Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date

Winkler, G. M. R.

English

NASA Technical Reports Server

N75 21174 
PRECISE TIME AND TIME INTERVAL (PTTI), AN OVERVIEW 
Gernot M. R. Winkler 
Naval Observatory 
ABSTRACT 
Present applications o f  precise time and 
frequency (T/F)  technology can be grouped 
as follows: 
1. Communications systems which require 
T/F for time division multiplexing and 
for using spread spectrum techniques. 
2 .  Navigation systems which need T/F for 
position fixing using a timed signal. 
3 .  Scientific-Metrological applications 
which use T / F  as the most precisely 
reproducible standard o f  measurement. 
4. Astronomical-Space applications which 
cover a variety of the most denanding 
applications such a s  pulsar research, 
Very Long Baseline Interferometry 
(VLBI) and laser/radar ranging. In 
particular, pulsar time-of-arrival 
measurements require submicrosecond 
precision over a period o f  one-half 
year referred to an extraterrestrial 
inertial system, and constitute the 
most stringent requirements for uni- 
form timekeeping to date. 
The standard T/F services which are avail- 
able to satisfy such requirements are 
based on an international system of time- 
keeping coordinated by the Bureau Inter- 
national de 1'Heure (BIH). The system 
utilizes the contributions from the major 
national services for standard frequency 
and time (USNO and NBS in the U S A ) ,  and 
it is implemented through a variety of 
electonic systems (HF, T/F signals, VLF, 
Loran-C, etc.). The performance of  these 
systems will be briefly reviewed. Several 
1 
https://ntrs.nasa.gov/search.jsp?R=19750019102 2020-03-19T21:03:25+00:00Z
of  the user systems (such as VLBI) can, 
in turn, be used as contributors to the 
global effort of T/F distribution. 
Moreover, PTTI is the one common interface 
of all time-ordered electronic systems. 
Time coordination (not necessarily synchro- 
nization) is the first necessary step for 
any wide scale integration and mutual back- 
up of such systems. 
OVERVIEW 
In this short overview we can only mention the major as- 
pects of time and frequency (T/F) which will be dis- 
cussed extensively in the papers of the Proceedings. The 
main concepts are as follows: 
1. Time of  Day -t UT1 (Rotation of Earth) 
2. Clock Time at Standard Meridian + UTC 
3 .  Synchronization 
4 .  Accurate Frequency 
5. Relativity: Local (Proper) Time, Coordinate Time 
6. Coordination: 1 ms + 10 ~.ls progres s  during the last 
10 years. 
For economical as well as practical reasons, SYNCHRONIZA- 
TION will usually be accomplished via clock time (UTC). 
Very accurate frequency cannot entirely be handled without 
also considering time. Relativity aspects must be consid- 
ered if precision of better than a few hundred nanoseconds 
is involved. Lastly, the various international time ser- 
vices are coordinated with the BIH to the order of 1 0  us - 
100 times better than required by pertinent CCIR recommen- 
dat ions. 
T/F has become a very active field during the last 10 years, 
due largely to the availability of commercial atomic clocks. 
Information is exchanged at a number of regular confer- 
ences, including the following: 
1. Annual Frequency Control Symposium*, Atlantic City 
2. CPEM, next meeting June 1976, Boulder (NBS-IJRSI-IEEE, 
3 .  PTTI Planning Meeting*, Washington, December (annu- 
4 .  URSI Commission 1 (National Meetings). 
(U.S.  Army), May. 
Conference Proceedings in IEEE Trans. IN). 
ally), NASA-DoD. 
2 
5. URSI Commission 1 (General Assembly (3  years), next 
6. IAU,  Commission 31, General Assembly ( 3  years) next 
7 .  International Congress f o r  Chronometry* (5 years), 
in Lima, August 1975). 
Grenoble, August 1976. 
last September 1974. 
*Proceedings available 
In addition there are regular training seminars by various 
groups, e.g., the NBS T/F seminars. 
Let us consider the uses o f  T/F: 
1'. UT1 (mean solar o r  sidereal time) which is related 
to angular orientation o f  the Earth is needed for navi- 
gation, space tracking and geodesy. Essentially this 
application group is concerned with the orientation of the 
Earth in space and its rotation around its axis. 
2 .  Other major uses deal directly with clock time. 
These applications come from a variety o f  time-ordered 
electronic systems: 
a. Communications Technology 
1) Time division multiplexing - channel packing, 
2 )  Reducing spread spectrum acquisition window. 
many stations on one frequency. 
b. Electronic Navigation in TOA Mode (Absolute) 
1) For improved geometry of position determina- 
2 )  For improved coverage - mixed systems. 
3 )  For integration with communication and iden- 
t ion (RHO -RHO) . 
tification systems. 
c. Metrology 
Time and frequency are by far the best control- 
lable parameters and can be used for measurement of length, 
voltage, pressure, temperature, etc. 
d. Astronomy - Space Technology. This last appli- 
cation has the most stringent requirements - fractions o f  
a microsecond over 1/2 year related to an inertial system 
(with relativity corrections for the movement o f  the Earth 
in the solar system). 
3 
These requirements f o r  precise time are being satisfied in 
a variety of ways; with time signals, publications and su- 
perpositioning of the timing capability on existing elec- 
tronic systems. 
We have available the following time information services: 
1. BIH Announcements. 
2. U.S. Naval Observatory publications, particularly 
. Time Service Announcements Series 1 through 17, 
and Almanacs (Ephemerides) (See Appendix) . 
3 .  National Bureau of Standards Time 6 Frequency 
Bulletins. 
These services refer to time as it is disseminated by the 
following systems: 
1. HF Standard T/F Signals: (WWY, CHU,  etc.). 
2. Timed electronic navigation signals: Loran-Cy 
OMEGA, Transit, and later Global Position System 
(GPS), etc. 
3 .  Wideband Communication links: Two-way. 
4 .  Portable clocks, Precise Time Reference Stations 
5.  Special systems, largely under R6D: TV, etc. 
(PTRS) . 
The HF signals provide a global capability (including a 
great number of coordinated foreign services) o f  1 ms pre- 
cision, if propagation and receiver delays (3 -5  ms depend- 
ing mainly on band width used) are taken into account. 
Item 3 is potentially the most accurate, but portable 
clocks remain our final "authority" to calibrate services. 
In greater detail, we could summarize the distribution 
capabilities as follows: 
1. HF radio time signals: 
2. Portable clocks: 
4. Loran-C/D 
5. Satellites: 
a. DSCS 
3 .  VLF-OMEGA: 
b. TACSAT 
c. TRANSIT 
d. GPS 
6 .  Television: 
Local : 
Long range: 
1 ms global 
0.5 p s  global 
1 - 3  ms phase track (Relative) 
0.5 us Northern Hemisphere 
0.1 ps "trunk line11,> 
0.5 LIS "intermediate" 
global j 2-way 
10 ns 
1 us 
4 
7 .  Microwave, laser 
8 .  Others: ( R e D ,  VLRI, power lines, etc.) 
(local) : 1 ns 
As an example o f  users who can a l s o  help in g l o b a l  timekeep- 
ing, we can mention VLBI which, as the following sketch 
shows, provides both synchronization and UT (also the polas 
coordinates, x and y). 
PRINCIPLE OF VLBI TIME DETERMINATION 
UT1 VERSUS SYNCHRONIZATION 
A USER AND CONTRIBUTOR 
Plane Parallel 
Wave Front Frsm 
3 ns Clock D i f f  
Incl. Path 
1 ms UT Error 
Redundant observations allow determination o f  time differ- 
ence, base line, UT1 and polar coordinates. 
Coordination of  Services: 
We have a system o f  international timekeeping in existence, 
coordinated by the B I H  which is located at the Paris Ob- 
servatory, and which is operated with some support from 
various international organizations. 
5 
The BIH operates under the auspices of the following organ- 
izations : 
STANDARDS -
INTERNATIONAL ORGANIZATIONS INVOLVED WITH 
STANDARD FREQUENCY AM) TIME 
1 : COMMITTEES i 
f J 
fUNEsC0 y 
I 
I 
SCIENTIFIC 
I S U P P O R T  FOR U T  1 
Abbreviations for PTTI 
BIH 
BIPM 
CCDM 
CCDS 
CGPM 
CIPM 
CCIR 
IAT 
FAGS 
IAU 
ICSU 
I TU 
SGVI I 
U.N. 
; STUDY GROUPS; 
- - - _ - - * - -  
- Bureau International de 1'Heure 
- International Bureau of Weights and Measures 
- Consultative Committee for the Definition of the 
- Consultative Committee for the Definition of the 
- General Conference of Weights and Measures 
- International Committee for Weights and Measures 
- International Radio Consultative Committee 
- International Atomic Time 
- Federation o f  Astronomical and Geophysical Services 
- International Astronomical Union 
- International Council o f  Scientific Unions 
- International Telecommunication Union 
- Study Group 7 
- United Nations 
Meter 
Second 
6 
UNESCO - United Nations Education, Scientific and Cultural 
URSI - International Union of Radio Science 
Organization 
The national organizations (time senices and/or stand- 
ards laboratories) provide basic data t o  the B I H .  The 
U.S. contributions from the U. S. Naval Observatory and 
the National Bureau of  Standards are given in detail in 
NBS Technical Note 649, "The Standards of  Time and Fre- 
quency in the USA". 
In the United States, the division of responsibilities can 
be briefly summarized as follows: 
T/F Responsibilities 
NBS USNO -
National Standard of Fre- National Standard o f  Time 
quency (Epoch, Date) 
Standard Frequency (and Control of Do9 T/F Trans- 
time) Broadcast missions 
Fundamental Research in Applied Research in Time 
T/F as Related to Clock as Related to Clock Appli- 
Time, Frequency Measure- cations, Astronomy, Geo- 
ments physics, and Navigation 
Synchronization Consultation and Management 
Consultation and Educa- lated to DoD 
t ion 
o f  PTTI Activities as Re- 
PTRS for USNO 
The i n t e r n a t i o n a l  c lock  time s c a l e s  IAT and UTC a r e  based 
on a number o f  i nd iv idua l  c locks ,  p r e s e n t l y  opera ted  by t h e  
fol lowing c o n t r i b u t o r s :  
CONTRIBUTORS TO T A I  (BIB) 
(SOURCE B I H )  
WEIGHT PER CLOCK 
89 
83 
78 
74 
56 
54 
48 
44 
37 
26 
13 
. 
4 
4 
3 
4 
24 
8 
6 
4 
3 
1 
1 
2 
2 
1 
1 
I 
- 
TOTAL, % 
% 
10 
9 
6.5 
8.2 
 - 
37 
12  
8 . 1  
5 
3 
0.7 
0 .4  
3 
4 
7 
5 
1 
2 
6 
8 
9 
10 
11 
The t a b l e  r e f l e c t s  t h e  August 1 9 7 4  s i t u a t i o n .  The Time 
Serv ices  i n  t u r n  keep t h e i r  coordinated s c a l e s  f o r  dissemi-  
n a t i o n  wi th in  about 1 0  us of t h e  B I H  by making very  small  
adjustments t o  t h e i r  s c a l e s  ( l O - I 3 ) .  The c locks  which con- 
t r i b u t e  t o  t h e  B I H  a r e  not  ad jus t ed .  
The graph T A I  - AT(i) d e p i c t s  t h e  performance of t h e  i n t e r -  
n a l  s c a l e s  o f  t h e  major c o n t r i b u t o r s  a s  der ived  f r o m  B I H  
r e p o r t s .  
8 
6 
4 0 
II II II II I t  II 
' I  I I 1  
a I I I I I I I I I I I I I I 
The 
let 
amp 
dat 
results of the timing operations are published in bul- 
ins which give actual clock differences. This is an ex- 
le from Section 3 of the BIH Bulletin, Circular D92 
ed 1974 July 4. 
COORDINATED UNIVERSAL TIME 
a. From Loran-C and Television pulses receptions 
Date 1974 
M J D  
&Y 2 May 12 May 22 
42 169 42 179 42 189 
Laboratory i UTC-uTC(i) (unit : 1 ps) 
DHI 
FOA 
IEN 
NBS 
NPL 
NRC 
OMSF 
ON 
OF 
ORB 
PTB 
RGO 
TP 
USNO 
VSL 
(Hamburg) - 1.3 
(Stockholm) + 38.5 
(Torino) - 10.5 
(Boulder) - 2.2 
(Teddington) - 36.4 
(0  t t awa) - 0.6 
(San Fernando) - 0.2 
(Neuchat el) + 20.5 
(Paris) + 1.7 
(Bruxelles) - 33.5 
(Braunschweig) + 0.3 
( H e r s  tmonceux) - 2.1 
(Praha) - 25.4 
(Den Haag) + 21.2 
(Washington) (USNO MC) + 0.3 
- 1.2 
+ 39.5 
- 11.0 
- 1.8 
- 36.8 
- 0.5 - 0.2 
+ 20.3 
+ 1.9 - 32.3 
+ 0.2 
- 2.3 
- 25.6 
+ 0.4 
+ 23.1 
- 0.G 
+ 40.A 
- 10.9 - 1.5 - 37.1 - 0.1 
- 0.3 
+ 20.2 
+ 2.0 
- 31.8 
0.0 - 2.6 - 26.5 
+ 0.5 
+ 25.0 
b. From clock transportations (un i t  : 1 us) 
From "Daily Phase Values", Series 4, No. 382, USNO 
San Fernando Naval Observatory, San Fernando, Spain: 
1974 May 16 (MJD = 42 183.3), UTC(USN0 MC) - UTC(0MSF) = 
- 1.2 2 0.1 
CONCLUSION 
PTTI technology offers capabilities which are desirable and 
useful in many modern electronic systems. With the availa- 
bility of high performance atomic clocks (cesium beam, ru- 
bidium-gas cell and hydrogen-masers), the system designer 
can allow remote stations to operate with a high degree of 
independence (e.g. the VLBI receivers need no link, only 
initial synchronization). 
10 
As always, such e x t r a  b e n e f i t s  e x t r a c t  a premium p r i c e  of 
a d d i t i o n a l  complexity and t r a i n i n g  of  ope ra to r s .  A s tand-  
a r d  t iming i n t e r f a c e  (1 pu l se  pe r  second, e t c ) ,  wi th  t h e  
l a r g e  number o f  systems which a r e  coord ina ted  with UTC, 
al lows some a d d i t i o n a l  b e n e f i t s  and/or redundancy. One 
can t h e r e f o r e  expect  an expansion o f  t h e  PTTI a c t i v i t i e s  
i n  t h e  f u t u r e .  
11 


Precise time and time interval (PTTI), an overview

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server