CGMS-41 NASA-WP-02
12 June 2013
Prepared by NASA
Agenda Item: WGII/9
Discussed in Working Group II
AN OPTIMIZATION ANALYSIS OF THE GCOM-C1 AND SENTINEL-3A MISSIONS
FOR IMPROVED OCEAN COLOR IMAGING COVERAGE
In response to CGMS Action 40.17
Brian D. Killough
NASA Langley Research Center, Hampton, Virginia
Executive summary
The Committee on Earth Observation Satellites (CEOS) System Engineering
Office (SEO) was asked to examine the coverage capabilities of a virtual satellite
constellation consisting of Global Change Observation Mission - Climate (GCOMC1) and Sentinel-3A and to determine what realistic steps could be taken to
improve the daily coverage available for ocean color imaging. The design orbits
of the satellites appear to be well matched but the difference in orbit repeat cycle
causes significant periods of complete overlapping coverage to appear. With
minor adjustments to the planned orbit of GCOM-C1 (4 km altitude, 2 minutes in
Local Time of Descending Node), the constellation can maximize the daily
coverage potential and remove the oscillations in relative spacing between
adjacent swaths that create periods of poor coverage. However, both creating
and maintaining the enhanced coverage pattern will require launch and orbit
insertion coordination between GCOM-C1 and Sentinel-3A as well as tighter orbit
maintenance than either mission team is currently planning. With the addition of
Sentinel-3B, a further optimization can be applied to the constellation to provide
full, daily global coverage or GCOM-C1 can return to its original design orbit as
most of the benefit of the formation flying would be lost.
Reference COVE tool: http://www.ceos-cove.org
CGMS-41 NASA-WP-02
12 June 2013
An Optimization Analysis of the GCOM-C1 and Sentinel-3A Missions for
Improved Ocean Color Imaging Coverage
1
INTRODUCTION
The Committee on Earth Observation Satellites (CEOS) System Engineering Office
(SEO) was asked to examine the coverage capabilities of a virtual satellite constellation
consisting of Global Change Observation Mission - Climate (GCOM-C1) and Sentinel3A and to determine what realistic steps could be taken to improve the daily coverage
available for ocean color imaging. The design orbits of the satellites appear to be well
matched but the difference in orbit repeat cycle causes significant periods of complete
overlapping coverage to appear. With minor adjustments to the planned orbit of GCOMC1 (4 km altitude, 2 minutes in Local Time of Descending Node), the constellation can
maximize the daily coverage potential and remove the oscillations in relative spacing
between adjacent swaths that create periods of poor coverage. However, both creating
and maintaining the enhanced coverage pattern will require launch and orbit insertion
coordination between GCOM-C1 and Sentinel-3A as well as tighter orbit maintenance
than either mission team is currently planning. With the addition of Sentinel-3B, a further
optimization can be applied to the constellation to provide full, daily global coverage or
GCOM-C1 can return to its original design orbit as most of the benefit of the formation
flying would be lost.
2
2.1
MISSION AND INSTRUMENT DESCRIPTIONS
GCOM-C1 SGLI-VNR
Set to launch in the middle of 2014, GCOM-C1 will carry the Second Generation Global
Imager (SGLI) which consists of a Visible and Near-Infrared (VNR) radiometer and an
Infrared Scanner (IRS). Only the VNR portion of the instrument is applicable to this
study. The satellite flies in a 796 km altitude orbit with a 10:30 AM Local Time of the
Descending Node (LTDN) and a 34 day orbit repeat cycle. The SGLI has a ground
swath width of 1150 km.
2.2
Sentinel-3 OLCI
Designed as a constellation of two satellites, Sentinel-3A will be the first to launch in
April of 2014. Each satellite will fly in an 800 km altitude orbit with a 10:00 AM LTDN
and a 27 day orbit repeat cycle. The satellites will be phased 180 degrees apart once
coordinated mission operations begin. The Sentinel spacecraft will carry both optical
and topographic instrumentation, but only the Ocean and Land Color Imager (OLCI) is
applicable to this study. The OLCI has a ground swath width of 1240 km.
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12 June 2013
3
ANALYSIS ASSUMPTIONS AND SETUP
Spacecraft propagations were carried out using a simple J2 propagator. An arbitrary
launch date was chosen and each satellite’s starting position was based on the desired
LTDN. Each satellite was propagated for 1 complete orbit repeat cycle and then the
data from that cycle was repeated as many times as needed to construct a propagation
that covered the desired time span. Each instrument swath was modeled using a
spherical Earth approximation and the altitude of the spacecraft at the Equator. The
OLCI instrument characteristics were taken from a presentation found at
www.vgt.vito.be website entitled “1210_15u00_BBerruti_Sentinel3.pdf”. Based on the
documentation, the OLCI sensor has a 68.6 degree field-of-view that is offset by 11.2
degrees to the right. This translates to an estimated ground swath width of 1244 km.
The SGLI instrument is based on the information provided in the first Research
Announcement for the GCOM-C1 mission available at http://suzaku.eorc.jaxa.jp. The
SGLI-VNR has a field-of-view of 70 degrees, centered at nadir, which translates to an
estimated ground swath of 1153 km.
4
ANALYSIS AND RESULTS
Figure 1. Base coverage pattern of the
notional GCOM-C1 mission (RED) and
the Sentinel-3A mission (BLUE) virtual
constellation. Significant overlap is
shown in this nominal case.
Based on a simplified starting point
defined by the arbitrary launch dates
discussed above, the nominal coverage
provided by the virtual constellation can
be forecasted (Figure 1). The coverage
provided by both satellites has
significant overlap due to the proximity
of their local crossing times and the
initial conditions of the propagations.
However, due to the differences in the
orbit repeat cycles, the pattern shown in
Figure 1 will not be maintained. The
ground swaths of each satellite will drift,
relative to each other, due to the slight
differences in the orbit characteristics of
the satellites. This oscillation will vary
the ground coverage pattern for the
virtual constellation over a 3 month period
between completely overlapping coverage
(Figure 2) and non-overlapping coverage
(Figure 3).
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CGMS-41 NASA-WP-02
12 June 2013
Figure 2. Complete overlap of the
GCOM-C1 (RED) and Sentinel-3A
(BLUE) virtual constellation. This is the
worst-case coverage scenario.
Figure 3. Non-overlapping swaths of
the GCOM-C1 (RED) and the Sentinel3A (BLUE) virtual constellation. This is
the best-case coverage scenario.
By making small modifications to the GCOM-C1 orbit, it is possible to remove the
oscillation in the ground coverage pattern produced by the virtual constellation. If the
GCOM-C1 orbit is raised by 4 km and a very small inclination adjustment is made
(<0.1°), then the coverage pattern of both satellites would remain consistent over the
entire 27 day orbit repeat cycle.
After moving GCOM-C1 to a 27 day orbit repeat cycle, a couple of options exist to
remove the remaining swath overlap; the LTDN can be adjusted to induce extra
separation between the ground tracks or the spacing between the satellites in the
shared orbit can be increased to allow the Earth’s rotation to provide the desired
separation. Because the change required in LTDN to accomplish the necessary
separation is small relative to GCOM-C1’s desired control band, it was decided to
adjust the LTDN by 2 minutes (10:32 AM) and retain the small temporal spacing
between adjacent acquisitions. With the proposed adjustments, the constellation’s
coverage changes to reflect the pattern shown in Figure 4. There is still a very small
overlap in the coverage near the Equator, which has been left as a potential coincident
scene measurement opportunity between the instruments. If this is not desired, an
additional small increase in either LTDN or satellite spacing will remove it.
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12 June 2013
While the required changes to GCOM-C1
are small, there are a few logistical
requirements that may prove more
challenging. The enhanced coverage pattern
shown in Figure 4 requires that the two
spacecraft are flying in a tight formation,
separated by only a few seconds in orbit. To
accomplish this, significant coordination will
be required between the GCOM and
Sentinel operations teams during GCOM-C1
launch, during orbital insertion, and
throughout the lifetime of the virtual
constellation. Maintaining this formation may
require tighter orbit control than either
operations team is currently planning to
enforce. This maintenance could also have
an impact on fuel consumption and therefore
mission lifetime, depending on the negotiated
orbit control requirements needed to achieve
the desired formation.
Figure 4. Optimized swaths of the
GCOM-C1 (RED) and Sentinel-3A
(BLUE) virtual constellation.
Once Sentinel-3B launches, significant
overlap will exist between the coverage
patterns of GCOM-C1 and Sentinel-3B.
Figure 5 shows the coverage pattern of the
optimized GCOM-C1 / Sentinel-3A
constellation with the addition of Sentinel-3B
phased 180 degrees apart from Sentinel-3A.
GCOM-C1 is filling in one of the two small
gaps that exist between the Sentinel swaths,
but unless there is a secondary benefit to the
ocean color community of redundant
coverage, either GCOM-C1 or Sentinel-3B’s
coverage is largely redundant. To close the
remaining coverage gaps, adjustments need
to be made to Sentinel-3B’s location relative
to the other satellites. If it is decided not to
pursue the three satellite constellation option,
GCOM-C1 can perform a small propulsive
burn to return to its original design orbit and
both teams can return to independent
operations.
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Figure 5. Optimized ground coverage
for GCOM-C1 (RED) and Sentinel-3A
(BLUE) with the addition of Sentinel-3B
(GREEN). Little or no gaps exist in this
scenario.