AOS Tandem Stereographic Camera Target Parameter List.pdf

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TANDEM STEREOGRAPHIC CAMERAS

INSTRUMENT CONCEPT AND PERFORMANCE TARGETS

The main purpose of the Tandem Stereographic Cameras is to provide information on low cloud dynamics. The related observing concept envisioned by AOS consists of two multi-angle instruments onboard two spacecrafts flying in close formation flight as illustrated in Figure 1 (one instrument per spacecraft). Each of the two instruments consists of two or more high-resolution cameras, one pointed at nadir and the others at an off-nadir angle intersecting the nadir pointing view of the high-resolution camera on the other spacecraft (see Figure 2). Each of the resulting sets of simultaneous stereographic images enable an instantaneous geometric measure of the height of cloud/aerosol plume-tops or surface features. The time-lapse between the two simultaneous stereographic images, allows one to derive cloud or plume-top motion vectors through tracking of image feature displacements.

Figure 1 Illustration of how two view angles (one at nadir) from tandem spacecraft provide stereographic and time-lapse imagery for retrieval of cloud-top heights, horizontal winds, and vertical winds.

Figure 2 Tandem Stereographic Cameras concept of operations with instruments on tandem spacecraft (S/C). The nadir and backward-viewing cameras on the leading spacecraft (green) and the nadir and forward-viewing cameras on the trailing spacecraft (blue) acquire images continuously along the dayside flight track. An optional third camera (shown in fainter hues) on both instruments enables S/C yaw maneuvers without interruption to Tandem Stereographic Cameras measurements. If the available power and data rate resources permit simultaneous operation of all three cameras, it could provide additional information about the observed scenes. Alternative approaches are allowed.

The Tandem Stereographic Cameras should be able to accommodate yaw maneuvers, in which the satellite is rotated 180 degrees such that forward facing cameras now face aftward and vice versa (see Figure 2), and enable the acquisition of two instantaneous stereo image pairs independent of the spacecraft orientation along its velocity (x) vector (see Figure 4). A symmetric three camera design is one possible solution, as illustrated in Figure 2, where the

2 | P a g e o f 1 1 optional third camera is depicted more faintly than the other two. Other designs can be chosen to accommodate yaw maneuvers as long as the science data quality cam be maintained.

The time separation between the two spacecrafts is expected to range between 30 and 60 seconds. The off-nadir camera’s view angle (defined as the along-track off-nadir angle of the camera boresight measured at the instrument) target is nominally set to -38° and +38° (if three cameras are used to allow for the yaw-flip maneuver as described above) to enable co-registration (at the center of the focal plane array in along-track direction) at the average time separation of 45 seconds.

A notional instrument design uses a focal plane detector area array with rapid readout of a single detector row and on-board summing of multiple reads enables the use of time delay and integration to achieve pushbroom images with an along-track ground sample distance (along-track GSD, spacing of successive images determined by the integration time and the number of summed readouts, not dependent on along-track view angle) and ground instantaneous field of view (GIFOV, cross-track pixel footprint projected onto ground, dependent on view angle across the swath) that meets the instrument performance targets. A benefit of using an area array with a windowing option is that the row to be read out, and the corresponding along-track angle within the field of view is selectable in-flight to assure image co-registration between the two spacecraft to compensate for static assembly errors in instrument pointing, offsets in spacecraft pointing, and possible changes in orbit separation ranging between 30 to 60 seconds (see Figure 3). Other approaches can be chosen as long as they meet the specified instrument performance targets.

Figure 3 Simplified depiction of the nadir (green) and forward off-nadir (blue) area array projected onto the ground.

The lines illustrate the along-track ground sample distance (GSD) and cross-track ground instantaneous field of view

(GIFOV). The red line indicates the instrument swath and the array readout locations providing co-registration between the nadir and off-nadir views in the case of fixed boresight angles (0° and 38°) and varying spacecraft separation of 45±15 seconds. In this case, the GSDs of the nadir and off-nadir views are matched and the larger off-nadir GIFOV would reduce image contrast, which would impact the geophysical retrievals.

The GSD and GIFOV are driving instrument performance factors to achieve the desired science data accuracy for cloud-top height and horizontal and vertical cloud-top velocity. The smallest possible GSD and GIFOV are therefore desirable while maintaining a swath near the target value of 100 km. For geophysical retrievals, it is desired for the nadir GSD and GIFOV to be 40 m or finer. It is also desired for both, the nadir and off-nadir GSD to be matched by using the same integration time and summed readout. If resource and cost savings can be achieved by using identical nadir and off-nadir camera (optical) designs, the off-nadir GIFOV can grow up to 60 m.

Another driving instrument performance factor is the ability to provide high contrast of clouds above ocean and land surfaces (free of snow and ice). Wavelengths dominated by either Rayleigh scattering, highly reflective vegetation (NIR), or strong atmospheric absorption would be less favorable. A solution centered around 620 nm was used in

FWDNADIR

GI

FO

V

GSD

Swath

GI

FO

V

GSD

GI

FO

V

GSD

S/C separa�on: 30 sec. 45 sec. 60 sec.

Flight direc�on:

3 | P a g e o f 1 1 previous scientific assessments. Imagery of bright clouds and snow shall not saturate up to 1.3 equivalent reflectance (defined as the product of π times the spectral band-averaged radiance, divided by the band-averaged solar irradiance) or be affected by polarization. Besides the signal to noise ratio (SNR) target values listed in Table 1, the bit depth of the digital data output should be adequate (preferably > 11 bit).

Table 1 Overview of signal to noise ratio (SNR) target values. Equivalent reflectance is defined in the text. The assumed top-of-atmosphere solar irradiance (at 620 nm) is 1610 W/m2/µm. The noise-equivalent delta radiance is the ratio between the upwelling radiance at the sensor level and the instrument SNR.

Equivalent Reflectance

Reflected Radiance [W/m2/µm/steradian]

NedL [W/m2/µm/steradian]

SNR target values

0.01 5.124 0.059 87

0.05 25.62 0.128 201

0.10 51.24 0.180 285

0.50 256.2 0.401 639

1.00 512.4 0.567 904

1.30 666.1 0.646 1031

Notionally, the proposed instrument should be pre-flight calibrated for linearity, flatfield, spectral and radiometric response, polarization, as well as geometry and temporal stability. On-orbit, absolute radiometric calibration is not required. However, relative radiometric stability is desired to maintain approximate radiometry between cross-calibration events using e.g. an imaging polarimeter on the same platform. It can be assumed that on-orbit geometric calibration will be performed regularly using ground control points to ensure image co-registration.

Summary of key instrument performance targets:

Spatial resolution

See text for definitions.

Nadir GSD: ≤ 40 m Nadir GIFOV: ≤ 40 m Off-nadir GSD: ≤ 40 m Off-nadir GIFOV: ≤ 60 m / ≤ 40 m (desired, see text)

View angles Defined as the along-track off-nadir angle of the camera boresight measured at the instrument.

Nadir: 0° Off-nadir: -38° and +38°

Swath ≥ 100 km Wavelength 1 band in the visible part of the solar spectrum is required. The wavelength for stereo imaging shall be chosen to provide high contrast between clouds or aerosol plumes, their surroundings, and the underlying surface.

Additional bands can be considered as a variant to enhance geophysical retrieval capabilities. Impacts on instrument cost and resource needs should be provided.

Bandwidth < 200 nm, and chosen to meet the SNR requirements without saturation over bright clouds or snow at

1.3 equivalent reflectance (see text for definition) while keeping the full bandpass for stereo imaging shortward of the surface vegetation “red edge”.

SNR ≥ 120 at 0.02 equivalent reflectance; ≥ 1000 at 1.3 equivalent reflectance. See Table 1 for additional values.

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INSTRUMENT RESOURCE ALLOCATION TARGETS

The AOS team has developed target spacecraft resource allocations for the Tandem Stereographic Cameras based on responses received through Request for Information (RFI). From this information the mission systems team developed spacecraft concepts commensurate with allocations as found in Table 2. Those values are not requirements but rather notional resource needs currently envisioned by the AOS team. Exceedance of these values are acceptable and expected, especially in the event of enhanced performance capability.

Table 2 Tandem Stereographic Cameras Target Resource Allocations

Tandem Stereographic Camera allocations are given for each of the two identical instruments (per S/C).

Resource Units Target Allocation (Current Best Estimate)** Mass kg 18 Operational Power (Orbit Average) W 19 Envelope Dimensions (LxWxH) in Operational Configuration cm 75 x 20 x 40

Data Rate (Peak*) bits/second 1.6x107 (with lossless compression)

*Peak data rate is the nominal rate while the instrument is in its acquisition mode.

**Please provide both the Current Best Estimate (CBE) and the Maximum Expected Value (MEV) for these resources. MEV = [(100 + XX)/100] CBE where XX is contingency in percent.

Figure 4 Instrument reference coordinate system.