The GPS system works by timing,
how long it takes a radio signal to reach us from a satellite and then
calculating the distance from that time. The speed of Light (SoL)
plays an important role in aiding us. Radio waves travel at the SoL,
That is 186,000 miles per second. So if a satellite was directly
over head and sent of a signal it would reach us in 6/100th of a second.
GPS advanced timing is done in Nano seconds most of them. A Nano second
look like this .000000001 of a second. But with all that speed and timing
you still have to know when the signal left the satellite at the exact
moment that’s were Synchronize timing come into play and with this some
the Pseudo random code. The satellites and receivers must generate
the same code at the same time. The advantage of using a set of codes or
in the case of our analogy, a string of number is that you can make the
time measurement any time you want.
This is a picture of the difference
in time.
GPS- use’s no numbers but complicated digital codes; the receiver reads these codes and translates them in to coordinates that we can apply to a map. Seeing exactly were a house is or following a series of coordinates to see a trail or a pond. With the aid of pathfinder office we are able to look at these coordinates in a map form then apply then in arc view on to a map to see were every thing is.
Pathfinder office program- is a program developed by Trimble to be used with their receivers in aiding the process of retrieving data on the receiver units. This program allows you receive and transfer data with the rover and the computer. It also lets you create data dictionaries, which is a vital part in the data collection field. A data dictionary is a list of items that you wish to collect data on when placed in the right slot all data is nicely organized and is easily found, read, and comparable to other data. You also can save, load, and edit maps that are created in pathfinder all this is necessary for viewing data understandably. Transferring data and receiving data are two other functions of the pathfinder program. If you want data from the rover receiver all you need to do is hook up the necessary cables and select data transfer under utilities. Form here you can receive data from the rover, send data to the rover and identify files size. After you get all the files and view them you need to differential correct them to do this you need correction files from an Internet site with the correct time that the data was collected and with the correct date. Then you will get and accurate map that is not off by much.
Arc view is the next step in creating
accurate map of the area that you are in. with the data that you collected
from the field then transferred into the pathfinder program you now have
a map but have nothing to compare it to. Once in the arc view program you
can labels all the changes in the map by creating a legend. When you map
starts to look like something you see in National geographic you know you’re
on the right track. Back to gps.
· The distance two a satellite
is determined by measuring how long a radio signal take to reach us form
that satellite.
· We assume that both the
satellite and our receiver are generating the same Pseudo-Random code at
exactly the same time.
· We know how long it took
for the satellites signal to get to us by comparing how late its Pseudo-Random
code is, compared to our code.
If the SoL is 186,000 MpS (miles per second) and we were off by 1/100th of a second that distance could be 1,860 miles. That is a largely considerable distance, but to fix it we need a very accurate timepiece luckily every GPS satellite has four. These are atomic clocks now they don’t run off atomic power; they get their name form oscillations of a particulate atom as their metronome, which is the most accurate way of keeping time. Now if you want a clock that is never off and this is what you looking for maybe you should think twice each of these clock’s carries a price tag of $100,000.00 ouch I think my Timex is good enough.
If the signals were off there are a few ways to check one is Trigonometry by using an extra satellite it could cause an imperfect sync, theoretically three satellites are plenty so there should be no error. Trig says, “ if three perfect measurements locate a point in three dimensional space then four imperfect measurements can eliminate and offset timing. But to add another item in the mix, our receiver clocks are no were near an accurate as the atomic clocks on board the satellites.
Pseudo-ranges caused by fast clock. The Pseudo range is used in GPS circles to describe ranges that contain errors (usually in timing) the receiver get the measurements that are bad and uses algebra to find out how much it’s off and corrects it self.
When the GPS system does not have four satellites above the horizon due to the formative years. You can integrate it with another navigation system the LORAN system. It gives near-GPS accuracy. Loran can semi-accurately record data when satellites are below the horizon. GPS systems when fully implements will consist of 24 satellites so at least four will be visible at all times. The need for four measurements has greatly impacted the way the receivers are designed. Now they have four channel receivers so one channel can be devoted to one satellite and no other interference can acquire. With a one channel receiver it takes a great deal of time to generate a position while it communicates with only one satellite at a time and each point takes two minutes and thirty seconds to compute, which although not to day is plenty of time to wait. Disadvantage- “system data message” a 30 second interrupts even in new satellite readings.
· Accurate timing is key
to measuring distance to satellites.
·Satellites are accurate
because they have atomic clocks on board.
·Receiver clocks don’t have
to be perfect because of trigonometry trick can cancel out receiver clock
errors.
·The trick is to make a
fourth satellite range measurement.
·Needing four measurements
affects receiver design.
Satellite position, satellites are
11,000 miles above the earth putting them clear of the atmosphere, make
orbits precise so they can be easily tracked. The air force puts the satellites
on orbit in precise locations according to the GPS master plan. Some ground
receivers have an “ Almanac” programmed into them, so they know were any
satellite is at any given time. DoD- departments of defense are constantly
monitoring satellite movement and function. They run on a non-geosync orbit
time, passing over the DoD twice a day to accurately measure altitude,
position, speed and satellite health. Their looking for “Ephemeris errors”
caused by little gravitational pulls from the moon, for sun by solar radiation
pressure. Then after computing all the data the DoD send the necessary
information for the satellites to correct the orbits.