Tuesday, December 1, 2015

All about that base (station)... more Trimble!

Hey readers! Today's blog is brought to you by both of our Antarctic reporters Emma AND Natalie! 
Natalie and Em rocking down our Trimble GPS base station.
Yesterday we surveyed the moraine. It was a long grueling day, but I think all of this heavy lifting is a good thing in the long run. Surveying the moraine includes using a GPS to map it's latitude and longitude coordinates, as well as elevation. We took both one long continuous (or mostly continuous) run of the moraine in order to better place it on our maps. We also took point measurements where each cosmogenic nuclide sample was, so we know exactly where all of our data comes from, and how old each part of the moraine is. This, as you can imagine, involve A LOT of hiking. (In total it was about 10 miles.) Plus to be more efficient, while Drew used the GPS, we picked up and carried the cosmo samples all the way back to camp. So, we carried rocks. On our backs. For miles. On rocky terrain... It was a fun day! Very tiring, but very productive as well. Now it's time for a little lesson on GPS. The more we learn about it, the more fascinating it becomes, so we thought we'd share!


Drew and Natalie start up the Trimble for the day.
"Onto Natalie for the details of how it works..."
"Thanks Emma!"
A Global Positioning System (GPS) works through the communication between satellites in space and a receiver on the ground. The U.S. Government has 24 satellites which continually orbit the Earth. These satellites are always moving in a precise route, so we can track where they are at a specific time. On average, a satellite orbits the Earth two times per day at about 7,000 miles per hour.
To collect high-precision geospatial data we first need to set up a base station. We set up our base station uphill from our Mt. Discovery camp where it had a clear view of the sky. This unit always stays running in the same position, level to the ground. The base station constantly communicates with satellites in orbit using radio frequencies and calculates its exact location. Then when we want to survey a particular feature or point, we set up a rover unit which attaches to a backpack so we can move around with it. This unit also communicates with the satellites. After we are back from Antarctica we can process the data collected by the rover and compare it with the data from the base station.  The position of the rover unit is important, but knowing its position relative to the base station, allows for higher precision surveys of points and features.
Leveling the base station.
Completed base station positioned by Mt. Discovery. (Bottom left corner)
Solar panel that powers the base station.We rocked it down to prevent it from blowing away.
The actual process in which the satellites communicate with the GPS is a little more complicated. A process called triangulation occurs when 4 or more satellites can see and communicate with a receiver station through line of sight.   The station can't be blocked by buildings, dense trees, nearby cliffs or canyon walls.  Each satellite produces a "pseudo random code" which is essentially a series of on and off switches through the radio. These pseudo random codes give the satellite a unique coding system so the unit can differentiate between satellites.
Close up of the base station's receiver.
The receiver needs to see at least 4 different satellites in order to get a more narrow range of coordinates. While we were surveying, the most satellites we could reach at once was 10. If only one satellite is visible, it can collect data over a wide area. If two satellites are visible that area can be smaller and more precise. If there are three satellites you can get data on a 2-dimensional location (latitude and longitude). With four you can get data for a particular 3-dimensional point (latitude, longitude, and elevation). Because each satellite is orbiting on its own path, each one is on a different orbit around the Earth at a specific time. Therefore, when all 4 satellites are communicating, there can only be one possible location with the correct angles and distances from each of those satellites. The greater the number of satellites within view of the GPS, the more accurate the measurement.
But how does a receiver station actually measure the distance from the Earth to the satellite when it moves at such a high velocity?
Drew surveying the moraine with the rover.
Natalie and Drew taking a break from surveying to do some shielding.
"Ooh I'll take this one"
"Go for it Em"
"Thanks Nat"
It comes down to math. A simple equation of  Velocity x Time = Distance. So, if we know how fast the radio signal from the satellite is moving, and how long it takes to reach the GPS device on Earth, we can find the distance it traveled. It's a really tricky process though, because radio signals travel at the speed of light, and since it only takes about 0.06 seconds for the code to travel from the satellite to Earth, you must use really accurate clocks.

So, since we always know how fast the radio signals are traveling, the key to finding distance is time. As we learned before, the satellite is emitting it's own special pseudo random codes. If the receiver emits the same code, and both start at the same time, they should be in sync, right? Wrong. Because the code has to travel from the satellite to Earth, there is a delay. You can then tell how long it took the code to reach the Earth by how much the two codes are out of sync. When you multiply the delay time (travel time) by the velocity you get the distance. Since we are dealing with such small numbers, the more accurate the clock, the more accurate the measurement will be. The clocks on the satellites are called atomic clocks, and they are very accurate. GPS receiver devices have less accurate clocks, as atomic clocks are very expensive.  This inaccuracy is ok because the GPS devices will communicate with multiple satellites, and the errors can be offset. So, the more satellites the GPS can see, the more accurate the timing. The more accurate the timing, the more accurate the distances. And finally, the more accurate the distances, the more accurate the GPS coordinates that we use every day.
Waiting for the GPS to find itself.
"Now back to Natalie for some interesting facts on the history of GPS"
"Thanks Emma."
GPS, also called NAVSTAR, was created by the U.S. Department of Defense in the late 1970s for military purposes. The first satellite was launched in 1978. Eventually, in the late 1980s the GPS system was released for civilian use. Each satellite produces two pseudo random codes on different frequencies: L1 and L2.  L1 codes are more complex and therefore more accurate. It is strictly for military use. L2 has simpler codes and is slightly less accurate. L2 is the frequency we use everyday on our phones, in our cars, and other commerical GPS devices. It's also the one that we use to collect scientific data on our Trimble device.

In 1994 the last satellite was launched into orbit, completing our US constellation of 24 satellites. Each satellite is built to only last about 10 years, so replacements are constantly being constructed and placed into orbit. Each satellite orbits so that five can be seen from every location on Earth.
The US constellation of GPS satellites.
GPS is not strictly an American system. Many countries have their own satellites and constellations in orbit. For example, the Russian system is called GLONASS, and the European system is called GALILEO. Each satellite weighs about 2,000 pounds and is 17 feet across. That's a lot of junk floating up in orbit!
Sample image of a satellite.
Well, now that you know a little bit more about GPS systems, and how they work, we hope you can appreciate the complexity that goes into determining where you are on a map.
Until next time,
-Natalie Robinson and Emelia Chamberlain

(Research and some graphics from the Garmin, and Trimble websites)





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