SeaHawk/HawkEye ▸ On-Orbit Changes
HawkEye On-Orbit Changes Over Time
by Alan Holmes
The Hawkeye Instrument been in orbit since December 3, 2018. The optics were covered by a solar panel and protected somewhat from the orbital environment up until 3/21/2019, when the solar panel was deployed and a first light image of California collected. Problems with stability and pointing prevented collection of useful ground data (for assessing calibration) until an image of Baja California on 5/24/2019 was captured. This image geometry was repeated on 5/23/2021, with very similar spacecraft attitude, pointing and sun angle, providing a good test of stability.
An analysis of dark frame data from the instrument, captured with the shutter closed, has shown no changes in CCD dark currents or offsets, nor any accumulation of dead pixels.
The most obvious change with the CCDs is the accumulation of a few dust particles over time. Since the CCD is windowless, a speck of dust landing on a 10 micron pixel can cause a large shift in sensitivity, and appears as a vertical dark line in the final image. Transfering the instrument from the manufacturer in California to integrate with the spacecraft in Glasgow, Scotland and back to Vandenberg Air Force Base, as well as the launch to space resulted in 20 to 30 new “lines” appearing in several bands due to such particles. Since launch a dust particle has appeared or shifted about once every two or three months. The impact of these particles on the data are corrected by updates to the flat field calibration tables.
Very rarely a dark line will appear in the data resulting from a radiation hit on a pixel in the dark data portion of the image. This has only been noted a few times, so radiation events seem to be rare, less than once per hundred images. A single bright pixel or two in the ground data would not be noticed, in general.
Currently the software looks for anomalous dark frame pixels and filters them out. CCD vulnerability to radiation has not been a problem, nor has exposure to direct sunlight, which likely happened many times as the spacecraft tumbled. The bandpass filters attenuate the bulk of the incident sunlight, eliminating any thermal damage.
Dark current in the CCDs in the typical instrument exposure time of 4.1 milliseconds is small, and easily corrected by use of the dark frame.
Loss of Sensitivity
Two images of the same ground location two years apart were collected, starting only two months after solar exposure began. We have looked at the ADUs detected from a dry riverbed in Baja California. Figure One shows the change detected over two years.
It is apparent that some loss of sensitivity has occurred in the blue. We have data at the one year point (that was not as closely positioned) that is consistent with this observation, but implies most of the degradation (75%) occurred in the first year in orbit. So, we expect smaller degradation going forward from this time, in the 5% per year range in the blue. One theory for the degradation is that outgassing from either the Z306 black paint or the 2216 epoxy used in the manufacturing of the instrument and condensed on the optics. Direct solar UV illumination then blackened the condensate.
Another problem is that a ghost image has appeared in each band, most noticeably in the red and near infrared bands. The figure below illustrates the ghost image of Cape Cod, Massachusetts appearing above and to the right of the parent scene.
This ghost was not visible in the initial images from orbit, was 1.5% on June 15th, 2020, and 2.9% on May 24th, 2021. It will probably max out at about 4%. It is believed to have been caused by the erosion of the antireflection coating on the exterior surface of the polarization scrambler. It is in a similar position in all bands, but not identical. It cannot be removed by processing since spacecraft motions over the 900 rows separating the ghost and the parent mean that the ghost position is not stable. In retrospect I should have used a double wedge scrambler, and purposely oriented any potential ghost to be near the parent source – at least near the same scan line. Then it could have been largely corrected. It was only 0.1% when shipped. Fortunately, using CZCS processing to correct for atmospheric transmission and sunglint reduces its effect on the chlorophyll derivation.
Signal to Noise Performance
The instrument was designed to sum 3 channels of data to improve the signal to noise ratio (SNR). However, the blur created by 3 samples 9 pixels apart, given spacecraft stability as it is at present, is not quite acceptable, so current images only use one channel, with a final SNR 60% of the design goal. However, the small pixels allow summing of nearby pixels to produce an image with resolution and SNR comparable to much larger ocean color instruments in areas of open ocean.