Lots of Eyes in the Sky

          On Saturday, September 11, 2010 Japan launched its first GPS satellite the "Michibiki", whose name means showing the way.  This is the first in a series of satellites that will have the intent to improve  accuracy of satellite navigation in the country (more specifically in the islands).  Scientists hope satellite accuracy will be increased as the current satellites strength is diminished by mountains or skyscrapers. This will be a feature which will be widely used by drivers in Japan.  Japan's system (QZSS) is only one of a number of satellite networks in the works by other countries to reduce dependence on the United States built and controlled NAVSTAR GPS network. Because of safety systems and commerce many countries have begun to take control of their own systems; including  Russia's GLONASS, the European Galileo and even China intends on turning its COMPASS network into a global system.  India plans to build a service as well. This isn't to surprising understanding all the growth that is occurring globally.  Foreign nations want to depend less and less on the United States.  This is a lot going on in the sky and makes you think about the United States and our position globally.  Makes me wonder if it's possible to have contention over satellite position in space.


Article

How do GPS devices operate?

          

          As summer comes to an end, days become shorter and the bright green leaves change to golden, scarlet, orange and brown. Fall is one of my favorite times of the year to enjoy nature by embarking on outdoor adventures, especially hiking.  Some of my favorite hikes close to home include Flat Rock, Tumbling Run, Pine Grove Furnace, Leigh Gorge and Ricketts Glen (just to mention a few).  Each time I reach the peak of one of these hikes, I look across the mountain ranges and wonder what the elevation is, where I am in relation to home and even how they compare to the mountains I ski during winter. Recently, I was gifted an all-purpose Garmin GPS to use on hikes; even though I only had a chance to bring it along on my most recent hike to Leigh Gorge, it has given me a better idea of the scope of the hike in terms of elevation, distance, longitude and latitude. The top elevation I recorded on this hike was approximately 1,400 feet.  This sparked me to wonder actually how the GPS actually works.  The Global Positioning System depends on 27 satellites 24 of which are in operation and 3 additional satellites in case of failure. These satellites continuously orbit the Earth and transmit the following data:  Time information (each satellite has an internal atomic clock which is accurate to the billionth of a second), Precise orbit information, Rough orbits of other GPS satellites, and System health.

         Each satellite weighs 3,000 to 4,000 pounds and circulates the Earth about 12,000 miles which is 2 rotations daily.  The orbits are arranged so that there are always 4 visible satellite signals.  After these signals reach the GPS device the receiver calculates the distance to each satellite; which is a process called trilateration.

         Does trilateration mean that we only need three satellites to receive an accurate location? If you know your exact elevation you would only partially be correct because there is still the time factor. GPS’s must receive input from four or more satellites (along with elevation) for your position to be be as accurate as possible.  Signals from satellites are three demonsional.  You can think of them as spheres.  The intersecting areas become your location.  The more satellites the more accurate your location. 

           Understanding trilateration is important to understanding how GPS devices operate.  This video on trilateration was helpful to me.  It might be a little long but does a good job of explaining trilateration.  I really think the new GPS gift is going to keep me occupied for a while as I discover all its capabilities.

Around the World

          I recently bought a new used car with about a hundred-thousand miles on it.  In the life of a Subaru that's really not all that many miles if you follow the maintenance guidelines.  For most people mileage on a car is really just a number, an indicator of when to get an oil change or a timing belt replacement.  I myself never thought of it as more than a number until the used car salesman mentioned “ya cars been around the earth about four times”.  I just thought, “Wow!  I'm buying a car that has done that?  The circumference of the earth is 24,902 miles.  Most miles are just driving around town with occasional weekend trips or even possibly a road trip across the country.  I'm sure if you mapped peoples driving habits it really wouldn't be all that interesting.  Just a bunch of really dense lines with random single lines jetting out into open space.  However, the salesman’s comment really made me think a little more about the circumference and spatial details of our planet.

          The circumference of the Earth after Aristotle argued the Earth was round was figured out by Eratothenes circa 200 B.C. He figured the circumference to within a half percent of accuracy.  This really astonishes me.  Not only the accuracy but the simple fact that it was done far before any one had traveled the circumference or without any modern technology to assist him.  To determine this he took measurements of shadow heights in two different parts of Egypt at specific times and then calculated the angle between the two.  From this he was able figure out how big a circle was needed to complete 360 degrees.  Eratosthenes thought that precise mapping really depended on accurate linear measurements.  I really don't think he could have been more correct in this thought.  The amount of perception and mathematical skill Eratosthenes had was truly amazing.  He was also the first person to use the word geography.  In my mind, Eratosthenes made one of the most significant if not the single most important contributions to modern cartography as we know it.