Imager Selection

Due to the desired high ground resolution, the optics for the cubesat had to be carefully selected. Many of the commercially available cubesat cameras simply did not have the requisite resolution or field of view requirements to make them viable on-orbit camera solutions. Instead, constructing a customized optics train (lens, sensor, and image processor combination) using commercially available products provides a means of more carefully tuning the optical performance. Read on to learn how the selected optical components are able to achieve a ground resolution of 7 meters-per-pixel.

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Components

Kowa LM100JC [1] -
Focal Length: 10 cm
Aperture Diameter: 4.2 cm

Sensor: Aptina MT9F002 [2] -
Pixel Size: 1.4 x 1.4 microns
Active Pixels: 3288 x 4384

Processor: ISIS On-Board Computer [3] -
Clock: 400 MHz
Storage: 16 GB, SD card, redundant

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Optical Theory

Optical performance metrics can be calculated using the chosen component parameters. Of particular interest are the instantaneous field of view (IFOV) and total field of view (FOV). The IFOV indicates the field of view of one pixel on the sensor in angle-space, so to achieve high resolutions IFOV should be as low as possible, emphasizing a long focal length and a small pixel size. However, if the pixel field of view is too small, more images will be required to achieve the desired coverage. Therefore, another desirable metric is a high number of pixels on the sensor to keep the IFOV small while the total FOV of the imager is large. This combination minimizes the number of images required  while maximizing the resolution-per-pixel.

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Optical System Performance

Given the selected components and the metrics described above, here are the key performance metrics. The ground resolution metrics are computed by projecting the field of view onto the surface of the Earth from our chosen altitude (500 km) to determine the covered area on a per-image and per-pixel basis.

Image Specifications (pre-compression):

• Bits Per Channel: 8
• Channels Per Pixel: 3
• Pixels Per Image: 14,414,592
• Bits Per Image (uncompressed): 345,950,208

Optical Specifications (horizontal x vertical):

• IFOV: 2.887 x 2.887 arcsec
• FOV: 3.517 x 2.637 deg
• Image Area Covered: 30.70 x 23.02 km
• Pixel Resolution: 7 x 7 m

The optical system specifications informed much of the design of the remainder of the system, including:

• Minimum Satellites Required (86) = Earth circumference (40,075 km) / Image Width (30.70 km)
• Image Frequency (3.02 sec) = Image Height (23.02 km) / Orbital Velocity (7.61 km/s)

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References
[1] - Kowa LM100JC Product Page - http://www.kowa.eu/lenses/en/LM100JC.2805.php
[2] - Aptina MT9F002 Product Page - https://www.aptina.com/products/dsc%1Cdvc/mt9f002/
[3] - ISIS Space On-Board Computer Brochure - http://www.isispace.nl/brochures/ISIS_ISIS%20OBC_Brochure_v.15.6.pdf