A system capable of pointing a balloon-borne telescope at selected celestial objects to an accuracy of approximately 10 arc minutes for an extended period (weeks to months) without reliance on telemetry is described. A unique combination of a sun/star tracker, an on-board computer, and a gyrocompass is utilized for navigation, source acquisition and tracking, and data compression and recording. The possibilities for intelligent activities by the computer are also discussed

Lewin, W. H. G.

Ricker, G. R.

English

NASA Technical Reports Server

N75 12743
PAPER 2.2
AN ORIENTABLE, STABILIZED BALLOON-BORNE GONDOLA
FOR AROUND-THE-WORLD FLIGHTS
George R. Ricker
Walter H. G. Lewin
Center for Space Research and Department of Physics
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
ABSTRACT
A system capable of pointing a balloon-borne telescope at selected
celestial objects to an accuracy of ^  10 arc minutes for an extended period
(weeks to months) without reliance on telemetry is described. A unique com-
bination of a sun/star tracker, an on-board computer, and a gyrocompass is
utilized for navigation, source acquisition and tracking, and data compression
and recording. The possibilities for "intelligent" activities by the computer
are also discussed.
DISCUSSION
The success of NCAR's Project Boomerang in demonstrating the possibility
" of around-the-world balloon flights has ushered in a new era for scientific
ballooning. The phrase "low-flying satellite" which has been used to describe
this new concept in superpressure ballooning is most apt, even though not
strictly accurate, because of the comparable observation times attainable from
both around-the-world balloons and true satellites. Unfortunately, the com-
parison also extends to many of the same experimental difficulties. For an
experiment that requires pointing a gondola-mounted telescope with a narrow
field of view, among these difficulties are finding answers to three rather
fundamental questions:
• Where is the gondola? (Longitude and latitude)
• In what direction is the telescope pointed?
(Elevation and Azimuth)
• What time is it? (UT)
An ideal system for long duration ballooning should make these determina-
tions without assistance from a ground controller because of the potential
difficulty of maintaining a secure (though not necessarily continuous)
telemetry link for weeks or even months at a time.
The last question is the easiest to answer: low power, accurate (i.e.,
stabilities of a few parts in 109-1010) flights clocks are commercially
available. The first part of the second question is also relatively easy,
since we can use gravity for a vertical reference. A simple tiltmeter system
will suffice, at least for coarse aiming. The remaining questions - determin-
ing the gondola geographic location and telescope azimuth - are fundamentally
more difficult to answer. During the last year at MIT, we have been working
on a system which answers these difficult questions by linking together three
components that are not normally present on balloon gondolas: 1) Combination
Sun/Star Tracker; 2) Gyrocompass; and 3) Flight Computer. This system that we
envision will not require any ground commands once it is in "orbit". It will
be capable of an absolute pointing accuracy of a few arc minutes. This level
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https://ntrs.nasa.gov/search.jsp?R=19750004671 2020-03-23T00:40:56+00:00Z
of accuracy is sufficient for the experiments which we are planning in infra-
red and x-ray astronomy, where we contemplate fields of view ranging upward
from 6 arc minutes.
An on-board computer opens up a number of other possibilities for such
things as in flight data compression or automatic cutdown over a predeter-
mined recovery area. This point will be discussed in more detail later.
Figure 1 is an overall view of an around-the-world gondola that might use
the system which we envision. The torque motor is the driving element for
stabilizing the entire gondola in azimuth. The sun/star tracker is the ulti-
mate reference for this azimuth stabilization. It is mounted on a platform
that is coupled by rigid bars to the torque motor housing (above) and to the
main gondola (below). The tracker will be gimbled only about an elevation
axis. During the day, the side of the gondola on which the solar cells are
mounted will be continuously pointed in azimuth toward the sun. This arrange-
ment greatly simplifies solar panel pointing, as a straightforward lead screw
will suffice to point the panel normal toward the sun. The principal scien-
tific detector, an x-ray or infrared telescope, will be offset-pointed in
elevation and azimuth with respect to the gondola frame. A gyrocompass
(discussed below in more detail) will provide a continuous readout of platform
geographic azimuth, including any oscillatory component.
Figure 2 is a detail of the combination sun/star tracker, shown in a top
view relative to Figure 1. Schematically, the situation shown is one that
might exist near sunrise with the sun rising in the east and two stars setting
in the west.* The elevation axis is pointing north-south. Two independent
optical sensors sharing a common focusing mechanism are shown. Both are Fair-
child Semiconductor CCD201 charge-coupled device area image sensors. At
night, only the star tracker would be active; conversely, during the day, the
sun tracker would be active. The relevant operational characteristics of this
tracker are given in the following table.
STAR/SUN TRACKER
(Fairchild CCD201, 10,000 Element Area Image Sensors)
Field of View (FWHM)
Resolution
Accuracy
Signal/Noise
(Per resolution element)
Weight
Power
Star Mode Sun Mode
1.5° 5.0°
1' 4'
1' 2'
10:1 =^3,03) > 100:1
*This situation is somewhat hypothetical, as the sky brightness for a solar
elevation >0° would probably be too great for the system to simultaneously
detect both the sun and stars.
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72
TO RIGGING
TORQUE MOTOR
X-RAY or IR DETECTOR
(Azimuth and Elevation
Drives/Encoders)
GYROCOMPASS "5
(Azimuth Output)
SUN/STAR TRACKER
(Elevation Drive/Encoder)
SOLAR CELLS
2 m2 for 200w day-
time, lOOv average)
LEAD SCREW
(Elevation Drive/Encoder)
Figure 1. Around-The-World Gondola (Daytime Mode)
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73
Stor Tracker Lens
(150 mm. f/1.7)
\
Elevotion Encoder
i\ Y////A
Elevation
Motor -
_ Stor .
Tracker
Focus Motor
f (Rack and Pinion )
-***— Sun _
Tracker
Sun Tracker Lens
(35mm.f/22)
\CD20I Sensors
(IOOxlOO)-2req'd.
10 cm
Figure 2. Sun/Star Tracker (Top View)
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Next, we consider how the overall system (tracker plus gyrocompass plus
computer) would be used to determine the longitude and latitude for the gon-
dola during the night and during the day. Figures 3A and 3B depict the earth
during the night and day, respectively. The gondola is over point G on the
earth's surface. The vector G&1 is apparent north determined by the gyro-
compass. At night, the star tracker is then used to measure the zenith angle,
<£, and the azimuth, y» relative to the gyrocompass azimuth (in the gondola
frame) for two stars. The substellar points on the earth for these two stars
are SSj and SSz. The two small circles are the loci of points on the earth's
surface at which the stars are observed at the measured zenith angle.
From these measurements and the Universal time, we can determine the longi-
tude and latitude of the gondola at night, after the computer has
identified the two stars; this will be a rather straightforward operation as
we will only use bright stars. We can also determine our gyrocompass error
£NS from this same data. Obviously, the on board computer is vital for all
these determinations in order to perform the required coordinate transforma-
tions .
In the daytime, we have only one star to work with - the sun. Therefore,
we must rely on our gyrocompass to narrow down the arc along the small circle
of constant solar zenith angle where we might be. We are limited in our
ability to do this by the gyrocompass error, ENS- This error is composed of
several position-, velocity-, and acceleration-dependent components, all of
which are systematic (and hence correctable) except for a small random drift.
The magnitude of this small random drift is such that it could accumulate to
^ 10 arc minutes between a sunrise and a sunset (^ 12 hours). At night, we do
not allow it to accumulate because of repeated star sightings, but during the
day we must accept it as an irreducible error; there is only one sun.
Figure 4 is a block diagram for the entire system. The central role of
the computer is immediately evident from its origination. The computer that
we are planning to use would be based on an Intel 8080 microprocessor with an
instruction cycle time of < 2 ysec. This is an 8 bit machine, and we would
probably require *\» 8 kbytes of memory to run our system.
The main telescope (x-ray or IR) would input its data to the computer,
and would receive azimuth and elevation commands from the computer. The roll
and pitch tiltmeters would tell the computer whether the gondola were out of
level (DC-wise). The torque motor servo would have an offsettable azimuth
null under computer control. The gyrocompass is a Gyrosystems Model 800,
which is a 2 axis gimballed system with a full 360° azimuth range. Its basic
resolution would be 1.3 arc minutes (14 bits). Its weight is approximately
4 kg, and it requires 20 watts of power. The sun/star tracker has already
been discussed. Data from the computer would be recorded on board and a
selection from it telemetered to earth continuously on an HF radio link. This
data would be of lesser quality than could be obtained from a VHF/UHF line of
sight link which would be used when and if the gondola happened to pass over a
suitable ground station. A data uplink (through the command receiver) could
also be used to modify the observing program or restart the computer system or
digital clock in the event of a power loss of other temporary malfunction.
The on board computer can provide a number of functions in addition to
those discussed above. These roles can be, somewhat arbitrarily, subdivided
into cogitative and incogitative categories. The incogitative roles are the
rather straightforward tasks which, regardless of the system, the computer
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75
A. Niqhf (Star Tracker) Day ( Sun Tracker)
North
Pole
Define: Y, - » M' - C - SS,
- Gyrocompass azimuth Co Star 1
Y
» - * N' - G - SS2
• Gyrocompass azimuth to Star 2
Nighttime Calculations:
Gondola Longitude, Latitude —
a. B)c *> f [<X, 8)SSj. *,, (X. 6)SSj. »2. Y,. Y2
* f (»,. *,. Y, - Y,. OT; a,, «,. a,. 6,
Gyrocompass Error --
£HS * * (Y» or Y*'
•v- » l(X. B)c. (X. B)SSi0r SSi. Y.ory,]
Daytime Calculations:
(A. S)G •>• /' [», Y. £NS. "T; O^UT).oe(UT)]
Figure 3. Longitude/Latitude Determination
2.2-6
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X-RAY OR IR
TELESCOPE
Detector
Signals
Azimuth
motor/encoder
Elevation
motor/encoder
SOLAR CELLS
- Elevation Encoder
- Elevation Motor
COMPUTER (INTEL 8080)
- 2ysec Instruction cycle
time
- 8K byte memory
VHF/UHT
Gyrocompass
Azimuth Encoder
SUN/STAR TRACKER
- Elevation
motor/encoder
- CCD Signals
Figure 4. Aspect/Data System
KEP?"~UCE:lLnY OP THE
ORIGINAL PAGE IS POOR
2.2-7
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would have to perform. The cogitative roles are more sophisticated ones
which would be limited by memory size and computing speed. First, the in-
cogitative:
INCOGITATIVE ROLES FOR FLIGHT COMPUTER
1. Gyrocompass corrections (GNS - Azimuth error; "north steaming
error" - velocity, acceleration dependent)
2. Source list look up and coordinate transformation:
[(a, 6) -" (Az, El)]
• Sun
- Coarse: Solar Cell Panel
- Fine: Sun Tracker
• Bright Stars
• X-Ray or IR Sources
3. Data compression (for on-board recorder)
4. "Flywheel" for day/night transition
5. Inflight gondola re-leveling (tiltmeter sensors)
6. Sun/star tracker focusing
7. Star tracker protection (shutter activation)
8. Mode switching (instrument calibration, operational checks, etc.)
For the cogitative roles, one can think of a "computer soliloquy" after
the fashion of an experienced human observer using a ground-based telescope.
Among the questions such a computer-observer might ask itself are the
following (where "I" = The Computer):
COGITATIVE ROLES FOR FLIGHT COMPUTER
1. Observing Program Selection
a) What sources are potentially visible now (i.e., sufficiently
above the horizon but not behind the balloon) ? Of these,
which have the highest pre-flight priority?
b) What sources have I not looked at recently? Of these, which
did I detect the last time I looked at them? Of the candi-
dates not examined recently, which should I look at again?
c) If I am observing a given source, am I detecting a positive
flux? If not, how much longer should I continue trying to
see it? If I am detecting it, is it behaving in such an
"interesting" manner (e.g., time or spectral variability)
that I should continue taking data on it?
2. Data Dump Decision (VHP or UHF Link)
a) Am I within range of a receiving station?
b) How long has it been since I last made a dump?
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c) Is the wind velocity such that I will have time for
a complete dump?
3. Cutdown Decision
a) Have I been in "orbit" long enough?
b) Am I over a suitable recovery area?
c) Is it daylight?
d) Have I turned on my tracking beacons and lights?
e) Have I received an override command from a ground
station?
The practical number and effectiveness of the algorithms for these computer
decisions will be largely determined by the skill and creativity of the human
programmer. Implementing all of the options listed here would likely require
a very large computer memoryj but with the steady decrease in price/bit and
the increase in bits/chip for low power C/MOS memory elements, this is not
likely to pose a significant problem.
In summary, the system described here should be able to achieve the high
degree of flexibility and pointing accuracy which balloonists have come to
expect from ground-controlled gondolas, without requiring a telemetry command
"umbilical", a link which could be very tenuous for around-the-world flights.
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DISCUSSION SUMMARY - PAPER 2.2
In response to questions about progress on this system, the speaker said
that the star tracker and the computer have been started.
It was suggested that the star tracker could be used for the same purpose
as the 20-watt two-gimbal gyrocompass and therefore save power. The conclu-
sion of the MIT Group is that at $5,000 and eight pounds per gyrocompass that
they represent a less expensive trade-off. These units are expected to have
ten arcminutes of drift per 12-hour period. It is expected that in the future
gyrocompasses will be available for $10,000 to $20,000 with absolute accuracy
of ±2 arcminutes for indefinite time periods.
The computer planned for this system is inexpensive, lightweight, uses
less than a watt of power and uses a CMOS-type memory.
Some of the advantages of the CCD detector planned for the star tracker
were mentioned. These are: no mechanical chopping required, location of
spatial elements known very well and high quantum efficiency. The cost is
about $500 to $1000 per unit. This star tracker is expected to have a signal
to noise ratio of ten for a third magnitude G star using a 1/30-second inte-
gration time.
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80
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An orientable, stabilized balloon-borne gondola for around-the-world flights

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