The AIROscope (Ames Infrared Observatory telescope) telemetry system is described from signal conditioning on the gondola to display and storage at the ground station. All analog and digital data from the systems and experiments on the gondola go to a PCM encoder which formats the data into 10-bit words. Therefore, 0.1-percent resolution is inherently available for experimental data. The coded bit stream directly modulates the carrier of the FM transmitter. To insure reliable transmission over a 650-km range an 11-watt FM transmitter operating at 1483.5 MHz is used on the gondola. Modulation is narrow band FM with a maximum deviation of plus or minus 500 kHz. The maximum modulation frequency is determined by the data bit rate which could be as high as 500 kbps. Presently, a rate of 20.48 kbps is used. The ground station receiving system includes a steerable antenna (19-db gain), preamplifier (21-db gain) and receiver. All data are recorded on a tape recorder. The recorded signal then can be played back through the PCM decommutator unit at a later time for detailed data analysis by the experimenter

Pitts, K. J.

English

NASA Technical Reports Server

N75 12762.
PAPER 5.2
AIROSCOPE TELEMETRY SYSTEM
Kenneth J. Pitts
Ames Research Center
ABSTRACT
The AIROscope (Ames Infrared Observatory telescope) telemetry system is
described from signal conditioning on the gondola to display and storage at the
ground station. All analog and digital data from the systems and experiments
on the gondola go to a PCM encoder which formats the data into 10-bit words.
Therefore, 0.1-percent resolution is inherently available for experimental
data. The Bi $-L coded bit stream directly modulates the carrier of the FM
transmitter.
To insure reliable transmission over a 650-km range an 11-watt FM trans-
mitter operating at 1483.5 MHz is used on the gondola. Modulation is narrow
band FM with a maximum deviation of ±500 kHz. The maximum modulation frequency
is determined by the data bit rate which could be as high as 500 kbps. Pres-
ently, a rate of 20.48 kbps is used.
The ground station receiving system includes a steerable antenna (19-dB
gain), preamplifier (21-dB gain) and receiver. The output from the receiver
is a Bi 4>-L bit stream like that on the gondola. This signal goes into the
PCM decommutator unit which "locks" on and provides data dispaays (decimal
digits, analog meters, or bit lamps) through several data word selector units.
Also, selectable data words are routed to strip chart recorders for permanent
data storage. All data are recorded on a tape recorder. The recorded signal
then can be played back through the PCM decommutator unit at a later time for
detailed data analysis by the experimenter.
GENERAL
This paper describes the telemetry system used on AIROscope. Topics
covered will be signal conditioning, the RF down-link, data encoding and decod-
ing, formatting, ground station data recovery and display, and tape recording/
playback.
GONDOLA DATA GATHERING AND PROCESSING
All data, both scientific and engineering, are routed to a pulse code
modulation (PCM) encoder unit to put them in a form that can be transmitted to
the ground station. These data indiude:
(a) Science/experimental data
(b) TV star field data (see DeBoo, 1974)
(c) Battery power
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https://ntrs.nasa.gov/search.jsp?R=19750004690 2020-03-23T00:39:57+00:00Z
(d) Subsystem power s t a t u s  
(e) Telescope o p t i c a l  alignment and d i rec t ion  
( f )  L i m i t  switch pos i t ions  
(g) Subsystem temperatures 
(h) Optics cont ro l ,  i .e . ,  focus and collimation 
( i )  Command v e r i f i c a t i o n  (see Barrows, 1974) 
Some o f  t h e  da ta  a r e  generated by CMOS (Complementary Metal Oxide Semiconduc- 
t o r )  logic  which produces bi- level  (OV, 5V) d i g i t a l  s ignals .  Other da ta  come 
i n  analog form (0 t o  10V) which a l so  can be encoded. With a 10-bit encoding 
scheme, a resolu t ion  of  about 10 mV can be obtained. A ha l f  s c a l e  level ,  o r  
5V,is telemetered t o  give a convenient check on overal l  system operation. 
The encoder can accommodate up t o  290 channels, has a through-put r a t e  
of up t o  500 kbps, and uses a format-controlled programmable read-only memory. 
Analog t o  d i g i t a l  conversion is performed where required. A t  present ,  approxi- 
mately 150 analog and 100 d i g i t a l  channels a r e  processed a t  20.48 kbps. 
Figure 1 i l l u s t r a t e s  the  gondola da ta  handling system. 
In  order  t o  be decoded when the  s ignal  reaches the  ground s t a t i o n ,  the  
da ta  must be transmitted i n  a standard format. This formatting is a l so  done 
by t h e  encoder. The present  encoder format has 10 b i t s  per word, 32 words per 
main frame. In  addit ion,  f i f t e e n  of the  main frame words a r e  divided i n t o  
two-word subframes ( fa s t ) ,  nine of t h e  words a r e  a l l o t t e d  t o  eight-word sub- 
frames (medium) and t h r e e  main frame words a r e  used fo r  sixteen-word subframes 
(slow). Using the  20,480 bps r a t e ,  the  following r a t e s  a r e  obtained: 
Word rate - 2048 wps 
Main frame r a t e  - 64 fps 
Fast  subframe r a t e  - 32 sfps 
Medium subframe r a t e  - 8 sfps  
Slow subframe r a t e  - 4 sfps 
Two channels a t  the  main frame r a t e  a r e  avai lable  f o r  s c i e n t i f i c  data.  TV data  
a r e  cornmutated a t  the  f a s t  subframe r a t e .  A l l  other  da ta  a r e  handled by the  
o ther  subframes. The first two words i n  each'main frame a r e  used f o r  frame 
synchronization. Also, one word i s  used f o r  a subframe I.D. code and is  incre- 
mented a t  the  frame r a t e  f o r  immediate subframe ident i f ica t ion .  
The output of  the  PCM encoder is a non-return t o  zero level  (NRZ-L) s e r i a l  
b i t  stream a t  20,480 bps. This da ta  stream goes t o  a code converter which con- 
v e r t s  t h i s  s igna l  t o  bi-phase level  (Bi @-L) code. This s ignal  then d i r e c t l y  
modulates the  RF c a r r i e r  of  the  transmit ter .  B i  @-L is used because synchroni- 
zat ion is  e a s i e r  when the  da ta  may contain a long s t r i n g  of  1's o r  0 's .  
RF TRANSMISS ION 
The L-band t r ansmi t t e r  on t h e  gondola provides a 1.4835 GHz c a r r i e r  which 
is narrow-band frequency modulated by t h e  incoming data.  Several of  the  more 
pe r t inen t  t ransmit ter  parameters a re  l i s t ed :  
Typical power out  - 11 watts 
Typical input  power - 28 VM: a t  1.3A 
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Carrier frequency stability - ±0.01 percent
Operating frequency - 1.4835 GHz
Modulation characteristics
Type - Narrow band FM
Deviation sensitivity - 500 kHz/VRMS
Modulation bandwidth - to 500 kHz 0 dB, -0.4 dB
Spurious emissions - Down 70 dB or better
Transmitter power is applied to a 1/4 wave stub antenna which provides approxi-
mately 3 dB gain and a broad polar pattern for reliable down range reception.
The RF link just described has the capability of accepting 500 kbps and
therefore can easily accommodate the 20,480 bps data rate for AIROscope. This
rate is determined primarily by the digitized TV data (see DeBoo, 1974).
Most other data vary at much slower rates.
GROUND STATION
The signal is received by a 19-dB parabolic antenna, amplified in an L-band
preamplifier and presented to the FM receiver. See Figure 2.
The receiving antenna can be positioned so as to maximize the incoming
signal. Some important receiver specifications are listed:
Type - Double-conversion, crystal-controlled FM
Frequency stability - ±0.003 percent
Sensitivity - 97 dBm at 10 dB quieting
Spurious signal rejection - 60 dB or better
Noise figure - 10 dB maximum
Demodulated bandwidth - 10 Hz to 150 kHz, ±1.5 dB
Operating frequency - 1.4835 GHz
The receiver has the capability of adjustable output level and carrier detec-
tion. In addition, this receiver contains a spectrum analyzer so that the
data spectrum and possible adjacent channel interference can be examined over
a 6 MHz bandwidth centered at the first IF of 50 MHz. This bandwidth may be
reduced to 100 kHz for increased spectral resolution. Selectable IF bandwidths
and video (demodulated) bandwidths are available to optimize signal reception
under adverse conditions. The combination of the receiving and transmitting
systems is such that at a 650 km maximum range, a signal-to-noise ratio of 4 to
7 dB is expected.
The demodulated output of the receiver is of course contaminated by noise.
This output is coupled to the bit synchronizer of the PCM decommutating unit.
The bit synchronizer locks on the incoming signal and provides a "clean" serial
bit stream and regenerated clock to the code converters. One converter provides
NRZ-L to the format synchronizer. The other provides Miller code or DM for tape
recording, which was selected because it eliminates the necessity to record
DC which can occur when NRZ-L is used.
The format synchronizer operates in one of four modes: Search, Verify,
Lock, or Check. Only when the synchronizer is in the Lock mode will valid
NRZ-L data be routed to the rest of the ground station. For a flow chart
description, refer to Figure 3. When in the Lock mode, the format synchronizer
applies frame and word I.D., parallel data, and a strobe to a word select and
5.2-4
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RECOGNIZABLE
FRAME
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Figure 3. Frame synchronization strategy.
5.2-6
304
monitor unit. This unit will display the 10 bits of a selected data word on
a binary lamp display and a meter. Also, three other word selectors are driven.
Each of two of these will select, by thumb-wheel switches, two words and dis-
play them on a four decimal, 7-segment readout as well as on lamps and a meter.
The third word selector will provide 20 words and is controlled by a punched
card program for maximum flexibility. Three of these words are routed to the
star field display electronics and the remaining 170 bits are distributed to
the status panels and consoles. Therefore, at any one time, a total of 25
10-bit words may be selected by the operator. (See Figure 2.)
The PCM decoding/decommutating unit and associated word selectors utilize
Series 7400 TTL and Series 830 DTL Integrated Circuit Logic.
Selected data can be displayed in four areas of the ground station racks,
in addition to the word selector displays. These are:
(a) Bit Status Light Bank — The Bit Status Light Bank provides a quick-
look at the health of AIROscope by examining data words on a bit-
by-bit basis.
(b) Engineering Control — The Engineering Control Panel displays various
housekeeping data such as optics alignment and subsystem power status.
(c) Pointing Control — Data displayed on the Pointing Control Panel is
associated with the status of telescope pointing for target acquisi-
tion. For example, the telemetry will tell the operator whether the
telescoping is moving left, right, up, down, or diagonally.
(d) Filter Spectrometer Control — Status of the IR experiment is dis-
played on the Filter Spectrometer Control area. Here, the experi-
menter monitors the state of a filter wheel and IR amplifier gain,
as well as other experimental parameters.
In addition, the ground station contains a magnetic tape recorder, strip
chart recorders and other devices for recording information and determining the
performance of the system. See Figure 2.
When the operator places the input code selector switch on the PCM Decom-
mutating Unit in the "DM" position (see Figure 4) the tape recorder can be used
to play back a taped mission through the ground station. In this mode, the
Miller Code serial data from the tape is converted back to NRZ-L and all the
ground station data displays function as previously described.
CONCLUSION
The AIROscope telemetry system offers a large and expandable capacity, the
capability to encode analog and digital data, and a variety of safeguards for
reliability of data transmission and command verification. This paper has shown
the versatility of the AIROscope system. Its versatility may be utilized to
accommodate expanded scientific requirements in future balloon-borne infrared
astronomy missions.
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REFERENCES
Barrows, W. AIROscope Command System. These proceedings, 1974.
DeBoo, G., Hedlund, R., and Parra, G. AIROscope Stellar Acquisition System.
These proceedings, 1974.
5.2-9
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DISCUSSION SUWARY - PAPER 5.2
No discussion.
5.2-10
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Airoscope telemetry system

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