105
data throughput, many combinations of such schemes are used. As the number of
keying schemes and the number of frequencies available increase so also does the
data throughput rate. It must be remembered that data throughput rate varies enormously and such high throughput rates are only necessary for very demanding
applications such as streaming video or audio or synoptic satellite imagery. Many
applications such as hourly buoy instrument data rely on simple GSM text messaging (see below).
5.2 Electromagnetic Data Transmission for Coastal Ocean
Observing
Electronic satellite data transmission and relay functions using electromagnetic
means are performed today at frequencies between 300 MHz and 300 GHz, in the
high band radio and microwave spectra. These bands are preferred since they allow
greater data density, directionality, and smaller antenna dimensions. Greater innate
data density is due to the higher frequency. In addition, while VHF-UHF may be
reflected by the ionosphere, the higher frequency microwaves reliably penetrate this
atmospheric layer and are thus essential for satellite communications. Microwaves
propagation is strictly line-of-sight. Obstacles between the Tx/Rx pair will impede
communication. The low wavelength of microwaves (0.1–100 cm) allows the use of
moderately sized parabolic reflectors which impart directionality to the resulting
radiation beam greatly increasing the useful range. Omnidirectional antennae are
used for transmission from the instrument platform source to the receiving antenna.
Microwave relay such as from a satellite to an earth station is achieved using highly
directional feed horn or parabolic reflector arrays.
Microwave oscillators, electronic devices that convert direct current to microwave frequencies, generate the carrier wave at the desired frequency. The first oscillators developed for radar transmission during the Second World War were the
klystrons, bulky vacuum tube device. Much more compact and reliable solid-state
devices incorporating field effect and junction-bipolar transistors are in use today
(Khanna 2006).
5.2.1 Satellites in Terrestrial Orbit as Communication
Platforms
Robust data telemetry, platform positioning, and vehicle tracking are essential for
sustained, autonomous ocean observing. Cellular networks now provide effective
low-cost data telemetry but range limited to about 35 km restricts this application to
nearshore areas. Satellites in earth orbit on the other hand offer widespread data
coverage at moderate cost. The era of satellite communications dawned with the
1962 launching of the TELSTAR satellite that for the first time provided an extended
5.2 Electromagnetic Data Transmission for Coastal Ocean Observing
data throughput, many combinations of such schemes are used. As the number of
keying schemes and the number of frequencies available increase so also does the
data throughput rate. It must be remembered that data throughput rate varies enormously and such high throughput rates are only necessary for very demanding
applications such as streaming video or audio or synoptic satellite imagery. Many
applications such as hourly buoy instrument data rely on simple GSM text messaging (see below).
5.2 Electromagnetic Data Transmission for Coastal Ocean
Observing
Electronic satellite data transmission and relay functions using electromagnetic
means are performed today at frequencies between 300 MHz and 300 GHz, in the
high band radio and microwave spectra. These bands are preferred since they allow
greater data density, directionality, and smaller antenna dimensions. Greater innate
data density is due to the higher frequency. In addition, while VHF-UHF may be
reflected by the ionosphere, the higher frequency microwaves reliably penetrate this
atmospheric layer and are thus essential for satellite communications. Microwaves
propagation is strictly line-of-sight. Obstacles between the Tx/Rx pair will impede
communication. The low wavelength of microwaves (0.1–100 cm) allows the use of
moderately sized parabolic reflectors which impart directionality to the resulting
radiation beam greatly increasing the useful range. Omnidirectional antennae are
used for transmission from the instrument platform source to the receiving antenna.
Microwave relay such as from a satellite to an earth station is achieved using highly
directional feed horn or parabolic reflector arrays.
Microwave oscillators, electronic devices that convert direct current to microwave frequencies, generate the carrier wave at the desired frequency. The first oscillators developed for radar transmission during the Second World War were the
klystrons, bulky vacuum tube device. Much more compact and reliable solid-state
devices incorporating field effect and junction-bipolar transistors are in use today
(Khanna 2006).
5.2.1 Satellites in Terrestrial Orbit as Communication
Platforms
Robust data telemetry, platform positioning, and vehicle tracking are essential for
sustained, autonomous ocean observing. Cellular networks now provide effective
low-cost data telemetry but range limited to about 35 km restricts this application to
nearshore areas. Satellites in earth orbit on the other hand offer widespread data
coverage at moderate cost. The era of satellite communications dawned with the
1962 launching of the TELSTAR satellite that for the first time provided an extended
5.2 Electromagnetic Data Transmission for Coastal Ocean Observing