We are currently experiencing a period of high solar radiation combined with wide short-term fluctuations in the radiation. The short-term fluctuations, especially when combined with highly energetic solar flares, can adversely affect the mission of U.S. Space Command's Space Surveillance Center (SSC) which catalogs and tracks the satellites in orbit around the Earth. Rapidly increasing levels of solar electromagnetic and/or particle radiation (solar wind) causes atmospheric warming, which, in turn, causes the upper-most portions of the atmosphere to expand outward, into the regime of low altitude satellites. The increased drag on satellites from this expansion can cause large, unmodeled, in-track displacements, thus undermining the SSC's ability to track and predict satellite position. On 13 March 1989, high solar radiation levels, combined with a high-energy solar flare, caused an exceptional amount of short-term atmospheric warming. The SSC temporarily lost track of over 1300 low altitude satellites--nearly half of the low altitude satellite population. Observational data on satellites that became lost during the days following the 13 March 'solar event' was analyzed and compared with the satellites' last element set prior to the event (referred to as a geomagnetic storm because of the large increase in magnetic flux in the upper atmosphere). The analysis led to a set of procedures for reducing the impact of future geomagnetic storms. These procedures adjust selected software limit parameters in the differential correction of element sets and in the observation association process and must be manually initiated at the onset of a geomagnetic storm. Sensor tasking procedures must be adjusted to ensure that a minimum of four observations per day are received for low altitude satellites. These procedures have been implemented and, thus far, appear to be successful in minimizing the effect of subsequent geomagnetic storms on satellite tracking and ephemeris computation

Bredvik, Gordon D.

English

NASA Technical Reports Server

N91-17084
"Procedures for Minimizing the Effects of High Solar Activity
on Satellite Tracking and Ephemeris Generation"
by
Gordon D. Bredvik
ABSTRACT
We are currently experiencing a period of high solar radiation
combined with wide short-term fluctuations in the radiation. The
short-term fluctuations, especially when combined with highly
energetic solar flares, can adversely affect the mission of U.S.
Space Command's Space Surveillance Center (SSC) which catalogs and
tracks the satellites in orbit around the earth.
Rapidly increasing levels of solar electromagnetic and/or
particle radiation (solar wind) causes atmospheric warming, which,
in turn, causes the upper-most portions of the atmosphere to expand
outward, into the regime of low altitude satellites. The increased
drag on satellites from this expansion can cause large, unmodeled,
in-track displacements, thus undermining the SSC's ability to track
and predict satellite position.
On 13 March 1989, high solar radiation levels, combined with
a high-energy solar flare, caused an exceptional amount of short-
term atmospheric warming. The SSC temporarily lost track of over
1300 low altitude satellites--nearly half of the low altitude
satellite population. Observational data on satellites that became
lost during the days following the 13 March "solar event" was
analyzed and compared with the satellites' last element set prior
to the event (referred to as a geomagnetic storm because of the
large increase in magnetic flux in the upper atmosphere). The
analysis led to a set of procedures for reducing the impact of
future geomagnetic storms. These procedures adjust selected
software limit parameters in the differential correction of element
sets and in the observation association process and must be
manually initiated at the onset of a geomagnetic storm. Sensor
tasking procedures must be adjusted to ensure that a minimum of
four observations per day are received for low altitude satellites.
These procedures have been implemented and, thus far, appear to be
successful in minimizing the effect of subsequent geomagnetic
storms on satellite tracking and ephemeris computation.
Introduction
On 13 March 1989, one of the stations which report three-hourly
values of geomagnetic flux (Ap), Fredricksburg, Virginia, reported
a level of flux averaged over all eight three-hourly measurements
which was the highest recorded value since 1960 (Reference 3).
This major geomagnetic storm was believed to be linked to a very
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strong solar flare observed on the surface of the sun three days
earlier, on I0 March, which was pointed almost directly at Earth.
Also on 13 March, another strong flare occurred; the x-ray
radiation from this flare, classified as an x-level, or highest
energy-level flare, undoubtedly contributed to the intensity of the
geomagnetic storm already in progress. Additional x-level flares
occurring on the 14 th, 16 _, and 17 t" of March extended the duration
and effects of the storm, which included dramatic displays of the
aurora borealis as far south as the Caribbean (Reference i).
Other, much publicized effects of the storm included disruption of
shortwave radio communications, power outages such as the one which
blacked out parts of Montreal and the province of Quebec for as
long as nine hours, and the spurious opening and closing of
automatic garage doors (Reference 2). The storm had an even more
dramatic effect on US SPACECOM's Space Surveillance Center (SSC),
which lost track of over 1300 low altitude satellites--over half
of the low altitude satellite population. Several days of
intensive around-the-clock manual analysis effort was required to
"catch-up" with the lost satellites and reduce the "lost list" to
a marginal level. Several more weeks of manual effort was required
to reduce the list back to nominal levels.
Rapid changes in solar radiation, such as occurred on 13-17 March,
1989, produce large unmodeled drag effects on low altitude
satellites and can defeat automatic observation processing and
element set maintenance. Prediction accuracy is thereby degraded,
thus causing problems in identifying and tracking low altitude
satellites.
Analysis of satellite observations and the SSC's observation
processing during and after the March Event led to a set of
software procedures for minimizing the effects of future such
events (large changes in solar radiation) on the SSC. A summary
of the analysis and a description of the procedures themselves
follows, preceded by a brief discussion of the effects of changes
in solar radiation on the orbits of low altitude satellites.
Effects of Solar Radiation on Earth's Atmosphere and Low Altitude
Satellites
Solar radiation warms the Earth's upper (tenuous) atmosphere
through two primary effects: photoionization, caused by
electromagnetic radiation, and ionization caused by collisions
between solar wind particles (mostly electrons and protons), and
air molecules. Increases in solar radiation increase atmospheric
warming, which in turn causes the tenuous atmosphere to expand
outward. This expansion increases atmospheric density in the realm
of low altitude satellites, producing increased drag. Conversely,
reductions in solar radiation cause the tenuous atmosphere to
contract, thus reducing drag on low altitude satellites.
190
The SSC measures the amount of drag recently encountered by low
altitude satellites and assumes this value remains constant as it
predicts future satellite positions. (There are exceptions to this
rule, such as when high interest satellites are maintained using
special perturbations theory. For these special cases,
measurements of solar radiation are incorporated in ephemeris
prediction). If changes in drag are gradual and drag measurements
are taken frequently enough, the assumption of constant drag
provides satisfactory accuracy. However, when large, rapid changes
in solar radiation produce similarly large, rapid changes in
atmospheric drag, measurement of drag can lag significantly behind
what is actually being experienced, and orbit prediction accuracy
can deteriorate.
There are both long and short-term variations in solar radiation.
The ll-year solar cycle is an example of long-term variation. A
solar flare, which can last from minutes to hours, is an example
of a short-term variation. Another example of short term variation
is the fluctuation in total solar radiation that occurs during the
peaks in the eleven-year cycle; these fluctuations can run their
course in as little as a few days. The SSC's method of satellite
drag prediction easily accommodates long-term fluctuations in solar
radiation. However, solar conditions such as those which occurred
10-17 March, 1989, in which the solar wind from a major flare
arrived at the earth at the same time that a peak in the short-
term fluctuation in total radiation was occurring, can cause drag
to change significantly in a matter of half a day or less. For
example, a spike in Ap, such as occurred in 10-17 March 1989, can
cause a 45 nm displacement in a near earth satellite's predicted
position in just 12 hours (perigee = 185 km). It was this rapid
increase in drag on low altitude satellites (generally, satellites
with periods less than ii0 minutes) which caused the SSC to
temporarily lose track of over 1300 satellites.
Overview of the SSC's Satellite Observation Processinq and
Satellite Element Set Maintenance Seqments
The SSC maintains the element sets of nearly 7000 satellites in a
file called the SATF. The element sets are updated each time
observations are received through a process called differential
correction, similar to a Kalman filter or, in effect, a seven
dimensional least squares fit. A complete differential correction
(DC) is not performed each time an observation is received;
instead, a simpler "sequential" DC is performed until a preset
period of time expires, at which time a full DC is performed using
all of the observations received during a period of time referred
to as the Length of Update Interval (LUPI). The LUPI varies from
5 to 14 days for low altitude satellites, depending on the apogee
and perigee heights of their orbits. The sequential DC permits
only relatively small changes to the satellite's element set but
has the advantage that it is much faster than a full DC, since it
does not attempt to do a "least squares" fit of all LUPI
191
observations (it performs a simple "update" of the covariance
matrix) and thus requires only one "pass". Once the update
interval for a given satellite expires, or the sequential DC fails,
or the element set accuracy declines past a given point, a full DC
is performed for that satellite's element set. A full DC requires
a minimum of seven observations.
Element sets from the SATF are periodically transmitted to
spacetrack sensors in the Space Surveillance Network. The sensors
use the latest element set received from the SSC to predict look
angles for satellites for which they are tasked to provide
observations. The sensors compare their observations against the
predicted positions of the satellites using the latest element set
received from the SSC. If the comparison is within established
association criteria, the sensor will tag the observation with the
requested satellite number and send it to the SSC as a routine
observation. If it does not meet the association criteria, the
sensor will tag it as an Unknown Object (UO) and send it to the SSC
tagged with a 9XXXX number in place of the satellite number. The
processing of UO observations within the SSC is much more tedious
and time consuming because they have to be compared against every
satellite in the SATF file.
Deterioration in the Observation Processinq and Element Set
Maintenance Segments during the March 1989 Solar Event
Prior to the Solar Event of March 13-17, 1989, Air Force Space
Command (AFSPACECOM) had anticipated that problems might be
encountered during the upcoming peak in the solar cycle and had
tasked Kaman Sciences (on contract to maintain the SSC software)
to examine ways to minimize the effects of changing solar radiation
on the SSC's mission of satellite tracking and ephemeris
prediction. Thus, at Kaman's request, satellite observation data
taken before, during, and after the March Solar Event was saved for
analysis. Using this data, Kaman recreated the March Solar Event
scenario in AFSPACECOM's Off-Site Test Facility using computers and
software similar to the SSC's. The analysis showed that
deterioration occurred in three chronological phases.
Phase i. During the initial phase, low altitude satellites
began reflecting the effects of the increased drag, caused by
the solar flare-induced geomagnetic storm, through relatively
large differences between their observed and predicted
positions. Although still within SSC and sensor association
limits, the magnitude of the difference caused the satellites
to fail sequential DC updates or fail the SSC's internal
accuracy check; they were then scheduled for a full DC update.
During normal periods, this process is able to "catch" and
update delinquent element sets before they degrade further.
However, during this solar event, the full DC typically either
rejected the latest observations because they were too far
from "nominal", or, when too many recent observations were
192
available to ignore, the full DC simply failed. In either
case, the element set was not updated with the new
observations. When the full DC failed, it failed either
because it could not accept the large change in drag and mean
motion suggested by the newest observations, or the full DC
could not meet convergence criteria, or both. The convergence
criteria could not be met because the disparity between the
old (before flare) and new (post flare) observations prevented
an acceptable "fit".
Phase 1 - Sugqested Fixes. To correct the problems in this
phase, the following steps suggested themselves.
O Reduce LUPI. If the Length of Update Interval (LUPI)
were reduced to the point where most or all of the
observations to be used in the DC update were new (post
flare), then the DC would not be permitted to ignore the
post flare observations. Further, convergence criteria
could be achieved since the preponderance of the
observations were post flare. Testing showed that
convergence could be achieved with a LUPI of three days
in nearly every case. Any further reduction greatly
increased the likelihood that less than the minimum
number of observations (seven) would be available for the
full DC.
O Increase Parameter Chanqe Limits for Full DC. To prevent
full DCs from failing because drag and mean motion
changes exceeded normal limits, these limits were
increased. Typically, the effects of the increase or
decrease in satellite drag caused by changes in solar
radiation are in-track. For example, the effect of
increased drag is to drop the satellite to a lower orbit,
thus reducing the semi-major axis of the orbit and
increasing mean motion. Other orbit parameters will
remain largely unchanged. The orbit plane will remain
the same, and only very slight changes in altitude and
eccentricity will occur. To the spacetrack sensor, the
orbit will appear the same, except the satellite will be
ahead of its predicted position (case of increased drag),
or behind its predicted position (case of decreased
drag).
Thus, during periods of high solar activity, the full DC
should be permitted to accept wider changes in drag and
the parameter which describes in-track motion, mean
motion (n, revolutions per day). Testing indicated that
the limit to changes in drag should be increased by a
factor of ten and the limit for changes in mean motion
increased by a factor of six.
193
O Increase Observation Flow Rate. If the daily observation
flow rate is not increased when the LUPI is decreased to
three days, the number of observations available for the
full DC will be drastically reduced, close to, or even
below the minimum of seven. To prevent the full DC from
failing because of a lack of the minimum number of
observations, and to maintain an acceptable level of
accuracy in the full DC, sensor tasking must be increased
to ensure that a minimum of four observations per day are
received for all low altitude satellites. If a still
higher number of observations were received, accuracy
could approach that of normal solar conditions with
normal LUPI values.
Phase 2. During Phase 2, the satellite element sets were so
inaccurate that the SSC could no longer associate routine
observations with their element sets. During this phase, many
of the sensors were still able to associate the observations
with the drag-displaced satellites because the sensors have
considerably wider association criteria than the SSC. In this
phase, the SSC typically detags properly tagged routine
observations and places them in an Unassociated Observations
File as unknown observations.
Phase 2 - Suqqested Fix: Increase In-Track Multiplier. This
problem can be corrected by increasing the association
criteria of the SSC to equal that of the sensors in the in-
track direction. Analysis showed that if the in-track
association multiplier were doubled, i.e. increased from 3 to
6 for low altitude satellites, the SSC's in-track association
criteria would approximate that of the Eglin phased array
sensor. (Association criteria is not the same for every
spacetrack sensor. Eglin was selected because, as a dedicated
spacetrack sensor, it provides more observations than any
other phased array sensor).
Phase 3. During Phase 3, the element sets are so old that
neither the SSC nor the spacetrack sensors can associate the
observations with the drag-displaced satellites. Thus, all
of the observations on these satellites are tagged as Unknown
Objects by the sensors. When they arrive at the SSC, they
eventually wind up in the Unassociated Observations File. At
this point, trained Orbital Analysts must manually retrieve
the observations from this file and attempt to associate them
with element sets in the SATF using various software tools
currently available. This process is very tedious and time
consuming and greatly increases CPU usage. However, this
intensive manual interaction was necessary 24 hours a day for
several days after the March Solar Event. Once this phase is
reached, there is no known method for enhancing the manual
recovery processes already known to SSC analysts and crew
personnel. The objective of the fixes recommended in the two
194
previous phases is to prevent Phase 3 from ever being reached•
Summary of SSC Performance Parameters Durinq the March 1989 Solar
Event
During the three phases described above, post-event analysis
revealed that the percentage of observations that were tagged as
Unknown Objects increased from 18% to over 40%; satellite catalog
accuracy dropped drastically; the success rate for full DCs dropped
sharply; total observation flow rate increased dramatically; CPU
usage rose to the point of saturation; and, as was previously
mentioned, the satellite "lost list" increased to over 1300
satellites. The normal level for this list is below i00
satellites.
Summary of Recommended Software Procedures
The following procedures were recommended for implementation in the
SSC. They apply to low altitude satellites (non-payloads) with
periods less than ii0 minutes:
. Increase spacetrack sensor tasking to ensure a minimum
flow of four observations per day. Maintain this tasking
until solar maximum subsides in late 1994.
• When Ap rises over I00, or a sharp change in F10 (solar
radiation measured at 10.7 cm wavelength, in units of
10 .22 watts/M2/Hertz) occurs, implement the following
steps:
a) Decrease LUPI to three days;
b) Increase the in-track multiplier for each satellite
to 6;
c) Increase DC change limit multiplier for drag to I0;
d) Increase the DC change limit multiplier for mean
motion to 6.
Return the above parameters to normal approximately three
days after solar activity returns to normal.
Conclusion
The software procedures described in this paper have been adopted
by USSPACECOM's Space Surveillance Center (SSC) as standard
operating procedures during periods of high solar activity since
late June, 1989. Their objectives are to maintain the satellite
"lost list", the Unknown Object (UO) rate, and CPU (Central
Processing Unit) usage at near normal levels during such periods.
Thus far, these objectives have been achieved.
195
REFERENCES
io
•
3.
Marcia Bartusiak, "The Sunspot Syndrome"; Discover, November
1989, p. 45.
Leon Jaroff, "Fury on the Sun"; Tim____ee,July 3, 1989, p. 46.
"Preliminary Report and Forecast of Solar Geophysical Data",
published weekly by the Joint NOAA-USAF Space Environment
Center, SESC PRF 707, 21 March 1989.
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