Chapter-3

Telemetry Applications
outlines
 Telemetry Systems Overview
 Elements of Telemetry System
 Telemetry Transmission Medium
 Airborne and Ground System

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 3.1. Telemetry Systems Overview
 The term telemetry is derived from the two Greek terms: “tele” and “metron”,
which mean “remote” or “far off” and “measure”, respectively
 Telemetry is defined as the sensing and measuring of information at some
remote location and then transmitting that information to a central location
 Telemetry is the process by which an object’s characteristics are measured (such
as velocity of an aircraft), and the results transmitted to a distant station where
they are displayed, recorded, and analyzed.
 Telemetry is the science of gathering information at some remote location and
transmitting the data to a convenient location to be examined and recorded
 Telemetry techniques becomes essential under two situations, or in other
words for two cases, given below.
 Distant location of the measurand: Telemetry uses electrical communication for
transmitting electrical signal representing the value of the measurand from the location of the
measurand
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 Cont..
Inaccessibility of the measurand: In such a situation, the electrical output of the
transducer (or sensor) sensing the measurand, cannot be accessed by conventional
method of connecting wires. Therefore, the electrical output is converted to a radio wave,
which is then transmitted to the user’s location. This type of telemetry is called Radio
Telemetry
In today's telemetry applications, which support large numbers of measurands, it is too
costly and impractical to use separate transmission channels for each measured quantity.
The telemetry process involves grouping measurements (such as pressure, speed, and
temperature) into a format that can be transmitted as a single data stream. Once received,
the data stream is separated into the original measurement’s components for analysis
Telemetry lets you stay in a safe (or convenient) location while monitoring what's taking
place in an unsafe (or inconvenient) location.
Data acquisition begins when sensors measure the amount of a physical attribute and
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transform the measurement to an engineering unit value
 3.2. Elements of Telemetry System
• Basic telemetry system elements are given below in the diagram.
• Location of the physical variable or measurand (from where the information,
that is, the value of the measurand is sent) is called the Sending End of the
telemetry system while the location of the user or the end device (where the
information is received and used) becomes the Receiving End of the
telemetry system.

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 (a). Transducer or Sensor
Converts the physical variable to be telemetered (that is, the
measurand) into an electrical quantity. This quantity in most cases is either an
electrical parameter (variable resistance, inductance or capacitance) or an
electrical signal (voltage or current)
(b) Signal Conditioner-1:
Converts the electrical output (which may or may not be a signal, as
explained above) of the transducer (or sensor) into an electrical signal
compatible with the next element, i.e. the transmitter.
The incompatibility could be either in the form (such as parameter versus
signal, voltage versus current, analog versus digital, etc) or in the magnitude
of the signal (that is, it is too weak to be used by the next element).

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 (c) Transmitter
• Its purpose is to transmit the information signal (a signal containing
information, i.e. a signal which is a function of the value of the measurand)
coming from the signal conditioner-1 using a suitable carrier signal to the
receiving end.
• It may perform one or more of the following functions
(i) Modulation: Modulation of a carrier signal by the information signal
(ii) Amplification: As and if required for the purpose of transmission
(iii) Signal Conversion: As and if required for the purpose of transmission.
For example, voltage to current conversion, or analog to digital
conversion, or electrical signal to radio wave conversion, or electrical
signal to optical beam conversion, depending on the nature of the
carrier signal and the signal transmission medium

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 cont….
(iv) Multiplexing: If more than one physical variables need to be
telemetered simultaneously from the same location, then either
frequency-division multiplexing (FDM) or time-division multiplexing (TDM)
is used
(d) Signal Transmission Medium
It is the medium or link that connects the sending or transmitting end
to the receiving end, over which the transmitter can transmit its
output signal to the receiver
(e) Receiver
 Its purpose is to receive the signal(s) coming from the transmitter
(located at the sending end of the telemetry system) via the signal
transmission medium and recover the information from the same. It may
perform one or more of the following functions
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 Cont….
(I) Amplification: Amplification of the received signal as and if required
for the purpose of further processing.
(ii) Demodulation: Demodulation of the received signal to recover information
signal. The demodulation process has to be complementary of the
modulation performed the transmitter.

(iii) Reverse Signal Conversion: This conversion is generally the reverse of

conversion performed by the transmitter. Thus the receiver is required to
perform current to voltage conversion, or digital to analog conversion,
or radio wave to electrical signal conversion, or optical beam to
electrical signal conversion, depending on the nature of the carrier signal
and the signal transmission medium.
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 Cont….
(iv) De-multiplexing: It refers to the process of segregating or separating
various information signals so that they can be delivered to their
respective end devices. The process in the receiver has to be
essentially the reverse of the multiplexing carried out by the
transmitter
(f) Signal Conditioner-2:
Processes the receiver output as necessary to make it suitable to drive
the given end device.
(g) End Device:
The element is so called because it appears at the end of the system.
Depending on the purpose of the telemetry in the given situation, the end
device may be performing one of the
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following functions
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 Cont…..
(i) Analog Indication: Analog indication of the value of the measurand through
the deflection of a pointer on a scale. The device used is very often a
permanent magnet moving coil (PMMC) meter.
(ii) Digital Display: Digital display of the value of the measurand on LEDs,
LCD, monitor screen etc.
(iii) Digital Storage: Storage of the digital value of the measurand in electronic
or optical storage device for a later use.
(iv) Data Processing: The digital values of the mesurand may be given to a
data
processor, such as a microprocessor, digital signal processor or computer, for
analysis etc.
(v) Closed-Loop Control: The analog or digital output of the signal conditioner-
2, representing the value of the measurand, may be fed to an automatic controller as
the feedback
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 3.3. Telemetry Transmission Medium

• The signal transmission medium can be one of the following types

1. Copper Wires
• A pair of copper wires (or conductors) provides a closed-circuit path for the flow
of an electrical signal from the transmitter to the receiver of the telemetry system.
2. Radio Link
• The space between transmitting and receiving antennas of the transmitter and receiver,
respectively, provides a radio link between the two ends of the telemetry system by allowing
the propagation of radio waves (high frequency electromagnetic waves) generated by
the transmitting antenna through it.
3 . Optical-Fibre Link
• An optical fibre, acting as a wave guide for the optical beam (in infra-red wavelength band)
generated by the transmitter, delivers the same to the receiver. It acts as an optical-
signal transmission link between the transmitter and receiver of the telemetry system .
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 Wire-Link or Wire Telemetry System
• The signal transmission medium here is a pair of copper wires.
• The transmitter comprises an audio-frequency (AF) AC modulator or pulse modulator,
as per the needs of the application, and an amplifier to strengthen the
modulated carrier signal before sending it on the copper wire-pair.
• Like the transmitter, the receiver also has only two elements, amplifier to carry
out necessary amplification of the attenuated signal received through wire-pair
and a demodulator to recover the information signal from the modulated carrier signal

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 Radio or Wireless Telemetry System
• The signal transmission medium here is a radio link, comprising a transmitting
antenna (TA), a receiving antenna (RA) and the space between the two used for
propagation of radio wave from TA to RA.
• The transmitter comprises a RF modulator (AM or FM type, depending on the
performance, bandwidth and cost considerations) and an amplifier.
• The receiver comprises an amplifier and a demodulator (AM or FM type as
required to match the type of the modulator)

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 Cont…
• Radio waves are propagated when the electrical energy produced by the radio
transmitter is converted into magnetic energy by the antenna.
• Magnetic waves can then travel through space. The receiving antenna then
intercepts a very small amount of this magnetic energy and converts it back
into electrical energy that is amplified by the radio receiver. Thus, sending
information through the air

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 Optical-Fibre or Fibre-Optic Telemetry System
• The transmission signal is a high-intensity narrow infrared optical beam.
• The signal transmission medium is an optical fiber, which works on the
principle of total internal reflection and thereby serves as the waveguide for
the propagation of the optical beam from the transmitter to the receiver.
• The transmitter includes
(i) a PCM modulator, which gets a digitized value of the measurand from
signal conditioner-1 and produces binary voltage pulses in a coded
sequence,
(ii) a voltage-to-current converter,
(iii) a light source, usually an injection laser diode (ILD) that converts the
binary current pulses to binary optical pulses, and finally
(iv) a01/06/2025
light-source to optical-fiber coupling unit.
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 Cont..
• The schematic diagram is as shown bellow

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 Cont..
• The receiver includes components performing the complementary
functions of the transmitter components in reverse order. These are
(i) optical-fibre to light detector coupling unit,
(ii) light detector, usually a PIN diode that detects the binary optical pulses it
gets from the optical fiber and converts them to binary current pulses,
(iii) current to voltage converter, and
(iv) demodulator, which delivers digital voltage signal to the end device
through signal conditioner-2
• In case the end device requires an analog input signal, the signal
conditioner-2 would include a digital-to-analog converter (DAC)

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 Modulation
• The information signal (that contains the value of the measurand) is very
often not suitable for transmission from the transmitter to the receiver and is,
therefore, used to modulate a carrier signal compatible with the given
transmission medium.
• The modulated carrier, which now contains the information, is transmitted by the
transmitter to the receiver.
• The following are the reasons for using modulation in telemetry
(a) To achieve compatibility with the given transmission medium:
• A carrier signal compatible with the given transmission medium is selected
for modulation.
(b) To improve signal-to-noise ratio
(C)To achieve multiplexing
Different carrier frequencies are used for modulation by different information signals
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frequency division multiplexing (FDM)
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 Modulation Methods
• In regard to the modulation, there are three possibilities and
accordingly there are three categories of telemetry systems as follows
a) DC Telemetry Systems
• These telemetry systems use no modulation. The information signal which
varies very slowly and is considered as a DC signal is transmitted as such.
• The transmission signal for DC telemetry systems is either a DC voltage or a
DC current and the signal transmission medium is essentially a pair of copper
wires
b) AC Telemetry Systems
• These telemetry systems use an AC carrier, which is modulated using
one of the following AC modulation techniques
1. Amplitude modulation (AM) telemetry system
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2. Frequency modulationInstrumentation
(FM) telemetry
En,g
system 19
 c) Pulse Telemetry Systems
• These telemetry systems use a pulse carrier, which is modulated using
one of the pulse modulation techniques
1. Pulse amplitude modulation (PAM) telemetry system
2. Pulse width modulation (PWM) telemetry system
3. Pulse phase modulation (PPM) telemetry system
4. Pulse frequency modulation (PFM) telemetry system
5. Pulse code modulation (PCM) telemetry system

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 Direct Voltage Telemetry System
• Transmission signal for this telemetry system is a direct voltage (DC voltage)
signal and the signal transmission medium is essentially a copper wire line
Sending-End Scheme: The transducer (sensor) converts the input physical
variable (measurand) to an electrical quantity, which is either an electrical
parameter or an electrical signal. This output is processed by appropriate
electronic circuits (signal conditioner unit) to yield a voltage signal, typically in
the range of 0-10V. This voltage signal is then suitably amplified to a value Vdc1
and to the copper wire link.

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 Cont…..
Receiving-End Scheme: To maintain simplicity of the system, the end
device at the receiving end is a permanent-magnet moving-coil (PMMC)
voltmeter. This type of meter has two important advantages of high
sensitivity and scale linearity.
• The meter measures the voltage at the receiving end of the line, Vdc2
• Transmission Error: The voltage at the receiving end is given by
Vdc2 = Vdc1 – I R, where I=line current and R is the
resistance of the line(wire)
Merits/ Advantages: The merits of direct voltage telemetry system are
1. Simplicity of the system and its components
2. Low cost of the system as there are no specialized components
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 Cont….
• Demerits/ Disadvantages: The demerits of this telemetry system are
1. It can be used only for short distances as both the error and the cost
of line increase with the length of the wire line.
2. As the line current is small, the leakage currents could become comparable
and thereby cause a large error in the meter reading
Direct Current Telemetry System
• Transmission signal for this telemetry system is a direct current (DC current)
signal and transmission medium is a copper wire line.
• The most commonly used current signal is 4-20mA
• Has a voltage to current converter while the direct voltage system uses a
voltage amplifier
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 Cont…
• The end device is a PMMC milli-ammeter as it has to read the value of the line
current at the receiving end, Idc2, which is in milli-ampere range. Its scale is
calibrated in terms of the measurand (M), so that the user can read the value of
M directly.

• The line current at the receiving end is given by

Idc2 = Idc1 – Ileakage
where Idc1 is the line current at the sending
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end and Ileakage is the small current
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leaking
 Cont…
• The following measures can be taken to minimize transmission error in
the direct current telemetry system
(a) The insulation resistance between the wires of the line and that
between each wire and earth should be maximum so as to minimize the
leakage.
(b) The telemetry system should be used for short distances only, because the
value of Ileakage is the product of the leakage current per unit length of the given
wire cable and its total length.
(c) Only an underground cable, as against open wires, should be used as the
former is not directly exposed to the variations in ambient temperature.
Merits/ Advantages: The merits of direct current telemetry system are
 Simplicity of the system and its components
Low cost of the system as there are
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Instrumentation En,g components 25
 Cont….
Demerits/ Disadvantages: The only demerit or limitation of this telemetry
system is that it can be used only for short distances because
(a) the error due to leakage can become substantial if the length of the wire
line is large, and
(b) the cost of the line increases directly with its length
Application:
Because of the above merits and limitation, this type of telemetry
system is very popular for in-plant telemetry where the distances are
generally short.

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 AC Telemetry Systems
• The transmission signal for AC telemetry systems is a modulated AC (sinusoidal)
signal.
• The type of modulation is either amplitude modulation (AM) or frequency
modulation (FM).
• The signal transmission medium is either a pair of copper wires, which would
use an audio-frequency (AF) carrier, or a radio link, which would need a radio-
frequency (RF) carrier
AM Telemetry System
 Transmission signal for this telemetry system is an amplitude-modulated AC
signal
 The amplitude of high frequency carrier wave is changed in accordance with the
intensity of the signal
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 Cont…
• Figure below shows the principle of amplitude modulation.Fig. (i) shows the
electrical signal whereas Fig. (ii) shows a carrier wave of constant amplitude.
Fig. (iii) shows the amplitude modulated (AM) wave

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 FM Telemetry System
 Transmission signal for this telemetry system is a frequency-modulated AC
signal.
 Generally a RF sinusoidal signal is used as the carrier and a radio link as
the transmission medium

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 Cont….
• Since the information (value of M) resides in the frequency, and not the amplitude of the
transmission signal, no telemetry error results from the attenuation or variations

• The merits of FM telemetry system are as under:

1. The most important advantage is that it can be used conveniently wherever it is difficult or
impossible to access the sensor output with wire leads.

2. The system and its components are quite simple.

3. The system is inexpensive as only ordinary/ standard components are used.

4. It can be easily extended to a multi-channel telemetry system using frequency division

multiplexing (FDM), in which case each channel uses a carrier of different radio frequency
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 Cont….
• Demerits/ Disadvantages
1. In outdoor telemetry applications, its performance can be problematic in bad
weathers
2. It can become expensive when used with long range-range
transmissions in commercial radio frequency band.

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 3.4. Ground and airborne systems

• Telemetry is applicable in flight testing, space exploration and rocketry, to

enable real-time monitoring of environmental conditions in flying objects.

• Space agencies use telemetry systems to collect data from satellites and other
space vehicles, providing engineers with real-time measurements of critical
system parameters required to develop and improve system performance.

• In-flight data transmission is particularly vital for missiles and space systems
due to the fact that these systems are often damaged or destroyed before they
complete the test flight
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 Cont..

• Airborne system is a system on the spacecraft to transmit and receive a signal to

the ground.

• Ground system is a system on the ground which is used to control flying objects
and to transmit and receive signal to and from the flying object.

• Command and Control

• Monitor spacecraft health

• Track

• Calculate spacecraft attitude

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 Cont……
• Telemetry is the only means for the spacecraft controller to know how the satellite is
functioning.

• Commands are generated from the ground station and transmitted to the airborne
system on the satellite.

• At the satellite the received signal is decoded and stored in memory .

• The command code is sent back to the ground station via telemetry links to check the
validity.

• If the command word is found correct, an execute command is sent for actual execution
of the function.
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