VFCT Digital Sun Sensor is perhaps the smallest and most accurate sensor for small
Satellites.
A simple, small (65 x 55 x 25 mm), low weight (0.12 kg), fully autonomous digital sun
sensor, suitable for small satellites, has been developed. It consists of two mutually
perpendicular linear image sensors mounted below a simple optical system for data
acquisition and processing electronics; all placed in one case. Communicating through
standard serial interface, it provides Sun vector with the accuracy about 0.05°. For the
linear CMOS image sensors may be used or CCD ones.
VFCT DSS has an accuracy of approximately 0.05°. It contains two mutually
perpendicular CCD or CMOS linear arrays and uses no lenses. Its advantages are low
mass (0.12kg) and low size (65 x 55 x 25 mm). It provides complete sun vector (two
perpendicular angles to the Sun) at a standard serial interface.
TECHNICAL DATA
Number of pixels: 2048
Field of view (FOV): 90° (± 45° )
Accuracy: ~ 0.05°
Electric interface: RS485 or RS422
Mass: 0.12 kg
Sizes: 65 x 55 x 25 mm
Power consumption: < 500mW
Digital Output: computed Sun vector or signal from all pixels
Supply Voltage: nominally 5V, by inserting internal DC/DC converter the
Supply Voltage can be extended up to 30 V
SENSOR DESCRIPTION
The principle of the measurement and construction of the DSS is schematically shown
in Fig. 1. A thin opaque layer with a narrow slit is placed above the CCD or CMOS
linear image sensor. Thus the Sun illuminates different pixels in dependence on the
angle to main sensor axis. Anti-glare coating coats the opaque layer in which the slit is
made to avoid unwanted reflections between filter and image sensor surface. The
distance between the image sensor and the slit defines the field of view (FOV) of the
DSS. To protect the image sensor against direct Sunbeams and radiation, and to fit the
appropriate exposure of the image sensor a special attenuation-filter is used as a front
window. There are two identical mutually perpendicular linear image arrays placed in
one case. Thus we get two angles between Sun and sensor axis measured in two
perpendicular planes what makes the determination of Sun vector possible.
The FOV of a single pixel is about 0.05° degree, and the light intensity of the Sun is
several orders higher than the intensity of the light reflected from the Earth. Hence the
reflected light does not deteriorate the accuracy of measurement unlike in the case of
cosine law sensor, where reflected light can be viewed at very large FOV by an
photosensitive element. As a result of that, the digital sensor using a slit sensor is
applicable for satellites in Low Earth Orbits.
Electronics and Data Handling
All electronics are placed in a small case 65 x 55 x 25 mm which provides the
timing for the linear arrays, computation of Sun vector and data communication with
the supervising computer or data collection system.
A block scheme of the sensor electronics is presented in Fig. 2. The analogue signal
from both linear sensors is tied to the multi-channel AD converter and by means of
Direct Memory Access (DMA) transferred in digital form to the SRAM memory. Both
the AD converter and DMA controller are the inner parts of the main microprocessor.
The auxiliary microprocessor provides the proper timing for image arrays, whereas the
main microprocessor is responsible for data processing. According to the regime chosen
a computed Sun vector (two mutually perpendicular angles), or signal levels of all
pixels are communicated serially to a supervising data collection system or board
computer. The latter regime is predominantly used in the case of testing the device.
An optional sub-board with DC/DC converter can be inserted to extend the supply
voltage range.
