The global positioning system is a spaced based satelite navigation system that provides location time information in all weather conditions , anywhere on or near the earth where there is an unobstructed line of sight to four or more GPS satellites.
Developed and maintained by the US Department of Defense (DOD)
2. What is GPS ?
(A very precise positioning system)
• Developed and maintained by the US
Department of Defense (DOD)
• Satellite Based
* 24 satellites
* 20,200 km high orbit
The global positioning system is a spaced based satelite
navigation system that provides location time
information in all weather conditions , anywhere on or
near the earth where there is an unobstructed line of
sight to four or more GPS satellites.
3. The current system is managed by the U.S Air Force for
the Department of Defense (DOD).
The current system became fully operational June 26,
1993 when the 24th satellite was lunched. GPS provides
specially coded satellite signals that can be processed
with a GPS receiver, enabling the receiver to compute
position, velocity and time.
A minimum of four GPS satellite signals are required to
compute positions in three dimensions and the time
offset in the receiver clock.
Accuracy and precision of data increases with more
satellites.
3
4. Characteristics of GPS
• Free
• Precise
• Reliable
• Anytime & anywhere
• All weather
• Unlimited user capacity
Almost!
5. Segments of GPS
Source:Trimble
1. Space Segment:
A constellation
of 24 satellites
• 24 GPS space vehicles(SVs).
• Satellites orbit the earth in 12
hrs.
• 6 orbital planes inclined at 55
degrees with the equator.
• This constellation provides 5 to
8 SVs from any point on the
earth.
6. This information is sent to the
three uplink stations.
The control segment comprises
of 5 stations.
They measure the distances of
the overhead satellites every 1.5
seconds and send the corrected
data to Master control.
Here the satellite orbit, clock
performance and health of the
satellite are determined and
determines whether
repositioning is required.
8. It consists of receivers that decode the signals
from the satellites.
The receiver performs following tasks:
Selecting one or more satellites
Acquiring GPS signals
Measuring and tracking
Recovering navigation data
9. How GPS Works ………
Uses measurements from 4+ satellites
Distance = travel time x speed of light
Source:Trimble
10. Requirements
Triangulation from satellite
Distance measurement through travel time of radio
signals
Very accurate timing required
To measure distance the location of the satellite
should also be known
Finally delays have to be corrected
11. Determining GPS Position
• Suppose the distance from
Satellite A to our position is
11,000 miles
• At this point we could be located
anywhere on the specified sphere
Satellite A
+
• Next, let us take another measurement
from a second satellite, Satellite BSatellite B
+
• Now our position is narrowed down to the
intersection of theses two sphere
12. Satellite C
+
Determining GPS Position
Satellite A
Satellite B
+
+
• Taking another measurement
from a 3rd
satellite narrows our
position down even further, to
the two points
• So by ranging from 3 satellites
we can narrow our position to just
two points in space
• These points are located where
the 3rd
sphere cuts through the
the intersection of first two spheres
13. Satellite C
+
How do we decide which one is our
true location?
Satellite A
Satellite B
+
+
• We could make a 4th
measurement
from another satellite to determine
the true point
• However, GPS receivers use a 4th
satellite to precisely
locate our position
• We can eliminate one of the two
points that gives a ridiculous
answer
• The ridiculous point may be too
far from the earth
OR
14. Distance to a satellite is determined by measuring how
long a radio signal takes to reach us from the satellite.
Assuming the satellite and receiver clocks are sync. The
delay of the code in the receiver multiplied by the speed
of light gives us the distance.
If the clocks are perfect sync the satellite range will
intersect at a single point.
15. How accurate is GPS?
Depends on some variables
• Design of receiver
• Relative positions of satellites,
technically known as PDOP (Position
dilution of precision)
• Post-processing
• Time spent on measurement
17. • Wide Area Augmentation System.
• It provides FREE GPS differential correction data for
visible satellites
• Developed & operated by the FAA (Federal Aviation
Administration) for flight navigation but it’s available
free to GPS users
• WAAS-enabled receivers can provide sub-meter level
accuracy anywhere in most locations of the US and
southern Canada.
18. • Differential corrections are computed from
ground stations and then uploaded to
geostationary satellites for broadcasting
• WAAS-enabled GPS receiver automatically
uses such correction data to enhance the
positional accuracy
20. Multipath
• When GPS signals arrive at the receiver
having traveled different paths
21. Satellite errors
Errors in modeling clock offset
Errors in Keplerian representation
of ephemeris
Latency in tracking
Atmospheric propagation errors
Through the ionosphere , carrier
experiences phase advance and the
code experiences group delay
• Dependent on
Geomagnetic latitude
Time of the day
Elevation of the satellite
Errors due to Multipath
Receiver noise
Good PDOP
Poor PDOP
22. Forces on the GPS satellite
Earth is not a perfect sphere and hence uneven
gravitational potential distribution.
Other heavenly bodies attract the satellite,but these
are very well modeled.
Not a perfect vacuum hence drag but it is negligible
at GPS orbits.
Solar radiation effects which depends on the surface
reflectivity,luminosity of the sun,distance of to the
sun. this error is the largest unknown errors source.
95% due to hardware ,environment and
atmosphere.
23. Common use of GPS
A. GIS data collection & mapping
B. Navigation
C. Recreation
24. GPS for Navigation
• GPS are getting popular in car.
• Comes with voice guidance.
25. GPS in recreation
• Hikers,outdoor adventures are now
depending upon GPS instead of
maps,compasses.
• As paper maps are outdated & compasses
may not provide precise location &
information necessary to avoid venturing
into an unfamiliar area.
26. GPS in Farmland
•Implementation of precision agriculture or site specific
farming could be done.
•Enables the real time data collection with accurate position.
•Helps in farm planning,field mapping,soil sampling,tractor
guidence.
•Helps farmers in low visibility field conditions as rain,fog.
Notas del editor
At the cost of 15 billion dollar of your tax money
Speed of the light: 186,000 mile/sec
300,000 km/sec
3x10^8 meter/sec
SNR is also known as signal strength
When SA was on, the position could be anywhere on 1st, 2nd, Alpine, or Spruce street. With SA removed, the position is located with in the road width of +/- 15 feet. (road = 35’, the circle is of 20’ radius). Before the location was within 100 meters, that is the point is about a football field away. Now it’s within the 10 meters. Removal of SA made GPS more useful and handy for our day to day use in cell phone, car, watch etc.
As aresults the code delayed and the distance measured is greater than the actual distance
Large delays with low elevation satellite( since they travle through more ionosphere)
More in day time( due to soalr radiation)
Delays more near the geomagnetic equator
Farm equipment capable of producing yield map during harvesting shows how crop yield varies across the field. Allowing farmers to plan fertilizer application for future crops.
