The MiDES is a state-of-the-art Earth horizon sensor combining the versatility of a conical scanner with the accuracy and reliability of a staring sensor. The resulting instrument is a light weight, low cost, low power alternative for LEO orbiting satellites.
Consisting of a single self-contained unit housing the optics, detectors, and all processing electronics, the MiDES operates on standard 28 VDC unregulated power and provides a digital output from its internal microcontroller via either an RS 232 or 422 interface.
The MiDES is specifically designed to provide horizon position for Earth orbiting unmanned satellites. The MiDES derives its information through the use of two pairs of pyroelectric arrays positioned 90 degrees apart in the imaging plane. Each 16 element array is spatially separated into two 8 element staggered columns to provide for sun/moon rejection and radiance compensation. Covering a range of 11 degrees, the pixels detect the temperature difference between the reference chopper, earth and space through measurement of the CO2 band's thermal gradient to 8 bit resolution. An algorithm calculates the position of the horizon based on the voltages obtained from the pixels which subtend the horizon gradient and the pixels that look at space and earth.
The heart of the MiDES is the Servo designed hybridized Lithium Tantalate (LTO) pyroelectric array. Servo Lithium Tantalate detectors are specially processed highly sensitive uncooled detectors which provide very stable outputs over a wide range of temperatures and operating conditions. Because of the nature of the pyroelectric material, a change has to be created between the detector and some known temperature source. In spinning spacecraft the change is due to the crossings of the detector from cold space to warm Earth, but in the 3-axis mode, where the sensors are essentially starring at the Earth's limb, this change has be made by the use of a chopper mechanism. On the MiDES, a tuning fork type chopper, or shutter mechanism is used, consisting of a flex pivot and driven by a resonant frequency driver. This produces AC signals which have an inherent long term stability and are insensitive to bias variations. Consequently the only moving part is the flex pivot, thereby eliminating any scanning motors, bearings and lubricants.
FAQS - Servo has compiled a list of Frequently Asked Questions concerning the MiDES.
The MiDES is a completely self contained unit housing the optics, detectors, and all processing electronics, thereby eliminating the need for remote electronic assemblies and cabling. One interface connector provides both power and output signals, thereby reducing the EMC susceptibility associated with separated subassemblies. The optics are mounted in a threaded barrel, affording a simple means of final focusing. Below the internal mounting plate is room for two electronic boards reserved for MiDES processing electronics and a microcontroller / interface.
The chopper is mounted on a flexible pivot. Two arms cover and uncover the pixels, while the third arm is actuated by the driver. The total -Q- of the system is designed to be highly resonant, minimizing power demands. The pivot is specified by the manufacturer to have infinite life under the expected conditions. These pivots have been used extensively for space applications, and there has been extensive research on their reliability. This data is available for review from the manufacturer. For further validation, life tests are presently under way at Servo.
Operational Range: +/- 5.5 deg.
Spectral Band: 14 to 16 micron
Weight: <1.4 Kg
Mounting: Mounting flange at base
Alignment: One unit pre-aligned
Power Input: 28 Volts DC +7/-6 VDC
Power Consumption : <800mW
Signal Output: digital word
Operating Temperature: -20 to +60 deg. C
Random Vibration: 28 Grms
Radiation: 50 Krads (design goal)
For further information please call 516 938 9700 ext 352 or email 
us.
