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Rugged Brookhouse NMEA 0183 Data Logger
General The Brookhouse NMEA data logger is a compact device for recording NMEA data such as water-depth, coupled to GPS co-ordinates. The data-logger has been designed specifically for recording data used as input for the creation of bathymetric maps, but other applications are possible. When surveying, circumstances often do not allow the use of a (laptop) computer or other electronics with complicated operating procedures. Simplicity is a key requirement. The Brookhouse NMEA data logger is small, rugged and very simple to operate. It has an LCD (Liquid Christal Display) and a sealed membrane keypad. There are 2 input ports for the connection of NMEA instruments: A GPS can be connected to one port and a depth sounder to the other. NMEA data is simultaneously read via the 2 ports and records are created with water-depth and latitude/longitude of the vessel’s position. It does not matter which NMEA data is input via which port and data may also be combined on one port. After all required data has been recorded, it can be downloaded to a computer. During the download process, extra computations are performed, such as corrections for tide, and unwanted data records can be filtered out. The next step is to create a regular grid from the randomly dispersed data, using interpolation techniques. From this regular grid data a contour map can be created.
The following links are useful for further information on bathymetric maps, contour maps and software to generate regular grids and maps from data, collected with the NMEA data logger. Surfer software: http://goldensoftware.com About bathymetric maps: http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html
Compatible GPS units and depth sounders Any GPS and depth sounder that outputs standard NMEA 0183 sentences at baudrate 4800 can be used as data source for the data logger. The data logger can also be connected to an integrated instrument system at 4800 or 38400 baud, e.g. to the output port of a Brookhouse NMEA multiplexer or to the output of a chartplotter/ fishfinder. The required quality and accuracy of the of the equipment depends on the application. If high accuracy positioning is required, a DGPS unit (differential GPS) will give best results. GPS position, GPS mode, HDOP and the depth can be displayed on the data logger's LCD. As no separate instrument is required to view the most important data during surveying, an active depth transducer and a sealed active GPS aerial can be used. This simplifies the equipment setup on small boats and is a cost-effective solution. GPS units based on e.g. SiRF StarIII chip technology provide good accuracy at low cost and good quality depth transducers that output NMEA 0183 are available from several manufacturers.
Elapsed time between GPS fix and sounding GPS's and depth sounders usually output NMEA data once per second, but not necessarily at exactly the same moment. If data is recorded aboard a moving boat, the latest received valid GPS co-ordinates may therefore be too old to be used as position data for a depth sounding. To determine if the latest lat/long co-ordinates as received from the GPS are applicable, it is important to know how long ago the latest valid GPS fix was received at the moment of the depth measurement, i.e. the elapsed time between GPS fix and depth sounding can be considered a measurement for accuracy, together with the GPS HDOP. For example, if the boat travels at 6 knots and the elapsed time between GPS fix and depth sounding is 1 second, the recorded lat/long would have an error of 3 meters. Depending on the application, this inaccuracy may be unacceptable. Therefore, the Brookhouse NMEA data logger computes the elapsed time in tenths of a second between the latest valid GPS fix and the moment of the depth sounding as well as the elapsed time between the moment op the depth sounding and the next GPS fix. Depending on which of the two times is the shortest, the lat/long of the preceding or following GPS fix is than paired with the depth and stored in the same record. The elapsed time is also included in the data record. When the data is later downloaded to a computer for further processing, the user can specify a maximum allowable elapsed time and records with a larger than acceptable time-difference can be filtered out. This is an important feature. Even if the most accurate DGPS and sounder are being used, the results can be inaccurate if this time factor is not taken into account.
Corrections for tide A bathymetric map based on a survey of tidal water that has spanned several hours can be very inaccurate if the depth soundings are not adjusted for tide. During the download process from the data logger to a computer, the data logger can optionally carry out sinusoidal tide computations and adjust the depth soundings accordingly. When the download is started, the time read from the first record is displayed and the user is prompted to enter the closest tide (high or low) and height before and after this time. The tidal adjustment is computed and applied for each subsequent record that falls in the tidal time range. When the time falls outside the range the user is prompted again (with audible warning). The original depth, adjustment for tide and adjusted depth are all included in the output records, so that the results can be checked. Most tide tables are based on local times, not UTC, while the GPS time is UTC. When the download is started, the data logger prompts the user to enter the local time offset and times are subsequently displayed in local time, which relates to the times in the tide tables.
Another adjustment the user is prompted to enter when a download is started is the transducer offset, i.e. the vertical distance in centimeters between the depth transducer and the water surface. The filtering and various adjustments ensure that the downloaded data is clean and accurate as possible and can be input directly by the application software such as Surfer, for best possible results. Waypoints During the download process, the data logger can also create waypoints in the standard $GPWPL format. The waypoint name consists of the word Depth: followed by the depth in meters at that location and stamp. The waypoints are output at 4800 baud from port 2 of the data logger. The user can specify if waypoints are selectively required for records with a certain "stamp" or for all stamped records. A stamp is a single keypad ASCII character added to the next record during recording if a keypad key is depressed. Stamps provide a convenient way of coding events or observations during recording. For example, stamp "1" may be used to indicate the presence a navigational buoy at the current location. Waypoints made from the stamped records can be used for various purposes. They can later be used for navigation purposes or to display depths in an existing electronic chart. For example, if observations are made that require closer inspection at a later time the waypoints of those locations can be uploaded to a GPS or chart plotter for easy navigation when returning to the locations later. The data logger functions are selected with the keypad keys * and #. After start-up, the user is asked via the LCD which function is required. The available functions for the bathymetric logger are:
Recording Data is written to flash memory. If the data-logger has been switched off, recording resumes at the end of the already recorded data. During recording, GPS position and the depth can be displayed on the data logger's LCD. The display can be changed by pressing the * key on the keypad. As no separate instrument is required to make the most important data visible, an active depth transducer and a sealed active GPS aerial can be used as data sources. Each time a data record is created, a short audible signal (beep) is given. This is a useful confirmation for the operator that the instruments are working and data is being recorded without constantly having to watch the LCD. GPS fixes with the valid flag not set are ignored. If no valid GPS fixes have been received for 30 seconds, recording stops, no more beeps are produced and the operator knows that something is wrong. This will prompt him to check the GPS and connections of GPS and sounder to the recorder. While recording, any NMEA 0183 sentences received via the 2 input ports are also transmitted from port 1. The NMEA data, which can be any NMEA sentence in any format, is output exactly as it has been received from the connected instruments. This feature provides a 2-channel NMEA Multiplexer functionality, which can be useful in certain system configurations. The recorded data records contain the following fields, separated by comma's (CSV format). Each record is terminated by CR-LF.
Although this raw data can be uploaded to a computer 'as is' for further processing, it is advantageous to use the special data logger upload function that contains important data preprocessing features. Upload with preprocessing In upload mode, the data logger is connected to a computer via serial port 1 and all data is transferred via the supplied cable. As the simple ASCII data-format CSV (Comma Separated Values) is used, uploading to any type of computer with any operating system is possible. No special software is required. For example, on a Windows-PC, data can be uploaded and stored in a disk-file with program Hyperterminal or any other terminal program with text capturing ability. All raw data fields are uploaded and in addition, the data logger performs the following functions:
The resulting extra data fields are added to the original raw data records during the upload process, so that both the original raw data and the corrected/adjusted values are available for further processing. The resulting records as received by the computer contain the following fields:
Upload of raw data only (fast upload) All data is compressed and transmitted as a .zip file. A program with zmodem protocol is required. This protocol is available in Hyperterminal. Upload of stamped records in Oziexplorer format. An extra upload-function is available, whereby only records that are "stamped" are transferred, in a format suitable for direct plotting of the soundings in a chart, as an Oziexplorer "points file". Stamped records are generated by pressing one of the keypad-keys during recording.
Hardware specification The data logger is based on a rugged, very small industrial computer. If the need arises, software updates can be uploaded in the field via the serial port, in "service mode". Different sizes flash memory modules are available. The standard logger comes with a 512 Mbyte module. This provides sufficient storage for over 1000 hours of recording without uploading. The logger is powered by 12V DC. Power consumption is 6W. Summary:
Pricing The Brookhouse NMEA data-logger is reasonably priced. It is possible to customize the hardware and software. Discuss your requirements and request a quotation by sending an email to sales@brookhouseonline.com
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