Knox, Robert G.

Middleton, Elizabeth M.

NASA Technical Reports Server

HyspIRI: Sampling and Time Series Issues
Robert G. Knox and Elizabeth M. Middleton 
(for the HyspIRI concept study team)
NASA’s Goddard Space Flight Center,
Biospheric Sciences Branch, Code 614.4
Greenbelt, MD
May 18, 2011
National Aeronautics and Space Administration
www.nasa.gov 
https://ntrs.nasa.gov/search.jsp?R=20110012869 2019-08-30T16:02:51+00:00Z
National Aeronautics and Space Administration
Introduction
Context
• VSWIR spectrometer observations: 
“The longest revisit time acceptable is a 
month or so; . . .” (NRC, 2007, p. 204)
- Trading off spatial resolution and temporal 
resolution, we arrived at a 60 m GSD (pixel 
spacing) and a swath width of ~150 km, suited 
to provide full global coverage in a polar, sun-
synchronous,19-day repeating orbit.
• “A multispectral imager similar to 
ASTER is required in the thermal data 
IR region. . . and a pixel size of less 
than 90 m.” (NRC, 2007, p. 115)
- Trading off requirements for spatial and 
temporal resolution, we arrived at a ~60 m 
GSD and swath width of ~600 km; suited to 
provide full global coverage in a sun-
synchronous, 5-day near-repeating orbit.
Outline
• TIR imaging opportunities
• Variation in frequency with latitude
• Overpass times/dates for example sites
• VSWIR imaging opportunities 
• Seasonal coverage
• Variation in overpass time with latitude
• Cross-track sampling geometry
• Generality of findings:
• HyspIRI reference orbit vs. 
other daylight sun-synchronous  orbits? 
vs. other Earth referenced orbits?
Knox & Middleton, HyspIRI: Sampling and time series issues 2
Equinox (20 March)
HyspIRI’s VSWIR swath 
(cyan), nested within TIR 
swath (red)
Annual TIR imaging opportunities in a 5-day near-repeating orbit, 
1 yr. simulation
Nominal orbit: average alt. 626.8 km, inclination 97.8°. TIR imager FOV: +/- 25.46° (60 m pixel GSD at nadir, 9272 cross-track pixels).
R.G. Knox, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v8.1.3, March 7, 2010. Plotted May 3, 2010.
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-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
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Latitude (N)
Annual TIR imaging opportunities in a 19/5-day repeat HyspIRI reference orbit
Swath: 50.92°, symmetric about nadir.  Sampled using a 1 by 1 deg. coverage. 
Maximum
Average
Minimum
(overall 
maximum:  
1845/1846 
80 N/S lat.)
Nominal orbit: average alt. 626.8 km, inclination 97.8°. TIR imager FOV: +/- 25.46° (60 m pixel GSD at nadir, 9272 cross-track pixels).
R.G. Knox, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v8.1.3, March 7, 2010. 
min. = 152, mean  > 166
5

TIR  accesses for 1 simulated year
National Aeronautics and Space Administration
Local time of TIR 
overpasses of 5 
FLUXNET sites, 
simulated for 1 year
Near the equator, overpass times 
are separated by 12 hours, on 
average. 
As the N latitude of the site 
increases, potential TIR collects 
are more frequent and less tightly 
clustered in local time.
Also, moving toward the north 
orbit pole (82.1 N) daytime 
collects are later and night 
collects earlier, whereas moving 
south the reverse is true (not 
shown).
Local Apparent Time (hour of day)
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National Aeronautics and Space Administration
VSWIR coverage frequency varies seasonally
Extracted from HyspIRI customer presentation to Team X, JPL, 3/23/2010 9
VSWIR: July 1-20, 2016
VSWIR: January 1-20, 2016
VSWIR coverage is limited by constraint: minimum 20  deg. Sun elevation angle.
Determined by 
the minimum 
solar elevation 
angle and the 
equatorial 
crossing time.
Annual VSWIR imaging opportunities in a 19-day repeating orbit, 1 yr. 
simulation, with a minimum solar elevation of 20°
Nominal orbit: av. alt. 626.8 km, incl. 97.8°. VSWIR spectrometer FOV:  2.8° E, 10.8° W (60 m pixel GSD at nadir, 2480 cross-track pixels).
R.G. Knox, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2, August 20, 2010. Note aliasing with sample grid. 
High 
resolution 
(1/8 degree) 
simulation of 
annual 
VSWIR 
coverage, 90 
x 90 degree 
(lat, lon) 
region
Nominal orbit: av. alt. 626.8 km, incl. 97.8°. VSWIR spectrometer FOV:  2.8° E, 10.8° W (60 m pixel GSD at nadir, 2480 cross-track pixels).
R.G. Knox, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2, August 17, 2010. Note aliasing at high latitudes. 
Strict repeating 
ground track vs. 
letting orbit 
drift, while 
controlling the 
crossing time?

VSWIR  accesses (SZA < 70°) for 1 simulated year
R.G. Knox & E.M. Middleton, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2
9:30
10:30
11:30
12:30
M A M J J A S O N D J F M
US-Brw:  Moist Tundra, Barrow, Alaska (71.32 N) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
CA-Ca1:  Campbell River, Maturing Douglas-fir,  British Columbia, Canada (49.87 N) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
US-SO2:  Sky Oaks Old Stand, Chaparral regrowing from 2003 fire, California (33.37 N) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
CR-Sro: Santa Rosa, Mosaic of pasture & secondary forest, Costa Rica (10.81 N) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
BR-Sa1: Santarem km 67 (LBA), Primary forest, Brazil (2.86 S) 
13
VSWIR  accesses for 1 simulated year
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
US-Bar:  Bartlett Experimental Forest, Deciduous broadleaf, New Hampshire (44.06 N) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
BR-Sp1:  Sao Paulo Cerrado, Savannah vegetation, Brazil (21.62 S) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
AR-Lac:  La Ciguena Santa Fe, Croplands, Argentina (29.26 S) 
9:00
10:00
11:00
12:00
M A M J J A S O N D J F M
AU-Tum:  Tumbarumba, Wet temperate sclerophyll forest, Australia (35.66 S) 
R.G. Knox & E.M. Middleton, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2
14
National Aeronautics and Space Administration
Daylight side of a sun-synchronous reference orbit, with 10:30 AM equatorial crossing (mean local 
time) at a descending orbit node. The sub-solar point (yellow) shows the location on Earth where 
the Sun is directly overhead, east of the ground track. Green line shows the plane of the ecliptic. 
3-D view shown in Earth Inertial Axes. 
20 Ma ch 12:59 UTC
HyspIRI near-polar sampling and illumination (late March)
R.G. Knox simulation with STK v9.2. Orbit: alt. 626.8 km, inclination: 97.8°.  Simulated mission days March 20-25, 2009. 
Earth graphics courtesy of Analytical Graphics Inc. 
HyspIRI’s nominal mid-morning (10:30 AM) crossing time is on a descending orbit (N-to-S) pass. Hence the 
observatory passes over sunlit terrain while descending east of the North Pole and while approaching west of the 
South Pole. TIR coverage is similar at both poles. (Ground tracks of sun-synch. orbits cannot pass directly over 
the poles.) Key: yellow arrows--Sun vector; white arrows--orbital motion; blue arrows--Earth rotation. TIR sensor 
swath (red) marked at 60 second intervals. 
N S
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National Aeronautics and Space Administration
Local time of VSWIR 
overpasses of 5 
FLUXNET sites, 
simulated for 1 year
Local apparent time, for a fixed mean 
local time, varies with the Earth’s orbit. 
(Blue bars are 6 minutes wide.)
As the N latitude of the site increases, 
the local apparent times of potential 
VSWIR accesses also increase.
Near the north orbit pole (82.1 N) the 
local time of potential accesses may be 
nearly 6 hours later than when crossing 
the equator (not shown).
Moving toward the south orbit pole 
local times are progressively earlier in 
the morning.
Local Apparent Time (hour of day)
US-Brw, Moist tundra flux site, Barrow, Alaska (71.32 N)
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9 10 11 12 13
0
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US-SO2, Sky Oaks Old Stand, Chaparral regrowing after fire, California (33.37 N)
F
re
q
u
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9 10 11 12 13
0
5
1
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BR-Sa1: Santarem km 67 (LBA), Primary forest, Brazil (2.86 S)
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re
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9 10 11 12 13
0
5
1
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AR-Lac:  La Ciguena Santa Fe, Croplands, Argentina (29.26 S)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
AU-Tum:  Tumbarumba, Wet temperate sclerophyll forest, Australia (35.66 S)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
R.G. Knox & E.M. Middleton, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2
Nominal orbit: av. alt. 626.8 km, incl. 97.8°. VSWIR spectrometer FOV:  2.8° E, 10.8° W (60 m pixel GSD at nadir, 2480 cross-track pixels).
R.G. Knox, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2, Aug. 20, 2010.; hi-res subset Aug. 17.
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Latitude (N)
Annual VSWIR Imaging Opportunities by Latitude in a 19-day repeat HyspIRI Reference Orbit
Swath: 13.62°, pointed 4° off-nadir; Local solar elevation angles > 20°; 56 km sample spacing
(Hi-res sampling was a 90-by-90 degree region, sampled at 1/8 degree.) 
Maximum
Average
Minimum
Hi-res
Hi-res Max.
Hi-res Min.
(overall 
maximum:  246 
near  82 N lat.)
18
HyspIRI ground tracks shortly after completing a 5-day near repeat pattern: 
(a) blue – descending (day) passes and orbit track; 
(b) red – ascending (night) passes and orbit track.
R.G. Knox simulation with STK v8.1.3. Orbit: alt. 626.8 km, inclination: 97.8°.  Earth graphics courtesy of Analytical Graphics Inc. 
25 March 2009 11:21:30 UTC
19
National Aeronautics and Space Administration
14 potential image collects for 1 simulated month (equatorial site)
Knox & Middleton, HyspIRI: Sampling and time series issues 20
Example of potential HyspIRI TIR data within 10 days of VSWIR coverage, 
for a near-equatorial study site in Brazil
Daylight accesses (cyan) include 1 coincident with the SWIR coverage and 3 other dates. 
Potential night data (red) include 5 overpasses, 1 within 13 hours of the SWIR coverage.
National Aeronautics and Space Administration
How general are these findings?
• Simon Hook asked (August 26, 2010):
“Which of these findings are specific to the particular orbit 
chosen for HyspIRI [reference orbit]?  And which are true of 
broader classes of candidate orbits?”
• The HyspIRI reference orbit: 
 Circular low Earth orbit (LEO), 
 Repeating ground-track (19 days, with 5-day near repeat),
 Combination of orbit average altitude (626.8 km) and inclination (97.8°) 
was chosen to be sun-synchronous, with a nominal equatorial crossing 
time of 10:30 AM on the descending node.
Knox & Middleton, HyspIRI: Sampling and time series issues 22
National Aeronautics and Space Administration
Local time of VSWIR 
overpasses of 5 
FLUXNET sites, 
simulated for 1 year
Local apparent time, for a fixed mean 
local time, varies with the Earth’s orbit. 
(Blue bars are 6 minutes wide.)
As the N latitude of the site increases, 
the local apparent times of potential 
VSWIR accesses also increase.
Near the north orbit pole (82.1 N) the 
local time of potential accesses may be 
nearly 6 hours later than when crossing 
the equator (not shown).
Moving toward the south orbit 
pole, local times are 
progressively earlier in the 
morning.
Local Apparent Time (hour of day)
US-Brw, Moist tundra flux site, Barrow, Alaska (71.32 N)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
US-SO2, Sky Oaks Old Stand, Chaparral regrowing after fire, California (33.37 N)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
BR-Sa1: Santarem km 67 (LBA), Primary forest, Brazil (2.86 S)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
AR-Lac:  La Ciguena Santa Fe, Croplands, Argentina (29.26 S)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
AU-Tum:  Tumbarumba, Wet temperate sclerophyll forest, Australia (35.66 S)
F
re
q
u
e
n
c
y
9 10 11 12 13
0
5
1
0
R.G. Knox & E.M. Middleton, NASA GSFC, Biospheric Sciences Branch, Code 614.4. Simulated with STK v9.2
National Aeronautics and Space Administration
Earth-referenced Satellite Orbits
• Geosynchronous: Orbit period = Earth’s period of rotation (approx. 23 hr. 56 min.). 
Occurs with circular orbit average altitude of 35,786 km.
Special variant: Geostationary (GEO) orbit is an equatorial geosynchronous orbit that maintains a nearly 
fixed position over the equator. Common examples: GOES weather satellites, most commercial 
communication satellites, NASA tier II mission GEO-CAPE.
• Low Earth Orbit (LEO):  Orbit altitude remains below ~1000 km 
(below the inner Van Allen radiation belt)
Special variant: Repeating ground track, where subsatellite trace repeats.
• Direct (prograde) Orbit:  Orbit moves eastward, with the Earth’s rotation. 
Rotation of the launch site supplies up to ~464.5 m/s (cf. 11.18 km/s escape velocity at 0 km altitude)
Examples: most human spaceflight, International Space Station, low-cost experimental missions, GEO. 
• Polar Orbit: Ground track passes near (i.e., within 10 degrees latitude) Earth’s N and S poles.
Examples: ICESat, perpendicular orbit planes selected for CLARREO, most EOS platforms
• Retrograde Orbit: Orbit moves westward, against the direction of Earth surface’s rotation.
Special variant: Sun-synchronous–orbit rotates to maintain approx. constant orientation with respect to Sun. 
The oblate spheriod shape of the Earth accelerates satellites approaching the equator and decelerates 
them approaching the poles. For near-polar, retrograde orbits < 5000 km altitude, there are orbital 
inclinations where the nodal precession approximates the apparent mean motion of the Sun.
Examples: Landsat series, Terra, Aqua, EO-1, POES series, most Earth resources satellites
Knox & Middleton, HyspIRI: Sampling and time series issues 24
Orbital inclination (97.9 ) represents the angle between the Earth’s equatorial 
plane, and the orbit plane, measured (by convention) at an ascending node.
National Aeronautics and Space Administration
Sun-Synchronous Orbits
For Sun-synchronous orbits the node precession rate is 0.986 
degrees/day, to match the average apparent motion of the Sun. 
Useful LEO solutions are near-polar, retrograde orbits with orbit 
poles at least 4 deg. latitude away from the Earth’s N and S poles.
(Data points from Wertz & Larson, eds. 1999. Space Mission Analysis and Design.) 
Knox & Middleton, HyspIRI: Sampling and time series issues 26
National Aeronautics and Space AdministrationH l -at-th -p le More dense coverage near
the N & S latitude limits.
Ascending and descending passes 
intersect, thus cross-track TIR scans 
are in different orientations for 
daytime and night data.
Sun-synch. LEO with 
observations about 
(+/-) nadir. 
National Aeronautics and Space Administration Knox & Middleton, HyspIRI: Sampling and time series issues 28
National Aeronautics and Space Administration Knox & Middleton, HyspIRI: Sampling and time series issues
GEO Platforms: Continuous (or no) access, with wide range of  
fixed view geometries; all possible solar illumination geometries
A single GEO platform (e.g., GEO-CAPE) provides a static pattern of viewing angles but can 
sample the full range of solar illuminations. Contrast this with the restricted illumination geometries 
from sun-synch. orbits. 
29
National Aeronautics and Space Administration
Some observations & conclusions
• Sun-synchronous low Earth orbits provide denser coverage near the poles, which 
may partially offset seasons with low reflected solar radiation.
• The interval between day-night TIR emissive observations decreases near the poles.
• The local apparent time for overpasses is commonly not 10:30 AM.
• A mission concept to meet 5-day and < 20 day sampling requirements (the reference 
orbit and instrument concepts) also provides more highly sampled areas: e.g., high 
latitudes, overlapping swaths. (Longitudes with more frequent overlap might shift over the 
mission life—depending on specs. for maintaining orbit repeat tracks.)
• TIR data might be used to interpolate ecosystem functional properties (e.g., 
seasonal canopy leaf phenology) at resolutions coarser than 60 m.
• An orbit designed for repeated global coverage compromises other attributes (e.g., 
continuous coverage, frequent opportunities for sensor inter-calibration).
Knox & Middleton, HyspIRI: Sampling and time series issues 30
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HyspIRI: Sampling and Time Series Issues

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