104
(NCEP) collect this GTS data and format it into BUFR (Binary Universal Form for
the Representation of meteorological data) for their processing needs. After processing, the BUFR is transmitted to the National Center for Atmospheric Research
(NCAR) for archival and additional processing into the International Maritime
Meteorological Archive (IMMA) format. The IMMA format has been adopted by
many organizations as the preferred format for marine observations. The data consists of basic observations taken from ships, buoys, C-MAN (Coastal Marine
Automated Network), and tide-gauge stations. Observations may include air and
sea surface temperature, wind direction/speed, waves, sea level pressure, etc.
All such transmissions are performed by superimposing digital signal data upon
established radio or microwave carrier frequencies through electronic modulation,
an integral part of the signal conditioning process.
Radio AM and FM
Readers are probably acquainted with analog audio radio amplitude modulation (AM) and frequency modulation (FM). In the former, the amplitude of a
high power carrier wave passing across a transistor at a fixed frequency is
modulated by applying the small microphone signal to the gate acting as an
electronic valve to modulate the amplitude of the high power carrier wave.
The carrier signal is applied to the transistor source and transmitted from the
drain to the antenna. In the more complicated FM circuit, the amplitude is
invariant but the frequency is modulated. Since it is the frequency that is modulated, a spectral band of some width is required. For commercial analog FM
radio transmission in the USA between 87.8 MHz and 108.0 MHz, for example, individual station band width is set at 0.2 MHz allowing only 101 stations
across the effective 20.2 MHz FM band.
Digital data transmission makes use of frequency modulation schemes. Some
internal data recording CTD instruments, for example, use frequency shift key modulation (FSK) to generate hexadecimal data (a numeral system with a 16 digit base)
for rapid instrument data download (the ASCII representation of hexadecimal
numerals is a set of numbers (0–9) and letters (A-F) adding up to a total of 16 characters). More commonly, data is transmitted as binary code in sequences of ones
and zeros. For such applications, a four-key FSK scheme (i.e., four different adjacent keyed frequencies) can be used to represent two bits per key (00, 01, 10, 11)
thus increasing data density by a factor of 2.
Carrier wave phase is an additional property of electromagnetic radiation that
can be exploited for data transmission in addition to amplitude and frequency.
Various schemes are available for phase shift key (PSK) modulation. Amplitude
shift key (ASK) modulation is possible, but AM radio bands are notoriously noisy
and are avoided for radio transmission in data telemetry. Nevertheless, some optical
data transmission modes through fiber-optic cable do implement ASK. As the object
of data transmission is to transmit data with a minimum of error while maximizing
5 Signal Conditioning, Data Telemetry, Command Signaling and Platform Positioning…
(NCEP) collect this GTS data and format it into BUFR (Binary Universal Form for
the Representation of meteorological data) for their processing needs. After processing, the BUFR is transmitted to the National Center for Atmospheric Research
(NCAR) for archival and additional processing into the International Maritime
Meteorological Archive (IMMA) format. The IMMA format has been adopted by
many organizations as the preferred format for marine observations. The data consists of basic observations taken from ships, buoys, C-MAN (Coastal Marine
Automated Network), and tide-gauge stations. Observations may include air and
sea surface temperature, wind direction/speed, waves, sea level pressure, etc.
All such transmissions are performed by superimposing digital signal data upon
established radio or microwave carrier frequencies through electronic modulation,
an integral part of the signal conditioning process.
Radio AM and FM
Readers are probably acquainted with analog audio radio amplitude modulation (AM) and frequency modulation (FM). In the former, the amplitude of a
high power carrier wave passing across a transistor at a fixed frequency is
modulated by applying the small microphone signal to the gate acting as an
electronic valve to modulate the amplitude of the high power carrier wave.
The carrier signal is applied to the transistor source and transmitted from the
drain to the antenna. In the more complicated FM circuit, the amplitude is
invariant but the frequency is modulated. Since it is the frequency that is modulated, a spectral band of some width is required. For commercial analog FM
radio transmission in the USA between 87.8 MHz and 108.0 MHz, for example, individual station band width is set at 0.2 MHz allowing only 101 stations
across the effective 20.2 MHz FM band.
Digital data transmission makes use of frequency modulation schemes. Some
internal data recording CTD instruments, for example, use frequency shift key modulation (FSK) to generate hexadecimal data (a numeral system with a 16 digit base)
for rapid instrument data download (the ASCII representation of hexadecimal
numerals is a set of numbers (0–9) and letters (A-F) adding up to a total of 16 characters). More commonly, data is transmitted as binary code in sequences of ones
and zeros. For such applications, a four-key FSK scheme (i.e., four different adjacent keyed frequencies) can be used to represent two bits per key (00, 01, 10, 11)
thus increasing data density by a factor of 2.
Carrier wave phase is an additional property of electromagnetic radiation that
can be exploited for data transmission in addition to amplitude and frequency.
Various schemes are available for phase shift key (PSK) modulation. Amplitude
shift key (ASK) modulation is possible, but AM radio bands are notoriously noisy
and are avoided for radio transmission in data telemetry. Nevertheless, some optical
data transmission modes through fiber-optic cable do implement ASK. As the object
of data transmission is to transmit data with a minimum of error while maximizing
5 Signal Conditioning, Data Telemetry, Command Signaling and Platform Positioning…