The A-Train: How Formation

Flying Is Transforming

Remote Sensing

Stanley Q. KidderJ. Adam Kankiewicz

Thomas H. Vonder Haar Curtis Seaman

Lawrence D. Carey

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The Afternoon Train is lead by Aqua, with an ascending equator crossing time of 1:30 pm

The A-Train

Aura

PARASOLCloudSatCALIPSO Aqua

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AquaAn observatory-class satellite with six instruments:Moderate Resolution Imaging Spectroradiometer

(MODIS), Atmospheric Infrared Sounder (AIRS),Advanced Microwave Scanning Radiometer for EOS

(AMSR-E)Advanced Microwave Sounding Unit (AMSU-A)Humidity Sounder for Brazil (HSB)Clouds and the Earth's Radiant Energy System (CERES)

Yet there is so much more to do….

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CloudSat and CALIPSO

CloudSat carries the 94 GHz Cloud Profiling Radar (CPR)

CALIPSO carries532 and 1064 nm Cloud-Aerosol Lidar with

Orthogonal Polarization (CALIOP) Imaging Infrared Radiometer (IIR) Wide Field Camera (WFC)

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PARASOLand Aura

PARASOL carries POLDER (Polarization and Directionality of the

Earth’s Reflectances)

Aura carriesHigh Resolution Dynamics Limb Sounder (HIRDLS)Microwave Limb Sounder (MLS)Ozone Monitoring Instrument (OMI)Tropospheric Emission Spectrometer (TES)

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Formation Flying:Control Boxes

Aqua is maintained in a control box of ±21.5 seconds

CALIPSO is maintained in a control box of ±21.5 seconds

PARASOL is maintained in a control box of ±21.5 seconds

30 sec

Aura is maintained ~15 minutes behind

Aqua

15 s

ec

131 seconds

73 seconds

900 seconds

CloudSat is maintained 12.5 ± 2.5

seconds ahead of CALIPSO

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Formation Flying: Horizontal Separation

Equator

20°N

40°N

40°S

20°S

60°W 30°W 0°

CloudSat

CALIPSO

Aqua

PARASOL

Aura

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Formation Flying:Horizontal Separation

NIGHT

DAY

Aqua

CloudSat & CALIPSO215 km

Ascending Node

Descending Node

To avoid sun glint, CALIPSO and CloudSat are offset 215 km in the anti-solar direction (maximum 240 km) from Aqua’s ground track at the ascending node.

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Requirement on simultaneity of radar and lidar measurements: Measurements of the same

cloud fields taken 15 seconds

Requirement/goal on spatial overlap of radar and lidar measurements: Footprints must pass 2000

meters edge to edge Equivalent to controlling

CloudSat's groundtrack to being within ±1 km of CALIPSO's lidar track

Goal for footprints to overlap at least 50% of the time

Lidar footprint(Dia= 70m)

Radar footprint(Dia= 1400m)

CloudSat Groundtrack

Position of footprints relative to groundtrack

2000 m

15 seconds(

113 km)

Goalconditionmet

Time delayedLidar footprint

Science Requirements Relatedto Formation Flying with CALIPSO

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Mid-Level Clouds

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Cloud Layer Experiments (CLEX)Ten experiments since 1995

Optically Opaque Mixed-Phase

Region (~300-500 m deep)

Precipitating Ice Region

(~.2-2.5 km deep)

Generating Cells ~ 1-1.5 km in Length

Typical Particle Concentrations: 100-200 cm-3 (Liquid)20-150 L-1 (Ice)

SupercooledLiquid

Ice

=

=

What we have learned:

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Typical Mixed-Phase Cloud Structure

The vertical profile of LWC (red diamonds) and IWC (blue diamonds) during the 14 October 2001 straight-line ascent from 1440 to 1510 UTC.

Liquid Water on Top

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4

3

Hei

gh

t (k

m)

Tem

per

atu

re (

C)

o

-5

-10

-15

-20

0.00 0.05 0.10 0.15 0.20 0.25(g m )-3

Water Content

LWCIWC

Ice Below

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B

A

Mixed-Phase Clouds Viewed By MODIS/CloudSat/CALIPSO

7/21/06 22:55 UTCMODIS 11 µm

−166− 168− 170− 172− 174− 176− 178

− 22

− 24

− 26

− 28

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VIIRS Cloud Phase

Algorithm B

A

Mixed-Phase Clouds Viewed By MODIS/CloudSat/CALIPSO

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CloudSat Radar Reflectivity (dBZ)

CloudSat Radar Reflectivity (dBZ)

BA

Hei

gh

t (k

m)

0

5

10

Hei

gh

t (k

m)

0

5

10 CALIPSO 532 nm Backscatter

-40

-30

-20

-10

0

CloudSat & CALIPSO Data

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Hei

gh

t (k

m)

0

5

10

25

-35

TB

11 (

°C)

GEOPROF-Lidar Cloud Layers: Detected by CloudSat

Detected by CALIPSO

MODIS TB11

VIIRS Cloud Phase Algorithm (top of image)

A B

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The Future: More satellites

Joining A-Train in 2008(?) are GLORY and OCO

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-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0.5 0 0.5 1 1.5

LANDSAT 7EO-1

SAC C TERRA

Orbital Plane

Equator

The Future:More Trains

The A-Train was not the first Train:

• EO-1 flew 1 min behind Landsat 7

• SAC-C flew 27 min behind EO-1

• Terra flew 2.5 min behind SAC-C

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The Future: Overflyers

A-Train (705 km)

Overflyer (~824 km)

Satellites in the same orbital plane, but at different altitudes would leverage the extensive cal/val efforts of the A-Train satellites (or satellites in other trains).

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BACKUP SLIDES

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The Future

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Hei

gh

t (k

m)

0

5

10 CloudSat Radar Reflectivity (dBZ)

BA

Hei

gh

t (k

m)

0

5

10

Hei

gh

t (k

m)

0

5

10 CALIPSO 532 nm Backscatter

GEOPROF-Lidar Cloud Layers

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-35

TB

11 (

°C)

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OCO Joining A-Train in 2008

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Formations…

Their mean anomalies and arguments of perigee must be related.

Let t be the desired separation time. Then their angular separation must be:

T

tM ~360

Their right ascensions of ascending node must be related so that they travel over the same ground track:

tdt

d earth

Assumes a circular orbit, for which M =

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A-Train Orbital Parameters

Aqua ECT = 13:35:19

A-Train satellites make 233 orbits in 16 days and fly on the WRS-2 grid

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Constellations

Several identical satellites in cooperative orbits

Make possible new observing capabilities

Take advantage of economies of scale

Can reduce launch costs

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A Sunsynchronous Constellation

• 7 satellites• Observations each

101 minutes

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Abstract

The A-Train, consisting currently of five satellites—Aqua, CloudSat, CALIPSO, Parasol, and Aura—constitutes the latest and most advanced example of formation flying. In this paper we will detail how the A-Train has transformed the way we study mid-level clouds, which obstruct visibility, pose an icing hazard to aircraft, and are difficult to forecast. Although they have been studied for many years using satellite and aircraft data, we still do not know how many mid-level clouds there are, what their geographical distribution is, or how they relate to cirrus clouds above and liquid water clouds below. A-Train instruments, especially MODIS, CloudSat, and CALIPSO, are yielding answers to these and other questions that are unobtainable by other means. Finally, we will discuss what we see as the role of formation flying in the future of remote sensing.

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Outline

The A-Train (4 min)

Examples, including mid-level clouds (4 min)

The future (4 min)

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The A-Train

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The A-Train

CloudSat lags Aqua by a variable amount <120 s

CALIPSO lags CloudSat by 15 ± 2.5 s

CloudSat and CALIPSO fly about 220 km to the right of Aqua to avoid sun glint

PARASOL lags Aqua by ~2 min

Aura lags Aqua by ~15 minStephens et al., 2002: The

CloudSat mission: A new dimension of space-based observations of clouds and precipitation. BAMS, 83, 1771-1790.

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A-Train Control Boxes

15” Aqua Orbit

15 min max

Aqua is maintained in a control

box of ± 21.5 s

21.5”

15”

21.5”

Parasol is maintained in a control

box of ± 21.5 s

21.5”

Calipso is maintained in a control

box of ± 21.5 s

CloudSat is maintained 12.5 2.5sahead of Calipso

Aura is maintained

about 15 min behind Aqua

15”

Aqua, CALIPSO, and Parasol have independent control boxes CloudSat’s control box is slaved to CALIPSO when formation flying

CALIPSO positioned 73 s behind Aqua (CALIPSO is controlled to +/-10-km at the Equator crossing

measured along the equator = +/- 21.5 sec)

• Satellite positions in the A-Train and Control Box dimensions specified in theACOCP document

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CloudSat, Aqua, and CALIPSO in Formation

orbit

116 sec(870 km)

30 sec(225 km)

43 sec

(322 km)43 sec

Aqua Control Box Calipso Control BoxCirculation OrbitCirculation Orbit

CloudSat C.B.

Circulation Orbit

≈ 15 sec(112 km)

Aqua, CloudSat, and CALIPSO in their formation configuration. Aqua leads. CALIPSO follows but maintains its motion independent of Aqua within its control box. CloudSat is tied to CALIPSO's movement around its box. CloudSat follows a small circulation orbit, 2.2 seconds (16.5 km) along-track, positioned 12.5 seconds in front of CALIPSO.

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The original goal of the CloudSat formation flying architecture was to overlay the radar footprints on the lidar footprints of CALIPSO at least 50% of the time. Analysis indicates that the overlap occurrence of radar and lidar footprints >90%, exceeding the goal.

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Examples