Telemetering - Vital Link to the Stars
November 1959 Popular Electronics
- the remote sensing and reporting of system parameters via radio link
- was just coming of age in the late 1950s when this article appeared
in Popular Electronics. It was the age of space payload rocket development
(as opposed to artillery and fireworks rockets), high speed jet airliners,
and the Pioneer 1 space probe. There was a great need to collect
data during the developmental and operational engineering project stages
in order to ascertain causes for failures when they occurred and to
know what went right when success triumphed. A pinnacle of the newborn
telemetering era was Pioneer 1, which carried an image scanning
infrared television system to study the Moon's surface to a resolution
of 0.5 degrees, an ionization chamber to measure radiation in space,
a diaphragm/microphone assembly to detect micrometeorites, a spin-coil
magnetometer to measure magnetic fields to 5 microgauss, and temperature-variable
resistors to record the spacecraft's internal conditions*. Unfortunately,
the launching rocket experienced a malfunction that buggered the flight
trajectory, but the craft still managed to return some useful data.
In that instance engineers benefitted from both success and failure
November 1959 Popular Electronics
of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged.
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Telemetering - Vital Link to the Stars
By Earl Stowell
Of little importance until recently,
telemetering is now one of the lastest-growing Ilelds. in' electronics
A 100-ton 85·foot missile fights its way up into the sky, arches proudly
out over the ocean, falters, veers wildly, becomes a surging spectacle
of flame, then plunges into the ocean. After the scientists recover
from their disappointment, how can they find out what caused the failure?
The answer is telemetering - the process of taking measurements at one
place and simultaneously sending them to another place for interpretation
Telemetering should not be confused with remote
control. When you set the temperature control of a modern stove, you
are using a type of remote control to regulate the oven temperature.
But if the stove has a signal lamp that goes on when the correct temperature
is reached, this is a form of telemetering - because information (the
temperature) is measured at one place and is then sent to another for
Sending Back the News. Only in
recent years has there been a need for telemetering. In the early days
of flying, for example, when planes were much simpler than they are
now, a test pilot would take a new plane up for its initial run and
would then report back to the designers what changes should be made.
But nowadays things happen so fast on test flights that it's impossible
for the pilot to notice everything that's going on. And today's aircraft
are so expensive and complex that if an accident occurs, a method of
determining the reason for the accident is essential. Telemetering provides
the means for solving these very real and important problems.
Weather-study instruments developed in the Thirties provided a clue
for the design of telemetering equipment. A German scientist devised
a simple but effective system for determining atmospheric conditions
at various altitudes. He attached a battery-powered radio transmitter
to a balloon and then hooked up some sensing elements to it that delivered
varying voltages in proportion to altitude, temperature, and humidity.
As the balloon floated through the sky, a three-point rotating switch
connected each of the instruments in turn to the transmitter. Even today,
this simple technique forms the basis of many telemetering systems.
Instrumentation. Telemetering systems seem
complicated, but their complexity comes from the amount of detail involved,
rather than from their inherent circuit complications.
links in a telemetering chain are the measuring instruments, which are
designed to produce output voltages in ratio to their readings. For
instance, to measure temperature between 0° and 100°, a measuring instrument
with an output range from 0 to 5 volts would deliver no output at 0°
and 5 volts at 100°. A temperature of 50° would result in an output
of 2.5 volts. It is more common, however, to use a measuring instrument
that puts out ±2.5 volts, with zero representing half-scale; hence,
-2.5 volts would indicate a temperature of 0° and +2.5 volts would mean
These voltage variations from the measuring instruments
must be coded before they are fed into a transmitter. For example, information
can be indicated by varying the duration of the pulse; this method is
called PDM (Pulse Duration Modulation). Or the amplitude of the pulse
might be varied (PAM, or Pulse Amplitude Modulation). Another method
is PCM (Pulse Code Modulation) or its close relative PPM (Pulse Position
Modulation) in which the position of two short pulses with relation
to each other is the code.
For extreme accuracies, a digital
system is used. In this system, each measurement is changed to a binary
number which may then be handled with high accuracy - up to 0.01% if
required - and the output can be fed directly into a digital computer.
After being coded, the information from the measuring instruments
modulates the r.f. output of a battery-powered transmitter. Most telemetering
systems use FM transmitters which provide about 2 watts output in the
215-245 mc. band. Some transmitters, however, put out as much as 100
Final Link. At the ground station, specially
designed antennas pick up the r.f. signals which are demodulated, sorted
out, and turned into understandable form. In some cases, the signals
are decoded as fast as they are received; this is called "real-time"
telemetering. Most of the information is recorded by tape recorders
for later study. Some advanced telemetering systems feed selected information
directly into computers for immediate processing, as mentioned above,
making it possible to notify a pilot of danger developing before he
is aware of its presence.
Because telemetering installations
are usually tailored to a particular job, there are as many telemetering
systems as there are engineers with imaginations. But if you know the
general principles involved, you will be able to understand any telemetering
system with a little study.
Block diagram of typical telemetering system. Subcarrier
oscillators all modulate
transmitter simultaneously. SCO 4 does
multiple duty by sampling
four different measurements in order when
Cross section of telemetering pay load carried
by the "Pioneer I." Although
short-lived, the "Pioneer I"
provided a great deal of data.
Problem and Solution. Let's take a typical telemetering
problem and then follow through on its solution. Assume that we want
to send up a missile for testing. Since it's doubtful that the missile
will return to the ground in one piece, we must get the information
we need while the missile is in flight. Suppose we want to know its
angle of flight, speed, yaw and pitch rates, the level of gamma rays
encountered by the missile, and various other measurable data.
Now, how do we get this information? We start by building a battery-powered
FM transmitter into the missile. Then, if we have many measurements
to send back to the ground, we can use a combination of two frequency-saving
methods of feeding the transmitter.
is last link in telemetering
chain. High-gain antennas, such as the 60' "dish" above by Radiation,
Inc., feed signals into elaborate electronic "brains."
First two racks of Parsons ground system contain tape recording
equipment; in the third rack are receivers and test equipment; the
next two hold bandpass filters and discriminators; racks 7 and 8
are demodulators and patch-panel racks; the next-to-last one contains
oscilloscope and associated equipment; rack at far right holds five
First, we use subcarrier
oscillators to create multiplex subchannels. Some systems use up to
18 subchannels - all of which can be handled by the same transmitter.
For instance, if the first subcarrier oscillator (SCO 1) has a center
frequency of 400 cps, a ±2.5-volt input signal from the sensing element
will cause SCO 1 to vary from 370 to 430 cps. Accordingly, SCO 18 has
a center frequency of 70 kc., and its output frequency will vary from
64.75 to 75.25 mc.
The outputs from the SCO's all modulate
the FM transmitter simultaneously and the receiver on the ground sorts
out and decodes the different SCO frequencies.
or more SCO's can perform multiple service by means of a simple mechanical"
system. A rotating switch with multiple contacts can be wired up to
connect several sensing elements to a single SCO one at a time. As a
small external motor rotates the switch, each sensing element is sampled
in order. See block diagram on page 43.
In the event that the
missile should be recovered intact, it's a wise precaution to include
a small tape recorder in the airborne system. Should the transmitter
go off the air for any reason, the recorded tape would be vitally important.
Expensive but Economical. Although telemetering
systems are fairly expensive, in our rocket projects they more than
pay their way. Telemetering makes each test flight - no matter how apparently
disastrous - at least partially successful. Design weaknesses can be
analyzed long after the flight is over, and troubles can be ironed out
before another test is attempted.
In industry, too, telemetering
pulls its own weight. Recently, a major airframe company spent two million
dollars for telemetering equipment to be used in testing jet transport
planes. The saving in test time will easily pay for the system.
Telemetering offers a rapidly growing field to those who like something
new and exciting. To the rest of us, it promises safer flying and information
which will speed our conquest of outer space.