Instrument Configurations

SSUSI has gone through several iterations of development based on feedback from the on-orbit performance.  The following table lists the configuration of each SSUSI, as well as its approximate local time of ascending node (equator crossing time on the night/afternoon side).  The configuration of F20 is subject to change, but it is likely to be identical to the F19 SSUSI. 



DMSP Flight F16  F17 F18 F19 F20
Launch Date 18 Oct 2003 4 Nov 2006 18 Oct 2009 3 Apr 2014 N/A
Local Time of Ascending Node 1700 1730 2000 1830 -
SSUSI Serial Number 05 03 04 01 02
SIS Grating Hyperfine Ruled SSUSI Blazed Holographic GUVI Unblazed Holographic Hyperfine Ruled Hyperfine Ruled
SIS Aperture 250 mm2 500 mm2 500 mm2 500 mm2 500 mm2
Primary Detector SAIC Photek rev1 Photek rev1 Photek rev2 Photek rev2
   Mask installed no yes yes yes yes
Secondary Detector SAIC SAIC SAIC Photek rev2 Photek rev2
   Mask installed no no yes yes yes
Operational Slit Narrow Wide Medium Medium Medium
NPS Threshold 1X 2X 2X 2X 2X

The key changes in the SSUSI instrument have been with the grating and detector, as well as with the discriminator threshold in the NPS.


The SAIC detectors were found to have unstable (increasing) long-wavelength sensitivity on-orbit and were replaced with the Photek rev1 detectors used on GUVI for the F17 and F18 instruments.  The long-wavelength sensitivity on the F16 and F17 instruments that are operating the SAIC detectors is corrected in software.  Once the remaining Photek rev1 tubes from the GUVI program were used up, the SSUSI program elected to purchase a second run of Photek tubes that made several improvements to the image quality and mechanical reliability.  These are indicated as "Photek rev2."


Several alternative grating technologies were evaluated with the F16, F17, and F18 SSUSI instruments.  The F17 SSUSI employed a blazed grating produced by holographic process that was expected to reduce the background scattered and stray light.  Due manufacturing defect, this grating has poor response at the 135.6-nm wavelength, but has excellent response in the LBH bands used for auroral observations.  The F18 SSUSI used the spare GUVI grating which was unblazed, but produced by a holographic process.  The F19 and F20 SSUSIs return to the efficient, but "noisier" mechancially ruled blazed grating used in F16.

Other changes

The SSUSI nadir photometer system (NPS) utilizes photomultiplier tubes that are also sensitive to energetic particles in the South Atlantic Anomaly (SAA) region.  After this sensitivity was discovered on F16, the pulse discriminator threshold was increased on the later SSUSIs.  The NPS is used to generate SSUSI's SAA product.  The operational slit in imaging mode has been changed as a trade between the overall throughput of the instrument versus the spectral resolution.  The default for instruments since F18 (inclusive) is medium.