November 1957 Radio & TV News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
In-flight telemetry systems have in the past and still do play an
important role in aircraft development - both manned and unmanned.
Formerly almost exclusively in the domain of large corporations
due to its high cost and complexity, telemetry is now performed
routinely, often by people who cannot pronounce the word or even
know its definition. For a mere couple hundred bucks, you can buy
an off-the-shelf quadcopter 'drone' that
a wireless link back to the pilot and includes not only a
video feed of an onboard camera, but also information on battery
voltage, position, altitude, and other parameters. 'Real' model
aircraft pilots have flight telemetry available via relatively inexpensive
onboard transmitters that broadcast many channels of status on engine
or motor RPMs and temperature; airspeed and acceleration, battery
temperature, voltage and current draw; altitude and attitude; control
surface positions; and basically anything the builder can fit a
sensor for. Some large scale models these days, especially those
with turbine engines, can cost many thousands of dollars and represent
a thousand or more hours of building time, so investing a couple
hundred dollars in flight telemetry is easily justified.
Telemetry in Flight Testing
By Charles M. Downs
Engineering Dept., Convair (San Diego)
Part 1 of a two-part series on principles and applications of
telemetering as used in developmental aircraft testing.
The Convair F-102A all-weather supersonic jet interceptor.
Telemetering played an important part in the exhaustive
flight testing of this aircraft at Edwards Air Force Base
before the F-102A's became operational with Air Defense
The past few years a new application of electronics has "come
of age." Telemetering (meaning to measure from a distance or to
transmit a measurement) which has been used for the last ten years
in missiles and rockets is now extensively used in flight testing
military and commercial aircraft. With the advent of supersonic
aircraft there was a need for a measuring system that would operate
automatically with accuracy and precision without distracting the
pilot of he aircraft in any way. Various airborne recording devices
had the accuracy required but took up too much space, were too heavy,
or were too restricted in the type of data they could record.
As an example, one type of recording oscillograph used as an
airborne recorder weighs 70 pounds, records 26 functions and has
a frequency range of from d.c. to about 500 cycles. With additional
equipment (linear amplifiers) the frequency range is extended to
3000 cycles. A telemetering system weighing half as much can record
many more functions with a response on some subcarrier bands (see
Table 1) of from d.c. to speech frequencies. Safety functions such
as temperature, pressure, and acceleration can be instantly and
continuously monitored on the ground during flight. If safe limits
are exceeded, the pilot can be notified for corrective action.
When an aircraft crashes into the ground at 500 miles per hour,
there is little left from which to determine the cause of the crash.
Lives have been saved because records, safe on the ground, revealed
the cause of system or structural failure in telemetered aircraft.
Data from airborne recorders would have been lost.
Since recording oscillographs are an important part of the receiving
station, it should be pointed out that telemetering has not replaced
the oscillograph as a flight test recorder; it has only moved the
oscillograph down on the ground where its full capabilities can
An oscillograph uses small mirrors mounted in galvanometer movements
to reflect beams of light onto a moving strip of photosensitive
paper. Thus the waveform of the current fed to the coil of the galvanometer
is photographed as a continuous graph. Paper speed is variable from
about 0.5 to 100 inches per second. One of the factors which limits
the upper frequency response of an airborne oscillograph is paper
speed. Due to the short supply of paper that can be carried aloft,
the speed must be kept low if more than a few minutes of flight
are to be recorded. This problem is overcome when the recorder is
located in the ground station. Telemetered data stored on magnetic
tape can be played back as many times as desired and oscillograph
records may be made at any paper speed. Here it should be pointed
out that oscillograph records are usually employed to determine
which parts of the flight should be selected for further data reduction.
Theory of Operation
The theory of operation of an FM/PM telemetry system should be
easily understood by anyone familiar with audio circuits. Several
audio oscillators, operating at different frequencies, are frequency
modulated by their associated pickups. The frequency-modulated audio
signals are used to phase modulate a radio transmitter operating
in the range of 215 to 235 megacycles. The r.f. signal is, in turn,
picked up by a receiving station where the audio output of a receiver
is passed through bandpass filters which separate the subcarrier
oscillators (the audio oscillators in the aircraft) from the complex
audio signal. The output of each bandpass filter is then fed into
an audio discriminator which produces a varying d.c. voltage corresponding
to the frequency shift of the subcarrier oscillator. The current
in the discriminator output signal is the electrical equivalent
of the original modulating signal obtained from the pickup in the
aircraft. The discriminator output is then fed into electro-mechanical
recorders such as recording oscillographs and pen recorders. Tape
recorders, at the same time, record the audio output of the receivers
so that in case of failure of one of the discriminators or electro-mechanical
recorders, the data will not be lost. The tape playback can be fed
directly into automatic data reduction computers if additional data
reduction is required.
Table 1. The various sub carrier bands that are employed
in telemetering systems.
Let us now consider various components of a system and their
functions. Pickups and subcarrier oscillators (SCO's) fall into
three basic types: voltage-, resistance-, and inductance-controlled.
Of these three, the voltage-controlled oscillator is probably the
most widely used. A diagram of a simple voltage-controlled system
is shown in Fig. 1. Excitation voltage (in this case d.c.) is applied
across a precision potentiometer. The shaft of the potentiometer
is linked to an angular motion such as the rudder of an aircraft.
The signal voltage from the arm of the pot is fed through shielded
wire to the SCO. In one type of voltage-controlled SCO the input
signal is fed to a reactance modulator combined with a Hartley oscillator.
In another type, a free-running multivibrator is frequency modulated
by using the signal voltage to bias the grid of one half of the
oscillator. The frequency stability of either of these types under
steady-state input conditions is excellent.
The frequency deviation of a voltage-controlled SCO is an inverse
function of signal polarity shift. In other words, when the signal
is made more positive, the output frequency decreases. Two voltage
ranges are generally used: 0 to 5 volts and ±2.5 volts. Since very
sharp bandpass filters (down 60 db a few cycles past the bandpass
limit) are used in the receiving station, SCO bandwidth limits and
therefore input voltage limits must be carefully observed. Band
limits (deviation limits) are determined as follows. There are 18
SCO frequencies established by the Department of Defense's Research
and Development Board (RDB) now in use. (See Table 1.) Each SCO
is allowed to deviate 7.5% of its center frequency each side of
center frequency. As an example, given a 2.3 kc. seo with a voltage
range of 0 to 5 volts, the output frequency at 2.5 volts would be
2300 cps or ƒh. At 5 volts the frequency would be
2127 cps or ƒl. With 0 volt applied, the frequency
would become 2473 cps or ƒh To provide a "fudge
factor" and to reduce noise, many systems are operated at 80% bandwidth.
Bandwidth now becomes 80% of 7.5% of ƒc.
Special 15% units are available which are used when wide-band operation
So far, we have considered only a d.c. modulating signal. Although
a.c. can be used to modulate voltage-controlled oscillators, another
factor must be considered: that of the highest a.c. frequency that
can be applied. Naturally, it would be impossible to impress 2000-cycle
modulation on a 2300- cycle SCO. Generally, it may be said that
up to and including the 14.5 kc. band, maximum modulating frequency
should not be more than 1.5% of ƒc. Above 14.5 kc.,
3% is the maximum. These limits are conservative and can be exceeded
but a point will be reached where severe intermodulation will occur.
It should be remembered, however, that it is the instantaneous value
of an a.c. modulating signal and not the frequency of the signal
that causes the SCO to deviate.
Voltage-controlled oscillators are used to measure control surface
positions, gas and liquid pressures, vibration, acceleration, or
any function which can be made to produce varying d.c. or a.c. voltage.
A 70 kc. voltage-controlled SCO can be modulated by the pilot's
microphone to provide one-way communication from the aircraft to
ground. This can be used to record the pilot's comments which, for
security reasons, cannot be transmitted over the aircraft's normal
The second basic type of SCO is the resistance-controlled oscillator
which changes frequency when a resistance change occurs in one or
more arms of an a.c.-excited Wheatstone bridge. A basic resistance-controlled
oscillator system is shown in Fig. 2. Bridge unbalance causes phase
shift to occur in a phase-sensitive amplifier-oscillator loop. Sensitivity
is determined by the number of active arms. Strain-sensitive bridges,
precision potentiometers, and other variable resistance transducers
are employed with resistance-controlled oscillators.
A troublesome drawback is present in resistance-controlled systems.
The shunt capacitance of the shield wire used between the pickup
and oscillator causes the response curve of the oscillator to become
slightly parabolic. (See Fig. 3.) This can be reduced, however,
by shunting the bridge at the SCO with another capacitance to balance
the unwanted capacitance of the shielded wire.
The third widely used type of SCO is the inductance-controlled oscillator.
Mechanical motion is transferred to a Mumetal slug which is suspended
in the field of a coil. The slug is oil- or air-damped, depending
on the frequency response desired. The coil comprises the inductance
of a Hartley LC oscillator. Thus the frequency of the oscillator
is modulated by the mechanical motion. The pickup is not usually
mounted more than a few feet from the oscillator because the shunt
capacitance of the shielded wire can lower the frequency of the
oscillator to a point where it is out of the particular band being
used, or can actually prevent the SCO from oscillating. This is
overcome by mounting the pickup near the oscillator and running
tubing (in the case of a pressure measurement) from the oscillator-pickup
location to the measurement point. In the case of an acceleration
measurement, the accelerometer can be mounted with the oscillator
at the point where acceleration is to be measured.
Fig. 1. Block diagram of a simple voltage controlled
subcarrier oscillator system.
Fig. 2. Resistance controlled SCO system.
Fig. 3. Resistance controlled SCO response.
Many special types of pickups are on the market which are designed
to do a specific job such as the measurement of airspeed, altitude,
fuel flow, and fuel quantity. Some of these contain their own SCO's
while others convert hard-to-measure functions into easily handled
voltage, inductance, or resistance changes.
The Radio Transmitter
After the audio output of the SCO's in a system (most systems
use from ten to twelve SCO's) have been mixed through voltage dividers
into a common audio bus, the combined or "complex audio" signal
is applied to the modulator input of a crystal-controlled, phase-modulated
For those unfamiliar with the main difference between a phase-modulation
and a frequency-modulation system, it might be well to point it
out. The end result in either case is the same; the frequency of
the transmitter becomes a function of the modulation impressed on
that transmitter. However, that result is achieved by different
methods. An FM transmitter is actually amplitude modulated. That
is, the frequency change of the transmitter is a function of the
amplitude of the modulating signal. A signal of 10,000 cps would
cause the same frequency shift as a signal of 1000 cps if their
amplitudes were equal. The rate at which the frequency of the transmitter
is varying, however, is dependent on the frequency of the modulating
signal that is used.
In a phase-modulated transmitter, the amount of frequency deviation
of the transmitter is a function of both amplitude and frequency
of the modulating signal. Given a modulator with flat frequency
response, and a ten SEO system with equal levels for all ten SEO's,
the deviation of the transmitter caused by each SEO would be proportional
to the frequency of the SEO. However, the frequency response of
the modulator tube and circuit is far from flat. The modulator tends
to attenuate the level of the higher frequency SEO's. The result
of these two opposing response curves (the increasing deviation
of the transmitter with an increase in modulation frequency, and
the attenuation of the higher frequency seo's by the modulator tube)
is a decrease in deviation response with an increase in frequency.
It is therefore necessary to pre-emphasize the higher frequency
SEO's in order to obtain the correct modulation index for a given
The transmitter is a compact frequency multiplier with about
2.5 watts output. When used to telemeter vehicles or objects which
are fairly close to the receiving station (1 to 3 miles) the power
output of the transmitter alone is enough to deliver satisfactory
strength; but when used in missiles and aircraft, additional power
amplifiers are required. Telemetering power amplifiers usually
contain 1 or 2 tubes (4X150A and832B are commonly used types) and
produce from t15 to 100 watts output, depending on tube type and
plate supply voltage.
Antenna systems are more of a problem than usual when mounted
on supersonic aircraft. In addition to exhibiting a satisfactory
radiation pattern, the antenna must produce a minimum of drag and
turbulence. For this reason flush antennas such as notch exciters,
slot antennas, and quarter-wave dipoles imbedded in non-metallic
material are used extensively. The airspeed measuring system on
flight test aircraft usually consists of a short tapered tube, an
inch or two in diameter, extending from the center of the nose of
the aircraft. If insulated, the airspeed boom can serve as a quarter
wave "spike" antenna. Blade antennas, in the form of an airfoil,
can be mounted on the bottom of the aircraft where maximum r.f.
propagation is effected. A quarter wavelength at the frequencies
in use is about 9 to 11 inches which facilitates compact antenna
One system uses one antenna mounted in each wingtip to prevent
signal dropout during rolls and sharp banks. Duplexers are in use
which permit two transmitters of different frequencies to be loaded
into a single antenna; three duplexers may be used to load four
transmitters into one antenna system.
Convair's. receiving station at Edwards AFB flight test
facility. In right foreground is playback unit for airborne
tape. The next three racks contain sub-carrier discriminators.
Left racks contain receivers, test gear, and patch boards.
Telemetering systems are manufactured in "building block" form.
Due to the individual requirements of any one flight research program,
it would be hard to build a "package deal" to be installed in all
types of aircraft. Therefore, it is up to the engineers and technicians
working with the equipment to use their ingenuity in selecting the
components for their system. With these things in mind, a few design
considerations will be discussed here.
Every system has its drawbacks and telemetry is no exception.
Possibly the greatest problem encountered in a measurement system
is noise. Types of noise existent in a telemetering system can be
placed in two general classes: intermodulation and random or transient
noise. Intermodulation can be caused by applying a complex wave
to a non-linear impedance. Appearing as beats between the component
frequencies of the complex signal, it produces frequencies which
are not present in the original complex wave. Beat frequencies are
always present in a complex audio bus (the combined output of the
several subcarriers in a telemetering system) but the levels of
the beats are down 30 db or so in relation to subcarriers. If two
subcarrier levels are allowed to increase to a high enough level,
the beat between them will attain a high enough level to seriously
affect the other subcarriers in the system. Intermodulation can
be reduced or eliminated by the use of impedance matching devices,
extra insulation, and carefully balanced subcarrier levels.
Random and transient noise should, if possible, be eliminated
at its source. Relays switching reactive loads can produce a transient
which can be picked up fifty feet away. A large capacitor or neon
lamp placed across the contacts of hte relay can suppress much of
the noise. Ground loops can be avoided by grounding the shielding
at one point only and as close to the SCO as possible. Noise which
amplitude modulates the SCO envelope is reduced by the limiters
in the receiving station radio receivers. However, noise which frequency
modulates the SCO must be eliminated at its source if the frequency
response of the SCO is to be utilized. In the case of a d.c. or
low-frequency a.c. measurement, the recording device can be damped
so that its upper frequency limit falls below the noise frequency.
Low-pass filters in the discriminator output circuit are also useful
in noise reduction but again the high-frequency response of the
subcarrier is limited.
(Concluded next month)
Posted January 1, 2015