EARTH
SENSORS / HORIZON SCANNERS FOR MICRO-SATELLITES
FIGURE 1: EARTH SENSOR FUNCTIONAL OPERATION
FIGURE 2: PHOTO OF EARTH SENSOR
EARTH SENSOR DESCRIPTION:
The earth sensor is composed of four
thermopile-optical assemblies viewing four
fields of view through a single 13.5mm cut-on
optical filter/window, as is shown in Figure
1. Each earth sensor measures the attitude
of the spacecraft relative to its axis. For
Pitch and Roll attitude readout, two sensors
mounted 90° apart around the spacecraft's yaw
axis are required, as is shown in Figure 1.
Each sensor is mounted such that the lower
edges of its fields protrude 10° below the
nominal earth horizon at the orbit altitude.
The output of each field is a D.C. voltage
proportional to the earth radiance multiplied
by the solid angle subtended by the earth in
that detector's field. Two additional
detectors are used to correct for earth
radiance variation as a function of season
and position in the orbit. One of these
fields views cold space continually, while
the other is always below the earth horizon
and thereby measures the earth radiance in
the vicinity of the spacecraft's position in
the orbit.
These four D.C. analog signals for each
sensor measured relative to ground are
amplified and routed to a multiplexer and A/D
converter, which subsystems are normally part
of the spacecraft computer. Using a
proprietary algorithm supplied with the
sensors, the spacecraft computer converts
these digitized voltages into Pitch and Roll
attitude measurements based upon the
calibration provided by Optical Energy
Technologies Inc. for various earth horizon
temperatures.
By compensating for the earth horizon
radiance variation as a function of season
and position in the orbit, the algorithm
provides a relatively constant slope of pitch
and roll attitude output for pitch and roll
attitude variations from null. Null offset
errors that would be caused by earth radiance
variations are also reduced by the radiance
compensation algorithm. However, there is
still a small null offset error caused by
small differences in the spatial location of
fields 1 and 2 (and 3 and 4) shown in Figure
1 relative to the radiance compensation field
between them.
A computer algorithm developed at OETI, using
a Kalman filter, will allow the spacecraft
computer to compute the absolute temperature
and radiance of the earth horizon in fields 1
and 2 and provide corrections for this
remaining error.
When combined with OETI's two axis sun
sensor, which can periodically correct sensor
readout errors resulting from long term
changes in detector responsivities and
temperature gradients in the sensor housing,
the overall output error of the earth sensor
can be reduced to <±1°.
SENSOR PHYSICAL CHARACTERISTICS:
WEIGHT: less than 100 grams
POWER: less than 50 mwatts using a ±5 vdc. power supply
PRICE/SENSOR: $16,000- (in quantities of 2)
EARTH HORIZON SIMULATOR
EARTH SENSOR WATER ICE OR LN2 HORIZON SIMULATOR
PRICE: $3,000-
Call us or
email
us and we'll be glad to discuss your needs.
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