WISPR Science

Calibration

Lakin Jones — Wed, 16 May 2018 21:04:26 +0000

Calibration

We plan a limited set of observations for instrument checkout and calibration following instrument turn-on on the approach to each solar encounter. A few images per day will be taken for up to ten days while the spacecraft distance from the Sun is less than 0.5 AU on the inbound segment of each orbit. Some of these images may involve small off-points of the spacecraft from the Sun (up to a few arc minutes) to verify the stray light performance of the instrument. These data would need to be downlinked prior to the solar encounter period to be useful for planning purposes.

For photometric calibration, WISPR compares selected background stars in the images to star catalog positions and magnitudes. Thanks to the wide FOV of WISPR, no spacecraft maneuver is required to capture a standard set of calibration stars. These calibrations are used to verify the pre-launch ground photometric calibration and to monitor the WISPR telescope throughput loss during the mission. The final photometric calibration accuracy using standard stars is ∼3 %, based on procedures developed and used for SOHO/LASCO and STEREO/SECCHI. Between perihelion passes, a three-phase calibration sequence must be performed: (1) to determine if any degradation of the detector and/or the lenses occurred during the perihelion pass, where the instrument might be subjected to high radiation exposure, (2) to anneal the APS detector, and (3) to perform a calibration sequence to determine the pre-perihelion calibration. Photometry changes are fixed by the stellar transits combined with LED calibration lamp images.

WISPR Example Observing Program for SPP Orbit 24

WISPR Pub Number 1

WISPR Science

Nominal Science

Lakin Jones — Wed, 16 May 2018 20:45:49 +0000

Nominal Science

Routine observations to meet the science objectives occur during a window of ∼10 days duration centered on perihelion when the spacecraft is within 0.25 AU of the Sun (see Table 6). The standard image capture method takes short exposures (<20 seconds) and sums up to ‘N’ individual exposures to achieve the required integration time using on-board processing for image summing and “cosmic ray” scrubbing techniques that were developed and used on SECCHI/HI. The instrument is operated primarily in a synoptic observing mode, and similar observations are conducted each orbit using preplanned schedule blocks uploaded in advance of each encounter. Special observations tailored to specific science objectives are conducted on selected orbits (e.g. close to the minimum perihelion or with favorable geometries of Earth or other missions). Data are stored on the SPP solid state recorder (SSR) for transmission to the ground. A subset of the SSR data is transmitted at higher priority to facilitate planning for the next orbit.

Table 7 shows an observing program that is designed to fulfill the mission requirements for the final orbit in the nominal mission (Orbit 24). Many of the baseline science measurement requirements (including radial scene coverage, photometric accuracy, image cadence, and science observation days for the orbit and mission) depend on the instrument distance from the Sun. For this reason, the observing program over the solar encounter period is divided into the following four regions based on spacecraft distance from the Sun: Perihelion: <0.07 AU; Inner: 0.07–0.11 AU; Mid: 0.11–0.18 AU; Outer: 0.18–0.25 AU.

WISPR Operational Timelines

Mission Event	Duration	WISPR Operations
Launch and Early Operations	Launch to first Venus encounter (L + 6 weeks)	Initial power on, IDPU, camera and FSW checkout, door-closed commissioning
Approach to First Solar Encounter	First Venus Encounter to First Solar Encounter (L + 6 weeks to L + 3 months)	Checkout/commissioning to prepare for science observations
Approach to Subsequent Solar Encounters	10 days per orbit for 23 orbits (spacecraft to Sun distance <0.5 AU) on inbound segment of orbit	Checkout, detector annealing, and on-orbit calibration to prepare for science observations
Solar Encounters	10 days per orbit for 24 orbits (spacecraft to Sun distance <0.25 AU)	Synoptic and tailored science observations
Aphelion Orbit Segment	68–130 days per orbit for 24 orbits (spacecraft to Sun distance >0.25 AU)	None (data downlinked when spacecraft to Sun distance >0.59 AU)

The highest cadence, full-FOV and partial-FOV observations are taken over a 36-hour period centered on perihelion. At larger distances from the Sun, the image cadence is reduced to satisfy the Level 1 photometric accuracy requirement. The observing program, including science data, housekeeping data, and CCSDS packet overhead, is constrained to fit within the WISPR data volume allocation of 23 Gbits for each orbit.

Early Operations and Commissioning

During launch and early operations (until the first Venus flyby, ∼6 weeks after launch), WISPR anticipates only door-closed operations, consisting of initial turn-on of camera subsystems, flight software (FSW) checkout and a few calibration lamp images (see table above). The door remains closed during this time and throughout the SWEAP commissioning slew to permit outgassing of the instrument and spacecraft and to maintain survival temperature with minimal heater power. WISPR door-open commissioning operations are conducted in the interval between the first Venus flyby and the first solar encounter 6 weeks duration.

WISPR Pub Number 1

WISPR Science

Flight Software

Lakin Jones — Wed, 16 May 2018 19:09:39 +0000

Flight Software

The WISPR Flight Software (FSW) is developed by the APL IDPU team. It incorporates considerable heritage/commonality from other missions such as MESSENGER, MRO/CRISM, New Horizons, and Solar Orbiter/SIS. The common software makes use of heritage boot code, telemetry and command packet handling, macro (stored command script) implementation, memory management, autonomy, and reporting modules. The data interface with the spacecraft is SpaceWire and has a SPP-specific protocol with static bus schedule and Instrument Transfer Frames. Telemetry packets may be up to 4096 bytes and there are 64 APIDs available for WISPR to use for addressing destination. There is a critical status packet monitored by the spacecraft, which can request power off or power cycle. WISPR receives time and status from SPP at 1 Hz.

Software that is WISPR-specific includes camera control, image processing, observation scheduling including autonomous operations, and instrument health. There are 12 independently controlled operational heaters. The observation schedule time line is loaded prior to the start of the encounter. The concept is similar to the time line developed for the STEREO/SECCHI instrument. All the parameters of the time line are loaded—image time, exposure duration, number of images in the sum, subimage coordinates, image compression, etc. To smooth out the telemetry flow to the spacecraft a large memory buffer has been included on the instrument side of the interface. The size of the buffer is sufficient to store the data from more than one orbit. No ability to perform selective data transfers to the spacecraft data recorder is envisioned.

Camera operation involves loading, starting, and stopping various instances of “microcode”, and setting various registers which determine certain observations parameters. There are also calibration LEDs which need control. Most of the image processing is done in hardware (FPGA) and is orchestrated by the FSW. The possible image processing steps are: bias subtraction, clipping (max/min), cosmic ray scrub, divide by 2°ø, autonomous exposure control, pixel binning, image data compression (lossy or lossless), apply mask, and sum multiple images.

Image files received from the FPGA, including headers, are put in packets and assigned ApIDs to specify destination and downlink priority. The FSW must also manage 3 areas of image memory: (1) 156 Mb SRAM serves as an image output buffer (2) ∼3 Gb of SDRAM is available for image processing and secondary image output buffer (3) 64 Gb of Flash memory is available as tertiary image output buffer, if required. The SpaceWire output to spacecraft is limited to 350 kbps. For observation scheduling, the FSW must conduct activities such as load microcode, specify to FPGA what processing is to be done with each image received from the camera, implementing post-FPGA processing, and handle image telemetry priorities. Since this is an encounter mission with limited contact, the scheduling must be tolerant of instrument power cycles. Hence, the nominal mode of operations is “Autonomous Mode” where observations are resumed at the current mission experiment time. It is possible to specify that WISPR boot into “Manual Mode” where commanding is required to conduct operations.

WISPR Pub Number 1

WISPR Science

Mission Operations

Lakin Jones — Wed, 14 Feb 2018 22:11:53 +0000

Mission Operations

The detailed observing schedule will be uploaded prior to the beginning of each perihelion pass. However, there may not be sufficient time to modify the detailed schedule for the upcoming perihelion passage after the download of the SRR from the preceding pass. For this reason, the observing objectives are defined for the next two perihelia.

The cruise/downlink portion of each orbit is broken into either cruise operations or science downlink operations. For cruise operations, the instruments may be powered on if the Sun-spacecraft distance is less than 0.82 AU. Periodically, during cruise operations the instruments may be powered off to support routine and special spacecraft activities. During cruise operations the fanbeam antenna (via X-band) will be used for spacecraft communications. Downlink rates will be limited and there is no plan to playback the SSR data.

During science downlink operations all instruments will be powered off and the high-gain antenna (via Ka-Band) will be used for playing back the SSR and retrieving all of the science data collected in the previous encounter(s). During both the cruise and science downlink periods, real-time spacecraft commanding will be done as needed to support routine and special spacecraft activities.

The table below outlines the planned DSN contact frequency for all phases of the SPP mission. Part of the planning process entails assigning downlink priority to telemetry. The SPP supports up to 10 levels of priority for downlink; a small percentage of science data comes down relatively quickly (days) after an observing window. The majority of science data comes down at lower priority and could take months to reach the ground. These priorities may vary from orbit to orbit and are managed by the SWG.

Mission Phase	Contact Frequency	Duration
Launch & Initial C/O Spacecraft	Continuous	2 weeks
Early Commissioning	5 × 10 hr (per week)	4 weeks
Cruise Operations	3 × 8 hr (per week)	Weekly
Science Downlink	10 hr/day	Entire science downlink period (Varies in each orbit leg, ∼4–21 days)
Solar Encounter Phase	3 × 4 hr (per week)	Entire encounter period (∼2 weeks)
Venus Fly-Bys	5 × 10 hr (per week) 10 hr/day	V−5 to V− 1 weeks V−1 to V+ 1 weeks

Science Operations Center

The WISPR Science Operations Center (SOC) at NRL utilizes the GSEOS software suite provided through APL to send command files to the SPP Mission Operations Center (MOC) for uplink to the spacecraft. At the SOC, WISPR personnel utilize a Heliospheric Imager Planning Tool (HIPT) to model observation plans and translate them to schedule files that are uploaded to the WISPR IDPU.

WISPR Pub Number 1

WISPR Science

Processing and Analysis Tools

Lakin Jones — Mon, 01 Jan 2018 22:45:31 +0000

Processing and Analysis Tools

The radiometric calibration of the data will be performed using the pre-flight laboratory calibration data and calibration updates using observations of an ensemble of stable stars as used for SOHO/LASCO and STEREO/SECCHI. The calibration team monitors the detector telemetry and the images and provides periodic updates to the science calibration routines. IDL procedures will be provided in the Solarsoft library to convert the Level-1 FITS image files into higher-level calibrated data products. These procedures will permit the user to perform standard corrections such as removal of geometric distortion, vignetting and stray light, and photometric calibration, on-the-fly for the data of interest. All calibration data necessary for these corrections will be included as part of the Solarsoft distribution which is publicly available at http://sohowww.nascom.nasa.gov. This approach has been used successfully for both LASCO and SECCHI, and ensures that the user has access to the most up-to-date calibrations while avoiding repeated processing and redistribution of large amounts of data.

Software tools for common analysis tasks that are in use for LASCO, SECCHI, and SoloHI will be extended to incorporate WISPR data. These include image visualization, generation of movies, feature tracking, structure measurement, and combining datasets from multiple remote-sensing and in-situ instruments and spacecraft. Forward fitting of threedimensional models to heliospheric features such as streamers and CMEs will also be provided in Solarsoft. NRL will work with the Community Coordinated Modeling Center (CCMC) at NASA Goddard Space Flight Center to produce appropriate heliospheric model calculations for comparison with the WISPR data for each Carrington rotation, as well as for selected events of interest. The results of these model calculations will be made publicly available on the WWW.

WISPR Pub Number 1

WISPR Science