. E. M. Hicks, ?'An Innovative Method for Measuring Frequency Stability 
without Dead time, Sixth Cal-Poly Measurement Science Conference, 
December 1976. 
3. J. A. Barnes e t  al, llCharacteriza tion of Frequency Stability, I f  NBS 
Technical Note 394, Issued October 1970; IEEE trans. on Instr. & Meas., 
IM-20, 1971. 
4. P. Lesage and C. Audoin, "Characterization of Frequency Stability, 
IEEE Transactions on Ins tr. & Meas. , TM-22, June 1975. 
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Fig. 6-Computer Printout of Allan Variance Calculation 
DISPLAY IS THE AVERAGE OF 
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QUESTIONS AND ANSWERS 
MR. HUGO FRAUHOFF, Efratom Cal i f o r n i a :  
I j u s t  want t o  make sure t h a t  everyone knows t h a t  Efratom i s  teamed 
w i t h  Rockwell. 
MR. KOIGE: 
I t h i n k  I f o rgo t  t o  mention tha t .  
MR. ANDREW CHI, NASA Goddard Space F 1 i g h t  Center: 
I n  your presentat ion o f  the frequency p l o t  versus temperature o r  
temperature versus time, i n  one curve there was a sudden jump i n  
frequency. Could you exp la in  why there was a la rge  jump? 
MR. KOIDE: 
Yes, we don ' t  r e a l l y  have an apswer t o  tha t .  There ar2 many v a r i -  
ables t h a t  cause tha t ,  and we are s t i l l  look ing i n t o  it. So I d o n ' t  
want t o  go i n t o  that .  


Rubidium frequency standard test program for NAVSTAR GPS

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