Technical information on 3 amature satellites currently in orbit.
A Guide to the Architecture of UoSAT-B
The following attempts to provide a guide to the sub-systems on the UoSAT-B
spacecraft, now in orbit as UoSat-2 or OSCAR 11. Expanded specifications
of individual modules will be made available for those experiments which
supply data to the downlink directly or via the telemetry system.
Mechanical Framework
The spacecraft is constructed in a similar way to Oscar-9, that is with a
square-section central core supporting rigid top and bottom plates. Solar
cells are mounted on all four sides of these plates, enclosing a basic cuboid
of dimension 35.5 * 35.5 * 58.5 cm. Two stacks, each of two module boxes of
dimension 23.5 * 17.6 * 3.1 cm, are mounted on the outside of each face of the
central core. A 'wing' extends the base of the spacecraft symmetrically by 16
cm on each side in one axis to permit the mounting of two SHF helical
antennas, one on each side of the launcher attach fitting which is itself
mounted in the centre of the bottom plate. The Navigation Magnetometer
and Space Dust experiments are mounted above this wing, one on each side.
Solar Cells
Four solar arrays of dimension 49.5 * 29.5 cm are attached to the four sides of
the spacecraft. These are capable of supplying up to about 0.9A at 28v when
fully illuminated. The cells were manufactured by Solarex.
Battery
A solid octagonal block of aluminium, 14.9 * 14.9 * 10.2 cm, is fitted into the
centre core of the spacecraft and is drilled to accept ten 'F'-sized
Nickel-Cadmium cells, each 3.2 cm in diameter and 9 cm long. These cells, in
series, form a 12v battery of 6.4Ah nominal capacity and are charged when the
spacecraft is in sunlight in order to provide sufficient power to run the
craft during peak load demands and its eclipse periods.
Battery Charge Regulator
Two redundant BCRs are responsible for accepting the 28v supplies from the
solar cells (and a similar supply from the umbilical connector) and charging the
battery as required depending on the current drain, the battery voltage and
the battery temperature.
Power Conditioning Module
The PCM regulates the 12-14v battery bus supply to provide 10v, 5v and -10v
supplies for powering the spacecraft systems and experiments.
Power Distribution Module (PDM)
The PDM switches the various regulated and unregulated rails to all the s/c
systems and experiments, dependent on the commands which it receives
from the Telecommand system. Each switch has an individual current foldback
facility so that a faulty module is allowed to draw up to a pre-determined
current before it is latched off, necessitating a power-down under positive
command before resetting.
Telecommand system
The telecommand system comprises three uplink receivers, three data
demodulators, a command detector and sets of command latches which hold the
status of the command specified. The receivers are located in the 144MHz,
438MHz and 1268MHz amateur bands and the demodulators are robust devices
which do not depend on `hase-locked loops or other potentially unstable
techniques. A command detector scans the three receivers according to a
priority system and detects a valid set of command instructions, passing the
data contained therein to the relevant latch. Some latches drive a set of
multiplexer address inputs directly so that uplink and downlink path
selection may be performed immediately on the command latch board.
The 112 command latches drive the Power Distribution System, the
remaining spacecraft systems and experiment functions. There is a parallel
i/o port to the spacecraft 1802 computer for autonomous control of
spacecraft operations in addition to serial data links with `(O 1802 #+uter
and the DCE for backup operations.
145.825 MHz Beacon
The 145 MHz beacon on UoSAT-B is nearly identical to the one flown most
successfully on UoSAT-1. The modulation index has been increased in order to
ensure more optimum reception on most radio amateur receivers. Modulation is by
frequency-shift keying, as on UoSAT-1.
435.025 MHz Beacon
This beacon is a completely new design which generates its frequency standard
from a phase-locked synthesiser system. As a result, the DC to RF efficiency
is much improved. In addition to frequency-shift modulation, phase-shift
modulation is a switchable option.
2401.5 MHz Beacon
When the original supplier of the 2.4GHz beacon was unable to meet his
commitment, Colin Smithers, G4CWH, at the University of Surrey stepped in and
designed and built the transmitter and power supply in under four weeks.
The DC to RF efficiency has been improved by some 5 times over the UoSAT-1
implementation. Both AFSK and PSK modulation methods are possible.
Telemetry System
The basic output of the UoSAT-B telemetry system is very similar to that of
UoSAT-1. However, 60 analogue channels, digitised to 3 decimal digits, and 96
status points encoded into hexadecimal digits are available together with a
real-time clock for frame identification and the satellite identifier,
'UOSAT-2'. A checksum digit can also be added to each channel. A dwell
facility has been added so that up to 128 channels can be output in
rotation, combined with clock times and line feeds or frame ends in any
combination.
1802 Computer & Digitalker
The 1802 computer has been designed to support all the modules on the
spacecraft, as well as to control the overall scheduling and be usable for
specific communications experiments. To satisfy these requirements,
the computer has access to many modules via parallel interfaces, and to
some of the others and the receivers and transmitters via serial
connections. In addition, there is a real-time clock and a total of 48kb
of RAM for data storage. The Digitalker speech synthesiser is housed with the
1802 and has ROMs containing over 550 words. These will be used initially for
'speaking' telemetry.
Navigation Magnetometer
The Navigation Magnetometer is a three-axis flux gate device, much upgraded
from the one flown on UoSAT-1. Indeed, the 14-bit resolution is very similar
to that obtained from the much more complicated scientific magnetometer
on the previous craft. The Nav. Mag. will be used for determining the
attitude of the spacecraft during initial manoeuvres, as well as for
experimental measurement of magnetic field disturbances once the attitude is
stable.
Magnetorquers & Boom Assembly
Magnetorquers - coils of wire energised to act as electromagnets - are built
into all 6 faces of the spacecraft, wound around the edges of the honeycomb
panels supporting the solar cells and the top and bottom plates. The
fields created interact with the earth's magnetic field to produce a torque
which tends to rotate the spacecraft.
When the spacecraft has been positioned so that the CCD camera end is pointing
towards the earth - a long and complex process - a boom can be extended from
the top of the craft. The boom looks similar to a steel tape measure, although
nearly circular once it has been unrolled, and some 12 metres long. The boom
carries a 2.5kg mass on the far end and this, in conjunction with the spacecraft
body at the other end, creates a 'dumb-bell' configuration which naturally lines
up with the earth's gravitational field so that one end points downwards,
rather like a pendulum - it is, however, bi-stable! Any residual swinging
motion can be damped with further controlled applications of the magnetorquers.
Sun Sensors
The sun sensors are made with specially fabricated solar cell
substrates which are masked by grey-code stripes and illuminated by light
passing through a slit in a metal foil in front. The mask coding on the cells
can be used to derive the angle at which the incident light is falling on
the slit. 6 such sensors are mounted around the top plate to provide complete
360 degree coverage.
Horizon Sensors
Built by a first year student at the University of Surrey, the Horizon Sensor
is able to detect when only one of two photodetectors is illuminated.
The detectors are housed in two narrow tubes of 4mm diameter and mounted
at a small angle to each other so that the whole sensor thus detects the 'edge'
of an illuminated object. This will be the earth, the moon or the sun
and a fix can then be made on the object's position.
Digital Communication Experiment
The Digital Communications Experiment (DCE) was designed and built by AMSAT
and VITA groups in the USA and Canada. It has two serial ports which can
receive and transmit to the RF system and the 1802, as well as an NSC-800
CPU and nearly 128kb of CMOS RAM. The DCE will be used to investigate various
packet radio protocols for use with a future digital 'store-and-forward'
satellite being planned by AMSAT. In addition, the DCE has interfaces with
the navigation magnetometer and the telemetry system for long-term data
storage.
Space Dust Experiment
The Space Dust experiment was built by a group of students at the
University of Kent, England. It has a dielectric diaphragm which, when
punctured by a large particle, discharges the capacitance associated with it,
therby indicating the impact. In conjunction with a piezo crystal
microphone which detects particles of smaller size, correlation techniques can
yield a measurement of the momentum of the incident particle.
CCD Camera
The CCD camera is a re-designed version of the device flown on UoSAT-1. Indeed,
the CCD array at the centre of the camera is the same type as used before,
although the later batches of this part are substantially improved over the
early one used 2 years ago. This time the analogue electronics surrounding
the array are also greatly improved. The active area of 384 pixels by 256
pixels is stored with seven bits of grey-level, in 96kb of RAM in the
DSR experiment. The DSR is then responsible for the picture downlink, aadding addresses and error correction and detection information as required.
The DSR downlink is organised in packets of 128 bytes each, three across each
imager line, so that two may be selected for display (using an extra digital
filter) on existing UoSAT-1 CCD displays. The variable video amp gain and
integration period of the CCD imager have been set up to provide the latitude
required to photograph both land images and also auroral features, the
latter being of interest in conjunction with the particle detector experiments.
DSR Experiment
The DSR stores data from the CCD imager, particle counter experiment or computer
UART and outputs it in a checksummed format. The unit has 2 banks of 96k x 8
CMOS memory which can be used as two seperate banks or as one 192kb bank.
The output frame consists of a three byte sync code, a two byte frame address,
128 bytes of data and 5 bytes of error detection/correction code. The data is
sent in serial form with start bit, 8 data bits and selectable*Bor 3 stop
bityB The data can be output at 1200, 2400, 4800 and 9600 BPS.
Particle Detectors and Wave Correlator Experiment
Three Geiger counters, each with different electron energy thresholds, similar
to those flown on UOSAT-1, and a multi-channel electron spectrometer are
mounted on the spacecraft to serve as a near-earth reference for
magnetospheric studies to be carried out concurrently with the AMPTE & VIKING
spacecraft missions due for launch later in 1984, and for ground-based
studies of the ionospheric D, E and F regions being pursued with
riometers and EISCAT. Data will be available in either real-time or,
for more detailed analysis, from stored measurements over both polar auroral
regions to professional scientists and radio amateurs. A data-base of
the measurements acquired over the life of the spacecraft will be established
at Surrey in conjunction with the SERC and will be available to
approved experimenters.
The modulations imparted on particles, as a result of wave-particle
interactions in the magnetophere on auroral field lines, will be observed
by a Particle Correlator Experiment designed around an NSC-800 microprocessor at
the University of Sussex, England. The measurements will identify wave-modes
responsible for accelerating electrons into the auroral beam and will also
identify wave-modes which limit the further growth of the auroral beam.
Roger M. A. Peel, G8NEF, University of Surrey, England.
JAS-1 Amateur Satellite
Technical Description
Introduction:
JAS-1 is an amateur radio satellite, promoted by JARL as a joint venture with
NASDA. NEC constructed "system" units (space frame, power supply etc.), while
JAMSAT, with its selected volunteer JAS-1 project team, designed and built the
"mission" units (transponders, telemetry/command and house keeping
micro-computer) and ground support systems.
JAS-1 has been completed and has passed all the necessary tests. It is in a
clean room waiting for the launch, currently scheduled for August 1986.
The outline of this unique satellite is explained in the following.
Many thanks to Harold Price, NK6K, for his assistance in the preparation of this
article.
February 11, 1986
N6MBM/JA2PKI
Tak Okamoto
191 Pinestone,
Irvine, CA 92714
Hamnet : 72307,3224
Telemail : TOKAMOTO
JAS-1 Mission Objectives:
1. JAS-1 will provide reliable world-wide amateur radio communications.
2. JAS-1 will enable radio amateurs to study tracking and command techniques.
3. JAS-1 will offer an in-space "proving ground" for radio amateur developed
and built transponders and sub- systems.
4. JAS-1 will provide NASDA an opportunity to carry out a "multi-payload"
launch using their new "H-1" launcher. (NASDA has never engaged in a
multi-payload launch, thus the JAS-1 project will offer NASDA an excellent
opportunity by providing them with an active payload having its own
telemetry-beacon and transponder for ranging.)
1. Form and general dimensions:
The spacecraft takes the form of a 26-facet polyhedron, which measures 400 mm X
400 mm X 470 mm and weighs 50 kilograms.
2. Launch and Orbit:
JAS-1 will be launched into a circular low-earth orbit, which will be non-sun
synchronous and non-polar.
Launch vehicle : H-1 2 stage rocket
Launch nuber : Test Flight # 1
Launch fite : Tanegashima Is. Japan
Launch date : August 1986
Estimated inclination : 50 degrees
Estimated altitude : 1500 k.m.
Estimated period : 120 minutes
Estimated window per pass : 20 minutes/pass
Estimated passes per day : 8 passes/day
3. Designed life:
Estimated lifetime is 3 years
4. Special Features of JAS-1:
JAS-1 carries two separate mode J transponders. One is a linear transponder,
and the other is a digital "store-and-forward" transponder mainly for
non-real-time communication between stations located in different time zones.
The reasons for selecting mode J for this first Japanese amateur radio
communications satellite are:
a) It is becoming increasingly difficult to use 145-MHz for a satellite downlink
because of man-made electrical noise and other interference.
b) The planners of JAS-1 wanted to provide a successor to AMSAT OSCAR-8's mode
J, which was originally developed by JAMSAT's engineering team back in 1976.
c) 435 MHz is much quieter than 145 MHz as a downlink band, it is comparatively
free from man-made noise and sky-temperature effects.
The digital transponder will provide "error-free" information exchange.
5. Transponders:
a) The linear transponder = mode JA :
The passband will be 100 kHz wide. The transponder will have an output of 1
watt p.e.p. Ground stations will need an uplink power of 100 watts e.i.r.p.
The sidebands will be reversed, i.e., the uplink is LSB, the downlink is USB.
There will be a 100 mW c.w. beacon switchable to PSK when needed.
Uplink pass band : 145.90 MHz - 146.00 MHz
Downlink pass band : 435.80 MHz - 435.90 MHz
Beacon freq. : 435.795 MHz
Translate freq. : 581.80 MHz
b) The digital transponder = mode JD :
There will be four 145 MHz band input channels using Manchester coded FM for the
uplink. Ground stations will need 100 watts e.i.r.p. There will be one
downlink channel in the 435 MHz band using PSK, the output will be 1 watt RMS.
Channels are :
Uplink channel 1 : 145.850 MHz
,, channel 2 : 145.870 MHz
,, channel 3 : 145.890 MHz
,, channel 4 : 145.910 MHz
Downlink channel : 435.910 MHz
The data format is HDLC. The protocol is AX.25 Level 2 Version 2. The data
transfer rate is 1200 bps for both uplink and downlink.
The reasons for not using Bell-202 type FSK modulation are:
a) To reduce the parts count onboard JAS-1. Using Manchester coded FM for
uplink reduces JAS-1's onboard decoder chip count by 16.
b) To improve the downlink margins. Due to JAS-1's tight power budget, only 1
watt is generated by the downlink transmitter. A more efficient modulation
scheme like PSK is required.
JAS-1 will be a store and forward system but not a real time digipeater.
Digipeating is not an effective use of a low orbit satellite such as JAS-1,
which has a limited communication foot print and visibility time.
JAS-1 has 4 uplink channels for 1 downlink channel. This is because the
difference of channel efficiency between uplink and downlink. An uplink channel
is shared by several ground users. Since the ground users can't hear each other,
and are listening to the downlink channel anyway, the uplinks are subject to
packet collisions. This scheme is called "Pure ALOHA", and is known to have a
theoretical maximum channel throughput of 18.4%. The JAS-1 downlink is 100%
efficient, since only JAS-1 transmits there. To balance capacity, as well as
add redundancy, four uplink channels are used. The combined uplink efficiency
is then 4 * 18.4% or 76%. The remaining downlink time is used for general
messages and telemetry data.
JAS-1 will accept a connect from only one station at a time with the software
scheduled for initial use. Multiple connections will be supported in subsequent
software updates. General packet operation is scheduled to begin in November
1986.
6. Digital Hardware:
The microprocessor is a MIL-STD-883B screened NSC-800 running with a 1.6MHz
clock. This is the only processor on board. It controls the digital
transponder and also acts as an IHU (Integrated Housekeeping Unit).
The on-board memory has a 1.5MB physical storage capacity. 48 chips of NMOS
256K DRAMs are used. A hardware based error-detection/correction circuit is
incorporated to protect the entire 1.5 MB and provide an 1 MB error free memory
area. The system program occupies some 32KB, the rest is used for message
storage.
The memory unit is physically divided into four identical 256KB memory cards,
any one of which can be assigned as the system area. Up to three cards can be
turned off. This design provides system redundancy and allows command stations
to control power consumption without a total loss of service.
JAS-1 has five hardware HDLC controllers. Four of them are for the uplink
channels and one is for the downlink channel. In total, these controllers
consist of some 140 CMOS MSIs, yet their power consumption is less than that of
a single NMOS LSI HDLC controller like WD-1933.
JAS-1 does not have any ROM but has simple hardware boot strap circuit instead.
This design is to increase system flexibility and reliability.
7. Power system:
25 of JAS-1's 26 faces are covered with a total of 979 pieces of solar cells.
They will generate 8.5 watts of power at the beginning of life.
JAS-1 employs 11 Ni-Cd battery cells with a capacity of 6 AH. These supply 14
volts average to JAS-1's main power buss. The 14 volts is converted and
regulated to +10V, +5V and -5V.
8. Antenna system:
JAS-1 has three antennas.
2 m reception antenna
Slant 1/4 wave Mono-pole Isotropic -4 dBi gain
70 cm transmission antenna
Mode-JA : Slant Turnstile L.H.C.P. +Z axis +3 dBi gain
Mode-JD : Slant Turnstile R.H.C.P. -Z axis +3 dBi gain
9. Attitude control:
Forced shaking using the earth's geomagnetic field. JAS-1 has two 1 ATm sq.
permanent magnets in its Z axis.
10. Telemetry:
Analog system telemetry has 12 analog channels and 33 system status flags. This
telemetry can be sent without the help of the NSC800 microprocessor and will be
turned on automatically by the separation from the H-1 launcher. The telemetry
is sent on the 100mW beacon on 435.795MHz in CW, switchable to PSK.
Digital system telemetry has 29 analog channels and 33 system status flags.
This software driven telemetry can be sent in any format, and can include short
text messages. This telemetry can be sent on either the mode JD downlink
channel (435.910MHz) or the mode JA CW beacon (435.795MHz).
11. Command:
A simple 3-channel tele-command system is used for global control functions,
e.g. JA transponder "ON"/"OFF", JD transponder "ON"/"OFF" and independent
"ON"/"OFF" of the A-0 beacon. An additional 37 channels are available mainly
for controlling the digital transponder.
Onboard command from the NSC-800 is also available.
12. Ground stations:
Mode-JA:
A ground station setup which was used for Amsat Oscar-8 mode-J can be used for
JAS-1 mode-JA. A station with a 10 watt 2 m SSB transmitter and a 10dBi beam
for uplink; and a 70 cm receiver (with low NF) with a 15dBi beam for downlink;
should be adequate for this job.
Mode-JD:
In addition to the mode-JA set up, FM mode is required for the 2 m transmitter.
Since JAS-1 uses the standard AX.25 protocol and 1200 bps data rate, ground
stations will be able to use a TAPR-style TNC, a 2 m FM transmitter and a 70 cm
receiver without modification.
The JAS-1 modem, a special interface board, will be made available containing
the Manchester modulator and an audio PSK demodulator allowing connection to the
"modem disconnect" connector of a TAPR-style TNC. The modem also connects to the
audio input and PTT of the 2m FM transmitter and to the audio output and
frequency control (option) of a 70 cm SSB receiver.
Although JAS-1 will be available to individual access, the general amateur
community will benefit from "JAS-1 gateways". Messages relayed through gateways
can be sent worldwide and is as easy as sending messages to distant stations via
a W0RLI HF gateway.
13. Outline of project history/schedule:
November 1982 : Freezing of conceptual/preliminary design
December 1982 : Preliminary Design
April 1983 : Detail Design
- June 1984 Engineering Modules Integration & Test
Ground Support System Integration
July 1984 : Flight Model #1 Integration & EIC/MIC
- December 1984
January 1985 : Flight Model #1 General Test
- March 1985
January 1985 : Flight Model #2 Integration & EIC/MIC
- August 1985
August 1985 : Flight Model #2 General Test
- November 1985
November 1985 : Software development.
- ?
References:
JARL News, JAS-1 User's Guide (Those are available only in Japanese.)
AO-10 ANALYSIS, PROPOSALS
June 17, 1986
The purpose of this document is to:
1. Present the current status of AO-10.
2. Forecast events between now and September 1986.
3. Explore the limited alternatives available.
4. Present proposals and define probable benefits.
5. Solicit additional expertise, advice and comments.
= = = = = =
1.0 CURRENT STATUS OF AO-10:
AMSAT-OSCAR 10 was 3 years of age yesterday. Despite a beginning which seemed
to be ruled solely by Murphy's 3rd postulate, the S/C has performed as well as
could reasonably be expected, considering the bent antennas, less than optimal
orbit, frozen 'O' rings, etc.
The satellite was designed with reliability as one of the foremost objectives..
since previous birds had succumbed due to eventual battery failure, TWO sets of
batteries were placed on board; ten Main batteries and ten Auxiliary batteries.
To date, the Main cells have performed so well that there has been no need to
bring the Aux cells online. Premature charging of the Aux cells would merely
serve to start their 'lifetime countdown', therefore, they have never been
charged in orbit.
As the spacecraft aged, the effects of 4U Cigh perigee (4,000 km instead of the
desired 1,500 km) began to be noticed; at this altitude, the S/C spent
significantly more time traversing the radiation-filled Van Allen belts
surrounding the Earth. Each trip through this area resulted in continuous doses
of undesirable radiation being experienced by most onboard components. The
effects of such radiation are cumulative.. the overall level of radiation
induced charge keeps adding to the previous exposures.
The IHU (Integrated Housekeeping Unit.. speak 'onboard computer') memory chips
are the most susceptable to excess charge of all the onboard components, since
they function by storing a definable charge to represent a one or zero in a
particular memory location. Over a period of time, random bits throughout the
16k memory began to fail. This did not present a disaster, since the S/C
designers had included sophisticated error correction circuitry for just such an
expected eventuality. The correction circuitry could detect and 'repair' a
single-bit error in any given byte of memory. It would detect, but not repair,
a double-bit error per byte.
On May 17, 1986, the error correction circuitry was apparently overwhelmed by
the damaging effects of an influx of high energy particles from the Sun. The
software Operating System had lost control with the Mode B transponder locked on
and strings of meaningless bits being transmitted on the beacon.
As a result of many hours of diagnosis and attempts to correct the situation by
ZL1AOX and others, a limited function software system was reloaded.
Subsequently, limited memory tests were performed in an attempt to assess the
extent of the damage and suggest methods of bypassing the faulty areas of
memory.
Before these tests could be completed, the S/C was apparently subjected to yet
another bombardment of radiation which reduced even the minimal operating system
to an essentially useless state.
Under certain conditions, UNDER-charging CAN be of actual
benefit, as we shall see.
2.0 FORECAST OF EVENTS THROUGH SEPTEMBER 1986:
Given the current attitude of the spacecrast, the position of the orbital plane
and the orbital parameters, the sun angle will change from the current value of
approximately -8 degrees to -49 degrees by 7/31 and to the NO POWER condition of
-90 degrees on 9/11 as indicated by the following chart (courtesy G3RUH):
DATE SUN ANG (deg) ALON (deg) ALAT(deg)
---------------------------------------------------
1986 May 22 18 157.5 21.7
1986 Jun 5 4 156.1 21.7
1986 Jun 19 -9 154.8 21.7
1986 Jul 3 -22 153.4 21.5
1986 Jul 17 -36 152.1 21.4
1986 Jul 31 -49 150.7 21.2
1986 Aug 14 -62 149.3 21.0 ) 47% Illum.
1986 Aug 28 -75 147.9 20.7 ) 26%
1986 Sep 11 -90 146.5 20.4 ) OOPS!
1986 Sep 25 -76 145.1 20.1 )
"An attitude change is ESSENTIAL before the end of July" (G3RUH)
If no intervention occurs, the S/C will reach a power down condition sometime
prior to September 11. At first glance, this might seem to be a disastrous
event; let's analyze this condition a little more thoroughly.
Of the many events which will occur at or near the -90 degree sun angle, the
following are of most concern:
2.1 Thermal stresses.
2.2 Low/no power considerations.
2.3 Erratic IHU operation during transition period.
2.1 THERMAL CONSIDERATIONS:
From a sun angle of -45 through -90 and back to -45, the sun will primarily be
shining on the bottom of the S/C (rather than on the solar panels), resulting in
a significant heating of that surface, while the opposite surface will suffer a
deepfreeze effect. The resultant temperature of important internal modules
(IHU, batteries, BCRs, etc) will reach temperatures dependent on the thermal
transfer characteristics of their housings, mounting brackets, etc.
We already possess telemetry data of a similar event which took place right
after the initial launching of AO-10. Analysis of that TLM data is being
performed by Command Stations right now. AO-10's thermal design expert (Dick
Jansson, WD4FAB) will be contacted as soon as he gets back to the Continental
U.S. on Saturday. He should be able to shed valuable light on this important
subject.
NICAD battery expert John Fox (W0LER) advises that this should make little if
any difference whether the batteries are charged or discharged when they are
subjected to the expected thermal stress.
2.2 LOW/NO POWER CONSIDERATION:
From both a battery and an IHU long-term 'health' viewpoint, it appears that a
complete power down condition could well provide major BENEFITS.
2.2.1 BATTERIES:
The Aux batteries have never been charged; their condition should
remain essentially unaltered through a
forced power down situation.
By the time power totally fails, the Main batteries will likely
have developed the notorious NICAD 'memory' for partial charging.
Fortunately, if each cell discharges to a level of 0.2 volts or
lower, (2.0 volts for the total array of 10), all 'memory' will
be ERASED. In addition, laboratory tests by W0LER have shown
that up to 85% of original (new) capacity can be expected from
the aged cells when they are recharged once again.
W0LER further advises that he has NEVER witnessed polarity
reversal during such deep discharge/recharge cycles. (John's
wisdom was gained from a 5 year period of DAILY measurement and
painstaking record-keeping on this very subject).
Providing there are no disastrous temperature effects of which I
am unaware, it would appear that the Main batteries will actually
BENEFIT from the power-down situation.
2.2.2 IHU:
According to several knowledgable individuals in the computer
industry, there is a reasonable chance that the disabling excess
charge on the memory chips may actually BLEED OFF if power is
completely removed from the memory for at least a 24 hour period.
If this fortunate state is actually realized, we could
optimistically expect to end up with a rejuvinated memory when
the S/C powers up again (good for another 3 years?).
2.3 ERRATIC IHU OPERATION DURING TRANSITION:
Once the IHU supply voltage begins to fall, there is a rather narrow 'window'
that exists in the shadow region between the FUNCTIONAL and the STOPPED IHU
states. In tests on nearly identical (simulator) IHUs in a terrestrial
environment, operation was essentially normal down to the 6.0 volt level,
erratic and unpredictable from 6.0 to 5.2 volts and totally inoperative below
that supply voltage level.
The erratic window region does generate a certain amount of concern; in this
region, the CPU may do ANYTHING. It may perform anomalous jumps to erroneous
program steps, it may perform erratic I/O operations with potentially harmful
results; Murphy's law is strictly enforced in this region... the most harmful
thing which can be imagined will most likely be realized.
There are certain techniques which can reduce this hazard; they will be
addressed later. The major point to be made here is that the time spent in this
'transitional area' should be minimized by any means possible.
3.0 CORRECTIVE ACTIONS AVAILABLE:
3.1 Do NOTHING until after September 15, 1986.
3.2 Perform memory diagnostics and attempt to patch around
faulty areas with a reduced-function Operating System.
3.3 Power down as soo\ as practical.
3.1 DO NOTHING UNTIL AFTER 9/15/86:
If we merely wait until the inevitable occurs, we stand the very good chance of
even further memory deterioration with the attendant prospect of not being able
to do anything about S/C attitude or onboard conditions. Erratic IHU operation
WILL take place anyway; Main battery discharge WILL occur. The AMSAT satellite
user group will become increasingly frustrated and discouraged and begin to seek
other interests after we fought so hard to get their attention in the first
place. Knowing this organization, I don't expect many votes for this option.
3.2 PERFORM MEMORY DIAGNOSTICS & ATTEMPT A PATCHED OPERATING SYSTEM:
While there will probably be a significant amount of support for this
alternative, there are good reasons to perform some tough objective analysis
before embarking on this route. The time and effort to perform this task is
indeed formidable. The chance of long-term success in this direction seems
small, indeed. By the time a thorough memory analysis is performed (if it can
even be done at all), further radiation damage will probably have already
occurred, thus rendering the analysis useless. In addition, this activity would
necessarily involve personnel who are already swamped with Phase-3C activities.
Time stolen from Phase-3C could well lead to a situation of similar consequence
a few years from now with the next satellite.
3.3 POWER DOWN AS SOON AS PRACTICAL:
As long as the first 3 bytes of memory remain functional, we should be able to
uplink simple assembler language routines to perform one to a few functions at a
time. It would be necessary to periodically run a memory diagnostic on at least
a portion of memory as insurance. SoZe of the functions which are considered
most important are:
3.3.1 Memory diagnostics.
3.3.2 Limited telemetry.
3.3.3 Transponder and beacon control. (No transponder usage)
3.3.4 BCR service to control battery charge rates.
3.3.5 Minimal attitude & spinrate control.
These functions can probably be performed by the Ground Command Station (GCS)
group with only minimal assistance from the spacecraft development team, thus
freeing them to concentrate on 'hardening' the Phase-3C bird.
4.0 PROPOSALS, RATIONALE & PROBABLE BENEFITS:
With the information currently available to me, I propose that alternative (3.3)
be implemented under the following conditions:
4.1 Bring the spin rate up to 45 or 50 RPM for maximum long-term
stability.
4.2 Intentionally begin changing the S/C's attitude toward a -90
degree sun angle to shorten the total 'outage' period.
4.3 When the IHU supply voltage begins to drop below it's normal
10 volt level, activate the transponder and beacon, then load all
of memory with a benign instruction code and 'hang' the CPU in a
tight loop to minimize the chance for erratic behavior.
The purpose of activating the transponder and beacon is to hasten
the discharge process as much as possible, thus shortening the
amount of time the IHU will spend in the potentially dangerous
'erratic window' region of supply voltage. Selected users would
be encouraged to assist in this rapid discharge process by
uplinking signals with a 100% duty cycle.
The benefits to be gained via this method are seen to be:
4.4 We reduce the time span where the IHU might perform a highly undesirable,
unpredictable and uncontrollable action such as reducing the spin rate to 0 by
activating all magnet coils in a DC state, rotating the antennas away from the
Earth, overcharging the batteries by erroneously setting the BCR control
latches, etc.
4.5 We at least have a chance of 'complete' recovery in a relatively short time
frame which would serve to enhance AMSAT's stature in the eyes of the users,
benefactors and the space agencies.
4.6 We reduce the numbers of satellite enthusiasts who will tend to abandon all
hopes of AO-10's recovery and switch over to RS satellites as a permanent
alternative.
While (4.5) and (4.6) may seem superflous to the technical purist, in objective
terms, it must be remembered that.. without the support of these groups, our
satellite service would (will) not exist!
5.0 SOLICITATIONS:
Needless to say, there are many problems to be worked out and Murphy will see to
it that major hurdles will present themselves, no matter which alternative is
pursued. AMSAT consists of a diverse group of specialists covering a wide range
of expertise. Your comments and suggestions are solicited immediately. If you
feel your idea has merit, don't hesitate to send it along, no matter how 'wild'
the scheme may sound. I cannot promise to reply to each and every suggestion or
comment, but I DO promise to study each and every one and present them to the
appropriate parties.
73, Ron Dunbar (W0PN), 6012 E. Superior St. Duluth, MN 55804
218-525-6554
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