NPOESS Orbit: Mean Local Time of Ascending Node vs...
14
NPOESS Orbit: Mean Local Time of Ascending Node vs. Spacecraft Time in Shadow Michael J. Kavaya NASA Langley Research Center Working Group on Space-Based Lidar Winds Wintergreen, VA June 17, 2009 Kavaya-1
Transcript of NPOESS Orbit: Mean Local Time of Ascending Node vs...
NPOESS Orbit: Mean Local Time of Ascending Node vs. Spacecraft Time in Shadow
Michael J. KavayaNASA Langley Research Center
Working Group on Space-Based Lidar WindsWintergreen, VA
June 17, 2009
Kavaya-1
Motivation• Customers for wind mission are NASA, IPO/NPOESS, and DOD
• NPOESS orbit height is 824 km
• NPOESS mean local time of ascending node (MLTAN) (local time when crossing
equator from south to north) either or 1330 (C-1, C-3) (afternoon), or 1730 (C-
2, C-4) (morning) [inclination angle = 97.727 deg]
• Active remote sensors need power, hence sunlight;
hence sun-synchronous orbit
• Can Doppler lidar work with any node time?
• Typical orbit software is very expensive and complicated;
includes rigorous position vs. time
• We only need time in shadow
Kavaya-2
Simplified Approach in MS Excel• Neglect earth rotation, local time, and local position• Sun always on + y axis• +z axis always points north• Begin with orbit plane normal = +y axis• SS orbit precession keeps orbit plane normal within the yz plane
• For inclination angle i, tilt orbit plane about x axis (top of orbit away from sun by i – 90 deg.
• For seasonal tilt, tilt orbit plane instead of earth, about x axis (top of orbit towards sun in summer)
• For mean local time of ascending node, tilt orbit plane about z axis (6 am = normal within yz plane)
N+z
+y
+x
Kavaya-3
Earth Orbit Around Sun
polenorth at thesun the towardis tilt Positiveyear into months ofnumber
12/371m2cos5.23Tilt(m)
=
−
°=
m
π
Kavaya-4
y = 0 y = Res =1.49597870691 x 1011 m
De = Earth Diameter =1.2756 x 107 m
Ds = Sun Diameter1.392 109 m = 109 x earth diam.
Re = De/2 = 6.378 x 106 mRs = Ds/2 = 6.96 x 108 m
ees
es RyR
RRz +−
=3
ees
es RyR
RRz −+−
=4
ees
es RyR
RRz +−−
=1
ees
es RyR
RRz −+
=2
Sunlight, Penumbra, and Shadow
1. Step through time
2. Calculate (x, 0, z) of normal orbit
3. Rotate orbit about x and z axes
4. Look for positions in penumbra and shadow
x = 0 plane
Kavaya-5
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80 90 100
0.1
= Sh
adow
, 0.5
= P
enum
bra
Time (Minutes)
12 13 14 15 16 17 18 19 20 21 22 23 24
824 km, +0 deg axis tilt (Sept 21)Orbit Period = 101.2 minutes
MLT of ascending node = 12:00, 13:00, etc.Vary MLTAN
full sun
penumbra
shadow
Kavaya-6
824 km, +0 deg axis tilt (Sept 21)Orbit Period = 101.2 minutes
MLT of ascending node = 12:00, 13:00, etc.Vary MLTAN
NPOESS MLTAN = 1330 or 1730
0
5
10
15
20
25
30
35
40
12 13 14 15 16 17 18 19 20 21 22 23 0
Min
utes
MLTAN
0.00
0.20
0.40
0.60
0.80
1.00
1.20
12 13 14 15 16 17 18 19 20 21 22 23 0
Ave
Sun
light
Fac
tor
MLTAN
Minimum orbit average
sunlight factor = 0.65
1330 ~ 0.665
Kavaya-7
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80 90 100
0.1
= Sh
adow
, 0.5
= P
enum
bra
Time (Minutes)
12 13 14 15 16 17 18 19 20 21 22 23 24
824 km, +23.5 deg axis tilt (June 21)Orbit Period = 101.2 minutes
MLT of ascending node = 12:00, 13:00, etc.Vary MLTAN
full sun
penumbra
shadow
Kavaya-8
824 km, +23.5 deg axis tilt (June 21)Orbit Period = 101.2 minutes
MLT of ascending node = 12:00, 13:00, etc.Vary MLTAN
NPOESS MLTAN = 1330 or 1730
0
5
10
15
20
25
30
35
40
12 13 14 15 16 17 18 19 20 21 22 23 0
Min
utes
MLTAN
0.00
0.20
0.40
0.60
0.80
1.00
1.20
12 13 14 15 16 17 18 19 20 21 22 23 0
Ave
Sun
light
Fac
tor
MLTAN
Kavaya-9
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80 90 100
0.1
= Sh
adow
, 0.5
= P
enum
bra
Time (Minutes)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
824 kmOrbit Period = 101.2 minutes
MLT of ascending node = 1330 (1:30 pm)Vary Month (21st)
Kavaya-10
824 kmOrbit Period = 101.2 minutes
MLT of ascending node = 1330 (1:30 pm)Vary Month (21st)
NPOESS MLTAN = 1330 or 1730
33.4
33.5
33.6
33.7
33.8
33.9
34
34.1
34.2
34.3
1 2 3 4 5 6 7 8 9 10 11 12
Min
utes
Month N, 21st Day
0.659
0.660
0.661
0.662
0.663
0.664
0.665
0.666
0.667
0.668
1 2 3 4 5 6 7 8 9 10 11 12
Ave
Sun
light
Fac
tor
Month N, 21 st Day
Kavaya-11
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80 90 100
0.1
= Sh
adow
, 0.5
= P
enum
bra
Time (Minutes)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
824 kmOrbit Period = 101.2 minutes
MLT of ascending node = 1730 (5:30 pm)Vary Month (21st)
Kavaya-12
824 kmOrbit Period = 101.2 minutes
MLT of ascending node = 1730 (5:30 pm)Vary Month (21st)
NPOESS MLTAN = 1330 or 1730
0
2
4
6
8
10
12
14
1 2 3 4 5 6 7 8 9 10 11 12 0
Min
utes
Month N, 21st Day
0.800
0.850
0.900
0.950
1.000
1.050
1 2 3 4 5 6 7 8 9 10 11 12 0
Ave
Sun
light
Fac
tor
Month N, 21 st Day
Minimum orbit
average
sunlight factor = 0.87
Kavaya-13
Conclusions
• New excel tool allows quick examination of time in
shadow for different orbit parameters
• Mission trades can include very important parameter of
orbit average sunlight and electrical power
Kavaya-14
