S A P P H I R E
.S.T.A.N.F.O.R.D.
.A.U.D.I.O.P.H.O.N.I.C.
.P.H.O.T.O.G.R.A.P.H.I.C.
.II.N.F.R.A.R.E.D.
.E.X.P.E.R.I.M.E.N.T.
S A T E L L I T E S
Y S T E M O V E R V I E W
.SAPPHIRE CURRENT CONFIGURATION.
Sapphire's mission consists of three system elements: the
spacecraft, the launch vehicle, and the ground system. In this page you
will find the basic ideas of how each element works. The design log
details the original process of thought that went into the spacecraft
components (now being covered in the subsystem descriptions). Also
included is a link to the general issues facing the overall systems.
[ Systems Main Page ]
[ Design Log ]
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Payloads
There are three payloads and one
telemetry experiment aboard Sapphire:
Tunneling Horizon Detectors
- The primary payload, an effort by Prof Tom Kenny of Stanford's Mechanical Engineering
Design Division and JPL. These are a new generation of infrared sensors
that detect changes in IR emission. They are micromachined - they fit in
a chip and operate at room temperature. The THDs will see their first flight
aboard Sapphire.
Digital Camera - This is the Fotoman Plus a, commercially
available black & white camera from Logitech. We've modified it for
space flight (removing the flash, for example) and intend to get pictures
of North America (around 1km resolution if all goes well). The camera does
its own JPEG compression and could store 32 pictures, although we'll be
using our CPU memory for that.
Voice Synthesizer - RC Systems sells a complete
package; you input a text string and it outputs a computer "voice"
that you can route through speakers or, in our case, a transmitter. The
purpose of the Digitalker, as we call it, is to provide a payload of interest
to the Amateur Radio community, specifically in education. We hope to take
a hand-held radio to schools, and as the satellite flies overhead, have
students listen to it "speak" - giving geography lessons or an
uploaded message. Then we can illustrate basic satellite and radio principles.
Telemetry Experiment
- A pseudo-payload, designed by students, is to assess
how well we can determine our attitude using the solar panels as a differential
sun sensor.
Processor
The CPU is a Motorola 68HC332 sold
by Vesta. We have built up a board around it that provides analog to digital
conversion and a few other features. There is a watchdog timer and around
100K of RAM. (Not much, but we don't need much.).
Communication
The Comm Subsystem uses
modified Hamtronics kits. The transmitter, broadcasts with 2 W of power
at 437.100 MHz. The receiver is at 145.945 MHz. Our Terminal Node Controller
(modem) is a modified Kantronics model, with our own settings and some
new space-rated chips installed. We've also built a multiplexer to allow
the Digitalker to share the transmitter with the CPU.
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Power
The Power Subsystem looks the same
as most other spacecrafts':
Ten NiCd "D" cells in
series (split in two packs) to provide eclipse power. (These are space-rated
cells.) Twenty-four strings of twenty solar cells each (two on each side,
four top and bottom). These Gallium Arsenide cells were surplus from other
projects and assembled with significant assistance from Lockheed-Martin
of Sunnyvale. Twelve- and five-volt regulation.
Sapphire is expected to generate
16 Watts of peak power off its cells, averaging around 8. (Standby power
consumption, by the way, is 3 Watts.)
Structure
The structure consists of four
aluminum honeycomb trays, one to a subsystem (from bottom: Power, Communications,
CPU, and Payloads), with eight external panels (six sides, top and bottom)
upon which the solar cells are mounted. The entire hexagonal cylinder,
fully loaded and on the launch interface, weighs nearly 40 pounds.
The launch interface is student
designed (see the paper
describing it) and utilizes a shape-memory alloy,
the Frangibolt, built by the TiNi Alloy
Corporation.
Attitude Determination and Control
The ADC subsystem follows in the
footsteps of the early Oscar satellites to provide a "controlled tumble"
as follows:
Four ALNICO-V bar magnets are aligned
with the spacecraft Z-axis. Sapphire will now line up with the local North
direction of the Earth's magnetic field (providing near-nadir pointing
over North America for the camera, and giving the IR sensors full view
of the Earth over the equator). The four transmit antennas are painted
black on one side and white on the other to provide a radiometer spin.
It is estimated that the satellite will rotate at about 1/3 rpm. This gives
the IR sensors a changing signal to measure. Hysterisis rods are placed
perpendicular to the Z-axis to damp nutation about that axis as well as
to create a maximum spin rate.
As for sensors, some infrared phototransistors
are positioned to verify when the Fotoman sees the Earth, and the Sun sensor
experiment using the solar panels has already been described.
Thermal Control
Every box for Sapphire has been coated with a black
silicone-based paint to increase conductivity. Certain items, like the
IR boxes, are placed on top of thermally conducting mats. These passive
controls are enough to keep the spacecraft in its operating temperature
range.
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As simple -- or boring -- as it may seem, the ground station and
operations of Sapphire are actually major elements of SSDL research!
Sapphire will nominally be contacted by the large Yagi antenna on the
4th floor of the Durand building here at Stanford. That antenna feeds
directly to our tracking and data relay computers inside.
Now, in most systems, the information received from the
communications antennae is relayed to another site where the ground
controllers work. In our case, the data is relayed across the room
(about five feet) to our mission control center.
The MCC, as we in the acronym business like to call it, handles all
the experiment scheduling, data processing, and health management of the
spacecraft. Normally, teams of personnel are responsible for setting
the system schedule, coordinating with the principal investigators, and
performing maintanance and troubleshooting on the satellite. With
Sapphire, it may only take a few.
Now, that's the bare-bones setup. But since SSDL is performing
research in spacecraft operations, we intend to do much more.
Our primary approach is automation -- developing methods for computers
to shoulder the workload and improve performance. Not that computers
are the best solution to every problem -- not at all! -- but
in seeking to automate processes, we develop testable, quantifiable
methods to address the common problems in spacecraft operations. And
that's what research is all about.
Included below is a schematic of our operations system, which we call
ASSET. More information is available on our web site, should we ever
get that put together. Note that in addition to our single station at
Stanford, we are setting up partnerships with universities in Alabama,
Utah, Montana, Sweden, Moscow, Rome, and all over the world. We'll gain
global coverage for Sapphire, and they'll gain global coverage for their
spacecraft, too.
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