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. At the front it has a 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. Application software can now filter out unwanted data records and 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 8 software: http://goldensoftware.com
About bathymetric maps: http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html
Accuracy
Instruments and GPS output NMEA data every 1 or 2 seconds, but not at exactly the same moment. Also, NMEA sentences from the GPS may be marked invalid. 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/lon co-ordinates as received from the GPS are applicable, it is important to know how long ago the latest valid GPS fix was made at the moment of the depth measurement. The elapsed time between GPS fix and depth sounding is a measument for accuracy. When water depth is measured on a moving boat and there has not been a valid GPS fix for say 5 seconds, the co-ordinates are no longer accurate. Therefore, the Brookhouse NMEA data logger automatically includes the elapsed time between the latest valid GPS fix and the depth sounding in each data record. When the data is later processed, records with a larger than acceptable time-difference can be excluded.
The data logger functions are selected with the 2 push-buttons on the front of the unit. After start-up, the operator is asked via the LCD which function is required. The available functions are:
Record.
Upload all recorded data to a computer.
Upload "stamped" soundings in a format for direct plotting of depths in a chart.
Erase all recorded data.
Service mode for loading software.
Recording
Data is written to flash memory and if the data-logger has been switched off, recording resumes at the end of the already recorded data. During recording, GPS co-ordinates or the depth can be displayed on the LCD. The display can be changed by pressing the right push button. This display facility makes it possible to use a sealed active GPS aerial without separate instrument. Each time a data record is created, a beep-sound is generated. This audible signal is a useful confirmation for the operator that the instruments are working and data is recorded. 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 beep-sounds 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, the NMEA 0183 sentences received via the 2 input ports are also sent out via a RS232 output port. 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. The output of this port can be fed into a (laptop) computer, if required and if allowed by circumstances.
Recorded data
Apart from the depth sounding and latitude/longitude, the data recorded includes the time of fix (UTC) from the GPS NMEA data and the elapsed time in tenths of seconds between GPS fix and sounding. The data-fields in the records are variable length, dictated by the output of GPS and depth sounder and are separated by comma’s. Each record is terminated by CR-LF. This data format can be read directly by most popular mapping application programs, such as Surfer.
Uploading
In upload mode, the data logger is connected to a computer via a serial port and all data is transferred via the supplied cable. As a simple ASCII data-format is used, uploading to any computer with any operating system is possible. No special software is required. For example, on a Windows-PC, data can be captured to a disk-file with standard Windows program Hyperterminal. During uploading, the data logger performs a conversion of the lat/lon co-ordinates and adds two fields to each record. The recorded GPS co-ordinates are in degrees and minutes. This is converted to decimal degrees. The co-ordinates in decimal degrees are added to each record. However, the original GPS data is also retained.
A second 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. Stamped records are generated by pressing the right pushbutton during recording.
Hardware specification
The data logger is based on a rugged, very small industrial computer. If the need arises, it can easily be re-programmed via the serial port, in "service mode". Different sizes flash memory modules are available. The standard logger comes with a 8Mbyte module. This provides sufficient storage for approx. 35 hours of recording. 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