Nimbus satellite - pulse-code-modulated telemetry subsyte

Adkins, F. L.

Griffin, C. E.

English

NASA Technical Reports Server

15. NIMBUS TELEMETRY
By R. Golden, GSFC; F. L. Adkins
and C.E. Griffin, Radiation, Inc.
https://ntrs.nasa.gov/search.jsp?R=19630008739 2020-03-11T16:53:34+00:00Z
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15. NIMBUS TELEMETRY
ILLUSTRATIONS
Pulse-Code-Modulation Subsystem - Unit A,
Block Diagram .................
Pulse-Code-Modulation Subsystem - Unit B,
Block Diagram .................
Telemetry Electronics Subsystem ........
Pulse-Code-Modulation Subsystem Module ....
Module Board and Tray ............
Module Board and Tray ............
Video Input From Telemetry Receiver,
Nimbus PCM Ground Station ...........
Bits Without Error vs. Signal-to-Noise, in DB,
Nimbus PCM Ground Station ..........
Nimbus PCM Ground Station Equipment ......
Nimbus PCM Ground Station, Block Diagram
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15. NIMBUS TELEMETRY
By R. Golden, GSFC; F. L. Adkins
and C. E. Griffin, Radiation, Inc.
The purpose of the Nimbus pulse-code-modulated (PCM) telemetry
subsystem is to gather, store (if required), and transmit to ground
stations, the engineering informatinn needed to evaluate the detailed
performance, environmental conditions, and general function of all
spacecraft subsystems. In addition, the Nimbus PCM telemetry sub-
system provides all attitude information needed to determine the exact
location of meteorological pictures.
The choice of pulse-code modulation over other types of telemetry was
made because of its accurate and reliable data-transmission capabil-
ity, and the ease with which the data can be reduced.
The Nimbus spacecraft will carry two electrically independent PCM
units. Unit A handles 544 channels, which it continually records on
tape for readout on command; unit B delivers IZ8 channels by direct
transmission.
The PCM telemetry subsystem will accept information from all the
spacecraft subsystems for "housekeeping" purposes and will also
acquire control-subsystem attitude data. Information can be of the
yes-no type or in analog form, stored by the unit A recorder for
command readout, or for direct transmission by unit B upon inter-
rogation.
Unit A consists of a 7-bit analog-to-digital converter, 544-channel
multiplexer, subcommutation, gates and registers (Figure I). It uses
less than 0. 75w of continuous power to monitor 542 data channels for
information such as voltages, current, and temperatures, and to con-
tinuously check the performance of other spacecraft subsystems.
These channels are time-multiplexed and serially recorded on tape at
a bit rate of 500 cps for playback at 15, 000 cps, a 30-to-i ratio of
time compression.
Frame composition is 64 words, including a synchronization word
composed of all "ones". Maximum number of bits per word is eight,
including word synchronization. Subcommutation is used with the
8-bit synchronization word following the frame sync. Subcommutated
groups 49 to 64 may be expanded to a total of 3Z subframes simply by
adding the necessary gate modules and timing circuits.
Unit A coder, similar to that used in unit B for redundance, accepts
asymmetric inputs from zero to -6.4v; channel input impedance is in
excess of 100,000 ohms.
Unit B operates only upon ground command, using less than 0. Z5w
operating power to scan the IZ8 channels for a 1. 75-minute reporting
period (Figure Z). This unit employs a bit rate of 10 bits/sec, with a
maximum of IZ8 words, including synchronization with three words in
the form of all "ones" followed by all "zeros", followed by all "ones"
After completion of one frame, the unit is deenergized.
Unit B consists of an analog-to-digital converter similar to that of
unit A, a multiplexer, gates, and a phase-shift modulator. Output of
unit B phase-shift-modulates a 5000-cps coherent subcarrier. Modu-
lator output is a 5000-cps sine wave. A 180-degree phase relation
exists between a "one" and a "zero" when compared to a reference
signa I.
Units A and B both are controlled by signals from the Nimbus clock
subsystem which furnishes 500-cps and 10-cps square waves, respec-
tively, to the units.
For test purposes, telemetry data at 500 cps (not stored) can be read
out from the PCM subsystem.
The non-return-to-zero (NRZ) output of unit A is converted in the "A"
modulator, located in the telemetry electronics (TME) package
(Figure 3). One cycle of a 500-cps sine wave is generated for each
"zero" bit of information. The modulator output is stored on tape and
may be read out directly. The TME unit also contains record amplifiers
which furnish the record signal and the 8000-cps erase and bias signals
to the tape recorder heads. Playback amplifiers, also located in this
unit, provide an output to amplitude-modulate the beacon transmitter
80 percent. Dual record and playback amplifiers are available for a
redundant tape recorder.
The TME unit includes a tuning-fork oscillator to provide 500-cps
signals for the PCM unit A bit rate and "A" modulator, as well as a
two-phase 100-cps signal for one tape recorder, in case the clock
signals should be interrupted.
Command switches for the PCM, TME tape-recorder amplifiers,
standby clock, tape-recorder functions, and transmitter input are
located in the TME package. Clock signals and unit B signals are
also routed through the TME for control and output to the beacon trans-
mitter. Special test equipment is provided to check out all units of the
PCM telemetry system.
The PCM and TME subsystems employ a unique packaging concept
known as the "solid mass" approach to electronic packaging, which
means that all portions of the system, including the components them-
selves, contribute to the rigidity of the package. By holding the inter-
nal parts in compression, the side panels need be only thin lightweight
sections since they experience only tension forces. This results in a
very high "electronics weight" to "box weight" ratio, as compared with
other types of satellite electronic packaging. The PCM module
(Figure 4) weighs less than Z3 pounds; the TM]E (Figure 3), 5 pounds.
The printed circuit cards and module layout are very important as
integral parts of the package. A novel concept is the face-to-face
interlocking of module-card assemblies, not onlylresultingin struc-
tural rigidity, but also doubling the surface area of the available
printed circuit boards, thus providing more room for module inter-
connections.
The interlocked module-card assemblies (Figure 5), or trays, are
insulated from each other by a layer of cast polyurethane foam. The
trays are interconnected through a special high-density lightweight
connector developed for this special application by Radiation, Inc. and
U. S. Components, Inc.
The modules are welded assemblies, potted with rigid polyurethane
foam (Figure 6).
System testing begins with a module test. Following this initial test,
the module is potted, X-rayed, and vibrated. The second test is
performed with modules temporarily connected together as a system;
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the first temperature cycling is done under this condition. Upon
completion of these module-level tests, the system is assembled and
the system test is also conducted over the temperature range. Not
until the system has passed these tests is it put into the package and
the acceptance tests started.
Environmental tests consist of the Nimbus subsystem sinusoidal and
random vibrations, and thermal vacuum tests. The thermal vacuum
tests are conducted with the entire telemetry system, including the
Hughes beacon transmitter and the Raymond Engineering tape recorder.
To date, a qualified prototype and flight-model PCM and TME have been
delivered.
Two units, the Nimbus PCM tape recorder and the beacon transmitter,
which complete the telemetry subsystem are extremel_ important in
data transmission.
The Nimbus PCM tape recorder is a 4/0 pressurized module,
weighing i0 pounds; power input is I. 6 watts during the record cycle,
and 10 watts in the playback mode. A Zl6-foot, i/4-inch-wide lubri-
cated mylar endless-loop tape is used as the storage medium for the
PCM "A" data. The tape unit records at a speed of 0. 4 inches/sec
and plays back at 12 inches/sec to give the 30-to-i time compression.
The 100-cps rate signal required for the drive mechanism is provided
by the clock subsystem or the PCM emergency oscillator. The record-
er module also contains motor-drive amplifiers for record, playback,
and momentum compensation.
An internal automatic-turnoff device assures that the tape unit will
return to its "record" mode after a playback of slightly more than the
full length of tape, or approximately 3. 6 minutes. The recorder util-
izes a single head for record and playback, with switching of the re-
spective amplifiers.
The physical distance between the erase and record heads provides a
blank area on this magnetic tape at the moment the tape unit is inter-
rogated. Knowing the exact time of the tape "gap" from command time,
all data on the tape may be correlated to time with the known PCM bit
rate.
The beacon transmitter continuously transmits Minitrack time except
when PCM interrogation is commanded; during this period, PCM data
are telemetered to a ground station. The transmitter, a solid-state
device using a pre-aged crystal which assures a frequency stability of
0. 005 percent, has a minumum CWoutputof 250 mw. The AM modu-
lator's frequency response is flat within 3 db from i00 cps to 60 kc,
and within 1 db from 200 cps to 30 kc. Distortion is less than Z per-
cent at 50-percent modulation. The transmitter output excites four
quadraloop antennas located on the periphery of the Nimbus spacecraft.
To date, three beacon transmitters have performed successfully with
PCM subsystems.
The Nimbus PCM ground station has been designed to accept all video
outputs of the PCM telemetry receiver, and to process, record, and
display the PCM telemetry data received from the satellite. The
equipment will accept and speedily process the data, allowing an im-
mediate review of information gathered during the satellite orbit. The
station is designed to display the information visually and to prepare
it for relay to remote facilities; it also can present visual displays of
selected data on oscillographs for a quick look at the information as it
is being received. In addition, it contains a computer complex for real-
time and delayed editing and processing of the information.
The ground station will, while completely unattended, acquire data
from the satellite with no commands from external equipment. It will
recognize,- program, acquire, and process any of the three PCM data
formats in any sequence.
Figure 7 presents characteristic video waveforms of the data formats.
The "B" PCM signal is 10-cps bit rate, non-return-to-zero-inverted
(NRZI) on a 5-kc coherent subcarrier, phase-modulated 180 degrees.
Each frame of "IB" data consists of 128 data words. The "B" as well
as both "A" formats contain word and frame synchronization groups.
Both "A" PCM signals contain 64 channels in a primary or minor
frame. Thirty-two of the 64 channels are subcommutated to 16 minor
frames. As shown in Figure 7, both "A" signals are NRZ signals
amplitude-modulated on a coherent subcarrier. The real-time "A"
data is received at a 500-cps bit rate, the tape-playback "A" data at a
15-kc bit rate.
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System performance in the presence of noise is indicated in Figure 8,
which presents measured versus theoretical signal-to-noise ratio
versus bits without error plots for a 15-kc "A" data. The actual curve
is approximately 3 db away from the theoretical, and predicts one bit
error out of one million with a signal-to-noise ratio of 17 db.
Figure 9, a photograph of the ground station, indicates the major system
components. From left to right, the picture shows a teletype send-
receive set; a MinCom GI07 magnetic-tape recorder-reproducer; two
racks containing digital-to-analog converters and oscillographs; a two-
rack acquisition station_ and the computer subsystem, comprising a
high-speed printer, digital magnetic-tape recorder-reproducer, and
the computer console. Functions of each of these components are
shown in the block diagram of the station (Figure I0).
The video output from the telemetry receiver is routed to the demodu-
lators, which remove the subcarrier from the incoming signal and put
out regenerated serial data. The video output is also routed to the
MinCom tape recorder where a record of the incoming signal is made
for delayed processing and for backup.
The regenerated serial PCM from the three demodulators is also
recorded on the MinCom. Outputs of the demodulators are also routed
to the synchronizers, where the word and frame sync patterns are
recognized and the data is converted from a serial to a parallel form.
Therefore, the output of the synchronizers is 7-bit parallel binary
data, as well as the synchronization pulses and data-status signals
required to process and display the data,
As shown in Figure I0, all "B" data are routed to the teletype where
the raw data are printed out in decimal form in real time.
The output of the "A" data synchronizers is routed to two decommu-
tators which, by means of a patchboard program, can select any 3Z
channels from the real-time or tape-playback "A" data required to be
converted to analog information and plotted on the four- to eight-
channel oscillographs.
All the incoming PCM data are routed through the computer-input
circuitry to the CDC-160A computer console for processing, record-
ing, and displaying under control of the program in the computer.
An outstanding feature of the ground station is its simplicity; there are
no buttons or controls to be manipulated during a satellite pass, regard-
less of the sequence in which the PCM data are transmitted. Also, it
is complete in that all the equipment required to process, record, and
display the data is an integral part of the station.
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Nimbus Telemetry

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