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
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
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
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