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 telemetering.
November 1959 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
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Telemetering - Vital Link to the
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 or "read-out."
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 "read-out."
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
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.
Telemetering systems seem complicated, but their complexity comes
from the amount of detail involved, rather than from their inherent
The first 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
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.
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 watts.
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.
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
Block diagram of typical telemetering system.
Subcarrier oscillators all modulate
SCO 4 does multiple duty by sampling
four different measurements
in order when commutator turns.
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 strip recorders.
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.
from the SCO's all modulate the FM transmitter simultaneously and
the receiver on the ground sorts out and decodes the different SCO
Secondly, one 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
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.
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.