SSUSI NADIR Photometer System

Nadir Photometer System (NPS) Design and Operation

The NPS operates only on the nightside. It is intended to provide the height of the F-region ionosphere and to corroborate the characteristic energy and flux of precipitating electrons in the aurora as determined by the SIS. To do this, three detectors are required for the SSUSI photometer subsystem. The detectors are identical except for the optical filter characteristics. For 427.8 nm observations, one detector is required with a fixed wavelength filter at 427.8 nm with a bandwidth of 5.0 nm. Two detectors are required for the 630 nm observations because a correction must be made for the Earth albedo and the contribution from backscattered moonlight and starlight. One detector will have a filter with a center wavelength of 630 nm and a bandwidth of 0.3 nm, and the detector measuring background will use a filter with a center wavelength of 629.4 nm and a bandwidth of 0.3 nm.

Each photometer unit includes an integrated detector package consisting of a photomultiplier tube, high voltage power supply, and pulse amplitude discriminator electronics.

Photometer Performance Characteristics
Parameter Unit #1 Unit #2 Unit #3
Pixel Field of View - full angle, circular 2.0° 2.0° 2.0°
Spatial resolution at nadir 25 km 25 km 25 km
Center Wavelength 427.8 nm 630 nm 629.4 nm
Spectral Bandwidth 5.0 nm 0.3 nm 0.3 nm
Optic diameter 0.5 inch 2 inch 2 inch
Pixel Integration Time 1.0 sec 1.0 sec 1.0 sec
Sensitivity (cnt/sec/Rayleigh) 5 30 30
Maximum count per pixel 500,000 100,000 100,000


Photometer Detector Characteristics
Parameter Unit #1 Unit #2 Unit #3
Photocathode Bi-Alkali Tri-Alkali Tri-Alkali
Input Window Glass Glass Glass
Cathode diameter 7 mm 7 mm 7 mm
Wavelength 427.8 nm 630 nm 629.4 nm
Output signal pulse pulse pulse
Maximum count per sec 500,000 100,000 100,000
Dark count (maximum) 40 cps 40 cps 40 cps
HV inhibit 100 millisec 100 millisec 100 millisec
Power 0.5 Watts 0.5 Watts 0.5 Watts
Operating Temperature (in spec) -30°C to -20°C -30°C to -20°C -30°C to -20°C

Each photometer unit contains an interference filter in front of a collimator lens. The filter characteristics for the three units are listed in Table 6. The interference filter temperature coefficient must not exceed 0.03 nm/°C in order for the bandpass to remain constant and controllable for Units 2 and 3. The filters in all three units are mounted in a thermal control fixture to maintain wavelength stability. Each photometer filter employs a thermostatically controlled heater to maintain an operating temperature range of +25°C ± 1°C and the heater can be commanded on or off. The filter temperature is monitored with an accuracy of 1° C. Passive cooling is used to maintain an operating flight range of -30 °C to -20 °C for the detector in order to reduce the dark count rate.

The photometer baffle design received a good deal of attention because the NPS operates in a near-dusk environment, . This environment is particularly difficult to model. In order to be able to function at a solar zenith angle of 98deg, a two-dimensional model of the twilight Rayleigh scattering radiation field was developed 10,11. The photometer has a glint zone of ±25 degrees and is located on the shaded side of the spacecraft GLOB. The NPS has dual, redundant illumination sensors. The illumination sensor triggers on earth albedo and inhibit the photometer detectors by gating the HVPS. The illumination sensor field of view is 10° which provides an adequate margin for the near-terminator orbits.