PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = " 2023-09-01 Initial Version. " OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "JNO" INSTRUMENT_ID = "ASC" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ADVANCED STELLAR COMPASS" INSTRUMENT_TYPE = "CCD CAMERA" INSTRUMENT_DESC = " [Note: The following description has be excerpted from the Volume Software Interface Specification (SIS) document which is located in the DOCUMENT directory of this volume. For additional information, including the figures referenced in the text, please see that document.] Science objectives The primary role of the ASC instrument is of a functional nature. It was included as part of the magnetometer investigation (MAG = FGM + ASC) as an engineering instrument, to supply accurate attitude to the vector magnetometers at the end of the MAG boom and solar array panel. However, during cruise and the prime mission it was realized that data from the ASC camera heads has scientific value, namely the recorded images and indirect radiation measurements (particle hit counts). Sensors The Camera Head Unit (CHU) is a monochrome camera with a 19 degree x 13.5 degree field of view and a 752x580 pixel CCD sensor. Captured images can be saved for future transfer to the spacecraft memory and then to the DSN. By nature, CCD sensors are affected by charged particles. When a particle traverses the active surface of the sensor it leaves an ionization trail spanning one or more pixels, known as hotspots. By quantifying the number of hotspots, an indirect measure of radiation can be extracted (related to the fluence of the effecting particles within the CCD integration period). The four ASC CHUs are located on the MAG boom of Juno, one pair on each magneto-optical bench, as can be seen in Figure 1. The camera heads are designated A, B, C, and D. Electronics The Data Processing Unit (DPU) holds the necessary electronics for ASC operations. In addition to handling communications and normal operation procedures (attitude determination etc.), other functionalities are included, such as quantifying the number of hotspots. Measured Parameters Radiation counts The interline transfer type CCD consists of a photo-sensitive area (integration layer) and a readout layer - both are sensitive to energetic charged particle irradiation. The relative sensitivity has been established from ground calibrations (see section 2.6). When a charged particle, sufficiently energetic to pass through the electronics' mass shielding, traverses the CCD sensor's substrate, many electrons are liberated from the lattice structure and are captured within the CCD's charge wells. Consequently, when read out, the affected pixel(s) will have a false elevated photon count. This ionization is not damaging to the affected pixel(s). After being read out, the charge well will return to its nominal level and sensitivity to photons. The effect is therefore transient. In the area of the image formed around the particle's interaction, the affected pixel will stand out with an elevated count. Since the interaction expresses itself as an isolated bright pixel, the formed image object will be referred to as a hotspot in this document. To differentiate between damaged pixels that continuously have elevated counts, these pixels may in some contexts be referred to as transient hotspots. Counting the number of transient hotspots in an image provides an efficient means of deriving the surrounding particle flux for particles with energies above the mass shielding stopping power. The measured hotspot count is telemetered as part of the periodic attitude telemetry that otherwise provides orientation quaternions to support the JUNO MAG investigation. Images The MicroASC instrument under normal operation captures and stores an image at a rate set by the integration time. These images however are discarded after they have been processed. The instrument has the capability to store captured images (one at a time), which can be exported and downlinked on demand. These images are made available in the data set in their original, uncompressed format as transmitted from the instrument. The gain settings for each captured image (determined automatically by the instrument's automatic gain control) and the integration time are present in each label file. Operational Modes For optimal attitude performance during periods of high radiation, the CCD integration time (shutter time) is decreased. All radiation count products available at PDS have been calibrated taking into account the integration time used for each image. Also, during times when stray light from Jupiter entered the CHU, the radiation count measurements are not accurate and will not correctly correspond the radiation environment at the given time. Thus, images with Jovian stray light are not included in the archived radiation count data set. Finally, the sampling period depends on the distance from perijove. The data sets are collected at three different sampling rates: 7s, 1s and 0.25s. The most frequent sampling rate is used within a few hours from perijove passage. The middle sampling rate is available a few hours further out on both sides of perijove, and finally, the low sampling rate is used for the remainder of the JUNO orbit. The sampling rate is not expressed directly in the archived dataset but may easily be inferred from the time tag difference. The decimation is performed by suppressing individual samples from the telemetry. Hence, the sensitivity per sample is NOT dependent on this telemetry update rate. Note on operational modes: Following PJ-35 (July 21, 2021) the camera heads providing input to the radiation monitoring activities had reached the radiation dose design limit. Following this dose limit, the camera heads exhibited a strong decrease in sensitivity, inhibiting use of the camera heads for star tracking. The low sensitivity state also impacts the apparent sensitivity to ionizing radiation due to a decreased effective cross-section. A calibration effort allowing linking between the nominal and the post dose limit response is on-going. Example star tracker source images from nominal and post dose limit period is shown in Figure 2 While the nominal operation radiation products offer good sensitivity in the low to medium radiation flux regions, it tends to saturate in the high flux regions. The post dose limit products, on the other hand, do not tend to saturate in these regions making these products a valuable contribution to the mapping of the Jovian radiation environment. Ground Calibration Extrinsic Calibration The instrument provides raw hotspot count as measured from the images, i.e., the number of charged particles that were sufficiently energetic to pass through the mass shielding. This physical mass shielding is from the mechanical structure of the CHU, additional shielding at selected directions, as well as the surrounding shielding provided by the spacecraft. Hence, the total mass shielding becomes strongly anisotropic leading to a directional dependent sensitivity. At time of writing, work is ongoing establishing an absolute/extrinsic calibration supported by Total Ionizing Dose (TID) signature matching with an on-ground radiated specimen. Intrinsic Calibration The image sensor is an interline transfer CCD. Such devices consist of: - A photosensitive part (the integration layer) - A masked part (the readout layer) Both physical layers are sensitive to ionizing radiation with their own sensitivity. The radiation counts telemetered from the instrument are subject to several calibration activities including: - Saturation: The particle counting is aborted if too many stellar candidate objects are identified. - Trailing: The ionization trail of a particle may not be confined to a single image pixel but may extend to neighbouring pixels. This may lead to incorrect classification as a stellar candidate object. - Clustering: Particle interactions in neighbouring pixels will be reported as a single interaction. The probability of an incorrect count increases with higher radiation flux. - Electronic shutter usage: During some operational modes, the built-in electronic shutter is used to reduce the electron accumulation. The electronic shutter works by flushing the integration layer for electrons at a fixed time before the transport to the readout layer is commanded. The effective integration period is then from the time of flushing to the time of transport to the integration layer. During the integration layer flushing, all signals from radiation impacts are flushed in this layer as well. The net radiation count will be equal to the counts accumulated from the integration layer during the effective integration period + the counts accumulated from the read layer during the entire read out cycle. Note: - All particle count data is calibrated/adjusted per the description above. - All particle images are NOT adjusted, and the apparent particle signatures reflect the original observation. Inflight Calibration No in-flight calibration has been performed. References Connerney, J. E. P., Benn, M., Bjarno, J. B., Denver, T., Espley, J., Jorgensen, J. L., Jorgensen, P. S., Lawton, P., Malinnikova Bang, A., Merayo, J. M. G., Murphy, S., Odom, J., Oliversen, R., Schnurr, R., Sheppard, D., & Smith, E. J. (2017). The Juno Magnetic Field Investigation. Space Science Reviews, 213(1-4), 39-138. https://doi.org/10.1007/s11214-017-0334-z Denver T et al (in editorial phase). The Juno ASC Energetic Particle Counter " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CONNERNEYETAL2017" END_OBJECT END_OBJECT = INSTRUMENT END