The onboard data processor for OAO 3 flight applications executes instructions at the rate of 2 1/2 billion per day. The computer commands, processes data, and controls telemetry subsystem programs onboard the spacecraft

Taylor, T. D.

English

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THE ONBOARD PROCESSOR ON OAO-3 
Thomas D. Taylor 
Figure 1 is a picture of the onboard processor, or OBP, which was developed here at 
Goddard by the Spacecraft Data Management Branch. This computer was developed 
specifically for flight applications, and I would like to address myself to the first application 
of this computer on OAO-3, or Copernicus, which was launched on August 21 of this year. 
Before talking about the boxes, I would like to mention that the computer was turned on 
the day after launch, and has been executing instructions at the rate of 2% billion a day. 
We have executed 2 X  10'l instructions since launch. This is an 18-bit, parallel, medium- 
scale computer. The four boxes on the right are four memory units. Each memory unit is 
4096 words. There is a central processor unit, an IO unit, and off to the left is the power 
converter for the computer. It is a 40-watt machine. Its capability lies somewhere between 
that of an XDS-920 and 930, to compare it with the capability of a ground computer. It 
has a ten-microsecond add time and a hardware multiply/divide capability. 
In interfacing the computer on the OAO spacecraft, we could only connect to signals which 
were on connectors, but we were still able to gain access to the spacecraft telemetry, and 
Figure 1. The OAO-3 onboard processor (OBP). 
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so have access to all spacecraft status data. There are four analog signals out of the 
computer, three of which control the fine wheels and one controls the power regulator 
unit on the spacecraft. The computer can also issue commands, and therefore, the com- 
puter has powerful interface with access to all spacecraft telemetry and the ability to 
send any OAO command. 
Next, I will discuss the applications of the computer, but I wanted to say in passing that 
the design of this computer was tailored for space applications and it is a very worthy 
design. Such features as hardware memory protect, the ability to load and dump the 
memory without program execution, and the very powerful interrupt structure have 
proved useful and made it easy to program. These features have increased its utility as 
well as its security. 
There are three main application areas: commanding, data processing, and as in Figure 2, 
worker programs, or subsystem programs. In the commanding area, the hard-wired 
command handler on OAO stores 256 commands. One day’s operation requires the 
execution of about 1000 commands. The computer provides an additional 1024 command 
storage capacity, so that the spacecraft can be loaded with commands once a day, sending 
256 to the wired command handler, an additional 1024 to the computer, and then in the 
back orbit the computer transfers commands in blocks of 128 to the h’ard-wired command 
handler. This has reduced the need for ground station coverage for OAO. Commands are 
loaded at only Rosman. There is no need to load commands at remote sites and ground 
station requirements have been reduced by 40 percent with this feature. 
a 
a 
DATA 
a 
a 
AUXILIARY COMMAND STORAGE 
SCRATCH-PAD COMMAND STORAGE 
PROCESSING-STATUS 
DUMPED ONCE/ORBIT 
OBP COMMAND ACTIVITY 
S & C MODE CHANGES 
ATTITUDE-FIX DATA FOR GROUND 
POWER SNAPSHOTS AT D/L/D 
PER IODIC SNAPSHOTS 
ALARMS 
VALUABLE FOR TRENDS AND ALARM 
Figure 2. OBP applications. 
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There is also what is called a scratch-pad command storage. This essentially augments the 
command execution of the command handler on the spacecraft. Commands may be sent 
to the computer rather than being sent to the command handler. The computer imitates 
the command handler, in that it compares the execute time of the command with the 
spacecraft time. When a time equality exists, the computer formats the command as 
though it were coming from the ground and sends it to the command handler for immedi- 
ate execution. This affords the interleaving of commands from preprogrammed command 
memory with the computer. It is a very important feature which has been used success- 
fully many times in orbit. 
The scratch command feature is used for engineering tests in whick the engineering test 
commands are loaded in the computer and it performs the engineering test and does not 
interfere in any way with normal experiment operations. Also, it is a good emergency 
feature so that should a problem be detected during a ground station contact, corrective 
commands which must be issued in the back orbit can be sent to the computer and it will 
execute them in the back orbit without interfering with the normal command memory 
load. 
In the data processing area, there is a 320-word block of memory, which is continually 
stored into during the back orbit of the OAO. In there, the computer records its command 
activity, and stabilization and control mode changes, some attitude data for the ground to 
assist in attitude determination of the spacecraft, and snapshots of the power system at 
dark-light and light-dark transitions. 
The 320-word block of memory has a very good synopsis of the condition of the space- 
cratt, and also a list of the significant events of the spacecraft in the back orbit. This data 
is dumped once an orbit, and a realtihe printout is given to the ground controllers. It has 
virtually eliminated the need for tape recorder playback and analysis which has a two- to 
three-week turnaround time. 
In the subsystem area we were fortunate in having access to all telemetry data in the 
computer, because this contains data from all the subsystems on the spacecraft, coupled 
with the ability of the computer to send any spacecraft command. With that interface, 
the computer is able to perform a broad range of functions in the various subsystem areas 
(Figures 2 and 3). 
For example, in the case of the Princeton Experiment Package spectrometer temperature 
control, the computer can provide a thermostatic function better than a thermostat, 
because even though it is a trivial function, the threshold is adjustable and the hysteresis 
can be diminished as much as thermodynamics allows. 
Heat pipe control is a program which turns heaters on and off to maintain a constant 
power to the bay in which there are heat pipes on the spacecraft. It is a complicated 
algorithm. The computer duty cycles the heaters to maintain a constant power, and all 
that’s needed is a program to do this function. The interface allows it and it would require 
considerable hardware to do the function on the spacecraft. 
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WORKERS [COMMANDING] 
0 THERMAL 
PEP SPECTROMETER TEMP CONTROL 
HEAT PIPE CONTROL 
0 POWER 
MAX POWER 
0 S & C  
HOLD WHEEL SPEED 
O WORK-AROUND PROGRAMS 
BST STAR SEARCH 
FINE GUIDANCE BACKUP 
RELIEF VALVE (GAS LINE) 
UNDER VOLTAGE DETECT 
SUN AVOIDANCE 
I-STAR INSTABILITY 
Figure 3. OBP applications (continued). 
In the power area there is a max-power algorithm in which the computer controls the 
voltage regulator. The computer essentially sets the operating point of the array to main- 
tain the maximum power from the array. 
There are several workaround programs which will be called into use should that need 
arise. One of them, the undervoltage detect, is going to be committed to operations this 
week, because the level is adjustable if the function is done from a computer. It has 
turned out on OAO that the undervoltage level does not permit the optimum charging of 
the batteries, so the project is going to commit to computer operation of this point. 
So we feel that the computer on OAO has enhanced ground operations, and has made 
them simpler. We feel that the computer has improved the performance of the space- 
craft. And we feel that we have proved that a central computer on a spacecraft can do 
subsystem jobs in a cost and performance effective way. 
MEMBER OF THE A UDIENCE: 
Have you used the arithmetic unit portion of the computer? 
MR. TAYLOR: 
Yes. The heat pipe control program has been our main use. That is a complicated 
algorithm. We are multiplying, dividing - in fact, we do the logic in double-precision 
arithmetic. 
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The onboard processor on OAO-3

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