Steven Marc Hill

The NOAA GOES x-ray calibration program

NOAAs Geostationary Operational Environmental Satellites (GOES) monitor the solar X-ray activity that enable the NWS Space Environment Center to perform operational specification and forecast of the space environment. The disk-integrated solar X-ray flux has been recorded for more than two decades by the GOES X-ray Sensor (XRS). Since 2003, GOES Solar X-ray Imager (SXI) has provided real-time images of the Sun and lower corona. On 2004 October 15, a sounding rocket launched from White Sands Missile Range marked an important milestone in the first-ever attempt at on-orbit response calibration of GOES X-ray instrumentation. This paper provides an overview of this effort, which includes participation of NOAA, NASA, University of Alaska, and University of Colorado. In addition, results of initial data reduction and analysis for the XRS, SXI, and the sounding rocket are presented.

GOES N-P SXI in-flight calibration using the Crab Nebula

A potential calibration strategy for the N-P series Solar X-ray Imager (SXI), on the Geostationary Operational Environmental Satellite (GOES), that uses an astronomical X-ray source (the Crab Nebula) is analyzed below. The Crab Nebula is one of the brightest X-ray sources in the sky, and is located near the ecliptic, making consideration of such a calibration possible due to its annual proximity to the sun. The results of these analyses show that in-flight calibration of SXI N-P using the Crab Nebula is possible. Using the longest single exposure which the instrument is capable of making (65 sec) yields a signal-to noise-ratio somewhat insufficient to meet SXI N-Ps 20% photometric accuracy requirement. However, summing several 65 second images would increase the signal to noise ratio, making such a calibration readily possible. This analysis demonstrates that effective calibration of SXI N-P could be carried out with only the relatively small cost of operations and analysis to the government, using approximately 12 hours of observing time per year. Expensive calibration underflights, using sounding rockets, would not be needed for SXI N-P. It is important to note that if the GOES R SXI, the follow on series to GOES N-P, uses a normal incidence primary mirror design, an astrophysical calibration with the Crab nebula will not be possible because of the change in instrument spectral response. However, other astrophysical sources could be examined.

Avalanche photodiode measurements of the solar soft x-ray irradiance

The Avalanche Photodiode X-ray Spectrometer (AXS) is an optics free spectrometer operating in the 400 eV - 10 keV energy range. The purpose of the instrument is to measure the solar full disk irradiance from .1 to 2 nm with a spectral resolution on the order of ΔE/E equal to approximately 15%. This spectral region is a key and highly variable energy source to the lower thermosphere. The instrument was developed for sounding rocket use and, in addition to the science objectives, is used for underflight calibration of National Oceanic and Atmospheric Administration (NOAA) Geostationary Operational Environmental Satellite (GOES) X-ray instruments. Photon events from an avalanche photodiode produce electron showers that are detected by analog electronics. Pulse height analysis yields the energy of the impacting photon. By recording the number of events per pulse height bin, the AXS produces a spectrum. This instrument has been developed at the University of Alaska (UAF) and was flown on a sounding rocket on October 15, 2004. Calibrations were performed at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) III facility in Gaithersburg Maryland. In this paper the instrument design and calibration are discussed as well as both laboratory and rocket flight measurements.

GOES-13 SXI Initial On-Orbit Performance Results

On 2006 May 24 NOAAs Geostationary Operational Environmental Satellite (GOES) 13 satellite was launched with the next generation Solar X-ray Imager (SXI) aboard. This instrument represents a significant step forward in performance over the previous SXI flown on GOES-12, even before that instrument suffered serious degradation. Like the previous instrument, the new instrument uses a grazing incidence optical design, but with a new detector and other improvements, it has about 10 times the sensitivity, twice the spatial resolution, and greatly reduced wide-angle scattering compared to the GOES-12 SXI. The GOES-13 SXI completed its 6 month checkout period in December 2006. Performance tests included dark current, flat-field, spatial response, scattered light, pointing stability and jitter. We present initial analyses and results of these tests as well as comparisons to ground test results. In addition, GOES-13 solar observations are compared to solar observations by other instruments. When it enters operations, the GOES-13 SXI will provide continuous, real-time observations of the X-ray Sun at 1-minute cadence.