Monthly Newsletter

February 2001

Tom Woods, participated in the Living With a Star Workshop, GSFC, Greenbelt, MD, January 25-26, 2001.

Brian Boyle and Bill Ochs, SORCE Mission Manager, attended the Alenia Tranceiver CDR at Dulles, MD, February 7-8, 2001

The SORCE IV &V planning meeting was held at GSFC February 21, 2001.

The March SORCE Monthly Review will be held March 22, 2001 at LASP.

The Chandler Rocket review will be held in Chandler, Arizona, April 3-4, 2001.

The SORCE Mission Operations Review will be held at LASP, Boulder, CO April 23-24, 2001.

SORCE is planning a Science Team Meeting July 9, 10, 11 2001 at the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado.

SORCE February 2001 Project Summary
The EOS SORCE Mission is on schedule and within budget. Launch is scheduled for July 2002. No mission descopes are required at the present, but TIM Hitchhiker will likely not be manifest on shuttle until 2002-2003 and therefore build been lowered in priority to accomodate SORCE scheduling. SORCE continues to closely monitor and adjust all schedules to meet program needs. Instrument build is well underway and problems discovered through test are addressed immediately. Ongoing program concerns are diminishing slack, additional procurement costs, IFOG manufacture and reliability, orbit trajectory, and late delivery of the transceiver and the spacecraft bench. All of these issues are "front burner" and solutions and fixes are developing.
A Science position will be offered in the near future to replace Tony Canas, who left the SORCE program in early January. SORCE is planning to hold two science meetings per year in the future, one summer and one winter. The summer 2001 meeting will concentrate on the results of SORCE instrument calibration and characterization. The winter 2001 meeting, to be held after the delivery of the SORCE observatory to Orbital Sciences Corporation, will be of a broader science context, and may be held in conjunction with the Fall AGU meeting.

TIM Nears Completion
The SORCE Total Irradiance Monitor (TIM) is nearly assembled . This instrument is designed to measure the total solar irradiance (TSI) to an absolute accuracy of 100 parts per million (ppm), reporting four such TSI measurements per day. Previous spaceborne TSI instruments, such as those currently flying on ACRIMSat and SOHO, have established a 22-year record of solar irradiance, recording the small fluctuations

due to solar activity about an average 1367 W/m2. The SORCE's TIM will be continuing these measurements through its goal 5-year mission life.
The TIM uses four electrical substitution radiometers (ESRs), each behind a precision aperture, to measure solar irradiance. A shutter modulates incident sunlight to a selected active ESR, while electrical heater power maintains a balanced ESR temperature with a reference sensor. Phasesensitive detection of the electrical power supplied to the active ESR at the shutter frequency measures TSI and reduces sensitivity to thermal drifts and noise. A digital signal processor (DSP) controls the instrument by timing shutter positions, maintaining the ESR temperatures via heater power cycling, and formatting instrument science and housekeeping data for telemetry downlink. These components are currently being assembled and tested at LASP.

Most component-level TIM characterizations are complete. NIST measured the 0.5 cm2 precision apertures to stated accuracies of 25 ppm. LASP measured the ESR reflectances at several wavelengths spanning the visible and near infrared, and selected the flight ESRs based on their low (~100 ppm) and stable reflectances. SORCE has recorded the resistances of all ESR electrical leads to the needed accuracy level . The TIM flight instrument is currently undergoing electrical and DSP functionality tests.
Over the next couple months SORCE will continue characterizations and tests of TIM. "First Light" on the completed instrument assembly will be in early March, along with tuning of the thermal control circuitry. Thermal vacuum and shake tests will (hopefully!) verify instrument robustness.
LASP is installing a heliostat that will facilitate end-to-end tests of the instrument and measure intra-instrument consistency by (nearly) equally illuminating adjacent TIM ESRs. TIM and the other three SORCE instruments are scheduled to be installed on the spacecraft optical bench in late spring, followed by co-alignment and functional tests. During integration and test on the spacecraft, we will finalize the preflight TIM measurement equation parameters, getting the science data analysis software ready for the SORCE July 2002 launch and subsequent TIM TSI measurements.

SIM Status
Significant progress has been made on the SIM Féry prism drive, CCD electronics and DSP code development. One flight CCD experienced a failure anomaly and the piece is at Goddard for analysis. CCD build will resume when this analysis is complete. Additional progress had been made in the focal plane, calibrator system and the sunshade and baffling design. The SIM case is assembled for vacuum test. Slit calibration has begun. The SIM thermal design was reviewed and resulted in only 3 minor RFA's (requests for action).

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