A system is described which uses the Tracking and Data Relay Satellite System (TDRSS) itself to compare the user satellite clock with a clock at the White Sands station that is referenced to Universal Time Coordinated (UTC). No command of the spacecraft by the system is required, and actual on-board clock corrections are made by the spacecraft control center at its discretion. Computer models were constructed using basic orbital parameters for user and TDRS satellites. With only first-order corrections and simple averaging techniques for constant clock rates, error measurement precision of better than one microsecond was obtained. More sophisticated computations should allow considerable improvement over this

Downs, R. E.

Mcintyre, J. W.

Whitworth, G.

English

NASA Technical Reports Server

CE R 
TR SA 
Gary Whitworth, J. W. McIntyre, and R. E. Downs 
The Johns Hopkins University Applied Physics Laboratory 
Laurel, Maryland 
ABSTRACT 
During the  1980's, a l l  NASA-supported low e a r t h  
o r b i t a l  spacecraft  w i l l  be supported by the  Tracking Data 
and Relay S a t e l l i t e  System (TDRSS) . User spacecraf t  w i l l  
have the  capabi l i ty  of time-tagging science and house- 
keeping data on board the  spacecraft .  
requires t h a t  the on-board t i m e  s ca l e  be r e l a t ab le  t o  a 
standard time sca le  such a s  Universal Time Coordinated 
(UTC). 
t o  compare the  user s a t e l l i t e  clock with a clock a t  the 
White Sands s t a t i o n  t h a t  i s  referenced t o  UTC. No command 
of t he  spacecraft  by the  system i s  required, and ac tua l  
on-board clock corrections a re  made by the spacecraf t  
control center a t  i t s  discret ion.  
This a c t i v i t y  
A system is described which uses the TDRSS itself 
Computer models were constructed using basic  o r b i t a l  
parameters f o r  user and TDRS s a t e l l i t e s .  
f i r s t -o rde r  corrections and simple averaging techniques 
fo r  constant clock r a t e s ,  e r ro r  measurement precis ion of 
be t t e r  than one microsecond was obtained. More sophis- 
t i ca t ed  computations should allow considerable improve- 
ment over t h i s .  
With only 
NASA supported spacecraft  i n  t he  fu ture  w i l l  require on board t i m e  
tagging and operat ional  timing requirements i n  the  microsecond and sub- 
microsecond range. Such precis ion is  not achievable using ground based 
clocks. It  can only be obtained by having high qua l i ty  clocks on board 
the  spacecraft  and a means of closely maintaining the spacecraf t  time 
sca le  t o  a universal  standard such a s  UTC. 
supported by NASA's Tracking and Data Relay S a t e l l i t e  System (TDRSS), 
which is scheduled t o  become operational in 1984. 
These spacecraf t  w i l l  be 
This paper describes a proposed system f o r  high precis ion main- 
tenance of user spacecraft  clocks which is planned t o  be a service of t he  
TDRSS. 
This timing system determines coarse and f i n e  spacecraft  clock error with 
Detailed descriptions have been given i n  an e a r l i e r  report  (1). 
39 
https://ntrs.nasa.gov/search.jsp?R=19830027082 2020-03-21T00:52:12+00:00Z
respect to UTC. Coarse error is det 
time code to one second resolution. 
in the once per second clock "tick." 
To begin, a brief description of the TDRSS and its communications 
link to user satellites is in order. 
of the TDRS system. Here the communications paths b 
ground station at White Sands New Mexico and a low e 
spacecraft via the geosynchronous Tracking and Data Relay (TDR) satellite 
are shown. 
station and user spacecraft. 
referred to as the "forward" link. 
ground station via the "return" link. 
Figure 1 shows the basic geometry 
The TDR satellite is transparent to data between ground 
The command link to the user spacecraft is 
Spacecraft telemetry returns to the 
Three main elements make up the TDRS communications system, shown in 
Figure 2. A user spacecraft is commanded by and returns telemetry to a 
Project Operations Control Center (POCC) , which is located remotely from 
the White Sands station. Command and telemetry data streams between the 
POCC and the TDRS ground station are carried over NASCOM (NASA Communi- 
cations) circuits which may involve any of several private and commercial 
communications paths including communications satellites. (The resulting 
unpredictability of time delay renders virtually impossible the precise 
monitoring of a user spacecraft clock from the POCC.) 
As the forward (command) and return (telemetry) data streams enter 
and depart the White Sands station, they are monitored for quality. 
Capability exists for passively and noninterruptively monitoring the data 
flow between any and all of the user POCC's and spacecraft. 
The TDRS ground terminal accepts command data as it enters from a 
POCC, modulates it on an S- or Ku-bank RF link, and transmits it to the 
user spacecraft via the TDR satellite. 
the spacecraft via the TDR satellite is demodulated and routed to the 
POCC . 
Likewise, telemetry returned from 
A prime function of the TDRS Bystem is to measure the radio range of 
the user spacecraft from the White Sands station via the TDR satellite. 
This is done by means of a pseudo-random noise (PRN) code which is modu- 
lated on the forward RF link along with the command data. At the user 
spacecraft, the third element of the communication system, the PRN code 
is received along with command data in a transponding receiver. 
A second PRN code, exactly like that received from the ground in 
length and chip rate but different in bit pattern, is modulated on the 
return RF link along with telemetry by the user spacecraft transponder. 
The code epoch, i.e., the defined "starting" point of the return code, is 
made to occur either simultaneously or in very precisely controlled 
synchronism with the received code epoch. 
40 
en the  re turn  code i s  received a t  t h e  i t e  Sands s t a t i  
r i p  delay and thereby an estimate of o -way range t o  t 
spacecraf t  may be determined by measuring the  delay between the  forward 
and return l i n k  code epochs. 
continuously on a user  spacecraft  during support periods,  the  forward and 
re turn  PRN codes w i l l  s t i l l  be present.  
Although ranging may not be performed 
In addition t o  a TDRS compatible transponder, a user 
have command and telemetry processing equipment. Also, s i  
cussion assumes the  presence of a r e l a t ive ly  s t ab le  o s c i l l a t o r  and clock 
on board the user  spacecraft ,  these are included i n  Figure 2. 
The ready-made PRN code provides the TDRSS with the  bu i l t - i n  capa- 
b i l i t y  f o r  prec ise  time monitoring of a user  spacecraft .  
r a t e  of these codes ( b i t  period equal t o  approximately 325 nanoseconds) 
provide the wide frequency bandwidth required t o  observe small increments 
of time. 
the  ground and spacecraft  receivers, provide the  high signal-to-noise 
r a t i o s  required t o  make precise  measurements. 
The high b i t  
The high autocorrelat ion function of the  codes, performed by 
The basic  time e r ro r  calculat ion t o  be performed i s  t h a t  typ ica l  of 
a l l  PRN time t r ans fe r  systems. 
involved, i n  an oversimplified form. 
t i c k  i s  indicated a s  the  cha rac t e r i s t i c  sho r t  pulse. 
mark of the user spacecraft  clock, with e r r o r  E ,  i s  shown as  the t r ans i -  
t i o n  of the 2O (i .e. ,  un i t )  second b i t  i n  the  spacecraft  p a r a l l e l  t i m e  
code. Measurements a t  both ground and spacecraf t  begin with a clock one 
second mark and end with a code epoch. Repeating code epochs a re  repre- 
sented by a series of short  pulses ,  one PRN chip wide. 
code period i s  approximately 85 milliseconds. 
Figure 3 shows the timing relat ionships  
The ground clock once per  second 
The once per second 
The primary TDRSS 
The time in t e rva l  between the  ground clock one second t i c k  and a 
pa r t i cu la r  code epoch, indicated by the dot ,  i s  measured a s  A . The 
dotted epoch propagates t o  t h e  user spacecraft  via  the  forwara l i nk  and 
i t s  time of a r r i v a l  measured a s  A . The coherently transponded code 
epoch i s  transported t o  the  g r o d  v ia  the  re turn  l i nk  where i t s  t i m e  of 
a r r i v a l  i s  measured a s  A 3' 
To a f i r s t  approximation, the  forward path delay tf i s  equal t o  the  
re turn  path delay tr. 
e r r o r  calculat ion t o  be 
This approximation permits the simplest  clock f i n e  
2 
41 
I t  can be seen t h a t  there  m i x t  be timing system e 
the spacecraft  and a t  the ground s t a t i o n  i n  order t o  p 
ments and calculat ions.  
communications system as  shown i n  Figure 4, 
ground s t a t ion  Time Transfer Unit (TTU) w i l l  have access t o  the  re turn  
telemetry streain by way of t he  data qua l i ty  monitoring equipment. 
with the  l a t t e r  equipment, t he  TTU only passively obse 
is  pa r t i cu la r ly  important t o  note t h a t  the  TTU a t  no t 
or takes control  of t he  command data .  
This equipment f i t s  i n t o  the 
The minicomputer-controlled 
As 
To minimize the  burden t o  the  spacecraft ,  a l l  mathematical manipula- 
t i ons  a re  performed i n  the ground s t a t ion  TTU. 
required only t o  measurea 
necessary t o  compensate f o r  r e a l  world complications not  included i n  t h e  
simple model shown (such a s  how t o  ident i fy  the  dotted epoch). 
a l l y ,  the spacecraf t  equipment must provide these measurements and log ic  
r e s u l t s  t o  the  spacecraf t  telemetry system. 
In addition t o  the  telemetry data,  t he  ground TTU a l so  receives the  
The spacecraf t  TTU is  
and perform ce r t a in  other log ic  operations 2 
Addition- 
ground s t a t i o n  transmitted and received code epochs re la ted  t o  the space- 
c r a f t  whose clock i s  being monitored. 
a r e  precision d i g i t a l  s ignals .  
p rec ise ly  known t o  within a predictable  tolerance within the  TTU. 
TTU uses these epochs t o  ul t imately determineAl andA3. 
corrects  fo r  known biases  throughout the e n t i r e  system, and determines 
the  e r r o r  between the  spacecraft  clock and the  ground s t a t i o n  clock which 
i s  maintained with respect t o  UTC. This operation can be performed a s  
frequently as  once per  second. Averaging and curve f i t t i n g  techniques 
a r e  then used on a set of such data t o  r e f ine  the  determination of clock 
e r r o r  and clock e r r o r  ra tes .  Error predict ion calculat ions can a l so  be 
performed. This information i s  then t i m e  tagged and sen t  t o  the space- 
c r a f t  POCC over NASCOM l ines  by the  TTU. A t  i t s  d iscre t ion ,  the  POCC may 
simply note t h e  clock e r ro r  o r  send correct ive commands t o  the  spacecraf t  
immediately o r  a t  some future  t i m e .  
These code epochs a t  t h i s  po in t  
The 
The delays between the  TDRS and TTU a r e  
The ground TTU then unravels the  p rac t i ca l  complications involved, 
We now examine the  two TTU's i n  functional form. The ground s t a t i o n  
TTU, Figure 5, i s  under operational control of the Computer and 
Controller t o  which a l l  hardware i s  connected by a standard data and 
control  bus. 
v i a  NASCOM. 
a l l  spacecraft ,  which may vary widely i n  b i t  r a t e ,  frame length and 
format. 
enabling the spacecraf t  TTU. 
A bus extender provides f o r  communications with the  POCC 
The Telemetry and Command Monitor i s  able t o  monitor data  of 
Command data  is  monitored only t o  observe POCC commands f o r  
Gating and Counting Logic i n  the ground TTU controls both the  t i m e  
i n t e rva l  measurements and performs the necessary manipulations of timing 
42 
tion in telemetry~ to determine cloc 
station clock time code 
r and controller for determining coarse time 
The spacecraft TTU, shown in Figure 6 consists basically of Gating 
and Logic Circuitry and a holding register. 
code epoch loads the holding register with the contents of a timing 
counter driven by the spacecraft clock. The contents of the 
register, which is reset to zero by the transition of the 2* 
of the spacecraft time code constitutes a measure of A2, and this is 
entered into the telemetry stream. 
The arrival of a received 
it 
Note that the resolution of the &;.measurement depends on the 
frequency of the spacecraft oscillator. 
spacecraft time code has a resolution of only microseconds and thus is 
driven at 1/10 the oscillator frequency, while the A measurement reso- 
lution is 0.1 microsecond. If the time code and A2 ieasurement resolu- 
tions are the same, the timing counter could be eliminated and the time 
code read directly into the holding register. 
As shown in Figure 6 the 
It is necessary that the spacecraft time code for unit ( 2 O )  seconds 
and above be entered into telemetry to monitor coarse time. 
In general, the code epoch timing, telemetry data and frame rates, 
and the clock one second tick will be asynchronous. 
has been designed with this in mind, but this in no way precludes the use 
of synchronous timing aboard the spacecraft. 
The timing system 
Also of course, the real world is not as simple as the pictures just 
discussed. 
return link code and telemetry data. 
ence marks in the telemetry are used to identify the particular epoch 
(i.e., the dotted epoch of Figure 3) to which A2 was measured. Differ- 
ential delay occurs at the ground station due to delays in decoding the 
telemetry which has been convolutionally encoded on the spacecraft (a 
TDRSS requirement). This differential delay can confuse the ground TTU 
unless a sufficient number of logic cues are provided in the telemetry. 
Supplying these cues is the major function of the spacecraft Gating and 
Logic circuitry. 
One of the major complications is differential delay between 
This problem arises because refer- 
A second major complication is doppler frequency shift which 
necessitates modification of the basic propagation time calculation. 
also alters the period of the PRN code, T , which is important in the 
resolution of range ambiguity caused by t6e short code length. These 
complications are all worked out by the ground based TTU. 
It 
Without exploring further the equipment or processing involve3, the 
precision to be expected from this system is now considered. 
pected that all TDRSS equipment and transmission delays will be known to 
It is ex- 
43 
within a tolerance su f f i c i en t  t o  meet the specif ied 
of 250 nanoseconds.. User spacecraf t  delays should b a1  
tolerance.  
w i l l  vary from one service i n t e r v a l  t o  the  next due t o  equipment switch- 
ing and spacecraft  location. 
These uncer ta in t ies  w i l l  r e s u l t  i n  systematic e r ro r s  which 
Random e r ro r s  w i l l  be caused primarily by RF system noise-induced 
j i t t e r  on the code epochs. 
by t h e  averaging of many measurements, i s  estimated t o  be a few percent 
of the  code b i t  period of 325 nanoseconds. 
This e r ro r ,  which w i l l  generally be f i l t e r e d  
The remaining e r ro r s  a r e  those caused by the  degree of refinement of 
t he  measurements and calculat ions.  To obtain information on the  perform- 
ance t o  be expected, a computer simulation was performed. I n  t h i s  simu- 
l a t i o n ,  asynchronous counters were run t o  simulate ground and spacecraf t  
clocks, code period, and telemetry frame ra t e .  Also, delay parameters 
which mimic the  d i f f e r e n t i a l  delays were incorporated. Time in t e rva l  
measurements were made t o  one-quarter microsecond resolution. The posi- 
t i ons  of the White Sands s t a t ion ,  the TDR S a t e l l i t e ,  and the  user space- 
c r a f t  as  a function of  time were simulated by describing the  s a t e l l i t e  
o r b i t s  with standard Kepler parameters i n  an ea r th  centered i n e r t i a l  
coordinate system. 
s l i g h t l y  oblate spheroid with a ro ta t ion  period equal t o  one s iderea l  
day. 
The ground s t a t i o n  was modeled a s  a f ixed point on a 
Two sets of r e su l t s  of t h i s  simulation, which used only simple f i r s t  
order calculat ions t o  determine clock e r r o r  a re  presented here. 
f i r s t ,  Figure 7 shows the r e s u l t s  when the  user clock r a t e  was set t o  
zero, and the e r r o r  estimate calculated by averaging a set of individual 
measurements over a 10 second in t e rva l  (10 individual measurements). On 
the  l e f t  a re  shown the  r e su l t s  with no correct ion made f o r  doppler s h i f t  
f o r  cases of user  spacecraft  approaching and re t rea t ing  from the  TDR 
s a t e l l i t e  a t  maximum doppler s h i f t ,  and f o r  minimum doppler s h i f t .  
dot i s  the average e r r o r  and the  extremes a r e  maximum excursions of 
individual measurements. 
The 
The 
The r e su l t s  on the  r igh t  a r e  fo r  the  same conditions but with a 
simple f i r s t -order  doppler correct ion added. 
obtained from successive differences i n  the  round t r i p  range estimates 
(A - A ). Maximum deviations with doppler corrections a r e  seen t o  be 
wighin 6 . 1  microseconds. 
The doppler correction was 
The second simulation r e s u l t  i s  shown i n  Figure 1. 
clock r a t e  was set t o  a f ixed r a t e  with some i n i t i a l  e r r o r ,  and the  capa- 
b i l i t y  fo r  e r r o r  predict ion examined. 
two small sets of individual e r r o r  measurements separated by minutes o r  
hours. 
5 minutes pas t  t he  l a s t  data point .  Data a r e  shown f o r  r e l a t ive ly  high 
Here the user  
The simulation program gathered 
A l i n e  f i t  t o  t h i s  data was made and t h e  t i m e  e r r o r  projected t o  
44 
and low ra tes  of t he  user clock. 
a r e  obtained when more time i s  taken t o  determine the smaller ra te .  
I t  may be seen tha t  comparable results 
The r e su l t s  ind ica te  t h a t  the  system w i l l  be able t o  pred ic t  
short-term clock e r r o r  t o  within one microsecond. 
procedure changes, more sophis t icated computations should permit even 
b e t t e r  performance. 
With no measurement 
Although the  timing system is primarily intended t o  service ear  
Indeed time t ransfer  t o  fixed locat ions using t h i s  system 
o r b i t a l  spacecraft ,  service t o  surface o r  airborne vehicles i s  also 
possible.  
could provide precis ion comparable t o  r e su l t s  achieved i n  e a r l i e r  
ground-to-ground PRN timing experiments (2). One such appl icat ion is  the  
maintenance of t h e  ground s t a t i o n  clock i t s e l f  by l inks through the TDRSS 
with USNO or  NBS. 
This work was sponsored by NASA Goddhrd Space F l ight  Center. 
45 
1. McIntyre, J.W., G. G. Whitworth, and R. E. Downs, Time Correction 
2. Chi, A.R. and E. Byron, 
Synchronous Satellite. 
Time and Time Interval Planning Meeting, pp. 357-377. 
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Time maintenance of user clocks via the tracking and data relay satellite system

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