Accuracy of Niu's coordinates
written by Evan Rapoport , May 16, 2008

People often ask us about the accuracy of Niu's plotted coordinates. This is a great question with a very lengthy answer. I'll give you the short version and some places to go if you're curious about the longer version. Believe it or not, as long as this is, this is the short version! Read more if you're interested...

We have two ways to compute Niu's coordinates: GPS satellites and Iridium communication satellites. In the ideal world, all of Niu's coordinates would be based on the GPS because the accuracy can be within about 10 meters (30 feet). Sometimes the rough conditions at sea make it impossible to connect with enough GPS satellites to get a good reading, which can reduce the accuracy to about 100 meters (300 feet) or higher. But that's not the whole story...

For a GPS reading, Niu must connect to at least three GPS satellites. If this isn't possible, we use our fallback plan: the Iridium communication satellites. Unfortunately, we've had to use these coordinates more often than we hoped (the next version of Niu, already being designed, will address these issues). When Niu sends its message to these satellites, they compute an estimate as to where the message was coming from (in other words, where Niu is located). These coordinates are provided along with a "computer error probability radius", or CEP radius. For example, this message might tell us, "There's an 80% probability that Niu is somewhere within a circle whose center is at [22.15205, -158.2968] and whose radius is 5 kilometers". The "80% probability" is a fixed number associated with Iridium's calculation, which is pretty complex.

So, we always hope to get the smallest CEP radius possible from the Iridium satellites (1 kilometer) because that gives us the best accuracy. But in reality, these numbers typically range up to about 10 kilometers (about 6 miles). Yikes! If we plotted these coordinates on the map, the errors in the data would make it look like Niu was zig-zagging all over the place (a lot more than what the wind and currents can do). But don't fear, math is here!

With some rather straightforward calculations, we are able to improve the accuracy and show a more realistic path on the map. This is done using a "weighted average" in which each point on the map is actually an average of the several points that preceeded it. Because points with a smaller error radius are more accurate, we give them more "weight" in the average than points with a bigger error radius. Once we've balanced all the numbers so that they have weights that correspond to their accuracy, we then weight them again so that the most recent points have more weight than the older points. We take the average of all these weighted numbers and voila, we have our point.

Of course, the more often we get points, the better the accuracy. We're currently getting points about once an hour, which will help us extend our battery life. When we see Niu's battery getting lower, we will reduce the communication rate to once a day so that we can extend the journey even longer.

What this all means, is that when you look at Niu's position and its path, you're looking at an estimated location that has some small error associated with it. In the near future we will add more details to the map so you know more about what this error is for each point. In the grand scale of the Pacific Ocean though, we have an extremely accurate knowledge of Niu's position.

If you'd like to learn more about these topics, please check out the following links:

Smithsonian: How Does GPS Work?
http://www.nasm.si.edu/gps/work.html

Wikipedia: Iridium Satellites
http://en.wikipedia.org/wiki/Iridium_(satellite)

Explaining the Point Clustering
written by Evan Rapoport , May 21, 2008

You may wonder why sometimes Niu's coordinates are all clustered together and then suddenly they jump a great distance. For those who read my previous blog on "The Accuracy of Niu's Coordinates", you likely know the answer. For those who didn't read it, here's the short summary.

When GPS satellites aren't available, we use an averaging of points based on less accurate measure of location based on communication satellites. The sudden jumps usually occur when GPS succeeds after several non-GPS points, since the non-GPS points will lag behind slightly due to averaging.

The savvy scientist may also notice that sometimes we go several hours without an update from Niu. Irrelevant to the GPS availability, this means that Niu had more time to travel between transmissions, causing the big gap. And of course, Niu might also just have moved faster during certain periods (though this doesn't account for the bigger gaps).

Niu leaves the tropics
written by Evan Rapoport , May 22, 2008

On Tuesday 5/20/08, Niu officially left the tropics, crossing over the Tropic of Cancer at about 23.5 degrees north latitude. In the "tropics" (between 23.5 degrees south and 23.5 degrees north latitude), the sun reaches its "zenith" in the sky (the highest point, or "straight up"). This occurs twice during the year in the tropics, except for at the Tropics of Cancer and Capricorn, at which it only occurs on the summer solstice.

The beautiful year-long warmth we get here in Hawaii is the result of the sun being higher in the sky for most of the year. Even in the "winter" (if 78 degrees counts as winter), the sun is still pretty high up. This gives us longer days and more direct sunlight, which keeps the temperatures perfect for that New Year's Day surf session.

For all you curious future scientists, I'll leave you with a question: Why are the tropics between 23.5 degrees north and south? If you can answer this, then you should also be able to figure out the significance of the Arctic and Antarctic circles.

Change in Niu's updates
written by Evan Rapoport , May 28, 2008

On 5/28/08, we sent a message to Niu to change its communications rate from once every hour to once every 18 hours. Since rough seas sometimes interrupt these messages, don't be surprised if Niu goes a few days without any updates.

This change was made to lengthen Niu's battery life. We will post updates on the expected battery life over the coming weeks.

General update
written by Evan Rapoport , June 05, 2008

Niu went silent for six days between May 30th and June 5th, which meant that 7 attempts failed occurred. During that time, Niu traveled about 52 miles.

Additionally, because we are now receiving points on a much less frequent basis and thus the distances traveled between points are much greater, we are not doing any averaging of the data as we did earlier. We've set the threshold for error of the points at about 6 miles. The associated error will be added to the site in the next revision.

No worries
written by Evan Rapoport , June 23, 2008

For those of you scoring at home, there hasn't been a new Niu point in over a week. But, don't worry, we did receive a message from Niu over the weekend. Unfortunately, the "probability of error" on the location provided by the Iridium satellites was about 40 miles! Usually, this is within 1-5 miles, so anytime the error is much higher than that, we don't post it online. If we didn't filter points like this, it would appear that Niu was jumping all over the Pacific!

New Niu Tracker
written by Evan Rapoport , July 28, 2008

Recently we released a new version of the Niu Tracker, which shows a lot more information. Don't worry, the maps and graphs can still be made bigger for more detailed viewing. We are also working on re-building the website so the tracker will be the homepage and will contain all the stories and info in this location. Keep checking back, a lot of exciting changes are coming soon!