1. The Instrument
 

CRISTA is a limb-scanning experiment that measures thermal emissions (4 - 71 µm) of selected trace gases with high spatial resolution in three dimensions (Offermann, 1993). By means of the measured limb radiance fields, dynamic structures of the atmosphere with small horizontal scales (300 km) can be resolved. The vertical resolution is in the order of 2 - 3 km.

The CRISTA instrument is mounted on the ASTRO-SPAS platform (Fig. 1) which is released from the Shuttle and operates at a distance of 20 - 100 km behind the Shuttle. For improved horizontal resolution CRISTA uses three telescopes that sense the atmosphere simultaneously at angles 18° apart (Fig. 1). To achieve high measuring speed and, consequently, high spatial resolution along the track, the detectors and the optics of the instrument are cooled by cryogenic helium. The incoming limb radiance is analyzed by four grating spectrometers of the Ebert-Fastie type with a spectral resolving power of about 500. Trace-gas emissions measured by CRISTA are listed in Tables 1 and 2 in the next section.

On Nov 3, 1994 CRISTA was launched aboard the Space Shuttle Atlantis into a 300 km, 57° inclination orbit and measured about 50,000 height profiles of limb radiance spectra by using a number of different measurement modes (including scans of different altitude regions of the atmosphere) and some calibration modes.

CRISTA raw data are uncalibrated detector output voltages, signals from tilting mirrors and gratings, and housekeeping data. These data are supplemented by spacecraft attitude and status data. The CRISTA data are converted to limb-radiance spectra on an instrument-sampling grid by means of calibration data derived from laboratory measurements and in-flight data.

The fast measurement speed of CRISTA results in a large amount of radiance data. The radiative transfer calculations, required to retrieve atmospheric temperatures and trace-gas mixing ratios, are very complex due to the large wavelengths and altitude ranges of the measurements (see Table 2). Since several iterations have to be performed, the quality of the retrieved data depends on the accuracy and speed of these calculations. To address the requirements of CRISTA, fast-forward models are used that are based on the Bandpak library (Marshall et al.,1994). The approach utilizes precalculated emissivity growth coefficients (see Gordley and Russell, 1981) that were generated by means of the Linepak algorithms (Gordley et al., 1994). The CRISTA emissivity data allow to calculate limb-radiance spectra to be calculated at the spectral resolution of the instrument. To retrieve atmospheric temperatures and trace-gas fields a multiple-emitter onion-peeling scheme is used.

REFERENCES

Gordley, L.L. and J.M. Russell III, Rapid inversion of limb radiance data using an emissivity growth approximation, Appl. Opt.,20, 807, (1981).

Gordley, L.L., B.T. Marshall, and D.A. Chu,Linepak: Algorithms for modeling spectral transmittance and radiance, JQRST 54, 563 (1994).

Marshall, T.B., L.L. Gordley, and D.A. Chu, Bandpak: Algortithms for modeling broadband transmission and radiance,JQRST 54, 581 (1994).

Offermann, D., CRISTA: A Shuttle experiment for middle atmosphere small scale structures, in: E.V. Thrane et al.(editor), Coupling processes in the lower and middle atmosphere, p. 389, Kluwer Academ. Publ., Holland (1993).

More can be found in Eos, Transactions, AGU, 76, 337-338, 1995 and on the CRISTA Homepage /
 
 

2. CRISTA 2: Instrumental Improvements and Mission Plan
 

For the second flight the CRISTA instrument and mission will have the following differences and special features:

• increased horizontal resolution (100 km x 100 km) in selected areas at the equator
• increased latitudinal coverage: 74° S to 74° N
• increased altitude range: scans below the tropopause
• increased sensitivity: improved measurements of CO, NO, temperature, and pressure
• increased validation measurements: by rocket salvoes, instrumented rockets, airplane with "zero-miss-distance"
• high-resolution measurements in the southern winter hemisphere

2.1 CRISTA-2 detectors and trace gases

CRISTA 2 has four spectrometers: SL, SCS, SR for left, center, and right telescopes and short wavelengths, SCL is for longer wavelengths and the center telescope only. The following tables provide the individual spectral channels or detectors assigned to these spectrometers and a summary of wavelengths and trace gases measured by CRISTA.
 

2.2 CRISTA-2 Mission Plan and measuring modes

Each CRISTA measuring mode is characterized by the altitude range and the step width for each spectrometer ("altitude scan mode"), and each mode also requires a special alignment of the ASTRO-SPAS satellite ("alignment mode"). The DASA and MAHRSI teams will be provided with the required alignment angles. .

Altitude scan modes are:
 

Mode S (Stratosphere)

Mode SL (Stratosphere Low)

Mode M (Mesosphere)

Mode T (Thermosphere)

Mode VAL (Validation)

Mode H (Hawk-eye, 90°/3D)

Alignment modes are:
 

"Ping-pong" (Standard for Modes S, SL, M, T)

Mode H ("Hawk-eye", 90°/3D)

0° orbit (=backward to flight direction)

Mode VAL (Validation, pointing at Wallops Island, Hohenpeissenberg, and Falcon aircraft comparisons)

"Normal” pointing is the so-called ping-pong mode with the center telescope pointing northward at ascending nodes and southward at descending nodes to increase the latitude coverage from 74° S to 74° N.

Mode H (Hawk-eye) is planned for a sequence of three orbits passing a low-latitude region. Typically, in the first orbit CRISTA will be directed roughly 90° to the right (northward, HnN), in the second 0° (backward to flight vector, HnB), and in the third 90° to the left (southward, HnS). This scheme will increase the horizontal and vertical resolution of the observations. The altitude scan mode is the same as for Mode S.

The validation mode (Mode VAL) is used to point the center telescope at Wallops Island, VA and Hohenpeissenberg, Germany, in order to perform the zero miss time and zero miss distance validation experiments as described in Chapter I. A special altitude scan mode during Mode S periods (V01-V06) will be used, but during Mode M periods (V07,V08) the altitude scan mode is also Mode M and will not change. For these last two validations the coincidence will be optimized for the SR spectrometer, which (along with SL) measures down to the stratosphere.

Additionally, the spectral scans can have a "standard” or "extended” spectral range (see Chapter II, Table 1). The following tables give the altitude ranges and step widths for the different modes for each spectrometer.
 
 

Mode S (Stratosphere) - x = 240 km; every 6^th profile with "extended” spectral range
 

Spectrometer	Altitude range  (km)	Altitude step  (km)	#steps
SL	11.0 - 55.0	2.0	23(+3)
SR	11.0 - 55.0	2.0	23(+3)
SCS	11.0 - 55.0	2.0	23(+3)
SCL	31.0 - 75.0	2.0	23(+3)

Modus SL (MET 1/21.0 to MET 2/7.3) - x = 255 km;

(Stratosphere Low: scans below the tropopause)
 

Spectrometer	Altitude range  (km)	Altitude step  (km)	#steps
SL	7.0 - 55.0	2.0	25(+ 3)
SR	7.0 -55.0	2.0	25(+ 3)
SCS	7.0 - 55.0	2.0	25(+ 3)
SCL	27.0 - 75.0	2.0	25(+ 3)

Mode M (Mesosphere) - x = 420 km; every 4^th profile with "extended” spectral range
 

Spectrometer	Altitude range  (km)	Altitude step  (km)	#steps
SL	15.0 - 105.3	2.15	43(+ 3)
SR	15.0 - 105.3	2.15	43(+ 3)
SCS	40.0 - 130.0	2.15	43(+ 3)
SCL	60.0 - 150.0	2.15	43(+ 3)

Mode T (Thermosphere) - x = 420 km
 

Spectrometer	Altitude range  (km)	Altitude step  (km)	#steps
SL	60.0 - 102.0	1.0	43(+ 3)
SR	60.0 - 102.0	1.0	43(+ 3)
SCS	60.0 - 165.0	2.5	43(+ 3)
SCL	80.0 - 185.0	2.5	43(+ 3)

Mode VAL (Validation) - x = 290 km; for special orbits pointing at WFF and Hohenpeissenberg
 

Spectrometer	Altitude range  (km)	Altitude step  (km)	#steps
SL	11.0 - 67.0	2.0	29(+ 3)
SR	11.0 - 67.0	2.0	29(+ 3)
SCS	11.0 - 67.0	2.0	29(+ 3)
SCL	31.0 - 87.0	2.0	29(+ 3)

Table 3: Detailed CRISTA-2 Mission Timeline (Subject to change; Status: June 1997, times approx. Mission Elapsed Time) a) General events and CRISTA altitude scan modes
 

from MET (day/hour)	to MET  (day/hour)	Event/  altitude scan mode	Remarks
0/0.0		Launch
0/3.0		Grapple
0/3.0	0/7.0	Checkout
0/7.0		Deploy
0/7.0	0/13.0	Checkout
0/13.0		Open MDA
0/11.0(15.0)	1/6.7	Mode M	M01,MVR01,M02
1/6.7	1/13.5	Coalignment 1	Jupiter, Moon, Sirius  MDA Cal
1/13.5	1/21.5	Mode T	T01
1/21.5	2/7.5	Mode SL	SL01
2/7.5	2/18.2	Mode S	S01
2/18.2	2/18.7	Mode VAL	V01, Wall./Hohenp.
2/18.7	2/22.2	Mode S	S02
2/22.2	2/23.3	Mode VAL	V02, Wall./Hohenp.
2/23.3	3/22.6	Mode S	S03,SVR01,S04
3/22.6	3/23.5	Mode VAL	V03, Wall./Hohenp.
3/23.5	4/21.4	Mode S	S05
4/21.4	4/23.8	Mode VAL	V04, Wall./Hohenp.,Falcon
4/23.8	5/9.7	Mode S	S06,SVR02,S07
5/9.7	5/11.2	STARE	STARE01
5/11.2	5/21.7	Mode S	S08,V05,S09
5/21.7	5/23.3	Coalignment 2	Moon  MDA Cal
5/23.3	5/23.7	Mode VAL	V06
5/23.7	6/22.4	Mode M	M02,MVR02,M03  V07, Wall./Hohenp.
6/22.3	6/23.9	STARE	STARE02
6/23.9	7/24.0	Mode M	M04, V07, Wall./Hohenp  M05.
8/0.0	8/5.7	Mode T	T02
8/5.7	8/6.2	STARE	RELAX
8/6.2	8/12.9	Coalignment 3	Jupiter, Sirius, Moon  MDA Cal
8/12.9	8/19.0	Mode T	T03
8/20.0	9/2.5	Rendezvous
9/2.5		Grapple

Further remarks:

SVRnn, MVRnn: For certain short periods during Mode S and Mode M, the spectrometers scan on each altitude scan twice (forward and backward, "VR"=Vorwärts/Rückwärts) to study relaxation effects on the detectors.

STARE01: "Staring", fixed altitudes (SCS: 18 km, SCL: ~30 km, SL, SR: 30 km); Alignment ~90° Southward; operation of multiple detector.

STARE02: ASTRO-SPAS looking to nadir.

Table 3: b) Special ASTRO-SPAS Attitude Modes (except Validation during Mode S which is included in the preceding table)
 

from MET (day/hour)	to MET  (day/hour)	CRISTA mode	Remarks/  ASTRO-SPAS attitude
2/7.5	2/11.0	Mode H	H1N, H1B, H1S
2/18.3	2/20.0	Mode S	0° orbit
2/20.0	2/22.3	Mode H	H2B, H2N
3/6.0	3/9.6	Mode H	H3N, H3B, H3S
3/19.8	3/22.2	Mode H	H4B, H4N
4/6.1	4/9.6	Mode H	H5N, H5B, H5S
4/18.3	4/20.8	Mode H	H6B, H6N
5/6.2	5/11.2	Mode H	H7N, H7B, H7S
5/17.0	5/18.8	Mode S	0° orbit
5/18.8	5/21.0	Mode H	H8B, H8N
6/21.4	6/23.8	Mode M	V07, Wall./Hohenp., Falcon
7/20.4	6/21.2	Mode M	V08, Wall./Hohenp.

 

Inflight line of sight calibration of CRISTA

To convert trace-gas emissions to volume-mixing ratios, it is necessary to know the exact measurement location in the earth's atmosphere. After the mission, the attitude reconstruction of the ASTRO-SPAS and CRISTA line-of-sight calibrations of the primary mirrors will be used to calculate an accurate tangent height (±200 m) for each atmospheric scan step. To check the alignment between the ASTRO-SPAS satellite and CRISTA during the mission, an inflight line of sight calibration ("Coalignment") is implemented in the mission using the planet Jupiter as a calibration source.

During two phases, at the beginning and the end of the mission (Coal #1, Coal #3, see table 3), the ASTRO-SPAS will operate in the inertial pointing mode for star measurements and point the optical axis of each of three CRISTA telescopes at Jupiter. The alignment of each telescope takes the time required for a complete orbit. At the beginning of the measurement the ASTRO-SPAS starts to slew slightly in the horizontal direction (= local horizontal during limb measurements, ±1° relative to the nominal direction) and the primary mirror of the telescope scans in the vertical direction ( ±0.5°). Vertical and horizontal alignment angles can thereby be determined. Additional alignments of the MAHRSI instrument on Moon and Sirius are also planned (each target during one orbit, Coal #1-3). When the planet or star has been located by CRISTA and MAHRSI and if there should be large misalignment angles, an adjustment of the alignment can be implemented during the mission.

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