Tag: GPS

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Mavis Peterson, June 28, 2005

NOAA Teacher at Sea
Mavis Peterson
Onboard NOAA Ship Fairweather
June 21 – July 9, 2005

Mission: Hydrographic Survey
Geographical Area: North Pacific, Alaska
Date: June 28, 2005

Weather Data 

Lat.:55 o7.2 N
Long.: 160 07.4W
Visibility:  1
Wind direction: 123
True wind speed: .9 knts
Swell wave height: 1
Sea water temperature: 9.15 C
Dry bulb 19.8
Wet bulb 9.0
sea level pressure: 1011.5
Cloud cover and type: cumulus overcast

Science and Technology Log 

I was fortunate to sit with some of the crew at the breakfast table this morning and was able to take part in a discussion regarding what we were doing today.  This gave me an opportunity to ask some questions.  I’m getting the idea of the science that is taking place here, but conversation today helped me understand the connections that I have been missing.  For example there are about eight programs that are used on board to gather and process the data. There are four important data gathering devices that are merged together: The exact time, the GPS location, the motion of the ship and the sonar.  Interestingly, as in many computer programs there is a “Bug” that they call the “Midnight Bug” that causes them to sometimes, not always, lose data for about half an hour.  The question is whether to stop and circle around and pick up what was missed or to continue. There are pros and cons and are decided by those colleting the data.  The information gathered on this ship is processed quickly.  This is an advantage because if there is an error or discrepancy the ship has not already moved out of the area so they can adjust or redo immediately.  Of course this allows for accurate information to be gathered.

An aside on time:  All NOAA ships use the same Greenwich Time no matter what time zone they are in. This saves confusion when crossing time zones.  Midnight here, in this time zone is 4:00 in the afternoon.  That is then the beginning of a new day.  There are three ways the ship can gather bottom data. (1.) When the main ship is “hydroing” as we will be doing for the next few days, 24 hours a day the ship is sonarring the bottom in a wide swath in deep water. (2.) When the water is too shallow for the ship to hydro that is when the launches are sent out to do basically the same thing, although they use less power because the water is not as deep.  (3.) The third way of collecting information is by shoreline observations, using the flat-bottomed small boat and physically eyeballing the area, taking notes and pictures and entering that data into the programs when they return to the ship. I discovered today that although all the ships in the fleet that are doing the same type of science use the same programs, they may not be using them in exact manner.  I would liken this to the example that although all fourth grade teachers must meet the same state standards of curriculum, they don’t all approach the task in the same way. An example is how the scientists “draw lines” of an area to be hydroed.  The FAIRWEATHER marks off polygon areas that need to be scanned.  This can be done in any fashion, across-up and down or in any pattern as long as the whole area gets covered. Other ships opt to draw in lines to follow in a selected area.  If they can’t follow the lines because of swells, or whatever, then they are out of luck as far as surveying that day. However because we are scanning the channel with the ship today, we are following lines.

I am going to observe a training demonstration at 1:00 P.M. in the boiler room and then tour the boiler room.  The training session went as planned.  It was how to use the oxygen mask and how and when to use the fire extinguishers.

Personal Log 

I spent a couple of hours listening and asking questions of those present, and then I worked on some lesson plans after lunch.  I also spent a little while at the bridge.  It is a beautiful day, bright and sunny. I will be going out on one of the smaller boats tomorrow.  I spent the rest of the day answering emails and working on lesson plans.  These are pretty heavy concepts for fourth graders, but I am getting some ideas simplified to their level.

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Mavis Peterson, June 23, 2005

NOAA Teacher at Sea
Mavis Peterson
Onboard NOAA Ship Fairweather
June 21 – July 9, 2005

Mission: Hydrographic Survey
Geographical Area: North Pacific, Alaska
Date: June 23, 2005

Scientific Log 

This is my first day out on the launch.  The computer on the launch stays there all the time and is loaded with the programs that are needed.  The sonar scanning devices, GPS and radar are also on the launch. The launch makes sweeping rows across the chosen area to be scanned overlapping each row slightly to prevent “holes” in the information gathered. The operators keep a close watch on the depth and if it gets too shallow, they leave that area for the smaller shore boats to gather information on.  I am learning so much, and trying to decide how I can share this information with my fourth graders.  Surely reading maps and following directions accurately will be lessons.

Personal Log 

The sea air has gotten to me in more ways than one.  Not only did I feel the wave action this afternoon, but upon returning, I am very cold so I am retiring early tonight after writing some notes, and checking out tomorrow’s schedule.

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Leyf Peirce, July 7, 2004

NOAA Teacher at Sea
Leyf Peirce
Onboard NOAA Ship Rainier
July 6 – 15, 2004

Mission: Hydrographic Survey
Geographical Area: Eastern Aleutian Islands, Alaska
Date: July 7, 2004

Time: 10:15
Latitude: N 57°31.730
Longitude: W 154°58.325
Visibility: 10 + m
Wind direction: 250
Wind speed: 18 knots
Sea wave height: 2 – 3 feet
Swell wave height: 2 – 4 feet
Sea water temperature: 10.6 °C
Sea level pressure: 1020.1 mb
Air temperature: 12.2 °C
Cloud cover: 2/8

Science and Technology Log

I talked more with P.S. Shyla Allen about how the multibeam echo sounders work on the ship to gather data about the depths of the ocean. Both the RAINIER and the launch ships use the following method to gather data. All of these vessels use echo sounders with anywhere from 120 to 240 beams that scan the ocean floor. The following diagram illustrates how this is done:

Peirce 7-7-04 Fig1
Figure 1: Multibeam Echo Sounding

Here, “z” is an echo sounding two-way travel time beam, and the multibeams are spread over the footprint distance of “f”. The size of the sound footprint, “f”, depends on the depth at which the measurement is taken, “z”. The greater the depth is, the greater the footprint is. However, the greater the footprint is also means less accuracy on the outer edges of the footprint. Therefore, the ship will run a “mowing the lawn” pattern across the given section to get desired overlapping of data:

Peirce 7-7-04 Fig2
Figure 2: Mowing the Lawn pattern

The width of these lines is determined by: width of x = 3 * z. By using this rough equation, the ship will be able to overlap the areas of least accuracy, i.e. the areas on the outer range of the footprint:

Figure 3: Ship running mowing the lawn pattern so the footprints overlap.

From this data, the depth and contours of the ocean floor can be determined. I also asked P.S. Shyla Allen about the problems and sources of error associated with this data collection. She responded by detailing three main issues that must be corrected when cleaning the data, i.e. the data must undergo three main correction factors before accurate readings can be analyzed. These three factors include: a) tide changes, b) sound velocity, c) the motion of the ship and GPS positioning. To correct for tide changes, the researchers must have accurate readings of the tides. Tide gauges are installed along the coastline at various points, and all readings are reduced to Mean Lower Low Water (MLLW). This basically gives the average of the lowest possible depth at a given location. To correct for sound velocity changes, which is the most important correction factor dealt with, researchers take measurements of water temperature and salinity level at the given depth reading. For every change of 1 ppm in salinity, there is a change of 3 m/s in sound velocity. Therefore, salinity is perhaps one of the most important factors. Finally, the motion of the ship and GPS position need to be corrected for. This includes correcting for the pitch, roll, and gyration of the ship as well as error in the GPS system. Because the ship uses Differential GPS (DGPS), this error is already accounted for. However, for the pitch, roll, and gyration of the ship, two antennas are used to on the port and starboard sides. These antennas, often referred to as Motion Reference Units (MRU), are very stable feed into the same computers that process the data. Therefore, the computer takes into account the readings from these antennas and combines this information with the corrections made for the tidal changes, sound velocity factors, and positioning of the ship. After cleaning the static from the data, a nautical chart can be produced. This method of charting the ocean floor is definitely more efficient than when researchers used lead lines—long ropes with lead that would be dropped down and then measured to determine the depth!

Personal Log

I woke up this morning after sleeping for about 12 hours—I think the seasickness medicine I took last night made me very sleepy. Luckily, however, all traces of seasickness are gone; I can even sit here at the computer and type without noticing the pitching of the ship very much at all. I think all of my muscles must be getting stronger as a result of reacting to the changing ground and all of the stairs I go up and down every day. I spent some time on the bridge this morning mostly asking questions about the tools used there and what various measurements mean. I find it very interesting that simply reading tiny numbers and using small switches and knobs will run this 231 foot ship. However, my experience aboard ships tells me that it is not even close to impossible. I know that even the slightest adjustment at the helm on a sailboat can change the course of the boat. I am reminded of sailing in the British Virgin Islands and the dispute over if it was more important to maintain the way point or try to make the boat go very fast. However, that is not an issue on this boat. We are supposed to reach the Shumagin Islands tonight, and tomorrow we will start the launches—I can’t wait!

Question for the Day

How many sets of data points must be filtered out before the data is considered clean? On what does this number depend? How does one determine if a data point is an outlier or and actual reading?