This paper was originally published in the January/February 1997 issue of Satellite Times magazine.
The Federal Communications Commission allow Radio Amateurs the privilege of using a myriad of digital communication modes in various forms on the ham bands. Tuning across amateur frequencies, one might hear communications utilizing continuous wave (CW) telegraphy, radioteletype (RTTY), AMTOR, PACTOR, ASCII, and packet radio, just to name a few. These digital signals are modulated onto RF carriers using such methods as on/off keying (OOK), frequency shift keying (FSK), and phase shift keying (PSK).
In the world of amateur satellites, early OSCARs used CW and on/off keying to transmit telemetry information to groundstations through low-powered beacon transmitters they carried into earth orbit. As technology advanced and home computers became popular, the digital communication modes, and modulation methods that supported them, changed. Soon, CW shared telemetry downlink time with RTTY, ASCII, and packet radio. Specialized satellites were later designed that contained only digital communication transponders. These satellites are setting an important trend in amateur satellites.
Digital communications have a significant and growing impact in the telecommunications industry, and the non-commercial world of amateur radio is no different. In fact, amateur radio provides opportunities for research and experimentation in the digital communications field not possible or practical in the commercial world. It is, therefore, important to have a basic understanding of digital communications and the OSCAR satellites that support them if the Amateur Radio Service is to continue to grow with advances in technology.
Basically, radio transmissions are accomplished by modulating a radio
frequency (RF) carrier with intelligence in a circuit known as a
modulator. The modulated RF carrier is then amplified, and radiated
into space through an antenna system. The intelligence the RF signal
carries may contain analog information, such as in the case of voice
or video, or digital information, such as in the case of morse telegraphy,
packet radio, or any other form of binary data sent in a serial fashion.
There are three aspects of an RF carrier that may be modulated in order
to carry intelligence. Either the amplitude of the RF carrier is modulated,
or its frequency is modulated, or its instantaneous phase is modulated.
In some cases, such as commercial broadcasts of color television, AM
stereo, or even modern telephone modems, a combination of phase, frequency,
and amplitude modulation is used to convey several discrete channels of
intelligence on a single carrier. However, this combination of modulation
methods is rarely seen in present day amateur radio communications.
The first form of amplitude modulation used by amateur radio operators
consisted of on/off keying of an RF carrier by morse telegraphy.
Commonly referred to as CW, morse telegraphy is still in wide use today
for both terrestrial and satellite communications. CW was the first mode
of communications adopted by OSCAR satellites for the transmission of
telemetry information through beacon transmitters carried on amateur
spacecraft, mainly because of its simplicity and communications
effectiveness. It is still widely used today.
Morse telegraphy is typically copied by ear. Satellite telemetry
conveyed in CW is interpreted through the use of look up tables and
calibration equations. One of the advantages of morse telegraphy is
that decoding is done by ear, and requires no data demodulator or
specialized electronics for reception.
During the mid 1970s, OSCAR satellites adopted more advanced techniques
for making telemetry transmissions. OSCAR 7, launched on November 15, 1974,
was the first amateur satellite to use radioteletype (RTTY) for the
transmission of spacecraft telemetry. Frequency shift keying (FSK)
modulation was used for the transmission of RTTY signals. Radioteletype
provided the advantage of transmitting information at a higher rate than
was possible using CW at speeds that could be comfortably copied by ear.
The speed at which data can be transferred between transmitter and receiver
is an important consideration for low-earth orbiting satellites with
limited access times. The disadvantage of RTTY is that it required
expensive or bulky terminal equipment for reception, and was later shown
to be relatively inefficient in terms of data rate per unit of power,
another important consideration when dealing with low-powered OSCAR
satellites with limited supplies of power. For this reason, RTTY
is rarely used today by OSCAR spacecraft.
During the 1980s, the FCC approved the use of the American Standard for
Information Interchange (ASCII) for communications on amateur frequencies.
This started a trend toward using more advanced data encoding techniques
directly compatible with home computers that were growing popular at the
time. This compatibility allowed PCs to be used to interpret, store, and
directly process data received from space. Like RTTY, ASCII required a
data demodulator to be used at groundstations to convert the signals
received from the satellite by the groundstation receiver to voltage
levels compatible with the computer performing the data processing
functions. Like RTTY, frequency shift keying was the modulation most
often used for making ASCII transmissions.
UoSAT-OSCAR-9, launched on October 6, 1981, was the first amateur
satellite to adopt advanced forms of digital transmissions such as
ASCII for transmitting spacecraft telemetry on its beacon transmitters.
UoSAT-OSCAR-9, and later its younger sister, UoSAT-OSCAR-11, assigned
audio tones of 1200 Hz and 2400 Hz to represent the digital ones and
zeros (marks and spaces) that comprised the ASCII serial data stream
used to transmit spacecraft telemetry information to groundstations.
This form of modulation is known as audio frequency shift keying, or
AFSK. The early UoSAT satellites used AFSK audio tones to modulate a
narrowband FM beacon transmitter, making groundstation reception as
easy as turning on a 2-meter FM rig. The problem, of course, was
demodulating the mark and space tones in ways that would be least
affected by noise, and converting those audio tones to the appropriate
logic levels suitable for interpretation by a home computer.
Also during the early 80's, AFSK-FM was adopted by early packet
radio pioneers who used FM voice transceivers and surplus Bell 202
telephone modems that were widely available at the time. This combination
provided a quick, easy, and low-cost method of transmitting digital
information using a commonly available voice-grade FM voice transmitters
without the need for making any circuit modifications to the communications
equipment.
There was, however, a price to pay for this simplicity. AFSK-FM is
essentially two FM signals in one, and as such, is terribly inefficient
in terms of bandwidth and communication effectiveness. AFSK-FM requires a
very strong signal for reliable communications, much higher than if other
modulation methods are used. Steve Goode, K9NG, studied this situation
with respect to 1200 baud packet radio communications and concluded that
a signal level that produced at least 25 dB of receiver quieting was
required for reliable packet radio communications using AFSK-FM. This
strong signal level is not easy to realize from OSCAR satellites
operating many miles from earth with only several hundred milliwatts
of transmitter power. Clearly, a better solution was needed if digital
communications were to be effective via OSCAR satellites.
For many years, a small group of amateur radio operators and low
frequency radio enthusiasts (LOWFERS) have been experimenting with
coherent continuous wave (CCW) communications. CCW permits the detection
of radio signals at levels too weak to be copied by ear, and allows
transmitter power levels to be reduced dramatically as a result. CCW
achieves its stellar weak signal performance by being sensitive to
not only the frequency and amplitude of the desired radio signal, but
also its phase. Coherent CW achieves high signal sensitivity while
remaining relatively immune to interfering signals and noise since
these undesired signals have random frequency and phase characteristics
compared to the desired signal.
Coherent CW is great for morse telegraphy communications, but it
can be shown that if the phase of the RF carrier were modulated
instead of its amplitude, an additional 6 dB in signal-to-noise ratio
could be realized over CCW's on/off keying technique without any
increase in transmitter output power. The 6 dB advantage comes when
the phase modulation is antipodal -- that is, it shifts by exactly
180 degree intervals. Phase shift keying comes in many different
varieties. Antipodal PSK is commonly referred to as binary phase
shift keying (BPSK).
BPSK is a very robust form of digital modulation and has been used
to good advantage in copying extremely weak signals from several
interplanetary deep space probes. BPSK was adopted by the AMSAT-OSCAR-10
and AMSAT-OSCAR-13 amateur satellites for telemetry transmissions, and
later by several other OSCARs carrying digital transponders.
The problem with coherent communications is that proper signal
demodulation requires a reference carrier with which to compare the
received signal in a synchronous phase detector. A phase detector
functions by comparing two signals and producing an output voltage
based on how well those two signals match in terms of frequency and
phase. If the frequency and phase of both signals match, a voltage
of a certain polarity is produced, while if both signals are completely
out of phase, an output voltage of opposite polarity is produced. Finally,
if both signals are 90 or 270 degrees out of phase, the phase detector
produces no output voltage.
Although BPSK offers a tremendous advantage in terms of signal-to-noise
ratio over FSK and on/off keying, its reception can be a bit tricky.
If viewed in the frequency domain, a BPSK signal may be thought of as
being a double sideband (DSB) suppressed carrier AM signal. This is
not too much different from single sideband suppressed (SSB) carrier
AM that is popular on the HF ham bands for voice communications.
Proper demodulation requires re-insertion of a locally generated
carrier having identical frequency and phase characteristics as the
carrier suppressed in the BPSK transmitter. This is similar to the
technique used to receive single sideband voice transmissions, except
that in the case of SSB, the phase of the locally generated carrier
(beat frequency oscillator, or BFO) is unimportant since only one
sideband is transmitted. Compounding the problem of requiring an
accurate frequency and phase reference for properly demodulating
BPSK signals is Doppler shift that is inherent in all amateur
satellite communications. Doppler shift causes the radio frequency
of the signal received from the satellite to constantly drift lower
in frequency as the slant range to the satellite changes during a
pass, making it even more difficult to perform proper phase detection.
Since the BPSK carrier is suppressed by the BPSK transmitter, the
key to demodulating BPSK emissions from satellites lies in the
regeneration of the needed carrier reference from the upper and lower
sideband components of the BPSK signal. Two popular methods exist for
doing this. One method uses a circuit known as a Costas Loop, while
the second method uses a Squaring Loop. In theory, the Costas Loop
offers slightly better performance at the expense of added circuit
complexity. The simpler Squaring Loop, however, can be made to provide
performance comparable to that of the Costas Loop with additional
filtering.
Other issues come into play when comparing the sensitivity of
a BPSK receiver. Radio signals received via satellite often fade in
amplitude as the slant range to the satellite changes, and signal
polarization changes occur during a pass. In order to accommodate a
wide range of signal levels, some BPSK demodulator designs incorporate
amplitude limiters. Limiters are often used in FM receivers, and are
responsible for a characteristic in FM communications known as the
"capture effect". If more than one signal is received at the same time
on the same frequency, the stronger signal will "capture" the limiter
and be the only signal detected by the receiver. If the stronger signal
happens to be noise, the desired signal will be completely lost.
Designing a BPSK demodulator with non-limiting automatic gain control
(AGC) has been shown to have a significant weak signal sensitivity
advantage over designs incorporating hard limiting. Furthermore,
post detection filtering that closely matches the bandwidth of the
transmitted signal to allows the detection of BPSK signals even if
they are received at a level below the ambient noise level.
BPSK signals are typically copied with receivers designed for
reception of single sideband (SSB) voice transmissions. SSB
receivers are essentially frequency converters that linearly convert
radio frequency signals to the audio spectrum where they can be copied
by ear. Virtually all groundstations that access the 1200 baud Pacsat
satellites use SSB receivers and BPSK demodulators for downlink
reception. Uplink transmissions, however, use FSK and a data
encoding technique named after its birthplace of Manchester,
England.
Pacsat satellites operating at a data rate of 1200 bits per second
(BPS) respond to FSK uplink signals that have been Manchester encoded.
Manchester encoding modulates the transmit data with a carrier equal
in frequency to the bit rate (1200 Hz). The resulting product is
actually BPSK with a suppressed carrier frequency of 1200 Hz, although
it is not necessarily handled as such. Manchester code is also known
as "split phase".
In terms of Pacsat communications, signals from a standard packet radio
terminal node controller (TNC) are combined to produce Manchester code
in a phase modulator, the output of which is low-pass filtered and used
to frequency modulate an FM voice transmitter, producing FSK. Manchester
encoding eliminates the DC component of a digital data stream, allowing
it to pass without distortion through the speech circuits of an FM voice
transmitter. A direct connection to the FM varactor diode is not needed
for 1200 bit per second Pacsat communications. Even phase modulated
voice transmitters can be used to generate Pacsat uplinks. The lack of
a DC component also makes a Manchester receiver immune to small changes
in carrier frequency such as those caused by Doppler shift, and allows
for easy bit clock extraction by the receiver.
The Pacsat receivers carried on-board the AMSAT-OSCAR-16, WEBERSAT-OSCAR-18,
and LUSAT-OSCAR-19 satellites were designed around a Motorola MC3362 dual
conversion single chip FM receiver, with one complete receiver chip
employed for each of the four uplink channels on each satellite. A single
GaAsFET-based frequency downconverter with an intermediate frequency (I.F.)
in the 40 to 50 MHz range is shared by all Pacsat uplink receivers.
The first I.F. in each FM receiver chip operates at a center
frequency of 10.7 MHz, and is preceded by a filter with a very sharp
skirt. The frequency response of the filter rolls off by approximately
20 dB at 20 kHz, and achieves a total of 70 dB of adjacent channel
rejection. Impedance matching techniques used on the input and output
of each I.F. filter minimizes passband ripple to approximately one
decibel.
Rather than use a final I.F. frequency of 455 kHz as is typical
in applications using the Motorola MC3362, Pacsat designers instead
used a final I.F. frequency of 1.8 MHz in an effort to realize a
highly linear FM discriminator characteristic across the entire
20 kHz bandwidth of the uplink channel. While Pacsat receivers
employ no automatic frequency control to compensate for Doppler
shift, groundstations employing approximately 3 kHz of peak
frequency deviation need not be concerned with Doppler shift
of their uplink signals.
High speed data communications by satellite is currently no
different than terrestrial high speed data communications, except
for the fact that satellite communications is full duplex, while
terrestrial communications, unless crossband, typically are not.
The same hardware is used in either case, and data is transferred
at 9600 bits per second (also referred to as "9k6") using frequency
shift keying (FSK). Because high speed serial data occupies a wider
frequency bandwidth than audio frequencies typically used for voice
communications, standard FM voice communications equipment can be
used for high speed data communications provided that direct
connections to the receiver's discriminator and transmitter's
varactor diode are made. These connections are needed to ensure
a response characteristic that is flat in terms of frequency and
phase across the entire bandwidth of the receiver. Such characteristics
are a necessity for high quality data communications.
It is interesting to point out that 9600 bit per second FSK
communications require no more RF bandwidth than that required by
1200 bit per second AFSK-FM. In fact, 9600 bps data communications
requires slightly less. However, in order to realize this bandwidth,
the transmitted data must be first processed through a low-pass filter
having a low group delay, and sharp cutoff prior to modulation. In
addition, high-speed data is often "scrambled" to randomize the data
pattern and provide for rapid bit clock recovery in the receiver and
minimize the DC component of the signal, making automatic frequency
control (AFC) both practical and effective.
As stated earlier, 1200 bps FSK data is Manchester encoded
to eliminate the DC component of transmitted data. In doing so,
however, it effectively doubles the bandwidth of the transmitted
signal. This is not a major concern in terms of 1200 bit per second
data communications, but carries a negative impact in terms of 9600 bps
communications. In an effort to conserve bandwidth, a different approach
is taken to reduce the DC component of high speed binary data. Instead
of Manchester encoding, high speed is randomized or scrambled prior to
transmission, and unscrambled during reception. Scrambling significantly
reduces the chances of transmitting long strings of zeros and ones, which
by virtue of their constant and unchanging polarity, carry a strong DC
component. It is the DC component of a digital signal that makes receiver
clock extraction slow and unreliable, data carrier detection difficult,
and receiver automatic frequency control (AFC) nearly impossible.
The Federal Communications Commission allows several forms of
data scrambling to facilitate high speed data and spread spectrum
communications on amateur frequencies. Note, however, that scrambling
is not used to obscure the meaning of the transmitted signal since the
methods used to scramble data on amateur frequencies are well published
and FCC approved for use by amateurs.
The method most often used to scramble high speed data normally
employs a cascaded string of shift registers. 17 such devices are
typically used. Taps made to the output of the fifth and 17th shift
register are combined with the 9600 bit per second data in a pair of
exclusive OR (XOR) gates. During transmit, the shift registers are
driven by a bit clock derived from the sending TNC. On receive, the
process is reversed, and the bit clock is derived from the clock
extraction circuits of the FSK data demodulator. It is interesting
to note that if scrambled data transmissions are monitored on a
narrowband FM voice receiver, the pseudo random nature of the data's
content makes the transmissions sound very much like the white noise
of an unsquelched FM receiver.
Data scrambling is not a panacea by any means. While scrambling
techniques reduce the DC component of serial data transmissions, they do
not eliminate it completely. As a result, a frequency response down to
5 Hz, or preferably DC, is required by the modem and the radio equipment
with which it is used. There is also some concern as to whether scrambling
reduces the bit error rate of a communications channel since an incorrectly
received bit fed into a string of shift registers will corrupt bits received
at a later time. If a corrupted bit is received at the end of one packet,
it can destroy the next packet received, even if the second packet is
received without error. Nevertheless, the data scrambling and unscrambling
technique described here is highly effective and is used by most 9600 bit
per second terrestrial and all satellite data communication links.
The purpose of this discussion was to show that there is a method to
the madness of amateur data communications, and that data transmission
protocols and modulation methods are not subjects that are shrouded in
secrecy. In some areas, modulation methods are selected for simplicity.
In other areas, they are chosen because of their performance superiority
over other methods.
The entire subject of digital communications and the OSCAR satellites
is far too detailed to effectively cover in this column. However, those
interested in a more in-depth understanding of this subject matter
are encouraged to consider reading "Packet: Speed, More Speed and
Applications", published by the ARRL. This book not only covers the
subject of data communications in depth, but also provides block
diagrams and schematics of projects that can further the understanding
of the concepts used to convey digital information on amateur frequencies.
See you on the birds!
Radio Basics 101
Morse: The Essential Language
Coherent Communications and BPSK
Manchester Encoding
Pacsat Receivers
High Speed Data Communications
Pseudo Random Scrambling
Conclusion