Real-time Drifter/Mooring:
Monterey Bay, California
Recent Drifter can being readied for Deployment
The importance of time series in oceanographic research is clear. Time series measurements of physical and meteorological properties, currently taken throughout the globe, have allowed investigators to resolve the important scales of oceanic and atmospheric variability. With a few exceptions the current time series are primarily physical in nature (i.e., temperature). Biological and chemical oceanographers are now looking to continuous observations of biological and chemical properties so they can also determine the spectrum of variability in these fields and when taken concurrently with the physical and meteorological observations determine the relation to climate and ocean variability. Spatial coverage will ultimately come from observations made from space, but high-frequency temporal and added vertical coverage will need to come from moorings and drifters with arrays of in-situ sensors. The paucity of biological and chemical time series has been due, in part, to the lack of this type of instrumentation, however, increased effort has recently been placed on the development of chemical and bio- optical instrumentation for the collection of these time series. Realizing that advances in ocean sciences are limited by the lack of instrumentation and systems capable of collecting these time series, the Monterey Bay Aquarium Research Institute (MBARI) has established a vigorous developmental program geared at making these observations possible.
One of the goals of the program was to develop a new set of control electronics and software that would allow for the collection, storage, and telemetry of data from any of a wide range of scientific instrumentation. The SOON Drifter is the focus of the present contribution. Additional impetus for designing such a system were the increased need for real-time environmental information, the need to easily add new instrumentation as it came forward and the need to test new instrumentation with rapidity and in an environment that was well documented with respect to other properties. The ability to add devices is key when new instrumentation is being developed at a rapid rate. Testing of new instrumentation is best carried out on an easily accessible platform where a basic set of observations are routinely taken (temperature, salinity, backscatter, CO2....) so that data from the new instrumentation can be readily analyzed and interpreted. The advantages of real-time telemetry are several-fold. It allows for quality control of data so as not to lose long, expensive drifter/mooring deployments. The data is immediately available for analysis, assimilation into models, and calibration of satellite sensors. The information provided in real-time by the system can be of tactical use for shipboard experiments, especially those geared at specific events.
Objectives
The initial scientific and technical objectives were:
1) Establish a a platform for the deployment of a set of unattended sensors and samplers.
2) Design a controller that collects data from scientific instrumentation via multiple interfaces and then telemeters the information real-time.
3) Make continuous observations of physical, chemical and biological properties in Monterey Bay and other regions' (Chile, Peru, EqPac, S. Atlantic,...) ecosystems so as to describe time-varying aspects with increased resolution and over long periods of time.
4) Contribute to the improvement of mooring and unattended sensor technology, with emphasis on biological and chemical properties.
5) Provide calibration for properties sensed from space.
System Design
The engineering objective of the drifter/mooring project was to develop a new set of control electronics and software that would allow for the collection, storage, and telemetry of data from any of a wide range of scientific instrumentation. Considering the rapid rate of instrument development it was important to design a system capable of multiple interfaces. A general-purpose controller was designed so that it could interface to scientific instrumentation via any anticipated method, including:
- RS 232 (110 to 38400 baud);
- Analog voltage;
- Digital logic (5 volt TTL or CMOS);
- Frequency measurements, including pulse modulation;
- Power control (turn device on/off);
The controller, centered around the Tattletale 8 (from Onset Computers) which has a Motorola 68332 chip acquires data from the configured sensors, stores the data as required, and telemeters the data back to shore via the Orbcomm satellite interface. The controller is easily extensible and programmable. By extensible it is meant that new devices can be easily added. It is programmable, allowing the user to change parameters such as sample rate, interface parameters and telemetry parameters.
The design of the board considered, in addition to extensibility, low power and reliability. Gernot Friederich has the specifications of the power budget.
The controller schedules a real time clock to put it to sleep and wake it up for its next task. There are 8 power slots (12V), 8 Serial Ports, and analog board connections
The Tattletale Controller, Licor CO2/Gas Analyzer, Orbcomm Modem, batteries, humidity sensor, and battery voltage sensor all reside in a PVC tube of 6" by about 32". Other instruments are attached via bulkheads located at the top of the drifter/mooring.
Software
There are two major software efforts for the drifter/mooring project. One effort consists of the code on the controller board on the mooring and the second involves the software used for the shore-side data analysis. On the drifter/mooring the programs running on the 68332 are written in C. The core of the code is a multitasking scheduler that controls wake up and interfaces with a series of drivers. There are drivers for the user interface, Orbcomm Satellite modem and each instrument. In this sense the programming is modular, so that if a new instrument is to be added, a new driver is developed and added the code. The current C code essentially schedules two different independent loops to run. One is called the telemetry loop, and the other is the sampling loop.
A typical set of operations is as follows. The clock wakes up the controller at a predetermined time. The code samples from the pre-configured instrumentation suite and stores the data in flash memory. If it's time to send the sample(s) via the Orbcomm interface, the desired sample(s) are given to the modem for transmission. Once communication is established with a satellite, messages are shipped via email in a first-in, first-out method - insuring that we get the latest data first. The data packets from the controller can be compressed to reduce the size of the files and therefore reduce satellite transmission time.
The collection of data from shore is automated by software running on a Hewlett-Packard workstation. The software is combination of VBScript, C executables, and cron jobs. A cron job is initiated every 15 minutes to request data from the "drifters@mbari.org" Exchange account. The VBScripts request data from the Exchange Server and then initiate a series of C programs that extract and decode the data files These programs decompress and append the data to daily files ASCII in drifter-specific directories.
Instruments and their interface
The majority of instruments are interfaced via RS-232 and a few are analog. The most common serial interface consists of power, receive, transmit and ground. Because each instrument has its own particular attributes each interface is different and some require more software than others.
The instruments currently available are: Licor CO2 Gas Analyzer (LI820 and LI800), air pump, Wetstar Fluorometer, Quake Orbcomm Modem (Q1500 and Q1400), HydroRad, SpecPRR , shutter, HydroSCAT, and CTD (3 diferent types). The interface characteristics of some of the core instruments are described in the software code section.
Telemetry
The telemetry system used by the SOON-Drifter/Mooring project is based on Orbcomm's Satellite Telemetry System. See the Q1400 and Q1500 for details.
The Quake modems use a packet based protocol over a 9600 baud serial interface. All messages are essential pushed and polled from the Tattletale controller.