The A-Train: How Formation Flying Is Transforming Remote Sensing Stanley Q. Kidder J. Adam Kankiewicz Thomas H. Vonder Haar Curtis Seaman Lawrence D. Carey

2 The Afternoon Train is lead by Aqua, with an ascending equator crossing time of 1:30 pm The A-Train Aura PARASOL CloudSat CALIPSO Aqua

3 An 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….

4 CloudSat and CALIPSO CloudSat carries the  94 GHz Cloud Profiling Radar (CPR) CALIPSO carries  532 and 1064 nm Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)  Imaging Infrared Radiometer (IIR)  Wide Field Camera (WFC)

5 PARASOL and Aura PARASOL carries  POLDER (Polarization and Directionality of the Earth’s Reflectances) Aura carries  High Resolution Dynamics Limb Sounder (HIRDLS)  Microwave Limb Sounder (MLS)  Ozone Monitoring Instrument (OMI)  Tropospheric Emission Spectrometer (TES)

6 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 sec 131 seconds 73 seconds 900 seconds CloudSat is maintained 12.5 ± 2.5 seconds ahead of CALIPSO

7 Formation Flying: Horizontal Separation Equator 20°N 40°N 40°S 20°S 60°W30°W0° CloudSat CALIPSO Aqua PARASOL Aura

8 Formation Flying: Horizontal Separation NIGHT DAY Aqua CloudSat & CALIPSO 215 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.

9 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) Goal condition met Time delayed Lidar footprint Science Requirements Related to Formation Flying with CALIPSO

10 Mid-Level Clouds

11 Cloud Layer Experiments (CLEX) Ten experiments since 1995 Optically Opaque Mixed-Phase Region (~ m deep) Precipitating Ice Region (~ km deep) Generating Cells ~ km in Length Typical Particle Concentrations: cm -3 (Liquid) L -1 (Ice) Supercooled Liquid Ice = = What we have learned:

12 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 Height (km) Temperature ( C) o (g m ) -3 Water Content LWC IWC Ice Below

13 B A Mixed-Phase Clouds Viewed By MODIS/CloudSat/CALIPSO 7/21/06 22:55 UTC MODIS 11 µm −166− 168− 170− 172− 174− 176− 178 − 22 − 24 − 26 − 28

14 VIIRS Cloud Phase Algorithm B A Mixed-Phase Clouds Viewed By MODIS/CloudSat/CALIPSO

15 CloudSat Radar Reflectivity (dBZ) B A Height (km) Height (km) CALIPSO 532 nm Backscatter CloudSat & CALIPSO Data

16 Height (km) TB11 (°C) GEOPROF-Lidar Cloud Layers: Detected by CloudSat Detected by CALIPSO MODIS TB11 VIIRS Cloud Phase Algorithm (top of image) A B

17 The Future: More satellites Joining A-Train in 2008(?) are GLORY and OCO

18 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

19 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).

20 BACKUP SLIDES

21 The Future

22 Height (km) CloudSat Radar Reflectivity (dBZ) BA Height (km) Height (km) CALIPSO 532 nm Backscatter GEOPROF-Lidar Cloud Layers TB11 (°C)

23 OCO Joining A-Train in 2008

24 Formations… Their mean anomalies and arguments of perigee must be related. Let  t be the desired separation time. Then their angular separation must be: Their right ascensions of ascending node must be related so that they travel over the same ground track: Assumes a circular orbit, for which M = 

25 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

26 Constellations Several identical satellites in cooperative orbits Make possible new observing capabilities Take advantage of economies of scale Can reduce launch costs

27 A Sunsynchronous Constellation 7 satellites Observations each 101 minutes

28 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.

29 Outline The A-Train (4 min) Examples, including mid-level clouds (4 min) The future (4 min)

30 The A-Train

31 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 min Stephens et al., 2002: The CloudSat mission: A new dimension of space- based observations of clouds and precipitation. BAMS, 83,

32 A-Train Control Boxes 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 = +/ sec) Satellite positions in the A-Train and Control Box dimensions specified in the ACOCP document

33 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 Box Circulation 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.

34 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.

35 Examples