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Big science from a small satellite
September 9, 1996
11 Min Read
During NASA's great days, both Congress and the Executive branch of government thought space important and well worth funding. The general public saw space as a new frontier and a great adventure, and supported space exploration enthusiastically. Satellites and spacecraft grew and became more and more capable. Billion-dollar satellites went roaring off the launch pads.
It's a different world, nowadays, isn't it? What's NASA's new slogan? Try "Faster, Better, Cheaper." The need to do more with less spawned NASA's Small Satellite Technology Initiative (SSTI). "The main reason NASA embarked on the SSTI was to reduce the overall risk of future spacecraft missions," says NASA SSTI Program Manager Marcus Watkins. This program seeks to produce satellites with payloads of 500 lbs or less, with a payload mass fraction of 50 to 70% of spacecraft weight, and lifetimes of three to five years. Also, the time from a Request for Proposal (RFP) to delivery of a vehicle should be greatly compressed.
Spacecraft designed to meet the requirements of SSTI will employ a standard structure and standard interfaces that can accept "bolt-on" payloads. This approach should reduce cost, simplify spacecraft design, manufacturing and test, permit use of off-the-shelf hardware and software, and allow concurrent payload and spacecraft development. Finally, the small spacecraft ought to prove quite muscular in their technical capabilities. NASA wants big science from these lightsats, according to Watkins.
Meeting the challenge. Ambitious goals, these, and two quite similar spacecraft named Lewis and Clark may help prove they can be met. The two are intended to demonstate a multitude of technologies while doing useful science. This article covers the Lewis spacecraft, which TRW delivered to NASA in June. "The RFP date was in early April of 1994," says Mark A. Folkman, of TRW's Space & Electronics Group in Redondo Beach, CA. "We wrote the RFP in one month and got the contract award about a month and a half after that." The $64-million SSTI contract specified that TRW would build a small spacecraft to carry three science payloads and deliver it to NASA in two years. NASA will launch Lewis late in this year.
The satellite consists of three major modules: the battery and propulsion module, the spacecraft avionics module, and the payload module. Its basic structure is made from K-1100 Graphite Fiber Reinforced Plastic (GFRP). This material provides good thermal conductivity in one direction, helping dissipate heat from onboard systems to external radiator panels. Prior to delivery, TRW ran a thermal vacuum test on the structure (without its primary payload onboard). This procedure effectively provided a 12-day structural bakeout. George Repucci of TRW's Space and Electronics Group describes Lewis' structure as built around a strong central cylinder embedded within the satellite.
Pie-shaped panels radiate outward from the central cylinder, and on the circumference of the pie-shapes TRW mounts six radiator panels. A lightweight propellant tank is mounted within the 2-ft-diameter central cylinder. "On Lewis we're running about 30% payload mass fraction," says Repucci. "We could put a taller payload on the top of the satellite, strengthen the upper module somewhat, and carry a much heavier payload." A payload mass fraction of 70% represents the maximum achievable, in Repucci's opinion.
Eighty inches tall, Lewis forms a 59-inch hexagon in cross section. At launch, it will weigh 850 lbs and provide 740W to onboard systems via eight conventional silicon solar panels. Small patches of advanced solar cells will fly on the spacecraft. Both GaAs cells and cascade junction solar cells will be demonstrated. Consisting of a germanium substrate overlain by GaAs and indium phosphide, the latter can offer efficiencies as high as 21%. A standard data bus simplifies interface with the payloads and subsystem units. One flight computer runs Lewis, and a redundant backup computer remains on standby to deal with a failure of the primary computer.
Lewis will fly at 523 km in a polar, sun-synchronous orbit (97 degrees inclination). In a sun-synchronous orbit, the spacecraft crosses the equator at the same local time every day. So it will pass over the same swathe of earth, under the same degree of solar illumination, every day.
"One of the things we're doing differently on this spacecraft is to use a crushable aluminum foil shaped rather like miniaturized galvanized roofing material to conduct heat from our electronic boxes to the satellite structure," says Repucci. It replaces the RTV rubber normally used beneath electronics. The foil makes removal of boxes and their replacement much easier by eliminating the need to clean up and replace RTV during integration and test.
Modular design enabled TRW to integrate the three modules that make up Lewis in parallel, both electrically and mechanically. Had the satellite been designed in series, Repucci says the effort would have stretched out over 14 to 18 months. Parallel development and test of the modules reduced the time needed for the integration of the satellite by three to four months. To accomplish this speed-up, TRW used on-board computers to test the modules in parallel. A third computer was used to develop and check out software before delivering it to the satellite. Integration of software and hardware proved quite straightforward, and Repucci gives the parallel process much credit for speeding that complicated procedure.
Although Lewis is small, building the spacecraft required a substantial technical team. "We have more than 20 subcontractors involved in the program and five NASA centers provided hardware or software to the satellite," says Repucci. Rapid and frequent communication between team members proved vital to meeting the tight development schedule. TRW formed integrated product development teams, and the members exchanged "massive amounts of e-mail," says Repucci. "When we made a decision, the group was involved in that decision and people were aware of what had been decided."
In addition, TRW made extensive use of technology from other smallsat programs, including technologies developed for such military programs as Brilliant Pebbles and Brilliant Eyes. Lewis' startrackers, solid state recorders, computer technology and cryocoolers all were advanced state of development items from military programs. In addition, the attitude control, data management, and electrical power subsystems used on Lewis drew heavily on proven technology.
Payload means payoff. Like the SSTI concept, the primary payload on Lewis, TRW's Hyperspectral Imager (HSI), pushes the envelope."We're not mission-specific with SSTI," says Watkins. "We allowed industry to come back to us and propose using instruments that they felt were ready to be flight tested." An example is the HSI, which Watkins describes as the first instrument of its kind to fly in space.
"Completing the hyperspectral imager was the biggest technical challenge we faced," says Repucci. LANDSAT, the current industry standard for commercial remote sensing, looks at seven spectral bands. "For a given resolution," explains Mark Folkman, "you've got to collect the same number of picture elements as older scanners, but you've got 384 spectral bands to send down."
"We see data rates of 450 MBit/second between the HSI and onboard solid state recorder," Repucci points out. The high data rate forced TRW to push the state of the art in the application of field programmable gate arrays. "We're using devices with 8,000 gates at utilizations of better than 90%, with nanosecond timing within the FPGAs," says Repucci.
Fundamentally, the HSI consists of a telescope, two spectrometers and a panchromatic camera (PAN). Referred to by TRW as the foreoptics, the telescope utilizes three mirrors and has a 1.048m focal length. A faceted mirror in the spectrometer section of HSI's optics splits the beam collected by the telescope into three rays, which are delivered to the panchromatic camera and spectrometers. The panchromatic camera provides high-resolution black and white images and helps sharpen the hyperspectral image produced by the two spectrometers.
One of the spectrometers operates in the Visible/Near Infrared (VNIR), the other in the Shortwave Infrared (SWIR). Each images spectra passing through an entrance slit onto a focal plane array (FPA). Each FPA has 256 pixels in the dimension perpendicular to the path swept across the earth by the satellite (cross-track). The VNIR spectrometer has 128 pixels in the dimension parallel to the satellite's path (along track), the SWIR 256 pixels.
Each data frame samples the cross-track line image spectra 384 times, covering 0.4 to 2.5 mum. The along-track motion produces the second dimension of the image. Data from the spectrometers can be arranged in a cube representing a space 7.7 km wide, 384 spectra deep, and as long as the number of data frames collected.
Folkman explains that titanium standoffs thermally isolate each FPA from the optomechanical subsystem, which houses the telescope and spectrometers. Gold-plated copper thermal straps extending from the PAN and VNIR arrays to their radiator remove resistive heating and allow operation of the VNIR array at 273K.
A custom silicon CCD forms the VNIR spectrometer's focal plane. Basic pixel size is 20 microns and the active image area is 768 x 384 pixels. As the spectrometer's electronics reads the data, the system arranges pixels in 3 x 3 groups, yielding a 128 x 256 image array of 60-micron pixels. The device has four output ports. Because the CCD operates at 240 Hz, the pixel rate at each output port comes to 2.4 MHz.
For the SWIR's focal plane array, engineers selected a Mercury Cadmium Telluride (Mercad) photodiode array and a CMOS multiplexer. Array format is 256 x 256 pixels, each 60 microns square. It operates at a frame rate of 240 Hz and the array's four output ports each run at 4 MHz. The SWIR focal plane assembly resides on a ceramic substrate. To help keep the detector at 115K the package mounts to a thermal standoff assembly that isolates the detector from the optical bench. Aside from the cryocooler that chills the detector, the remaining thermal path is through an electrical cable. It consists of a lamination of Kapton(R) and Constantan(R)/copper conductor layers that keep thermal conductivity low.
The panchromatic camera's FPA consists of a linear CCD with 2,592 pixels, each 10 microns square. On-orbit, the FPA reads out through two output ports at approximately 2 MHz. Given that pixel size is six times smaller than the VNIR and SWIR pixels, data frame rate runs at 1440 Hz.
In operation, the HSI functions as a pushbroom scanner. As the Lewis spacecraft moves around the earth, it sweeps the line of coverage forward like the head of a push-broom. "There are no moving parts except the cover that we open,"says Folkman. Opening the cover exposes the foreoptics (telescope) and admits light to the HSI.
Because the basic satellite structure (called the bus) employs graphite construction, HSI won't open its cover until after two weeks on orbit. During this period, the structure should finish outgassing.
Onboard Lewis, HSI data are stored in a solid state recorder. Although data compression or manipulation can be done by an onboard computer, early data sets will be sent straight to ground stations. Personnel on the ground merge telemetry from HSI and Lewis with the image data to interpret where the instrument pointed.
And what next? Watkins and his colleagues hope to move away from the large, billion-dollar satellite. One way of doing so may involve breaking up very big satellites into a number of smaller platforms. In future missions, NASA may demonstrate that stable instruments on smaller satellites can achieve co-registration of data. That kind of setup could permit a number of satellites to fly as a virtual platform.
The great days of big space programs may never return. But Lewis and the HSI demonstrate that space exploration still has a future. Small, relatively inexpensive spacecraft that carry capable instruments can enable us to continue to study the environment our earth traverses--space.
Versatility in a small package
More than 40 technology demonstrations and experiments will fly into space on the Lewis spacecraft. The 850-lb vehicle will attempt to show that small spacecraft can do significant work on orbit, and perform for a period of three to five years.
PARTIAL LIST OF LEWIS TECHNOLOGIES MISSION PAYLOADS
Linear Etalon imaging
Spectral array Diffuse EUV cosmic background spectrometer
PAYLOAD SUPPORT TECHNOLOGIES
Pulse tube cryocoolers
Optical pointing assembly
Gbit solid-state recorder
Lossless/lossy data compression
SPACECRAFT BUS TECHNOLOGIES
Spacecraft loads and acoustic measurements
GPS attitude determination
Wide field of view star tracker
Magnetically suspended reaction wheel assembly
Autonomous orbit maintenance
Metal matrix heat strap
Multi-junction GaAs and amorphous silicon solar cells
SPACECRAFT BUS SUBSYSTEMS
Integrated thermal control
Higher efficiency, long life
High-performance data processing and management
Reusable flight software
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