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MIT 2 693 - Lecture 1: Introduction to Class

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2.693 Oceanographic Instrumentation Lecture 1: Introduction to Class Jim Irish 7 February 2006 Oceanography is an observational driven science. That is, nearly all our knowledge of the ocean and the processes that occur there comes from observations made in the ocean. Therefore, this class will provide some guidance on information that you will need to make quality observations in/of the ocean. Objective of an Ocean Observing Program (experiment): 1. Describe the statistics of the ocean - • Mean temperature at a place, • Maximum and minimum temperature during a year, • Variance of currents at a location, • Covariance or correlation between temperature and atmospheric winds, • The average vertical density gradient in the North Atlantic, • The power spectral density of the internal wave elevation in the open ocean, • The tidal component of the sea surface elevation, etc. 2. Therefore, we need observations. To observe the ocean we need – • Platforms from which to make observations • Sensors to make measurements, • Data conditioning circuits, • Recorders with sampling programs in time or space and in quantity measured, • Power supplies (batteries, solar panels), • Storage media, • Analysis (processing) – such as calculating the density gradient and normalizing this to the Brunt Vaisala frequency, N² = g/ρ ∂ρ/∂z, from measurements of temperature, conductivity as a function of pressure (depth) using the Practical Salinity Scale of 1978 and the Equation of State of Seawater 1980) to get the potential density and estimate the scaled gradient. 3. Need to know what results you want to be able to design an experiment, select sensors, plan sampling (in both time and space) to get the required statistics/description. 4. All of these processes can be through of as “coloring” the data that you get – you are looking at the ocean through colored glasses, and you need to know what effects each of the steps has on the data, to assure that you are getting the statics that truly represents the ocean. This is what this class is about. To good information from and experiment you need to: 1. Place a sensor (transducer) in the ocean – Here you the need a platform from which to make the measurement – on a surface buoy or ship, on a mooring line, on a cable lowered from a ship, mounted on a bottom tripod, on an ROV, AUV, neutral buoyant float, satellite, aircraft, etc. 2. A transducer which changes the environmental signal (e.g. temperature) into something we can measure (e.g. resistance in the case of a thermistor). Note thisis a critical point. If you mess up here or in the next couple of steps, you cannot recover lost information! 3. Signal conditioning – A sensor generally consists of a transducer and signal conditioning. A transducer’s output needs further processing before it can be recorded. This may me a regulated voltage supply with a bridge with the thermistor in one leg, an operational amplifier that changes the resistance of the thermistor into a voltage for digitizing. At the same time the circuit can have a filter in it to remove unwanted high frequency signals (to prevent aliasing) so that only low frequency signals of interest remain, (and the sampling and storage requirements are simplified/reduced). 4. Sampling – this is making a measurement at discrete points in time for a finite record length. (This determines what frequencies you can obtain information about from your data). Also the sampling (e.g. an A/D converter) quantifies the signal into discrete intervals – e.g. temperature is digitized so that one count is equal to 1 millidegree C. The with a 16-bit digiter, the A/D will be off scale with its maximum value at 16.383 ºC. Therefore, you will need to make the resolution smaller or increase the number of bits in the A/D to cover the full oceanographic temperature range. Note that the size of the number from the A/D puts requirements on how many digits are recorded and therefore the size of storage needed. You should consider how many digits to digitize a measurement (you don’t want too much resolution that is unused) and the range of the digitizer so you cover the range of variations in the ocean you are measuring. 5. Internal processing – this is possible with the large selection of low power, high capability microcontrollers available now. This could consist of data compression by taking Fourier transform, averaging in frequency space and saving the results (much smaller space required). Filtering the data to eliminate some frequency, or to pick out some frequency (such as a tsunami). Calculating statistics and saving these rather than the time series. Conditional sampling that will alter the sampling rate depending on environmental signals present, so that you only sample occasionally when nothing exciting is happening, and then more rapidly during a high energy event. 6. Data storage. This can be internal to the instrument or transmitted to another location for storage. Data can be stored in the instrument on RAM, compact flash cards, hard disks, or EEPROMS. It can be stored elsewhere by sending the data over a radio link to shore, or a over wire to a ship for storage. For example: • A CTD sends its data up the wire to the ship using FSK techniques where it is stored on a PC on hard disk. • An offshore buoys sends data to shore via: a. GOES (Geostationary Operational Environmental Satellite) in GLOBEC on Georges Bank and currently by GoMOOS – The Gulf of Maine Ocean Observing System –www.gomoos.org in buoys far from the coast. The data capacityof GOES is limited to 1 m at 100 baud per hour. b. ARGOS (Polar orbiting satellite which also provides platform position for finding a buoy should it break loose from its mooring). Argos isalso used for tracking drifters, but only returns about a dozen messages of 32 Bytes each per day. c. Iridium satellite or the several LEOs (Low Earth Orbiting satellites) which provides two-way communications (at 4800 Baud) between a remote platform and the scientist (but is relatively expensive). d. Spread spectrum radio (900 MHz or 2.4 GHz line of sight), which also provides two-way communications to shore or ship. This can be used to download control files or even new software to the remote system. However, you need to provide the shore based station e. Cellular phone (as


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