In-Flight Operations

The in-flight operation of the VIS cameras can be described by considerations of the DE-1 image shown at the upper lefthand side of Plate 2. This image is taken at the visible wavelength 557.7 nm on 2 January 1982. The nightside aurora is shown along with the position of the terminator. This terminator is defined as a line at 8° to the nightside of and parallel to the terminator at Earth's surface. The responses due to bright emissions from sunlit Earth are suppressed by reverse voltage bias of the sensor photocathode as triggered by the protective circuitry monitoring the pulse rate output. At an exemplary radial distance of the spacecraft position of 8.4 R_e the apparent angular size of Earth is 13.7°. The field-of-view of the VIS low-resolution camera is also shown by the superposed square on the DE-1 image. The low-resolution camera provides 65,500 pixels within this field-of-view. For comparison the number of pixels in a DE-1 image as taken from this altitude and within the field-of-view of the low-resolution camera would be 576 pixels.

The field-of-view of the low-resolution camera is sufficient to provide monitoring of the nightside aurora in a single frame. An anticipated series of images of an auroral substorm as returned from this camera is shown in the bottom four panels of Plate 2. These anticipated images are taken from an actual sequence of images from DE-1. The frame repetition period for DE-1 is 720 s and is to be compared with that from the low-resolution camera, 12 s. If the medium-resolution camera is used to obtain an image within the solid angle shown in Plate 2 then a four-frame mosaic must be taken. The corresponding number of pixels in the mosaic is 262,000 pixels and the mosaic repetition period is 48 s.

A closer examination of the image in the upper left-hand side of Plate 2 reveals that the frame from the low-resolution camera is not centered on the direction to Earth's center, i.e., along the nadir direction. The axis of the platform rotation, or clock angle, is directed normal to the orbital plane. Thus the platform rotation can allow the cameras' axis to be directed to any position along a line centered and running vertical in the DE-1 image. The bi-axial mirror motion within the VIS instrumentation allows for angular positioning of the two cameras for visible wavelengths at any position within a 20° × 20° field-of-view. This large field-of-view not only allows mosaicing of the entire planet at radial distances beyond 5.8 R_e, but also the monitoring of apparent stellar positions for analysis of the attitude stability of the spacecraft despun platform. This stability can be monitored at high time resolution, if required, by telemetering ``postage stamp'' images of the star's position. It is also obvious that the apparent position of the auroral oval changes as Earth rotates. Compensation for this apparent motion during an orbit is also accomplished with the bi-axial mirror. As the local time of the orbital plane changes during the course of a year then the viewing geometry with respect to the fields-of-view also changes. Thus the appropriate field stop must be activated to exclude sunlit Earth from being viewed by the secondary optics during certain periods. All of these functions are performed by the instrument's control processor. For each orbit the orbital parameters are uplinked, along with the desired mosaicing, field stop positions, camera type, filters, and platform clock angles. Each orbit is considered to be an observational campaign.

The instrument processor performs two further special functions. This processor provides the signals to the sensor electronics for shuttering the photocathodes when the long antennas mounted on the rotating spacecraft body are within the cameras' fields-of-view. Secondly the processor examines the Earth camera images to ensure that the location of the limb is being correctly identified from the onboard computations based upon the uplinked orbit, orientation of the spacecraft spin axis, and platform clock angle. The field-of-view of the Earth camera, 20 20, is fixed with respect to the despun platform and coincides with the full solid angle range of the cameras for visible wavelengths. Images from the Earth camera can be telemetered every 12 s. The number of pixels is 65,500, and 5900 of these pixels lie within the low-resolution camera frame shown in Plate 2. At a given time the image from only one of the three cameras can be processed by the instrument subsystems. The instrument processor is responsible for formatting the data, receiving and distributing the command time line, and coordinating the activities of the four compression processors. It is anticipated that the code books for image compression will be optimized during the first month or two of in-flight operations.

The key parameter (KP) data to be supplied to the Central Data Handling Facility (CDHF) for initial correlative studies with other ISTP observations consist of one image of the nightside aurora every 5 minutes. The images are to be taken with the low-resolution camera for visible wavelengths at 557.7 nm. Anticipated examples are shown in Plate 2. The images are to be targeted at an area of frequent substorm onset, 67 magnetic latitude and 2300 magnetic local time.