The primary function of the ultraviolet spectrometer (UVS) is to measure
the density of nitric oxide between the altitudes of 100 and 200 km in
the terrestrial upper atmosphere by observing the (1,0) and (0,1) gamma
bands. The UVS design is similar to instruments flown on the Solar Mesospheric
Explorer (SME), Pioneer Venus, and several rocket flights. It consists
of an Ebert-Fastie spectrometer, an off-axis telescope, and two Hamamatsu
phototube detectors. The spectrometer has a focal length of 125 mm and
uses a 3600 l/mm mechanically ruled plane grating which produces a dispersion
of 1.8 nm/mm at the detectors. The phototubes each have fused silica windows
and a cesium telluride photocathode. The telescope is an off-axis parabola
with a 250 mm focal length and is used to image the spectrometer slit on
the limb. The combination of the spectrometer and the detectors produces
a spacing of 22 nm between the two channels and the exit slits are sized
to give each detector a 3.7 nm bandpass. The grating in the spectrometer
will be set to place the (1,0) gamma band ( 215 nm) on one detector and
the (0,1) gamma band (237 nm) on the other detector. Both channels have
a sensitivity of 450 counts/second/kiloRayleigh.
The UVS is mounted with its optical axis perpendicular to the spin axis
of the S/C. Its telescope images the entrance slit of the spectrometer
on the limb with the long axis of the slit parallel to the horizon. The
image of the slit on the limb is 3.5 km high, which determines the fundamental
altitude resolution of the instrument. The integration time of the is set
to 27 milliseconds. To minimize requirements on the S/C, data are stored
for the downward limb scan only. Allowing for some overscan, this produces
65 samples per spin from each channel. The storage operation is initiated
by a signal derived from the horizon crossing indicator in the ADCS. The
data are stored in a buffer which is emptied, time-tagged, and stored once
per spin by the S/C microprocessor.
The auroral photometer (AP) is a two-channel broad-band instrument that
is used to determine the energy deposited in the upper atmosphere by energetic
auroral electrons. It is similar to airglow photometers developed by LASP
and flown on OGO-5 and -6 in the late 1960's. The channels consist of two
Hamamatsu phototube detectors, a UV filter for each channel, and a field
of view limiter for each channel. Both channels have circular fields of
view, 11 degree full-cone. The detectors are identical phototubes with
magnesium fluoride (MgF2) windows and cesium iodide (CsI) photocathodes.
Channel A has a calcium fluoride (CaF2) filter placed in front of the detector
and channel B has a barium fluoride (BaF2) filter. The combination of the
CsI photocathode and the CaF2 filter produces a bandpass from 125 to 180
nm for channel A, allowing a combined measurement of the LBH bands, the
OI doublet at 135.6 nm, and the OI triplet at 130.4 nm. Channel B has a
135 to 180 nm bandpass, providing a measurement of the LBH bands and the
OI doublet at 135.6 nm with the exclusion of the OI triplet at 130.4 nm.
The sensitivity of channel A at 130.4 nm is 23 counts/second/Rayleigh and
the sensitivity of channel B at 135.6 nm is 26 counts/second/Rayleigh.
The AP and UVS photomultiplier electronics are identical, resulting in
significant economies in fabrication and operation.
As with the UVS, the AP is mounted with its optical axis perpendicular
to the S/C spin axis. The AP produces continuous data but at a much lower
rate than the UVS. Only the downward- looking 60° of each spin will
be stored. The integration time for each channel is set to 63 milliseconds
which provides 32 samples per channel per spin. Data from the AP are stored
in its buffer, which is emptied once per spin by the S/C microprocessor.
||SOLAR X-RAY PHOTOMETER
The solar X-ray photometer (SXP) measures the solar irradiance at wavelengths
from 2 to 35 nm in the soft X-ray to hard EUV (or "XUV") portion of the
solar spectrum. Each photometer channel consists of a silicon photodiode;
wavelength selection is accomplished by thin metallic films deposited directly
onto the diode surface. A door with a fused silica window covers the diodes.
When it is open, the diodes measure solar X-rays plus some visible light
contamination. When it is closed, the X-rays are blocked and only the visible
light is measured. By subtracting the door-closed measurment from the door-open
measurement, the X-ray fluxes are obtained.
Five photodiodes are flown. Coatings are selected so that overlapping
bandpasses can be used to isolate key parts of the solar spectrum at low
resolution. The coatings are: Tin, Titanium, Zirconium/Titanium, and Alumninum/Carbon.
The fifth diode has no coating - it is used to measure the visible light
transmission of the fused silica window. The approximate bandpasses of
the diodes are:
- Sn: 2-8 nm
- Ti: 2-16 nm
- Zr/Ti: 5-20 nm
- Al/C: 15-35 nm
The sensitivity of each channel is approximately 10 electrons per photon.
The diode current is converted to a freqeuncy at 50 kHz/nA and counted
by the same type of electronics used in the UVS and AP. The field of view
is 70 degrees full cone. The SXP takes 12 measurements per spin, centered
on the zenith, with a 63 second integration time. Thus, it obtains an integrated
solar measurement once per orbit, when the sun is near the zenith. The
nominal operations plan calls for a door-open measurement on every other
orbit. Data is stored in a buffer that is emptied once per spin by the
S/C microprocessor, in the same manner as the UVS and AP.
||GLOBAL POSITIONING SYSTEM
The GPS satellite system showing sample
lines of sight to the
The Global Positioning System (GPS) consists of a constellation of 24 Earth-
orbiting satellites split between six orbital planes. The satellites constantly
broadcast their positions down to Earth. The retrieval and decoding of
these signals using an antenna and GPS receiver allow anyone to locate
their position with a guaranteed 100 meter accuracy. GPS receivers on spacecraft
allow for orbit and attitude determination. The SNOE spacecraft will fly
a microGPS Bit-Grabber Space Receiver (BGSR) which will record data and
send it to the ground in the satellite playback data. No on-board processing
will be done. The GPS instrument was provided by JPL and the data will
be analyzed by both JPL and the University of Colorado. The entire instrument
is quite small and very low power. The antenna is a 7.3 cm square ceramic
patch antenna.The receiver is approximately 200 cubic centimeters weighing
less than .5 kg. The GPS instrument will sample GPS signals. It will take
and store short duration (several milliseconds) samples of the composite
GPS signal at the CA bandwidth. The samples will be programmable in duration
and frequency and will be post-processed to recover the GPS observables.
Nominally, the BGSR will sample several milliseconds of data on three selected
spins of every orbit. The data received from the BGSR will be processed
using ground-based software that can be run on a desktop computer. The
software will be able to produce spacecraft orbits, attitude measurements,
and clock fixes based on the telemetered GPS signal samples. As the mission
progresses, these data may be provided for uplink to the spacecraft computer.