The use of satellite-based and aerial-based imagery of the Earth is popular among government and commercial entities. Satellite images may be collected with multiple different sensors (for example in DigitalGlobe's WV-3 satellite) that at any given instant in time view different points on the ground. For example, a satellite may contain many separate sensors that are each line scanners.
Each sensor may have one or more bands (e.g., 3-15 bands). Further, one or more of the sensors may be populated with multispectral VNIR sensors, having a ground resolution of 1.24 meters. VNIR has a fairly standard meaning in the industry of the portion of the electromagnetic spectrum from roughly 400 to 1100 nanometers in wavelength. And multispectral refers to the use of multiple narrower wavelength ranges throughout the range. For example, it might refer to eight specific wavelength bands within the 400-1100 nanometer range (e.g., coastal (approximately 400-452 nm), blue (approximately 448-510 nm), green (approximately 518-586 nm), yellow (approximately 590-630 nm), red (approximately 632-692 nm), red edge (approximately 706-746 nm), near infrared 1 (NIR1) (approximately 772-890 nm), and near infrared 2 (NIR2) (approximately 866-954 nm)). Also, one or more of the bands in one or more of the banks may be populated with panchromatic sensors, having a ground resolution of 0.31 meters. Panchromatic has a fairly standard meaning in the industry of a relatively broad spectral band that may include all or most of the visible spectrum (450 to 700 nanometers) and possibly other regions adjacent to the visible spectrum (e.g., 450 to 800 nanometers). Also, one or more of the bands in may be populated with SWIR sensors, having a ground resolution of 3.7 meters. SWIR has a fairly standard meaning in the industry of the portion of the electromagnetic spectrum from roughly 1100 to 3000 nanometers in wavelength.
Further, the WV-3 satellite uses a line scanner that is thousands of pixels wide and has only a few such rows for each of the panchromatic band, several multispectral (MS) bands, CAVIS bands, and so forth.
With an integrated sensor containing each of these sensors, the integrated sensor field-of-view is typically swept across the Earth's surface in “push broom” fashion. Additionally, the attitude (angular position/orientation) of the satellite may be adjusted to view different areas on the Earth's surface. Necessarily, many if not all of the different viewing angles will be from a non-nadir position.
As described above, the panchromatic and MS bands each have their own section of detectors that simultaneously collect the image. All bands eventually overlay the target area, creating a complete image. The orbital motion of the satellite is factored into the scan profile, which results in an image that covers the target area. Exposure is controlled by the scan rate/line rate as well as the time-delayed-integration, which result in minimal pixel smearing during the scan. Scanning arrays are able to collect a large amount of area in a short time.
Area array image sensors are typically less expensive than line scanner image sensors, and they capture an image of a much larger ground area than image sensors. However, because of the amount of time necessary to transfer all of the image data off of an area array image sensor, and because of the length of time that a given pixel needs to collect photons from the Earth's surface, it has previously not been practical to use area array image sensors for remote sensing from satellites.
These area (or framing) arrays (as what is found in consumer cameras) capture a whole scene in a single snap. They are usually small in image size compared to a line scanner, which collects a longer image. The advantage of area arrays is that: (1) they are cheaper to buy and align in the camera during construction; (2) they are smaller, but useful for small satellites; and (3) all the pixels are aligned with one another, so it is easier to geo-locate the other pixels once one of the pixels is geo-located.
However, proper exposure time is needed. Because of the orbital motion of the satellite, the boresight of the camera has to be steadily maintained on the target during the image for the long-enough exposure, otherwise the image will be smeared.
It is against this background that the improvements disclosed herein have been developed.