position data in the time code to be updated with a shorter delay after a
satellite maneuver. However, the NOAA ephemeris files accessible to NBS apply
only to satellites being used for the visual imaging operations. In some
cases, when the visual imaging and time code dissemination functions are
performed by different GOES satellites, NBS must rely on its own position
predictions generated on the NBS computer system from the satellite orbital
elements.

E. NBS/Boulder

NBS/Boulder serves as the central control facility for the GOES time code
operations. In addition, this site is responsible for a variety of other
functions including overall system monitoring, communications, coordination
with the various NOAA groups, documentation of system performance, software
and hardware development and maintenance, position data generation, user
support, and the archiving of performance data.

Figure 3 is a block diagram showing the principal subsystems used at
NBS/Boulder in the time code operations. The monitoring subsystem includes a
minimum of six time code receivers, redundant data loggers, and microcomputer
hardware and software for analyzing, plotting, and archiving the satellite
data. The GOES/East and GOES/West time codes are each monitored continuously
by a set of three receivers. One of these is a version that receives the raw
time code without making use of the satellite position data to compensate for
path delay variations. The other two are more sophisticated versions that
function as primary and backup units to receive the code and use the satellite
position information to compensate for the path delays. The receiver outputs
are measured against a direct feed from the NBS atomic time scale system which
provides a direct measurement of the difference between the received signal
and UTC(NBS). As an aid in assessing overall system performance, the
corrected (for path delay) and uncorrected receiver outputs for both the East
and West satellites are plotted continuously on strip chart recorders as well
as being stored in electronic form. The monitoring system also includes an
oscilloscope display of the received 100 Hz clock waveforms which are
sensitive indicators of the current status of local interfering signals
generated by nearby land-mobile radio communications. A specially modified
time code decoder is alco used in the monitoring system to decode and display
the other information in the time code transmissions - e.g., the accuracy
indicators, daylight-saving-time status, leap-second status, DUTI value,
on-line TCG identifiers, and Wallops Island equipment alarm indicators.
Finally, the monitoring subsystems in Boulder include the capability to
monitor the status of a duplicate set of time code generators and data loggers
identical to that in use at Wallops Island. These units are maintained as
spares for the Wallops Island subsystems and, in addition, can serve as
testbeds for software/hardware upgrades or modifications before such changes
are implemented in the operational systems.

The NBS/Boulder facility also includes the necessary subsystem components for
producing the satellite position predictions in a form compatible with the
Wallops Island equipment. The primary computer processing is performed on a
large mainframe computer system running a version of NASA's Goddard Trajectory
Determination System (GTDS) orbit determination program. The eritical input

- 3.5 -


¢ sanity

SAYO 30 YSEWnNn
22st BQc1 O26 229 BBE B
T T —T T TT T

|
i

|

La Mi ak ad 7

1

9861/S1/70-2861/1B/18
(d3L34ed09) LSV3/S309-I1N

Q0e-

BB1-

Oot

22

SONOISSOYIIW

~ 5.9


4)

5)

6)

better. Also, for a period of about one year we had regular
opportunities to compare the time code performance when two
different versions of GIDS were used to generate the path delay
corrections. Although the two versions (one at NBS/Boulder and the
other at NOAA/Suitland) were obviously very similar in many ways,
they had been independently adapted for different mainframe
computers and operating systems. Generally, the NOAA/Suitland
results seemed to show somewhat lower diurnal variations, but the
actual computed path delays agreed to within at least 20
microseconds. Finally, our observations over many years show no
long-term systematic drifts in the values of GOES time code versus
UTC(NBS), suggesting that any errors being introduced into the path
delay computation process are at least not time dependent in any
significant way.

Transponder Delays. The time code signals are transmitted to the
GOES satellites from Wallops Island at S-band, changed in frequency
to the downlink frequency of about 469 MHz, and retransmitted to the
earth. The processing of the signals through the satellite
transponders and antenna systems results in some additional time
delay. Although the transponder delays are known from pre-launch
measurements on the ground and could, in principle, be treated as a
separate correction to be applied to the received data, it is
generally easier and more practical to consider them as part of the
overall equipment delays which can be estimated by comparing the
received time code initially with a known timing reference at the
user's site. Experience indicates that such delays can be
considered as a constant correction over long periods without
introducing significant errors.

Ionospheric/Tropospheric Effects. As the time code signals pass
through the ionosphere and atmosphere they are subjected to various
perturbations relative to a totally free-space path. These effects
have been thoroughly studied and documented in the radio propagation
literature. For the frequency range of interest, the typical paths
used, and the overall accuracy level of interest with the GOES time
code, these effects can be considered negligible.

Receiver Delays. The receiving equipment commonly used with the GOES
time code signals introduces additional delays into the signal path.
These delays are usually measured for each receiver at time of
manufacture and compensated for by setting switches in an
appropriate delay-compensation circuit. Provisions and measurement.
instructions are usually included with the receiver to permit the
user to check, and if necessary, readjust the delay compensation.
Comparisons made on many different receivers at NBS suggest that the
uncertainties associated with the manufacturers’ delay
determinations may be as high as 30 microseconds and are also
somewhat dependent on received signal strength and ambient
temperature. For most applications these delays may be treated as
constant in time.


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

7. FUTURE PROJECTIONS

A. Projections for the GOES Satellite System

As of October 1987 the future of the GOES satellite system appears to be
secure until at least the mid-1990’s. This assessment is based on several
current factors. First, NOAA has announced launch schedules and procurement
actions that will provide replacement satellites throughout that time period.
The present GOES/East and GOES/West satellites are relatively recent versions.

NOAA has recently issued a final contract for the design and construction of
the next group of three GOES satellites, which will be GOES-8, 9, and 10.
Those satellites are scheduled to provide the operational services during the
1990-1995 period. Thus, from the point of view of the space segment NOAA has
made a definitive commitment to continue the system for at least 8-10 more
years.

A second indicator of NOAA's intent to continue the system is the current
upgrade of the ground segment as it applies to the data collection system that
also carries the NBS time code. Approximately, $5,000,000 is being invested
in a major upgrade of the ground support facilities to enable the system to
handle data from many more data collection platforms. The system currently is
near saturation with more than 3000 data collection platforms in operation and
a significant expansion in capacity has been proposed, justified, and funded.
Although this upgrade does not directly impact our time code operations, it is
a further indication that the overall GOES system is viewed as a valuable,
long-term national resource.

B. Status of Forma IBS /NOAA ‘reement:

The joint effort involving NBS and NOAA to provide the NBS time code is
formalized by a Memorandum-of-Agreement between the two organizations. It was
originated in 1977 and signed by Dr. Ernest Ambler for NBS and Mr. David
Johnson for NOAA. The agreement sets out the general responsibilities of each
organization and states that the agreement remains in effect for a five or ten
year period that can be extended with the agreement of both parties. The
Memorandum-of-Agreement was so extended in 1982 for a five year period. In
August 1987 it was again renewed by both organizations, this time for a ten
year period until 1997.

Cc. NOAA Satellite Tracking Operations

Earlier in this report it was mentioned that NOAA revised its satellite
tracking procedures about two years ago with some apparently negative impact
on the NBS time code operations. In discontinuing the original trilateration
technique and converting to a less precise angle-tracking method, NOAA has not
been able to provide the same consistent high quality of their orbital

- 7.1 -


fraction of a second. In addition, the code also gives certain other related
information such as indicators for standard or daylight saving time, leap
years, the difference between UTC and the astronomical time scale UTI,
indicators for the current accuracy of the transmitted time signals, and
system diagnostic indicators. Full details on these features of the time code
format are made available to receiver manufacturers so that users can have
access to the complete information via the commercial receiver versions.

B. Link to the NBS Primary Standards and Time Scales

The GOES time code is designed and operated in such a way that the actual
received time signal at the user’s location is directly related to the primary
UTC(NBS) atomic time scale maintained by the NBS Time and Frequency Division
in Boulder, CO. Since this UTC(NBS) time scale is based on the primary NBS
Frequency Standard, users with access to the GOES time code essentially have
access to the primary NBS standards, though, of course, with appropriate
accuracy degradations due to the links to the users that are involved.

Since NBS does not own or control the overall GOES satellite system, there are
a number of constraints placed upon how the links to the users are implemented
and operated to minimize accuracy degradations. For example, NBS cannot
transmit its time signals directly from the Boulder, CO, location of the
primary standards to the satellites, but rather must perform this operation
from NOAA'S Satellite Control Facility at Wallops Island, VA. As a result the
time code system must be designed so that, effectively, a version of UTC(NBS)
can be established and accurately maintained at Wallops Island. Details of
how this is accomplished will be discussed later in this document, but the
basic method used is to maintain a set of independent atomic frequency
standards and clocks at Wallops Island and then to periodically measure them
in terms of UTC(NBS) and adjust their outputs accordingly to remain within
certain limits relative to UTC(NBS). Similarly, NBS is dependent on NOAA for
the basic satellite tracking data used to generate predictions of satellite
position for path delay compensation. NBS has no control over the quality of
the satellite tracking data, but continuously monitors timing errors in the
received time code and attempts to keep users informed when system errors are
larger than normal.

Based on many years of continuous monitoring of the GOES time code
performance, we can conclude that: (1) the time signals generated at Wallops
Island by the NBS equipment can be kept within 10 microseconds of UTC(NBS) at
all times; (2) the received time code can be used anywhere within the
hemispheric coverage area at an accuracy level of about 1 millisecond by
making only a constant correction to all data for the approximate geographical
location of the receiver; and (3) the received time code can usually provide
better than 10U-microsecond accuracy with respect to UTC(NBS) by using the
satellite position data to compensate for path delay variations.

C. Reliability and Performance Verification Aspects

NBS has designed the GOES time code dissemination system with the point of
view that the service should be essentially as reliable as the other NBS time
and frequency dissemination services, such as WWV, WWVH, and WWVB. In

- 2.2 -


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

4. OPERATIONAL PROCEDURES

From the user's point of view the GOES satellite time code can almost be
considered as a utility service that is conveniently and continuously
available over a wide geographical area without the need for detailed
knowledge about how the service is provided. The procurement and simple
installation of a suitable GOES time code receiver provides NBS referenced
time at the user’s site to within the published specifications with the
complex support operations and procedures conducted by NBS and NOAA being,
largely transparent at the point of use. In this section the supporting
operations that are critical to insuring that the delivered timing signals
remain within established limits will be briefly described. The operational
procedures to be described are grouped into several major categories
involving: (1) updating of the satellite position data; (2) verification of
the time and frequency performance of the Wallops Island equipment and the
related adjustments; (3) updating and verification of other data stored in the
Wallops Island systems and included in the time code transmissions; (4)
monitoring operations at NBS/Boulder; (5) monitoring of the GOES time code
performance at other sites; and (6) user notification channels.

In performing the various operational procedures involved with the GOES time
code, many different hardware and software subsystems are used, including
large mainframe computers at NBS/Boulder and at NBS/Gaithersburg,
microcomputer systems at Boulder and Ft. Collins, CO, commercial communication
software packages for the microcomputers, numerous items of commercial and
highly specialized NBS-built time and frequency equipment, a complex
satellite-orbit-determination computer program originally developed by NASA,
numerous special-purpose computer programs developed by NBS, and other
hardware and software systems owned by organizations such as NOAA and the U.S.
Naval Observatory. Since it is clearly impractical to describe in detail how
all this hardware and software operates, only a general description of the
various procedures used to support the service is given.

1. Procedures for Updating the Satellite Position Data

1.1. Generation of Updated Satellite Position Data

The procedure involves obtaining the most recent available satellite
orbital elements from NOAA, entering this and certain other
information as input into the GTDS orbit determination program,
running the GTDS program on a mainframe computer, and downloading
the resulting position data file to a local microcomputer system.
The sets of orbital elements for each of the two operational GOES

- 4


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

6. INTERNAL QUALITY CONTROL

A. Monitoring at Wallops Island for Internal Quality Control

In order to insure that the operating cesium standards and their associated
time code generator outputs remain within a few microseconds of UTC(NBS) at
all times, the 1 Hz outputs of the time code generators are monitored with
respect to both Loran-C and the GPS satellite signals. For Loran-C a
commercial timing receiver is used and the differences between each time code
generator and an appropriate Loran-C signal from the receiver are measured
once each hour and stored in the Wallops Island data logger system.
Measurement resolution is a small fraction of 1 microsecond. Since the Loran-
C delay was previously calibrated by portable clock comparisons and since
Loran-C system time is accurately known with respect to UTC(NBS) at all times,
these data provide a reliable link from the time code generators to UTC(NBS).
Even though the various Loran-C transmissions may depart from UTC(NBS) by a
microsecond or so on occasion, they are constrained through their control
links to UTC(USNO) such that any difference is bounded. Thus, a measurement
of the NBS Lime code generators versus Loran-C can generally be relied upon to
within at least a few microseconds. One operational difficulty, however, is
that re-establishing proper operation of a Loran-C receiver after a
malfunction is a complex task, not easily performed by the NOAA Wallops Island
staff, who are not time and frequency specialists. The result is that once
the Loran-C receiver becomes disabled for any reason, it may remain out of
service for some time before someone who can set it up again for proper
operation becomes available.

In recognition of the potential difficulties with Loran-C, NBS established
during 1985 an alternative monitoring system based on reception of the timing
signals from the Global Positioning Satellite System. Since the GPS timing
receiver was designed and constructed by NBS, there are ample servicing and
replacement possibilities. The GPS receiver is fully automatic and operates
for many months without attention. The receiver measures nominally ten GPS
satellite passes each day and for each one stores a 13-minute average of the
time difference between the GPS signal and the local time reference which is
essentially the 1 Hz output from the on-line time code generator. The time
differences typically have associated with them a standard deviation of 5-10
nanoseconds. Each GPS satellite timing signal contains information providing
the predicted difference between that signal and GPS system time, which is
known in terms of UTC(NBS) to within an accuracy of about 50 nanoseconds.
Thus, this monitoring operation provides a convenient way to keep track of the
time code generator outputs relative to UTC(NBS) to an accuracy that is much
better than that of the overall GOES timing system.

The time code generator measurements in terms of Loran-C and GPS are accessed
from Boulder and Ft. Collins several times per week using the normal dial-up
data link to Wallops Island. Adjustments are made as necessary to keep the

- 6.1 -


The new publications will present more technical detail than the
information that can be included in NBS Reports of Calibration.
In general they will also provide more detail than past
publications in the scientific and technical literature; such
publications, when they exist, tend to focus upon a particular
element of the topic and related elements may have been published
in different places at different times. The new series will
integrate the description of NBS calibration technologies in a
form that is more readily accessible and more useful to the
technical user.

The present publication, SP 250-30, NBS Measurement Services:
GOES Satellite Time Code Dissemination: Description and
Operation, by Bechler, Davis, and Milton, is one of approximately
20 documents in the new series published or in preparation by the
Center for Basic Standards. It describes calibration technology
and procedures utilized in connection with NBS Service
Identizication Numbers from 53110 to 53150 listed in the NBS
Calibration Services Users Guide. Inquiries concerning the
contents of these documents may be directed to the author or to
one of the technical contact persons identified in the Users
Guide (SP-250).

Suggestions for improving the effectiveness and usefulness of the
new series would be very much appreciated at NBS. Likewise,
suggestions concerning the need for new calibration services,
special tests and measurement assurance programs are always
welcome.

Joe Simmons, Acting Chief
Office of Physical Measurement Services

Helmut Hellwig, Acting Director
Center for Basic Standards

iv


effect of these maneuvers on timekeeping capabilities is to place
bounds on how far the path lengths and the corresponding variations
in path time delays can deviate from some mean value associated with
the nominal, assigned orbital location.

NBS advances the time codes at the Wallops Island origination point
by exactly 260 milliseconds (the mean path delay), so that the
uncorrected time code arrives at any point within the coverage area
on the earth with a maximum error of 16 milliseconds. If the user
is willing and able to compute, or otherwise determine, the
appropriate path delay for his particular geographical location and
apply this as a fixed correction to all received data, the only
remaining errors are due to the variations in the received signals
due to variations in the satellite position. The magnitude of the
changes in the satellite position depends largely on the exact orbit
of the satellite and, in particular, on the orbit inclination.

For typical GOES/East and GOES/West satellites, operating under
normal conditions, many years of monitoring have shown that time
code variations over periods of one day to several years rarely
exceed 1 millisecond due to satellite motion. However, under more
unusual circumstances, for example, when a satellite failure causes
the time code transmissions to be shifted to an in-orbit spare
satellite, larger variations may be produced. This may happen when
the spare satellite has a larger-than-normal orbital inclination or
when the orbit is not as well determined for some reason.

Experience has shown that, in the worst cases, the orbit inclination
may reach 2-3 degrees, resulting in time code path delay variations
of between 2 and 3 milliseconds. Such occurrences, though rare, do
happen occasionally and, unfortunately, are not predictable or
controllable by NBS. Time code shifts larger than the typical 1-2
millisecond range may also be produced during eclipse periods during
the Spring and Fall seasons each year, when time code operations are
sometimes transferred to a spare satellite that is located in an
entirely different orbital position for two hours each day. When
this operational procedure is followed by NOAA, the result may be
large changes of many milliseconds in the path delay for the
affected time codes during the two-hour periods each day. However,
because the shift is so large and occurs on a known schedule, it can
easily be identified and accounted for by the users. Beginning with
the launch of new-generation GOES satellites in 1987, this eclipse
effect should be eliminated, since the newer, higher-powered
satellites are designed to maintain normal operations throughout the
eclipse periods without requiring shifts of operations to spare
satellites. In the Fall 1987 eclipse season, for example, no time
code shifts to other satellites were necessary.

For those users needing accuracy better than about 1 millisecond,
NBS provides the means to make automatic compensations for any path
delay variations. The procedure, as discussed previously, entails
using the satellite position data transmitted along with the time
information to compute the magnitude of the path delay to the user's

-5.3-


€ aunbly

YN

dn-1vid SNOHda13aL

- 3.6 -

YOLINOW WV3YLS
11a 3009 3WIL SWAdOW a1vos 3WiL
(S@N) OLA
T 21a
o-
YOLINOW ale!
(a31044ux09) FONSYSSUIINI (a3.L0444O9)
YH3AI303Y PY ] Ih HSAIRZOSH 9901
dnyova dnyova
1 a DOL
(Q31L04Y4xOO) 439901 (d3.1L04yuuOO)
YaAIZ0aY viv Y3AIZ0aY YN v DOL
dan-wid SHOLINOW
ANOHdSTSL Wu 1V
(aa LOSYYOONN) 3040034] |YaqHOOaY (d3LOSYHOONN) 4
Y3AAIZO3Y LYVHO IHVWHO YAAISO3Y SW3GOW
isva/Si09 1SaMWS309 WALSAS LNAWdINOA
GNV1SI SdOTIVM SYvdS
W3LSAS WNNALNV

PPP

WALSAS ONIYOLINOW 3009 AWIL S309 HAG TINOG/SEN


allow frequency or rate adjustments to the basic standards. This is
accomplished indirectly in the time code generators by a provision
for adding or subtracting a selected number of 0.2 microsecond steps
per day, thus effectively adjusting the output frequency of the
cesium standard. The procedure simply requires an appropriate
command to be issued from Boulder or Ft. Collins by the
microcomputer system running in a terminal mode that modifies the
"rate word" stored for each of the independent TCG's.

Rate adjustments are normally made very infrequently. Many years of
experience has shown that, once the rate compensation is correctly
determined and implemented in the system, it generally is
unnecessary to revise it more often than about once per year.

During the interim periods, of course, occasional small time
adjustments are made to the 1-Hz outputs as described above to
maintain all reference signals within acceptable tolerances of a few
microseconds with respect to UTC(NBS).

Procedures for Updating and Verifying Other Information in the Wallops
Island System

The transmitted time codes on the GOES/East and GOES/West satellites
contain, in addition to the time and position information, other related
information for the benefit of time code users. This includes indicators
of the current difference between the UT1 and UTC time scales, impending
leap seconds, current accuracies of the timing information, daylight
savings time periods, and the current year. This information must be
manually loaded into the appropriate memory locations in the time code
generators from Boulder and revised as necessary. Other information must
also be loaded which provides default satellite position data to be
broadcast under certain conditions, such as when the satellites are in
eclipse, and which performs certain system configuration functions on the
time code generators and data loggers.

In each case the appropriate data are loaded into the Wallops Island
equipment by sending a command word from the local microcomputer system
that specifies the number of bytes to be loaded, the value of the data
bytes, the memory location where the data are to be stored, and a
destination indicator (particular time code generator or data logger).
Similarly, commands can be sent that cause the Wallops Island system to
respond with the values of the data currently stored in any memory
location.

In some cases the data become effective immediately when loaded; for
example, a new value for the system accuracy is reflected immediately in
the transmitted time code. In other cases the loaded data arms an
internal process which results in the revised values becoming effective

- 4.4 -


system has been established at the NBS WWV/WWVB radio station site near Ft.
Collins, CO under the control of the Engineer-in-Charge. The facility
includes a dedicated microcomputer system, communication links and software,
monitoring receivers for both the East and West satellites, and data storage.
This site performs all necessary functions in the absence of Boulder
personnel, in case of Boulder equipment malfunctions, and routinely on a
reasonably periodic basis to maintain skills and to verify proper operation of
the backup equipment and procedures. The Ft. Collins site is set up to
receive all GOES-related communications from NOAA (via Telex) independently of
Boulder.

G. Auxiliary Monitoring Sites in Washington, DC, and Kauai, HI

Because the position data transmitted in the time code may produce path delay
corrections that depend on geographical location in unanticipated ways,
independent time code monitoring facilities are maintained outside the
Boulder/Ft. Collins area. The GOES/East satellite is monitored independently
at the U.S. Naval Observatory in Washington, DC. A GOES time code receiver
operates continuously there and outputs its hourly measurements in terms of
the UTC(USNO) time scale to a file in the USNO computer system. These data
are then accessed from Boulder once per week and transferred to the NBS
archival files. Comparisons of these measurements with the corresponding data
from the Boulder systems are useful in detecting degradations in the quality
of the timing information as received, especially, during periods when the
satellite orbital elements are of lower quality than normal. In addition to
this monitoring role, as has been previously mentioned, the USNO facility is
used to house a computer file of current GOES system status information and
relevant announcements intended for users of the time code. This status file
is accessible to any user free of charge by simply dialing up the USNO system
with a suitable terminal and requesting the NBS file. The information in the
file is updated as needed remotely from Boulder.

For similar reasons an independent monitoring capability for GOES/West is
maintained at NBS radio station WWVH located in Kauai, HI. This receiver's
data are stored on floppy disks which are mailed to Boulder at two-week
intervals. The data are then transferred into the archival files using a
specially developed formatting program. The WWVH GOES measurements are
referenced to the local WWVH time scale system which is kept synchronized to
within five microseconds of UTC(NBS) at all times via higher-accuracy GPS
satellite comparisons with Boulder and by occasional portable clock
comparisons with other standards.

- 3.8 -


site. Since the position data are updated once each minute,
uncertainties associated with the path delay computation can be kept
relatively low. The achievable accuracy, of course, depends on the
quality of the satellite position information, which in turn depends
mainly on the quality of the satellite tracking data supplied to NBS
by NOAA. Once again, this critical component of the overall time
code system uncertainty is not under the control of NBS.
Furthermore, experience has shown that the quality of the tracking
data and resulting orbit determination uncertainties can vary from
time to time in unpredictable ways. Generally, however, such
problems do not cause time code variations exceeding 100
microseconds. Under some conditions, such as at the time of
satellite maneuvers, one expects that the resulting orbital
determinations may have larger-than-normal associated uncertainties
for a few days until new tracking data can be accumulated and
processed. The problem has not proved to be too severe, however,
since NOAA routinely issues sets of predicted post-maneuver orbital
elements prior to each maneuver and these are usually of sufficient
quality that the resulting position predictions after the maneuver
remain within our normal 100-microsecond limits. In unusual cases,
when the actual maneuver differs significantly from that projected,
the transmitted position data after the maneuver may produce path
delay computation errors of several hundred microseconds; these may
persist for up to one week until a revised orbit determination based
on actual observations becomes available to NBS. To minimize the
impact on users at such times NBS flags the abnormal-accuracy
condition by changing an appropriate bit in the time code data
stream. Some receivers are designed to detect this bit and alert
the user of the degraded system accuracy and to display the
approximate magnitude of the maximum error in the time code.

Even if the satellite orbit data contained no errors, some
uncertainties would be introduced by any imperfections in the large,
sophisticated GTDS computer program used to generate the satellite
position predictions. Although the GIDS program attempts to
incorporate the complex perturbing effects of the moon, sun,
planets, and other forces on the earth satellite, this cannot be
done perfectly for such a complex physical system. The
documentation for this program requires several large volumes and
the analysis of potential errors is well beyond the scope of this
discussion[3]. However, there are some observations that shed some
light on the level of possible errors introduced by GTDS. We
consistently observe a diurnal effect in the recorded measurements
of the received time code as corrected for path delay versus the
"standard," UTC(NBS). This very regular, sinusoidal variation
typically shows a peak-to-peak magnitude of 10 to 30 microseconds
during a period of one day (see the data plots discussed later in
this section). It apparently reflects some combination of
imperfections in the tracking data and the processing of these data
by the GIDS program. The fact that the diurnal variations do not
usually exceed this range gives us some confidence in the
capabilities of GIDS to perform at the 100 microsecond level or

~5.4-


Generally, the alarm circuitry is set to generate alarms if any two time code
generators differ by more than about 15 microseconds. Time adjustments can be
performed as necessary from Boulder in steps of integral multiples of 0.2
microseconds, The output of any of the time code generators can be selected
to provide the actual coded time information to the GOES/East and GOES/West
satellite uplink systems.

The satellite position data included along with the basic time information are
stored in each time code generator’s memory and are kept current by periodic
updates from Boulder via the dial-up link. Although the position data from
Boulder include new values only for each hour, the Wallops Island systems
interpolate between these values each minute so that the users of the received
time code can obtain path delay corrections that accurately reflect the
satellite's position at all times. The position data as transmitted include
the latitude, longitude, and a radial distance relative to a fixed reference
satellite orbit. These data, together with the known locations of the Wallops
Island origination point and the receiver, are sufficient for the computation
of the particular propagation-path delay with a resolution of less than 1
microsecond,

The Wallops Island equipment also includes: doubly redundant data loggers for
storing one month of daily measurements on each of 8 data channels and more
than 5 days worth of hourly measurements of each time code generator output.
These measurements are typically differences between the time code generators
and a received Loran-C navigation system signal. Since the Loran-C timing
signals are also monitored versus the UTC(NBS) time scale in Boulder, the
Wallops Island measurements can be used to indicate when the atomic standards
or time code generator outputs need to be adjusted in order to keep the
transmitted time code within acceptable limits relative to UTC(NBS). The
stored measurement results are periodically examined remotely from Boulder via
the dial-up link to assure proper system performance.

The Wallops Island time code generation software also contains provisions for
including other types of information in the time code as transmitted. This
includes: (1) accuracy indicators to alert users when the time code accuracy
is degraded for any reason; (2) indicators for daylight-saving-time periods;
(3) an indicator to alert users that a leap second will be inserted into
UTC(NBS) at the end of the current month; (4) position data for a spare GOES
satellite during eclipse periods when the NBS time code may be switched to the
spare satellite for two hours each day; (5) the current value of DUT1, which
gives the approximate difference between the astronomical UY1 and the UTC time
scales; (6) an identifier of the current "on-the-air" time code generators;
and (7) certain system health status indicators to alert NBS/Boulder personnel
of sclected abnormal conditions at Wallops Island.

Once the NBS time code signals are inserted into NOAA's GOES uplink equipment,
NBS has no further control over the dissemination of the time code. Clearly,
a high degree of cooperation is necessary between NOAA and NBS to insure that
the overall time code dissemination process functions smoothly and with the
performance and excellent reliability characteristic of other NBS services.
As mentioned previously this cooperative arrangement is formalized by an
NBS/NOAA Memorandum-of-Agreement initiated in 1977 and renewed periodically.

- 3.3 -


information for computing path delays; current accuracy indicators for both
the East and West time codes; an invalid-position-data indicator; a daylight-
saving-time indicator; a leap-second notification; and system status
information. The NBS equipment at Wallops Island also includes capabilities
for measuring and storing various time difference data, fault detection and
alarm circuitry, provisions for monitoring Loran-C and GPS satellite
transmissions as independent timing references, memory for storing 10 days
worth of position prediction data for each of the two operational satellites,
and modems for use with the Boulder-Wallops Island data link.

The time code is continuously transmitted from this system to the East and
West GOES satellites at S-band and is then downlinked on two slightly
different frequencies near 468 MHz in one of the meteorological satellite
allocated bands. This constraint to use meteorological satellite frequency
allocations for the GOES/East and GOES/West downlinks may result in
interference in receiving the time code transmissions in some urban areas,
since these allocations are shared with the very large and very active land-
mobile service. Furthermore, the land-mobile use is designated as the
"primary" one within the U.S. while the meteorological satellite use is
"secondary." In practical terms this means that if interference to the time
code is experienced from land-mobile transmissions, it must be tolerated.
Fortunately, time code receivers can be designed to effectively ignore much of
this type of interference when necessary. In general, the time code as
received from GOES/East is less affected by interference than that from
GOES/West because one of the land-mobile channels exactly coincides. with the
GOES/West frequency while the closest one to the GOES/East frequency is
somewhat offset.

The satellite position information included in the time code format is
generated from sophisticated orbit-prediction programs run on large computers
at NBS/Boulder or at NOAA in the Washington, DC, area. Data inputs for this
program include the satellite orbital elements which are determined from
satellite tracking data obtained by NOAA and/or NASA. The computer program
generates position predictions for each satellite for each hour during the
next ten-day period and these are further processed by the microprocessor-
based time code generation equipment at Wallops Island to generate updated
values each minute that are then encoded along with the time information.
Users then have the option to simply decode the received time information
achieving accuracies of about 1 millisecond or to also use the position data
to compute a path delay from Wallops Island to the user's location. The
position data are updated each minute to compensate for movements of the
satellites. In the latter case, timing accuracies of better than 100
microseconds can be achieved. GOES timing accuracy as transmitted from
Wallops Island is maintained to within at least 10 microseconds by continuous
monitoring relative to Loran-C and GPS satellite transmissions and by
occasional portable clock trips.

Reception of the GOES time code is possible on a continuous basis throughout
much of the Western hemisphere as shown by the coverage maps for both
satellites in Figure 1. Overlapping coverage is provided within the
continental U.S. and certain other areas. While there are also operational
satellites in the European (METEOSAT) and Japanese (GMS) regions that are part

- 1.2 -


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

5. ASSESSMENT OF UNCERTAINTIES
A. Introduction

In a one-way time dissemination system, such as the GOES satellite time code,
there are a number of error sources that can degrade the accuracy of the
timing signal as received at the user's site, even if the signal is assumed to
be exactly on time at the origination point at Wallops Island, VA. The
principal reason is that several components of the distribution system may
introduce additional time delays as the transmitted signal follows the Wallops
Island-to-GOES-satellite and satellite-to-user path. To the extent that such
additional delays are known or measurable and remain constant with time, it
may be possible to account for them without adverscly affecting the overall
performance of the system.

In the case of the GOES time code the major potential error sources include
the following:

1) Time differences between the Wallops Island reference clocks
providing the time code signals and the UTC(NBS) time scale;

2) Delays due to the electronic processing equipment at Wallops Island
that take the NBS time code signal as input and convert it into a
suitable form for transmitting to the GOES satellites;

3) The free-space signal propagation delay for the path from Wallops
Island to the user's location;

4) Additional delays introduced by the GOES satellite transponder as
the time code signal is received and retransmitted to the earth;

5) Effects on the signal as it passes through the troposphere and
ionosphere; and

6) Delays due to the user’s receiving equipment.

In Section C below each of these possible error contributions will be
discussed in more detail.

B. Quoted Uncertainties

NBS generally quotes three different accuracy capabilities for the GOES time
code technique. For users who wish to simply receive and decode the time data
without making any corrections for satellite path delay, the maximum error is
16 milliseconds, and the exact amount depends on the particular geographical
location. For users who are willing to calibrate the path delay one time by
some appropriate independent means, the time code variations will not normally
exceed 1 millisecond and the system may be used at this accuracy level over
long periods of time. For users with the most sophisticated receivers that
use the satellite position data in the time code to compute continually
updated satellite path delay corrections, the accuracy capability normally
remains within 100 microseconds and often stays within 50 microseconds for
long periods. In all modes of operation, however, the user must be aware that

-5.1-


A capability has been established to accumulate and archive
monitoring data from GOES/East as received at the USNO site as an
independent check on the GOES/East time code performance. It is
important to have access to independent monitoring data from a
different geographical location from Boulder since some time code
perturbations, such as effects due to poor quality satellite orbital
elements, are expected to be geographically dependent. The receiver
at the USNO monitors the GOES/East satellite continuously and trans-
fers its data to a file in the USNO computer system. The data are in
the form of hourly measurements of GOES/East versus UTC(USNO). The
file capacity is set to be about 10-11 days worth of the hourly
data, Data can be retrieved at any time by accessing the USNO
Automated Data Service and downloading the appropriate file. In
order to perform some error checking during the file transfer and to
store the resulting data properly in the NBS archival GOES data
files, a special downloading program has been developed.

NBS/Ft. Collins, CO, Site

Facilities are maintained at NBS radio station WWV at Ft. Collins,
CO to monitor the GOES/East and GOES/West time codes from an
additional site and to provide a real-time indication of the results
of the various control operations that are performed from Ft.
Collins in its role as a backup operational control and monitoring
center for the GOES time code.

Continuously operating time code receivers are used to monitor the
East and West codes. At present the measurements of the differences
between the received GOES satellite timing signal and the local
UTC(WWV) timing system are preserved in a data logger and in disk
files but are not archived in the Boulder data files. They serve
mainly as an indication of system performance when new position data
are inserted into the Wallops Island system from Ft. Collins.

Procedures involved for logging, displaying, archiving, and plotting
the received data are essentially identical to those uscd for the
NBS/Boulder system.

. NBS/Kauai, HI, Site

NBS also accumulates and archives GOES/West monitoring data from a
site at NBS radio station WWVH on Kauai, Hawaii. The data are
monitored continuously by a time code receiver and hourly
measurements are stored in a data logger. Twice each week the data
are downloaded to a microcomputer disk file. At intervals of
approximately one month, data are transferred from WWVH to Boulder.

~ 4.7 -


CONTENTS

DESCRIPTION OF SERVICE
A. Introduction ----
B. GOES Time Code System -
Cc. Recent Improvements to the GOES System ----------------

DESIGN PHILOSOPHY -

A. Intended Audience of User and Performance Levels
B. Link to the NBS Primary Standards and Time Scales
c. Reliability and Performance Verification Aspects
D. Support for Time Code Users

DESCRIPTION OF SYSTEM
Introduction ----
Wallops Island, VA -
. Space Segment
NOAA Ground Facilities -
. NBS/Boulder
Ft. Collins, CO, Backup Facility -----------------------
Auxiliary Monitoring Sites in Washington, DC, and

Kauai, HI ---

QmMnHUAwP

OPERATIONAL PROCEDURES --------~--~-------------------------

1. Procedures for Updating the Satellite Position Data -
1.1 Generation of Updated Satellite Position Data --
1.2 Transfer of Satellite Position Data to Wallops
Island -

2. Procedures for Verifying Wallops Island System Time
and Frequency Performance and Making Related Adjustments
2.1 Checking Performance of the Wallops Island
Clock System -----------------------------------
2.2 Checking and Resetting Wallops Island System
Date and Time --~------------------------5500557
2.3 Verifying and Resetting the Wallops Island
Glock Rate Words ------------------------------

3. Procedures for Updating and Verifying Other
Information in the Wallops Island System --------------

4. Monitoring Operations at NBS/Boulder -

4.1 Verification of Current Time Code Performance --
4.2 Archiving and Retrieving Past Time Code
Performance Data -------------~ -

4.3 Initialization of the Data Logger Systems -

vi


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

This document describes the GOES (Geostationary Operational Environmental
Satellite) satellite time code dissemination system operated by NBS to provide
time and frequency users with an NBS traceable reference signal for
calibration and general timekeeping applications. The discussion includes the
various subsystems to generate, monitor, and verify the timing signals located
at NBS/Boulder, NOAA/Wallops Island, VA, NBS/Ft. Collins, CO, and several
other dispersed sites. Some of the main operational procedures used in
providing a close approximation to UTC(NBS) at any user's location in the
Western hemisphere are described, along with an analysis of the uncertainties
associated with the GOES time code generation, transmission through the
satellite path, and reception at the user's site. Techniques and facilities
used to maintain internal quality control are also described.

Key words: calibration; frequency; GOES satellites; satellite; standards;
time; time code; time dissemination


C. Space Segment

The operational GOES satellite system normally consists of two operational
satellites serving as GOES/East and GOES/West, an in-orbit spare satellite
positioned approximately midway between these two, and a varying number of
partially inactive, older in-orbit standby spacecraft. This space segment is
totally owned and controlled by NOAA. The time code signals are normally
uplinked continuously to both operational satellites on an S-band frequency as
part of the interrogation channel used to communicate with the various Data
Collection Platforms in the environmental sensing system. The time codes are
then transmitted back to earth on downlink frequencies of 468.8375 MHz and
468.825 MHz for GOES/East and GOES/West, respectively. The time code
transmissions are then received on the earth at a signal level of
approximately -130 dBm.

During normal operations the time codes are transmitted continuously through
the GOES/East and GOES/West satellites. However, due to certain electrical
power constraints in the present generation of GOES satellites, some
modifications are sometimes invoked during two eclipse seasons each year when
the solar panels do not generate sufficient electrical power to maintain
normal operations. These periods are generally from 1 March to 15 April in
the Spring and from 1 September to 15 October in the Fall. Since the eclipse
period each day is centered at midnight at the local longitude of the
satellite, NOAA sometimes switches the time code operations from the normal
operational satellite to the spare satellite, located at about 100 degrees W.
longitude, for a two-hour period each day centered around 0500 UT for
GOES/East and 0900 UT for GOES/West. During the most recent period (Fall of
1987), NOAA was able to avoid such temporary switching operations due to the
availability of newer satellites, GOES-6 and GOES-7. All future generations
of GOES satellites are expected to have higher power capabilities and will
therefore not require special procedures during the eclipse periods each year.

D. NOAA Ground Facilities

In addition to the NOAA Satellite Control Facility at Wallops Island, VA,
other NOAA facilities are also important in the functioning of the NBS time
code dissemination system. The NOAA-provided satellite tracking data have
already been mentioned. In the normal mode of operation NOAA generates sets
of satellite orbital elements approximately once each week by processing the
tracking data derived from measurements made at Wallops Island. These orbital
elements are then distributed to interested users, including NBS, by Telex
from the NOAA Satellite Operations Control Center in Suitland.

NOAA's computer facilities at Suitland, MD have also proved helpful to NBS
under some modes of operation by offering an independent, alternative access
to computed satellite position data through the direct accessing of
NOAA-generated ephemeris data files maintained at Suitland. This procedure
can offer advantages relative to the NBS/BRoulder processing of the satellite
orbital elements in the sense that the NOAA ephemeris files are updated more
often (every few days) than the orbital elements that are distributed to user
organizations via Telex. The Suitland file access also generally allows the

- 3.4 -


GOES SATELLITE TIME CODE DISSEMINATION
DESCRIPTION AND OPERATION

1. DESCRIPTION OF SERVICE
A. ntroduction

In 1974 the National Bureau of Standards (NBS) began regular dissemination of
an NBS-referenced time code via two geostationary GOES satellites
(Geostationary Operational Environmental Satellites) operated by NOAA
(National Oceanic and Atmospheric Administration). The primary mission of the
GOES satellites and associated support systems is to gather a variety of
environmental data from various sources, including large numbers of remotely
located sensing platforms throughout the Western hemisphere, relay the
information via the satellites to a central processing facility, and make the
processed information available to the World Meteorological Organization and
other interested users. The NBS time code is interleaved into the data
collection platform interrogation channel, providing a continuously available
accurate time-of-day reference both for internal NOAA data handling operations
and for more general time and frequency applications throughout much of the
Western hemisphere. Commercial time code receivers are readily available for
a few thousand dollars that can provide received timing accuracies of better
than 100 microseconds over averaging periods of hours, months, or years.

The cooperative NBS/NOAA program to provide and disseminate the NBS time code
via the operational GOES satellites was formalized for a five-year period in
May 1977 by an NBS/NOAA Memorandum-of-Agreement, which was extended by both
organizations in May 1982 for an additional five-year period and again in
August 1987 for an additional ten-year period. Since 1974, significant
improvements have been made in the NBS time code generation and control
equipment at the GOES Satellite Control Facility at Wallops Island, VA; the
time code control and monitoring procedures used by NBS to assure overall
system accuracy and reliability; and in the newer generations of the GOES
satellites themselves.

B. GOES Time Code System

The GOES time code system consists of an NBS-owned time code generation,
monitoring, and control system at NOAA's Wallops Island site; the satellite
uplink facilities located at 75°W and 135°W longitude, respectively;
monitoring, computing, and data storage facilities at NBS/Boulder and at NBS
facilities in Ft. Collins, CO; two-way dial-up data communication links
between Boulder/Ft. Collins and Wallops Island; and support operations such as
NOAA's satellite tracking operations and additional monitoring facilities in
Washington, DC and Hawaii. The triply redundant time-code-generation system
is based on a set of cesium atomic standards. The time code used is specially
designed for compatibility with the GOES Data Collection System and has been
described in previous publications [1]. The code as transmitted via the two
GOES satellites includes complete time-of-year information; the current year;
DUT1 values -- i.e., estimates of the current difference between the UT1
astronomical time scale and UTC atomic time scale; satellite position

- 1.1 -


1.2.

satellites relaying the time code originate with the NOAA satellite
tracking networks and the NOAA satellite navigation personnel in the
Washington, DC, area. The orbital elements are distributed to
interested users such as NBS approximately once per week by Telex.
In addition to these routine updates resulting from more recent
tracking data, NOAA also generates and distributes sets of predicted
post-maneuver orbital elements just prior to any satellite maneuver,
such as those performed occasionally to maintain the satellites’
orbital location near their assigned longitudes.

The orbital elements provide input information needed by the GTDS
orbit prediction program to project satellite positions during the
next ten-day period. These data are inserted into the GTDS program
by running a special procedure program that requires only simple
data entry in response to prompts. The main GIDS program is then
run on the mainframe by a straightforward command from the local
microcomputer system operating in a terminal mode. Normally, this
operation is completed within a few minutes, resulting in the
creation of a file giving predicted satellite positions (latitude,
longitude, and radius) for each hour during the next ten days. This
file is then downloaded to the local microcomputer in an ASCII text
format that can be directly sent to the Wallops Island time code
generation system.

Transfer of Satellite Position Data to Wallops Island

The file of predicted satellite positions at hourly intervals is
then transmitted by a dial-up telephone link to the NBS time code
generation systems at NOAA's Satellite Control Facility in Wallops
Island, VA, by using a special-purpose data transfer program. The
program is designed to transmit in blocks of 24 hourly lines of
data, checking the received echo response of each character against
the original value. After each block further checks are made to
test transfer integrity by issuing appropriate commands to the time
code generators and observing the resulting responses. Any
discrepancies, such as are sometimes caused by poor telephone
circuit quality, cause the program to abort the load operation and
the process must be repeated until no errors are detected. The
valid data are then stored in designated memory locations within the
time code generators, where they are accessed at the appropriate
times for inclusion in the transmitted time code format along with
the time information. The time code generators also perform an
interpolation function between the hourly values, so that the
transmitted values are actually updated each minute.

- 4.2 -


NBS Measurement Services:

GOES Satellite Time Code Dissemination:
Description and Operation

ep w1s0°_ 120? ane 60° 30° 0° _ 30°60", NBS
i ct i" . Special

<= _, "ge Publication
I an 250-30

+ i
7° ELEVATION AwgLe i} i ELEVATION ANGLE
Lt eee eae

_ Bina) eee

Roger E. Beehlei
Dick Davis
John B. Milton

u.d. Department of Commerce
National Bureau of Standards


accessing and using the satellite ephemeris files generated and maintained by
NOAA in their Suitland, MD computer systems. This system normally contains
post-maneuver data concurrent with the actual satellite maneuver and thus
solves the problem at some slight cost of operational convenience. Some
previous tests of these procedures prior to the partial equipment failures in
GOES-4 and GOES-5 were very encouraging.

All new GOES satellites, starting with GOES-7, should have sufficient on-board
reserve electrical power to carry the normal operations through the two-hour
eclipse periods each day during the Spring and Fall eclipse periods. This
improvement should eliminate one of the main operational inconveniences for
some present users, the switching of at least the West time code back and
forth to the spare satellite during eclipse periods.

- 7.3 -


of the same worldwide meteorological satellite system as GOES, these
satellites do not currently include an identical or similar time code in their
broadcast formats. Several forms of commercial GOES time code receivers are
currently available which feature automatic operation with small antennas.
Prices range from about $3500 to $4500, depending on the accuracy level and
receiver features provided.

C. Recent Improvements to the GOES System

As the GOES time code system and operational procedures have evolved during
the past few years, several improvements have been incorporated. A second-
generation system that provides increased reliability through triple
redundancy, more elaborate diagnostic information available remotely to
NBS/Boulder personnel, independent electrical power backup, and the
replacement of rubidium standards with more stable (in long term) cesium
devices has been installed at Wallops Island. The information transmitted to
users has been expanded to include higher-resolution satellite position data,
UT1 time scale information, daylight-saving-time indicators, leap-second
notification, expected-accuracy indicators for each time code, the current
year, and system status information. Monitoring has also been expanded by
placing receivers at the U.S. Naval Observatory in Washington, DC, and at
radio station WWVH in Hawaii to provide better geographical coverage and by
acquiring dedicated backup receivers for the NBS/Boulder and Ft. Collins, CO,
backup systems. GOES status information is now available to interested users
via the monthly NBS Time and Frequency Bulletin [2] and, on a more current
basis, from the USNO Automated Data Service. The USNO system can be accessed
using a variety of terminals at either 300 or 1200 baud with even parity.
Telephone access numbers are: (202) 653-1079 (commercial); 653-1079 (FTS);
and 294-1079 (Autovon). After responding to the prompt asking for
identification, the GOES status information is obtained by requesting the file
“NBSGO" with the command "@NBSGO" followed by a carriage return. These status
reports are designed to report interruptions in service, temporary
perturbations in operations which result in reduced reception accuracies or
other problems, and accomplished or projected changes that affect the GOES
time code.

- 1A -


*seaue abeuarod
3S9M/S309 pue 3se3/S309 *L aunBy4

al TT ot

mete sa Al ee 379Nv WOLAVARTE L 17] 2
W/ Zee 7am NEB\NEam
Ze tt TA | _
[ 4\ \|& a
att ji tL AY
i aa
AT
inoalae :
\ naire

- 1.3 -

209 OT x) 30t 00 306 «OT OST A 0ST 3 0ST Tv


also maintained in Washington, DC (for GOES/East) and at NBS radio station
WWVH in Hawaii (for GOES/West). The Washington, DC monitoring data can be
related easily to UTC(NBS) using UTC(USNO) as a transfer standard. The Hawaii
data are measured in terms of the local WWVH UTC time scale system, which is
linked back to UTC(NBS) by regular GPS satellite comparisons.

c. Experience With the Internal Quality Control Mechanisms

Continuous monitoring of the received GOES time code signals has been carried
out in one form or another since the inception of the service in May 1974.
The results show that, for the vast majority of the time, users have had
access to these reference signals at accuracies of better than 1 millisecond
(when corrections are not applied for path delay variations) or 100
microseconds (when corrections for path delay are applied).

Keeping the time code generators at Wallops Island within a few microseconds
of UTC(NBS) has not proved to be a problem. The combination of cesium-
standard stability, reliable external checks against Loran-C and GPS, and
redundant automatic alarm systems has served well in this regard. On the rare
occasion when a cesium standard or time code generator has failed suddenly, it
has only taken a few minutes to have the Wallops Island station staff
reconfigure the system to use a different on-line time code generator system.

Experience has shown that the major cause for time code deviations exceeding
normal limits is the eclipse operations conducted during 45-day periods in the
Spring and Fall of each year. With the transfer of time code operations on
one or both GOES satellites to a spare satellite for two hours each day most
users experience either loss of signal or substantial time shifts. Although
NBS began transmitting correct position data (for the spare satellite) during
these eclipse periods a few years ago, many receivers apparently do not
automatically lock on to this satellite and therefore produce relatively large
errors. As discussed earlier, however, this problem has apparently been
eliminated as of Fall 1987 due to the use of newer version satellites that do
not require switching to standby spacecraft during eclipse periods.

The next most serious problem has turned out to arise from operations at times
of satellite maneuvers. The limitations in our version of the GIDS program
used to generate the new position predictions that prohibit the updates from
becoming available to the users until the start of the next day after the
maneuver sometimes result in time code errors building up to several hundred
microseconds for several hours after a maneuver. This type of problem occurs
on the average of once every few months. On more infrequent occasions the
post-maneuver predicted orbital elements supplied by NOAA prove inadequate for
the new orbit and time code errors of several hundred microseconds might then
persist for 4-7 days until new tracking data becomes available.

On one or two occasions over the past five years there have been extended
outages of the time code lasting for 1-24 hours due to complete loss of
electrical power at NOAA's Wallops Island facility. This type of occurrence
is beyond our control and, in principle, is very unlikely to reoccur since the
facility is now served by an improved backup electrical power system. As an

- 6.3 -


recent abnormal performance perturbations in the time code,
announcements of upcoming satellite maneuvers and their likely

impact on users, and notifications about revised operations during
eclipse periods.


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

REFERENCES

{1] NBS Publication TFS-602, "NBS time via satellites," available upon
request.

Hanson, D. W., Davis, D. D., and Cateora, J. V., "NBS time to the Western
hemisphere by satellite." Radio Science, vol. 14, no. 4, 731-740.

[2] NBS Time and Frequency Bulletin (monthly), available upon request.
[3] Goddard Trajectory Determination System (GIDS): User's Guide, July, 1975.

Mathematical Theory of the Goddard Trajectory Determination System,
April, 1976.

[4] Beehler, R. E., "GOES satellite time code dissemination," Proc. 14th
Annual Precise Time and Time Interval (PTTI) Applications and Planning
Meeting, December 1982, 57-87.

- R.1 -


The procedures for data collection and data logger setup are
essentially identical to those employed at Boulder and Ft. Collins.

To keep users informed of changes in the GOES system or operations
and to report significant perturbations to the time code performance
as observed at NBS/Boulder, one section of the monthly Time and
Frequency Bulletin is devoted to reporting such information. This
provides one means of keeping GOES time code users informed about
past, current, and future developments in the service but has the
disadvantage that the information may be more than a month old by
the time the Bulletin reaches the end user.

Past and current performance information is based on the extensive
monitoring capabilities described previously. Information about
future developments, such as revised operational modes during
upcoming eclipse periods or the anticipated launching of new
satellites, is obtained through periodic contacts with various NOAA
officials concerned with relevant aspects of the overall GOES

User Notification Procedures

6.1. NBS Time and Frequency Bulletin
satellite system.

6.2.

GOES Status Reports in the USNO Automated Data Service System

In order to provide a mechanism for informing users of the GOES time
code about the current status of the system on a more timely basis
than is possible by the NBS Time and Frequency Bulletin, NBS and the
USNO cooperates by maintaining a status file in USNO's Automated
Data Service that is accessible to any user at any time free of
charge. The GOES status file in the USNO computer system is updated
by NBS GOES personnel whenever there is a significant event to
report. Every effort is made to report changes and announcements as
soon as practical. The file is named "NBSGO" and users access it by
using a standard type of computer terminal or a microcomputer
operating in a terminal mode at 300, 1200, or 2400 Baud using even
parity and 7 data bits. The user is only required to dial the
appropriate access telephone number, answer a prompt with his name
and organization, and then transmit the command "@NBSGO" followed by
a carriage return in order to download the status file.

The procedure for updating this status file consists of preparing a
suitable text file using a word processor and microcomputer system,
accessing the USNO system by appropriate passwords, transmitting the
file to the USNO computer system, and then reading the file for
verification purposes. Examples of information reported include

- 4.8 -


ASSESSMENT OF UNCERTAINTIES -

A.
B.
Cc.

D.

Monitoring Operations at Other Sites ----
5.1 U. S. Naval Observatory (USNO), Washington, DC -
5.2 NBS/Ft. Collins, CO, Site -----------------------
5.3 NBS/Kauai, HI, Site -----------------------------

User Notification Procedures -----
6.1 NBS Time and Frequency Bulletin -
6.2 GOES Status Reports in the USNO Automated

Data Service System ----------------------------

Introduction
Quoted Uncertainties
Discussion of Error Sources
1) Wallops Island Reference Errors -
2) Equipment Delays at Wallops Island
3) Free-Space Propagation Delay
4) Transponder Delays
5) Ionospheric/Tropospheric Effects
6) Receiver Delays
7) Measurement Uncertainties
Summary of Performance-Monitoring Data -

INTERNAL QUALITY CONTROL -----------------------------------

A.

FUTURE PROJECTIONS -

vaAwP

Monitoring at Wallops Island for Internal Quality
Control
Monitoring of Received GOES Signals Versus UTC(NBS) ---
Experience With the Internal Quality Control

Mechanisms --------------------------------------------

Projections for the GOES Satellite System -
Status of Formal NBS/NOAA Agreements ----
NOAA Satellite Tracking Operations --------------------
Potential for Improvements to the Time Code

Performance ~----------~---------------------522 6-5-2277

REFERENCES ----------------------------- 222-222-222 ene e

vii

PEEPS
Naan

PS
© 0

S
©

BUNUHHUUHUHUUH
DANUUUNNNNEPRE

a
a


data for this program consist of the satellite orbital elements, supplied to
NBS by NOAA. Typically, the GIDS program is run about once per week to
generate position predictions 10 days in advance for each operational
satellite carrying the time code, The GIDS program outputs a file of position
data which can be transmitted directly to the Wallops Island subsystem. When
the alternative position-data-generation procedure is used, in which the data
is actually generated by NOAA’s Suitland, MD computer, the function of the
NBS/Boulder facility is to transfer the data files from Suitland to Boulder
over a dial-up link, reformat the data to be compatible with the Wallops
Island system, and then transfer it to Wallops Island. The various file
transfers involved in all of these operations are conducted using a dedicated
microcomputer system running commonly available commercial communications
software or, in the case of the Boulder - Wallops Islands transfer, specially
developed file transfer software to provide for necessary error-checking.

The file transfer capabilities via dial-up links are also used routinely from
Boulder to access the remote time code monitoring system in Washington, DC.
Data transfer from the Hawaii monitoring site is done by physical transfer of
data tapes, since the Hawaiian phone lines do not always support reliable data
transfers. All monitoring data are examined for any signs of problems and
then archived in computer files for later use. The archived data include
hourly measurements from eight different receivers in three different
geographical locations.

The NBS/Boulder system provides control functions by allowing various commands
and interrogation messages to be easily communicated to the Wallops Island
equipment. Capabilities include the loading of updated position data, making
time and/or frequency adjustments to the basic references, examining and
resetting the alarm systems, re-initializing the time code generators and data
loggers in the event of a total electrical outage, examining and transferring
the monitoring data files, arming the system for automatically updating the
value for DUT1 or inserting a leap second at a specified time, arming the
system for automatic transmission of position data for the spare satellite
during eclipse periods, initiating instantaneous measurements of any of the
1-Hz signals, and updating and/or verifying the contents of any portion of the
system's memory.

NBS/Boulder also serves as the focal point for contacts with GOES time code
users and equipment manufacturers. Information about the time code is
disseminated in a variety of ways. In addition to the normal responses to
correspondence, telephone calls, and visits, status information about the
system is made available to interested users by publication in the monthly
Time and Frequency Bulletin and, more currently, through a status file
maintained and updated regularly by NBS/Boulder as part of the U.S. Naval
Observatory’s computerized Automated Data Service. For more complete and
longer-term information dissemination needs periodic technical papers and
reports are presented at appropriate conferences.

F. Ft. Collins, CO Backup Facility

In order to provide backup equipment and personnel for performing all the
essential GOES time code monitoring and control operations, a complete backup

- 3.7 -


capabilities of the responsible agencies in maintaining good satellite
stationkeeping operations.

Figure 5 presents data for a corresponding period of about 4 years, but in
this case the received data represent daily averages of UTC(NBS)-GOES/East
after corrections are applied for satellite position variations. These data
are typical of what a user would observe with an automatic receiver that
decodes the satellite position information in the time code, computes path
delay corrections, and continually adjusts the output 1 Hz reference signal to
remain. close to UTC(NBS). Although it is apparent from the plot that
occasional brief periods exist (such as at times of satellite maneuvers) when
the received time signals depart from UTC(NBS) by 100 microseconds or more,
the great majority of the data fall within a 50 microsecond range with respect
to UTC(NBS).

An analysis of similar monitoring results as obtained from different
geographical locations, such as Washington, DC, and Hawaii, shows conclusively
that the overall system performance does not depend significantly on the
particular geometry involved. In other words, the GIDS program is very
effective in computing the proper satellite positions which, in turn, can be
used by the receiver algorithms to compute accurate path delays to any
location. From many years of such observations we feel safe in considering
the overall system accuracy to be within 100 microseconds, excluding the
relatively brief periods covering special situations such as satellite
maneuvers. Users are cautioned about these "abnormal" periods by a variety of
means, including published information about the GOES system, announcements in
the monthly Time and Frequency Bulletin, special alerts updated as often as
necessary in the GOES status information file maintained by NBS in the U.S.
Naval Observatory’s Automated Data Service computer system, and designated
accuracy indicator bits in the time code message.

-5.7-


y ean8ty

SAYO 40 YSaEWnN

gast _oozt oss sw EC,
4 gaat
Ph Wry + gag2
4 gone
9861/S1/r0-2861/ 10/10 |
(G3ALQSYNOINAD LSV3S3/SS09-91LN

Q2av

SCNODSSOY3DIW

- 5.8 -


at some specified future time. Thus, for example, it is possible to set
new values for. the DUT1 correction, the daylight-saving-time indicator.
the leap second warning indicator, and the operational mode during
eclipse periods, all of which affect the time code content at a later
time. In the rare case where a complete system outage has occurred and
all data must be re-entered into the systems as quickly as possible,
special "cold start" programs have been developed to automatically reload
all the current system parameters into the time code generators and data
loggers with minimal required involvement of the Boulder or Ft. Collins
operator.

In performing most of these aata loading operations the system operator
has. a choice of using either a manual or a partly automated procedure.
The manual technique requires the operator to accurately formulate the
command word, transmit it. to Wallops Island, and then to issue a "dump"
command to verify that it was completed successfully. In the
alternative, partly automated procedure the operator can make use of a
specially developed software program that has been designed for maximum
convenience. In using this completely menu-driven program the operator
selects which parameter he wishes to load into the system from a menu and
responds to prompts for the values of the data bytes to be loaded and any
other necessary information. The program then constructs the appropriate
command word, transmits the command to the equipment, issues a
verification command, and displays the results to the system operator.

In general, a totally experienced operator may prefer the manual approach
since it is quicker, while a backup less-experienced operator may prefer
the security and convenience of the automated software.

Monitoring Operations at NBS/Boulder

4.1. Verification of Current Time Code Performance

A capability 1s maintained both at NBS/Boulder and at the NBS Ft.
Collins site for monitoring the current and recent timing
performance of the received time codes from GOES/East and GOES/West
and the current operational status of the overall system. The
monitoring systems include a number of satellite time code receivers
that continuously receive signals from both satellites, chart
recorders to provide a continuous plot of the received signals
versus UTC(NBS), data loggers similar to those at Wallops Island, a
specialized decoder for decoding and displaying certain selected
bits in the serial time codes, and suitable microcomputer equipment
for processing and storing the data.

Normally, the chart recorders are observed several times each work
day to insure proper time code performance. Four plots are routinely
generated: GOES/East and GOES/West uncorrected for satellite

- 4.5 -


PREFACE

Calibrations and related measurement services of the National
Bureau of Standards provide the means for makers and users of
measuring tools to achieve levels of measurement accuracy that
are necessary to attain quality, productivity and competitive-
ness. These requirements include the highest levels of accuracy
that are possible on the basis of the most modern advances in
science and technology as well as the levels of accuracy that are
necessary in the routine production of goods and services. More
than 300 different calibrations, measurement assurance services
and special tests are available from NBS to support the
activities of public and private organizations. These services
enable users to link their measurements to the reference
Standards maintained by NBS and, thereby, to the measurement
systems of other countries throughout the world. NBS Special
Publication 250, NBS Calibration Services Users Guide, describes
the calibrations and related services that are offered, provides
essential information for placing orders for these services and
identifies expert persons to be contacted for technical
assistance.

NBS Special Publication 250 has recently been expanded by the
addition of supplementary publications that provide detailed
technical descriptions of specific NBS calibration services and,
together with the NBS Calibration Services Users Guide, they
constitute a topical series. Each technical supplement on a
particular calibration service includes:

o specifications for the service
o design philosophy and theory
o description of the NBS measurement system
o NBS operational procedures
© measurement uncertainty assessment
error budget

systematic error
random errors

o NBS internal quality control procedures

iii


elements as previously. As a direct result of these changes in procedures,
NBS degraded its accuracy-performance specification on the time code to 100
microseconds for applications in which the satellite position predictions are
used to compute path delay corrections. More recent experience (1985-1987)
has offered some optimism that the current techniques are being further
refined and that the new tracking procedures may not cause as big a problem
for the time code as previously thought. Although much more experience needs
to be accumulated, it seems likely that the current tracking operations will
continue to support a 100 microsecond accuracy specification with a good
possibility of maintaining better than 50 microseconds most of the time.

D. Potential for Improvements to the Time Code Performance

Further improvements to the time code performance are likely to be rather
minor in view of the built-in dependence on critical operations that are the
responsibilities of other organizations. Since the time code operations are a
relatively minor aspect of the overall GOES system operation, decisions on
issues such as tracking techniques will continue to be made by NOAA based
largely on only their own requirements. Although NOAA has been extremely
cooperative in many operational aspects of maintaining the time code service,
it is unrealistic to expect that NBS can have significant impact on future
GOES developments.

With respect to the time code system itself there will be a continuing process
of replacing and upgrading our own equipment from time to time as needs and
opportunities arise. The current time code system at Wallops Island is
basically a second-generation system and some ideas for the next generation
equipment are already under consideration. The recent improvement to the
monitoring capabilities through the use of GPS satellites appears to solve
those requirements for as long as the GPS signals remain available to the
civilian sector. Using GPS as a reference for the Wallops Island GOES system
contains a certain aspect of overkill, since GOES does not require the better-
than-100 nanosecond accuracy of GPS. However, this may turn out to be an
advantage, because, if the GPS capabilities are intentionally degraded by DOD,
the resulting accuracy will most likely still be sufficient for our needs with
GOES.

In mid-1987 the sensitivity of the NBS equipment to electrical power outages
at Wallops Island was reduced by installation of a dedicated, separate battery
backup power system. This system can provide electrical power for the NBS
standards, time code generators, and data loggers for several hours in the
event of loss of NOAA's primary electrical power.

One current limitation, the inability of the NBS version of the GTDS program
tor satellite position prediction generation to process data for the actual
day of a satellite maneuver, has been previously described. This prevents NBS
from being able to insert updated position data into the time code system
after a maneuver until 0000 UTC on the next day. Extensive cfforts have been
made by NBS in the past to isolate and correct the source of this computer
programming problem, but they were unsuccessful. In the future, after the
GOES system satellite configuration becomes more normal with the launch of
new, replacement satellites, we should be able to bypass this problem by

- 7.2 -


added precaution, however, the NBS equipment at Wallops Island has been
supplied with its own special battery backup power supply since mid-1987.

In general, the internal quality control procedures have proven effective and
major changes and additions are not considered necessary. The reliability of
the time code service has been maintained at a level consistent with the other
related NBS services such as WWV, WWVB, and WWVH.

- 6.4 -


practical terms this implies downtimes of less than 0.1%. To achieve this
level of reliability a number of features and operational capabilities have
been built into the equipment and control procedures. Double or triple
redundancy is used for all critical subsystems at Wallops Island, including
the atomic frequency standards, time code generators, and communications
components. Additional spare units are maintained and are regularly operated
and monitored in Boulder. Redundant data acquisition equipment is also used
at both sites. Complete diagnostic information on the Wallops Island
equipment is continuously generated automatically and can be accessed from
Boulder at any time via dial-up telephone lines. System "health" indicators
are also transmitted as part of the time code message and are routinely
monitored in Boulder. The time outputs from the three redundant time code
generators at Wallops Island are automatically compared and an
out-of-specification result immediately generates alarms at Wallops Island to
alert the NOAA staff of a problem and transmits an appropriate alarm signal
over the satellite link to the monitoring system in Boulder.

To further guard against errors in the transmitted time code, particularly
those that might be dependent upon the specific geographical location,
independent time code monitoring facilities are maintained at Ft. Collins, CO;
Washington, DC; and Kauai, HI in addition to the extensive monitoring at
NBS/Boulder. The system control capabilities that enable personnel in Boulder
to insert updated data in the Wallops Island equipment and to adjust as
necessary the basic frequencies and times involved via dial-up telephone lines
are also duplicated at Ft. Collins, CO for redundancy. Essentially all
monitoring and control procedures can be conducted equally well from either
the Boulder or Ft. Collins sites with trained personnel at each location.

D. Support for Time Code Users

NBS attempts to provide active support for GOES time code equipment
manufacturers and users. Timely responses are provided to requests for
information. Special publications and brochures are generated and widely
distributed. At appropriate intervals technical presentations are made at
conferences and meetings to inform users and potential users of the current
capabilities and performance of the time code system. Each month relevant
information about the time code system and performance, including performance
perturbations, scheduled changes in operations, and other status reports, is
published in the Time and Frequency Bulletin and distributed to a large
mailing list that is updated annually. For those users needing to know such
information on a more timely basis, NBS maintains a GOES status file in the
U.S. Naval Observatory’s Automated Data Service in Washington, DC. Interested
users can access this computer file of current information at any time free of
charge using any type of standard communications terminal.

- 2.3 -


@ eunbiy

S€N OL ANID

SHOIWHAN39
ANOHdaTaL dn-Ivid 4svIs 3009 3WIL
GNV1SI SdOTTWM WOouS
‘ | [ WSISAS
SH3AISOaY ZONVE Lo WHv1V
ONvHO! [21>] SYa9901 viv SW3GOW
BareYaTUP IT ato’
viva SONVAWOO [tmrmenrmennd H5\ISOIY
viva
Slo]? Sd9
Z16]a
2|8la|zis
2 a elm ol 1
312 /81812 “
Se oo
aio i=}
rom od ES be 1
OS|2lol>
EIZz{9
21515
ANG
a
GUVONVLS
wnis3o
ISAWS3O9 OL ~———J_—LINN 3 DOL _ .
HOLOSTaS @ dol]. 2. 8_Y
isva/S309 OL <—— oe] cs owuaNnao GYVGNVLS
V99L|"3q00 SWIL wnisao

WALSAS 3009 SWIL SA09 GNVIS! SdOTIVM


Library of Congress Catalog Card Number: 87-619906

National Bureau of Standards Special Publication 250-30
Natl. Bur. Stand. (U.S.), Spec. Publ. 250-30, 49 pages (Jan. 1988)
CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 1988

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325


7) Measurement Uncertainties. Typical applications of the GOES time
code involve the measurement of the time difference between the
received GOES time code reference and a local clock pulse. Although
the resolution of such a measurement can easily be maintained at
better than 1 microsecond with simple equipment, there are much
larger uncertainties introduced by the rather limited bandwidth
occupied by the time code signal. For the typical signal-to-noise
ratios available and a 100 hertz data rate for the time code,
fundamental considerations lead to uncertainty levels of about 10
microseconds. This is borne out by the observed scatter of
successive measurements of the received time code, using two
identical, collocated receivers in the NBS laboratory.

D. Summary of Performance-Monitoring Data

In view of the extreme complexity of the overall GOES time code dissemination
system and the difficulties discussed above associated with establishing valid
statistical uncertainties for the various components of the system, the best
estimate of actual system performance achieved is obtained from monitoring
data collected at a variety of sites. Since the ultimate objective of this
service is to provide UTC(NBS) at each user’s site, one need only to receive
the GOES time code at various geographical locations where a reference
standard with a known relationship to UTC(NBS) is available and to monitor the
time difference between the received GOES signal and the known reference. The
degree to which the received signal agrees with UTC(NBS) is a measure of the
overall uncertainty of the dissemination system, taking into account all the
delays and uncertainties described previously. The degree to which the
observed time difference, if any, remains constant with time is a measure of
the overall system stability performance.

Monitoring sites currently include: the main facility at NBS in Boulder, CO;
the backup operations control facility at the NBS radio station site in Ft.
Collins, CO; the U.S. Naval Observatory in Washington, DC; and the NBS WWVH
radio station site on the island of Kauai, HI. Monitoring data from these
sites are regularly accessed from Boulder, analyzed, and archived in permanent
computer files. Periodically, these results are compiled and published to
document the performance of the GOES time code system. One example can be
found in Reference [4].

Figure 4 is a plot of the observed daily averages of UTC(NBS)-GOES/East over a
period of about 4 years where no corrections have been made for satellite
position variations. The sudden shift in the time code data of about 1
millisecond near day number 1200 is not an actual change in the received data
but rather reflects a modification to the receiver at NBS which resulted in a
significantly different delay. This instrumentation effect could be corrected
for but has not been in this plot. This time code performance is typical of
that observed by users with receivers that do not make use of the transmitted
satellite position information. he conclusion is that the received code can
be considered as a reasonable representation of UTC(NBS) to within about 1
millisecond as long as a constant correction is applied to all data for the
user's approximate geographical location. The obvious lack of a long term
drift of the GOES time signal with respect to UTC(NBS) speaks well for the

-5.6-


Procedures for Verifying Wallops Island System Time and Frequency

Performance and Making Related Adjustments

2.3.

Checking Performance of the Wallops Island Clock System

By accessing the Wallops Island equipment using a dial-up telephone
link with a microcomputer system operating in a terminal mode, we
may command the system to display two types of performance data: (1)
a set of recent hourly and daily measurements of each of the time
code generator outputs versus a local reference standard, which is
normally a Loran-C timing receiver or a GPS satellite receiver; and
(2) current, instantaneous measurements of any selected TCG output
versus the Loran-C reference. Alternatively, the direct time
difference between any two time code generators can be observed by
issuing a suitable command. This procedure allows Boulder or Ft.
Collins operational personnel to monitor the performance of each
time code generator and atomic frequency standard system in terms of
external references (Loran-C or GPS satellites) that can be related
easily to UTC(NBS). Also, their relative performance can be easily
monitored so that adjustments can be made to individual subsystems
when the drift rates become excessive. Such measurements are
routinely monitored every few days and whenever any type of
automatic alarm is generated by the system itself. Currently, the
measurements of the on-line time code generator in terms of the GPS
satellite system time provide the best method for insuring that the
GOES time code, as transmitted, does not depart from UTC(NBS) by
more than a few microseconds.

Checking and Resetting Wallops Island System Date and Time

Provisions that allow a remote operator to easily check and, as
appropriate, reset the system's date and/or time are included in the
Wallops Island time code generation system. Using a microcomputer
in a communications terminal mode, the operator simply sends
appropriate commands to the equipment to adjust any portion of the
current date or time. This procedure is normally required only in
the event of a major electrical power problem at Wallops Island, a
serious NBS equipment malfunction, or occasionally to make a routine
adjustment of the 1-hertz outputs to compensate for the small drift
rates of the atomic standards with respect to UTG(NBS).

Verifying and Resetting the Wallops Island Clock Rate Words

Since it is generally not a good idea to actually adjust a cesium
standard's frequency by frequent C-field changes and since no NBS
personnel are available at Wallops Island to perform such tasks, a
remote capability has been provided in the system to effectively

- 4.3 -


NBS MEASUREMENT SERVICES:
GOES Satellite Time Code Dissemination:
Description and Operation

Roger E. Beehler, Dick Davis, and John B. Milton

Time and Frequency Division
Center for Basic Standards
National Measurement Laboratory
National Bureau of Standards
Boulder CO 80303

January 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary
NATIONAL BUREAU OF STANDARDS, Emest Ambler, Director


unusual situations such as eclipse periods and satellite maneuvers can cause
system accuracy to be temporarily degraded beyond these Limits.

CG. Discussion of Error Sources

4)

2)

3)

Wallops Island Reference Errors, The timing outputs of the time
code generators are remotely adjustable from Boulder in steps of 0.2
microsecond, and, in principle, could be kept within less than 1
microsecond of the UTG(NBS) time scale. However, since the GOES
time code system is designed to provide accuracies in the 10-100
microsecond range, the reference signals from the time code
generators are normally controlled only to remain within a few
microseconds of UTC(NBS). As discussed in more detail in other
parts of this document, the time code generator outputs are
routinely monitored against appropriate references to maintain this
accuracy level. This is easily performed by observing at frequent
intervals the measured time differences with respect to the other
time code generators themselves, the external references from Loran-
and the GPS satellite system, and most importantly, with respect
to UTC(NBS) directly by continuous reception of the GOES/East and
GOES/West satellite signals at Boulder and Ft. Collins, CO. These
monitoring and time-adjustment procedures limit timing errors at
Wallops Island with respect to UTC(NBS) to less than 10 microseconds
at all times.

Equipment Delays at Wallops Island. As the timing outputs of the
NBS equipment at Wallops Island are further processed through other
electronic subsystems, such as frequency translators and modulators,
to integrate them into the composite satellite uplink signal,
additional time delays may be introduced. Since experience has
shown that these delays remain constant over very long periods of
time and are difficult to evaluate precisely without an on-site NBS
staff, they are treated as fixed biases and compensated for by
advancing the originating timing signals by an appropriate amount.
The uncertainties and variations in these fixed delays are much
smaller than the overall accuracy of the time dissemination system.

Free-Space Propagation Delay, Following the propagation path from
Wallops Island to the GOES satellite and back to the user's location
on the earth, the time code signal is delayed by approximately 260
milliseconds. The exact amount, of course, depends on the exact
positions of the satellite and the user's location and can be
calculated using straightforward geometrical considerations. As the
satellite moves around in its geostationary orbit, the path length,
and thus the signal propagation time, also change by corresponding
amounts.

Since the GOES satellites are constrained to remain close to their
assigned orbital locations in the geostationary arc, stationkeeping
maneuvers are performed on the satellites as necessary to keep the
positions within acceptable tolerances. Typically such
stationkeeping maneuvers are needed several times each year. The

-5.2-


on-line time code generator within a few microseconds of UTC(NBS). In
addition, automatic alarm circuits continuously monitor the internal time
differences between pairs of time code generators and generate warnings if any
of these measurements exceed a preset limit (usually about 15 microseconds).
Since alarms cause audible warnings to the Wallops Island staff who
immediately contact NBS on a 24-hour-per-day basis, any timing errors are
usually corrected promptly. Experience has shown that such errors are
unlikely to occur more than once or twice per year.

B. Monitoring of Received GOES Signals Versus UTC(NBS)

The primary means for insuring that the GOES time code signals remain within
specified limits with respect to UTC(NBS) is the continuous monitoring and
measurement of the received signals at several locations having access to a
good approximation to UTC(NBS). At NBS/Boulder signals are received
continuously from both the East and West satellites, using an ensemble of six
or more receivers. The data are recorded on strip chart recorders for easy
visual monitoring and stored in a system of redundant data loggers. The
measurement reference in the Boulder system is one of the clocks in the
UTC(NBS) time scale system, thus providing a direct link to UTC(NBS). Twice
each week the data logger data are analyzed for proper system operation,
plotted as appropriate for permanent records, and then transferred to
redundant computer disk files for archival purposes.

To further insure proper system performance a special time code decoder system
in Boulder continuously monitors and displays all received bits in the
received time code, including position data, DUT1 correction, daylight savings
time and leap second status, the transmitted accuracy indicators for each
satellite, the on-line time code generator identifier, and diagnostic
indicators concerning any alarm conditions in the Wallops Island time code
generation system. Finally, a visual display of the 100 Hz clock and data
signal components from several of the monitoring receivers is available as an
indicator of local interference conditions and coarse signal parameters.

A smaller version of this monitoring system is also maintained in Ft. Collins
to provide an independent capability for assessing system performance. The
reference for this system is the local UTC time scale used to generate the
transmitted signals for radio stations WWV and WWVB. This local UTC system is
kept synchronized to UTC(NBS) by continuous automatic monitoring of GPS
satellite signals and regular comparisons with the corresponding measurements
being made in terms of UTC(NBS) at Boulder. A special GOES time code monitor
and alarm generator has been designed, constructed, and placed into operation
by the Ft. Collins staff at the radio station site. This system generates an
alarm condition if the received time code signal deviates from the local
reference time scale by more than a selected number of microseconds and
remains outside these limits for longer than a selected amount of time. Any
alarm that is generated is interfaced with the station's normal alarm-
monitoring system which causes a staff member to be notified of problems on a
24 hour per day basis.

Primarily as an aid in detecting any geographical dependencies that might
creep into the GOES timing system, simple automatic monitoring systems are

- 6.2 -


position versus UTC(NBS) and GOES/East and GOES/West corrected for
satellite position. The corrected plots should normally stay within
100 microseconds of UTC(NBS) except for brief periods following
major satellite maneuvers. An oscilloscope display is maintained to
show the status of interference that may be perturbing one or more
receivers, Not much can be done about the interference when it is
troublesome, but it can help oxplain changes in observed
performance. The data loggers may be accessed at any time to view
the most recent 128 hourly measurements of the received signals or
the most recent 30 daily values.

. Archiving and Retrieving Past Time Code Performance Data

Procedures have been developed to store the GOES monitoring data on
a hard-disk system and to retrieve any selected portion of the data
for examination or plotting. The routine monitoring data from the
NBS/Boulder and Ft. Collins systems are normally transferred from
the data loggers to the hard-disk files each Monday and Friday.
Since the data loggers store the most recent 128 hours of data, this
procedure insures that no data are lost. The retrieval and plotting
procedures are performed whenever such data are needed. Generally,
5-day plots are produced for examination every few days to check on
proper system operation and hardcopy plots are produced on a monthly
basis for archival purposes. Data plots for periods of up to 5 years
are generated occasionally for special publications and presen-
tations. Three special-purpose computer programs have been written
to store the data logger data in archival files, plot the current
data from the data logger, and plot any selected portion of data
from the archival disk files.

Initialization of the Data Logger Systems

The data loggers used in the monitoring operations at NBS/Boulder
have provisions for driving chart recorders as well as for storing
past measurement data digitally. Upon initial setup or after
electrical power outages an initialization procedure must be
executed to define start and stop pulses for each of the eight
channels and to define various parameters associated with the analog
outputs for the chart recorders. A special purpose software program
is available to initialize these parameters automatically or they
can be entered into the equipment manually by transmitting
appropriate command words from a microcomputer system acting as a
terminal.

Monitoring Operations at Other Sites

5.1.

U.S. Naval Observatory (USNO), Washington, DC

- 4.6 -


GOES SATELLITE TIME CODE DISSEMINATION
DESCRIPTION AND OPERATION

2. DESIGN PHILOSOPHY

A. Intended Audience of User and Performance Levels

The GOES time code dissemination system was designed with the intent of
providing continuously available, convenient-to-use timing signals for those
users needing better accuracies, coverage, and reception reliability than are
typically available from WWV, WWVH, and WWVB. This translates roughly into
accuracy capabilities of better than 1 millisecond, coverage extending well
beyond the U.S. mainland, and reliable reception 24 hours per day that is not
subject to serious degradation due to ionospheric and atmospheric propagation
variations.

A number of previous user surveys conducted by NBS have shown that the needs
for time and frequency reference signals span a very wide range of accuracy
requirements, specific types of information, geographical location, and
measurement application. It is also clear that very large numbers of users
and applications are involved. Since existing HF broadcast services such as
those from radio stations WWV and CHU have traditionally been very successful
in serving the large numbers of customers with relatively modest needs for
accuracy, the GOES time code was developed from the beginning to provide
relatively modest numbers of users who need timing accuracies in the 10-1000
microsecond range with continuous, reliable, and widely available timing
signals suitable for use with automated, relatively inexpensive receiving
equipment. Typical users or applications include: a common time base for
correlating scientific observations made at widely separated sites - e.g.,
monitoring ot geophysical events; the coordination of switching operations to
control power flow in large interconnected electric power networks; a timing
reference for analyzing the sequence of events before and during faults in the
electric power networks; the synchronization of various types of communication
networks and systems; calibration labs and other organizations needing an
NBS-traceable timing reference at an accuracy level of better than 1
millisecond; and a convenient, widely accessible source of "coarse" time for
systems such as electronic navigation systems that have internal capabilities
for synchronization at much higher accuracy levels.

The GOES time code is designed so that it can be used at two different
accuracy levels, depending on each user's particular requirements. If the
highest possible accuracies are not needed, only part of the time code
information need be used with a resulting reduction in equipment cost and
complexity. However, by making use of the satellite position data transmitted
along with the time information, a user has the option to achieve higher
accuracies but at the expense of a more sophisticated receiver.

In contrast to many other sources of timing signals, such as Loran-C
navigation transmissions, the GOES satellite time code provides complete UTC
time information, including the year, day-ot-year, hour, minute, second, and

- 2.1 -


GOES SATELLITE TIME CODE DISSEMINATION:
DESCRIPTION AND OPERATION

3. DESCRIPTION OF SYSTEM

A. Introduction

The complete system to generate, monitor, and evaluate the GOES satellite time
code includes subsystems at a number of different locations. The time code is
actually generated at NOAA's Satellite Control Facility at Wallops Island, VA.
This site also includes data acquisition equipment, alarm systems, and
communications equipment. The critical orbital element information, needed in
the computation of the satellite positions for path delay correction, is
generated from NOAA and NASA satellite tracking data and distributed from
NOAA's Satellite Operations Control Center in Suitland, MD, to NBS/Boulder and
NBS/Ft. Collins. After the orbital element information is used in Boulder as
input to a large satellite orbit determination computer program, the resulting
predictions of each satellite's position each hour for a ten-day period are
transmitted back to the time code generation system at Wallops Island for
inclusion in the time code message sent to the satellites. In addition to
generating the position prediction data, the NBS/Boulder site is responsible
for overall control of the time code system, updating the information being
transmitted in the time code as appropriate, monitoring the status of the
Wallops Island equipment, responding to any system abnormalities, continuously
monitoring the performance of the GOES/East and GOES/West time codes as
received in Boulder, archiving all monitoring data from Boulder and the other
monitoring locations, notifying users of the overall system status, and
coordinating all operations with the backup time code system operator at Ft.
Collins, CO. Time code monitoring operations are conducted at Washington, DC;
Kauai, HI; and Ft. Collins, CO, as well as in Boulder. The remainder of this
section includes more details on the various functions and equipment at each
of the sites mentioned.

B. Wallops Island, VA

Figure 2 shows a block diagram of the NBS subsystems located at NOAA's Wallops
Island Satellite Control Facility. The basic time and frequency reference is
obtained from a set of two independent cesium atomic frequency standards.
Their outputs drive three independent time code generators, designated TCG A,
TCG B, and TCG C. Provisions are included for inserting rate offsets for each
cesium standard (to keep the reference frequencies in agreement with the rate
of the UTC(NBS) time scale) and time offsets for each time code generator (to
keep the actual time outputs synchronized to the UTC(NBS) time scale). The
rate and time offsets can be entered into the system via a dial-up
communications link from Boulder. Because of the extremely high stability of
the cesium standards used, the rate corrections are very small (typically,
less than a few parts in 104?) and need to be revised only about once per
year. The outputs of the three time code generators are continuously compared
and, if normal limits are exceeded, generate an audible alarm to alert the
Wallops Island personnel to immediately inform NBS of the situation.

- 3.1 -