Independent PCM telemetry data signals received from missiles must be correlated to within + or - 100 microseconds for comparison with radar data. Tests have been conducted to determine RF antenna receiving system delays; delays associated with wideband analog tape recorders used in the recording, dubbing and repdocuing processes; and uncertainties associated with computer processed time tag data. Several methods used in the recording of timing are evaluated. Through the application of a special time tagging technique, the cumulative timing bias from all sources is determined and the bias removed from final data. Conclusions show that relative time differences in receiving, recording, playback and processing of two telemetry links can be accomplished with a + or - 4 microseconds accuracy. In addition, the absolute time tag error (with respect to UTC) can be reduced to less than 15 microseconds. This investigation is believed to be the first attempt to identify the individual error contributions within the telemetry system and to describe the methods of error reduction within the telemetry system and to describe the methods of error reduction and correction

Matthews, F. L.

Pickett, R. B.

English

NASA Technical Reports Server

PRECISE TIMING CORRELAT19N IN TELEMETRY RECORDI&~ 
AND PROCESSING SYSTEMS 
R. B. Pickett 
F. L. Matthews 
ITT Federal Electric Corporation 
Vaudenberg Air Force Base 
ABSTRACT 
Independent PCM telemetry data signals received from missiles must be corre- 
lated to within 1100 microseconds for comparison with radar data. Tests have 
been conducted at  the Space and Missile Test Center (SAMTEC), Vandenberg Air 
Force Base, California, to determine R F  antenna receiving system delays; de- 
lays associated with wideband analog tape recorders used in the recording, dub- 
b h g  and repdocuing processes; and uncertainties associated with computer pro- 
cessed time tag data. Several methods used in the recording oi  timeicg are 
evaluated. Timing error versus tape recorder head alignment is plotted. Tape 
recorder phase lead effects OD time code formats and data are given. The time 
bias associated with computers processing of data is presented. Sources of tim- 
ing errors and the calibration and operathg techniques available to minimize 
these er rors  are discussed. Through the application of a special time tagging 
technique, the cumulative timing bias from all sources is determined and the bias 
removed from final data. Conclusions from test data shmv that relative time 
differences in receiving, recording, playback and processing of two telemetry 
links can be accomplished with a rl microseconds accuracy. In addition, the 
absolute time tag er ro r  (with respect to E C )  can be re&ced to less than15psec. 
This investigation is believed to be the f irst  attempt to identi@ the individual 
e r ro r  contributions within the telemetry system and to describe the methcds of 
error reduction m d  correction. 
INTRODUCTION 
Missiles launched from Vandenburg Air Force Base in California are tracked 
by systems located throughout the world. Radars provide position and velocity 
data in terms of iang-?, azimuth, elevation and range rate, Telemetry stations 
receive information encoded and transmitted from the missile. For missiles 
configured with inertial guidauce systems, position and velocity information in 
terms of X, Y, Z and X, Y, z i s  transtdtted on the telemetry link. 
The continued improvement in ballistic missile performance places an over in- 
creasing accuracy requirement oa posi4.!on and velocity data. When sensor 
https://ntrs.nasa.gov/search.jsp?R=19750003211 2020-03-23T03:06:55+00:00Z
performance capabilities do not meet the accuracy requiremats d imt ly ,  
analysts use multistation mhtiom bo refme the trajectory cta& Such solutions 
may include inputs from widely spaced radars as well as the inertial guidance 
data. When mergiug the sources, timing offsets are particularly troublesome, 
as such errors  propagate in a complex manner during the merging. 
In order to avoid these pmblems, timing offsets are limited to G O O  micro- 
seam& (p s )  o r  less, This minces the position error due to timing alone to 
less than 2.4 feet, even at escape velocity. In practical terms, this means 
tbat data received at stations t k u g h a u t  the world must be synchronized to 
within i100 p s of UTC. 
Range Timing Spstem 
The SAMTEC timing system accuracy is directly traceable to the U. S. Naval 
Observatory's Coordinated Universal Time (UTC). This traceability is main- 
tained as shown in Figure 1 through the SAMTEC Precision Measdrement 
Laboratories (PML) Precise Time Reference Station (PTRS). The PML main- 
tains a calibration accuracy of e . 0  ps of epoch USNO-UTC master clock. The 
PML has the responsibility to maintain the four SABnTEC Central Time Signal 
Generators (CTSG) Cesium Beam primary frequency and cbdr standards in 
calibration and synchronization wit3 UTC (USNO). The CTSG systems are in- 
stalled at Vandenberg F i r  Force Base and Pillar mint  Air Force Station, 
California; Kaena Point, Oahu Island, Hawaii; and Canton ISM, Phoenix Island 
Group. Each CTSG is equipped with dual cesium standards, frequency dividers 
and clocks, alarm and transfer circuitry, Inter-Range Instrumentation Gmup 
(IRIG) Time Code Generadors and distribution amplifiers. At specified cali- 
bration periods, the PML transports a portable cesium "flying clocktt to each 
CTSG to calibrate and synchroni~e the CTSG cesiums with UTC (USNO). Any 
drift in the "flying clock" is accounted for in the PML 
As s b w n  in Figure 2, the distribution of the IRIG time code formats A, B, D, 
E and H and 1, 10, lOOpps and 1, 10 and 100- pulses is generally accomp- 
lished over telephone plant cable pairs o r  a UHF radio system. The timing cir- 
cuits provide radar, telemetry and other instrumentation systems with standard 
time code formats. Total delays a t  each site a r e  then determined with a portable 
frequency standards and a computing counter, as shown in Figure 3. 
Radar Synchronization 
As shown in Figure 4, the master timing pulse a t  each radar station is routed 
from the timing center to the radar area and then to radar equipment where 
inter~wgrte pulses are initiated. It is  these 20pps pulses which actually "freeze" 
Figure SAMTEC Instrumentation Systems u T C ~ R I G  Timing and 
Frequency Accurac~l Traceability to USNO 


Figure 4. Multiple Radar Interrogator Time Tag Data Synchronization 
the data registers. The data is then "dumped" and stored on tape, along with 
timing. The delay of the strobe pulse in relation to UTC is directly measured 
quarterly. This delay is added to the computed drift of the master clock to 
obtain an estimate of the total offset on each operation. 
As an additional precaution, a backup cesium standard is located a t  the radar 
site, The20ppspulses a r e  compared with the cesium 1 pps "on time" pulse 
prior to each operation. If the difference in time between the pulses is not with- 
in *25 p s  of the designated delay for the site, the actual difference must be 
logged by the operator. Thus quarterly measurements plus pre-operational 
checks assure that all radar data can be accurately aligned for merging. 
Telcrlletry Synchronization 
As shown in Figure 5, telemetry stations synchronize a time code generator to 
the  incoming IRIG signal. The generator then outputs "clean" IRIG timing which 
is recorded on magnetic tape. Also, data from the receiving system is recorded 
on adjacent tracks of the same tape. !luring post flight processing, the magnetic 
tape is replayed, data and timing signals a r e  shaped, and both a r e  inputted for 
computer processing. Part of the processing requirement is to correlate range 
timing to the time of data reception a t  the station to within one computer word 
time. (Typically 178 PS on critical programs). Unfortunately, e r ro r s  in the 
recording/rcproducing system may accumulate to several hundred microseconds 
and must be corrected on the final data product. The significant contributions to 
the accumulated er ror  have been measured, and a re  discussed below. 
a. Figure 5 shows the mean RF receiving system and recorder time delays 
from three antenna systems for both predetection and post-detection 
recording. 
b. Several models of tape recorders were tested. Figuri C illustrates 
that each type of recorder reacts differently to the IRIG A lOkHz and 
IRIG B 1 kHz carriers. It was found a s  shown in Figure 7, that a dubbed 
tape had  approximate:^ doubled the time delay of the orginal tape. 
c. Head azimuth alignment causes only 1 p s  of timing error ,  at3 shown in 
Figure 8, if heads a r e  aligned within IFUG specifications. 
d. As shown in Figure 9, and er ror  of up to 33 ps may result from allow- 
able even and odd head placement tolerances. 
e. During playback, the time code generatar decodes the IRIG timing signal 
and outputs timing pulses for computer use. As shown in Figure 10, it 
was found that jitter on the pulses was significant for IRIG B codes. 
T 9 a ANTENNA SYSTRW S# 
TI f DATA RECIIVRR soas 
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Figure 5. Absolute RF System Receive/Record Time Delays 
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Figure 6, Comparisonof Playback Time Delays of Different Brands of Recorder 
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f. An unmodeled e r ro r  occurs from time to time when the recorder IRIG 
timing i s  noisy. Operators may filter the data to obtain better lock. 
Such filtering introduces additional delay and amplifier inversion of the 
code may introduce an unexpected time delay of one half cycle (500 p sec 
for IRIG B). Such errors,  of course, destroy the carefully controlled 
timing correlations. 
Correction of Telemetry Errors 
The er rors  outlined above may accumulate to several hundred microseconds 
on a given data run. Fortunately, the er ror  i s  a reasonably constant value and 
can be easily determined a s  follows. 
Referring to Figure 11, the delay to the signal conditioner 1 is known to be less 
than 2 ps  and is ignored. The conditioner introduces a 1 bit data delay and 
its output i s  recorded on a redundant recorder track. Each 1 pps pulse from the 
time code generator switches the data off and inserts a pre-programmed word 
in the data. During processing, the special word can be easily identified and 
the last data bit prior tc  the word inserted is noted. This i s  the data bit which was 
received within 1 bit time of an even second mark (less the delay through the 
receiving system). The actual data produced from an uninterrupted (normal) 
input is then examined and the time provided by the computer for the reference 
bit determiiied. This time should be an even second. Any deviation i s  the accu- 
mulated timing er ror  which existed a t  the even second. The mean and standard 
deviation of all 1 second measurements is obtained. This provides the timing 
bias caused by the accumulated error.  The bias can be removed in subsequent 
processing operations. 
Future Requirements 
A range requirement may be received which requires that the data received on 
two separate links be correlated to within 510 ~ s .  From the data provided 
above, it can be seen that even if the two data streams a re  recorded on the same 
head stack and high frequency IRIG codes a r e  used, it will be difficult to control 
the computer ~nputs to within the required tolerance. Of greater significance, 
however, is  the software resolution of * 1 word time in assigning timing fftags'f. 
This resolution must be reduced to f 1 bit time to meet program objectives, 
Future Needs 
Although cesium standards have proven reliable, failures require that a "flying 
clockff be sent to the remote site between normal calibration intervals. What is 
needed is a technique for synchronizing remote standards to within *1 ps 

witbat reIying on a portable clock. Tbe application of satellites has been con- 
sidered aad the atxumcy appear8 to be satisfactory. However, fts application 
depedils cm the availabilitg of inexpensive receiving equipment which can com- 
pek, fram a cost effectiveness standpoint, with the flying clock concept, 
REFERENCES 
1. Pickett, R. B., "Improving Telemetry Timing Accuracy", Interna'donal 
Telemeterfng Conference Proceedings, 1971. 
2. Maaews, F, L. , **Timiag Accuracies of Range Instrumentation Systemsw, 
International Telemeter- Conference Proceedings, 1972. 
3. Mattbews, F. L. and Streich, R. G., "Timing Correlation in Telemetry 
Recording and Processing Systemsw, International Telemetering C d e r e n c e  
Proceedings, 1973. 
4. "Telemetry Standardsw, IRIG Document 106-73 (Revised May 1973). 
5. "Test Methods for Telemetry Systems and Subsystemsw, LRIG Document 
118-73 (Revised May 1973). 
6, **IRIG Standard Time Formats", IRIG Document 104-70 (Revised August 
1970). 
QUESTION AND ANSWER PERIOD 
MR. CHI: 
Are there any questions? 
DR. COSTAIN: 
You never mentioned the quality o r  band width of the lines that you were using. 
I found that IRIG-B was even beyond the capability of commercial telephone 
lines, and the error rate was unacceptable, and we went to the FSK coding 
modems. 
You a r e  talking about using 100 kilohertz. You must have pretty good lines. 
MR. MATTHEUS: 
If and when we do go to the 100 kilohertz, of course it  would be in the coax type 
line. 
Normally, our IRIG-B is  distributed, when it  i s  on telephone lines, on a 3 KC 
circuit, and IRIG-A requires coax. 
DR. WINKLER: 
I zm very impressed by the work which you have described. There a r e  several 
comments and ideas which I would like to explore here with you. 
Isn't it true that the absolute time difference which you have between your 
various channels on one and the same tape recorder, i s  less of a problem than 
the fluctuationtq which you have not only from gap misalignment, but even more 
so fmm changes in the tape tension, and c k n g e s  in the tape type? If you use a 
different tape, I would expect you to have qtilte a different delay from channel 
to channel. 
Don't forget that five microseconds a t  a speed of 120 inches per second amounts 
to just a few microns of difference in position a t  any me moment. 
For  that reason, it  appears to me that i t  may be usefel to make a test of trying 
to superimpose a low level sine wave, which i s  derived from your standard, on 
the very same channel on which you have your timing data comitg in, which are, 
of course code modulated a t  any rate. 
So, you could separate them electrically, but you would have the advantage of 
having pass& all the signals, the timing signals, and your information over 
exactly the same channels on the tape. 
Another possibility, of course, would be to use a wide band video recorder, and 
to put all your ckannels onto one carr ier ,  and use actually a car r ier  frequency 
type of recording system. 
But i t  is  definitely pushing the possibilities to the very end. 
\Veil, I failed to mention that we, in the alignment of our recorder heads, used 
the lissajous patterns to make our alignment, and have found it  very effective in 
aligning up the head, a s  to the azimuth alignment. 
h=R. KLEINKOPF: 
Jack Kleinkopf here, from NASA Flight Research Center, a t  Edwaxls Air Force 
Base. 
One thing that I didn't get clear -- maybe yoit covered it  -- was the discrepancy 
between your samp!e data system, the PCM system, and the sample rate on 
board, and the ground timing. 
How do you go about resolving that? 
MR. MATTHEWS: 
1 should say airborne timing is  cot available on the missiles that a r e  flown out of 
Vandenberg, and therefore all of the timing is  generated from the ground system. 
Does that answer your questior. ? 
MR. KLEINKOPF: 
Well, do you have an oscillator in your airborne sample data system, and do 
you have a frame of PCM data coming in, so that the post reduction analysis can 
go back and determine where the beginning of the frame of the PCM data i s  with 
respect to your ground base time. 
MR. MATTHEMS: 
Okay. Normzlly, there a r e  specific points in the flight that a r e  known very 
accumtely, and we can align our data up with time a t  these points, such a s  
wllen they go through staging. These represent speciffc time intervals which 
we know. At those points we know that a t  a certain time that, because of 
separation o r  attenuation, we get dropouts. U'e also know that these particular 
points in our data stream a r e  exact and that we can line the timing up with that. 
MR. KLEINKOPF: 
I see. You a re  saying this i s  all based on a launch time, o r  something like 
that, initialization point. 
MR. MATTHEWS: 
Ure record the liftoff time. But one of the points that I was pointing out, 1 believe 
on the last slide there, was that in order  to measure the absolute time, we need 
to know very accurately what the time delay difference i s  between UTC and this 
one pps pulse where we i n ~ e r t  the pre-programmed won] into the data stream 
with a special recorded track. 
That data is pulled off, put into the computer, and stored. It i s  also looking and 
averaging the time of the mZllisecond marker - that is, the IRIC time code goes 
into the time code translator. It then inputs the millisecond markers into the 
computer, and it knows a t  a certain point this one pps pulse takes place where 
that millisecond marker should be. We thexi correct for that millisecond 
marker. 
MR. CHI: 
Thank you again. 


Precise timing correlation in telemetry recording and processing systems

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