GLAS is a satellite laser altimeter designed to measure ice-sheet topography and cloud and atmospheric properties. The onboard Nd:YAG laser emits 1064nm and (frequency doubled) 532nm light, which is reflected from the Earth and atmosphere and collected by a telescope. A small portion of the light emitted toward the Earth is picked off by a free space to fiber optic coupling system and routed around the instrument by connectorized fiber optic cables. There is also a 2 km fiber optic delay line to provide for onboard timing calibration, a fiber optic pigtailed diode laser onboard test source and a fiber optic connectorized LED onboard test source. The stability of the signals through the fiber optic system must be maintained to 10% in amplitude and 20ps in timing for certain critical signal paths. It has been challenging to meet these requirements with the spacecraft resources available. Components from many commercial vendors have been used to satisfy these requirements. The system design, components selected and testing performed will be discussed in this paper.
GLAS is a satellite laser altimeter designed to measure ice- sheet topography and cloud and atmospheric properties. The onboard laser is an Nd:YAG with doubler, emitting 1064 nm and 532 nm light. The reflected light from the Earth and atmosphere is collected by a telescope and analyzed to produce the altimetry results. Fiber optics are used to route timing and calibration signals within the instrument as well as to provide calibrated delays. The fiber optic system includes free space to fiber coupling optics, cables and connectors, diode laser and LED fiber pigtails and a 2 kilometer fiber optic delay line. Since these signals are received and processed as analog values, it is necessary to preserve the magnitude, timing and pulse shape as much as possible in the thermal, mechanical and radiation environment of space. This is in contrast to many onboard fiber optic systems which are used for digital communication and can accept wide variations in these parameters as long as the thresholds for accurate detection of a bit aren't exceeded. The requirements include a stability of up to 1% in signal magnitude and 20 ps timing stability in the timing signals and fiber optic delay line over the whole mission, including all environmental effects. Meeting these requirements with the spacecraft resources available has proven challenging. Fiber optic components have been chosen and tested to accomplish this as best as possible. The system design, components selected and testing performed will be discussed in this paper.
The dual-rate 1773 (DR1773) fiber optic data bus (FODB) experiment is one of twenty four space fight experiments on the Naval Research Laboratory's (NRL) microelectronics and photonics test bed (MPTB). MPTB is an NRL satellite payload that will be composed of modern technology microelectronics and photonic experiments. The Goddard Space Flight Center (GSFC) experiment on MPTB will evaluate the in-flight performance characteristics of the Society of Automotive Engineers (SAE) Aerospace Standard (AS) 1773 FODB in the space radiation environment. AS1773 is a passive star coupled FODB that operates at data rates of Mbps and 20 Mbps. This protocol is commonly referred to as 'dual-rate 1773 (DR1773).' Two different DR1773 fiber optic transceiver designs are tested on the experiment. These devices are the Boeing DR1773 and the SCI DR1773 transceiver. The transceiver devices on the experiment are implemented in a star coupled system and are tested at the higher 20 Mbps data rate. It is estimated that DR1773 at 20 Mbps will meet approximately 80% of NASA's current data rate requirements. DR1773's predecessor, Mil-Std-1773, is currently being flown on several NASA spacecraft. Mil-Std-1773 operates at a single rate of 1 Mbps. Because DR1773 is based on an existing bus protocol, incorporating DR 1773 into the NASA spacecraft program would be both time and cost effective. Success of the DR1773 experiment will provide valuable data on the DR1773 FODB space radiation performance as well as proof of concept for use of the DR1773 FODB for future NASA spacecraft systems.
Kenneth LaBel, Mark Flanegan, George Jackson, Donald Hawkins, Cheryl Dale, Paul Marshall, Donald Johnson, Christina Seidleck, Rodney Bonebright, Jae Kim, Eric Chan, Thomas Bocek, William Bartholet
NASA Goddard Space Flight Center (GSFC) along with the Naval Research Laboratory (NRL) has been at the forefront of the space community in terms of the use of fiber optic data busses and links in the space radiation environment. Previously, we have described the ground radiation test program of the small explorer data system (SEDS) 1773 1 Mbps fiber optic data bus (FODB), as well as its associated in- flight space radiation-induced performance. Further work has also been presented covering higher speed photonic components utilizing III-V materials. Because of the success of the SEDS 1773 FODB coupled with the radiation testing of III-V devices, a second generation FODB capable of both 1 Mbps and 20 Mbps operation is being developed for spaceflight utilization. We present herein preliminary ground test radiation results of hybrid transceiver devices manufactured by Boeing Space Systems that perform the electro-optic and opto-electric translations in support of this medium rate FODB, the AS1773 bus. These devices, designed to be radiation hard (or rad hard), will be flying on NRL's Microelectronics and Photonics Testbed (MPTB) payload as a NASA experiment. This experiment is described in detail elsewhere in this proceedings.
The small explorer (SMEX) project at Goddard Space Flight Center (GSFC) launched the first spaceflight implementation of the MIL-STD-1773 fiber optic data bus on the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) in July of 1992. The Small Explorer Data System (SEDS), of which the MIL-STD-1773 data bus is a part, has been successful. The MIL-STD-1773 bus is the implementation of the MIL-STD-1553 protocol using fiber optics. Advantages of the fiber optic bus over the electrical bus include lower power, lower weight, and immunity from EMI/RFI. It does not radiate electrical or magnetic fields. It is a nonconductor so it cannot conduct electrical noise into or from a subsystem. This is particularly advantageous on a spacecraft with very sensitive instruments which are often susceptible to electrical interference. Although the MIL-STD-1773 bus is a 1 Mbps bus like the MIL-STD-1553 bus, the fiber optics also provide a path to the much higher rate systems required in upcoming NASA missions.
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