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Computer Cartography

What is GPS? A Space-Based Navigation System designed by the US military
What does GPS provide? Autonomous Geo-Positioning, 10-20 Meter Accuracy, Worldwide Coverage, Availability 24 hours per day, Military Security, Low End-User Cost, Receivers to Each Soldier, Installation on Every Vehicle
How accurate does GPS need to be for geography and anthropology? Under a foot
How accurate does GPS need to be for archaeology? Under an inch
Who do people usually refer to when referring to GPS, and why? The U.S. because it was the first available and is the best.
What does NAVSTAR stand for? Navigation System with Timing and Ranging
When did NAVstar become completely available 24/7 worldwide? July 1995
What does FOC stand for? Full operational capacity
What does IOC stand for? Intermediate operational capacity
What are some of the civil applications of GPS? Land/sea/air/space navigation, Intelligent Vehicle Highway Systems, Search and Rescue, Mapping/GIS, Surveying, Recreation
What three major segments does GPS consist of? Space, Control, and User
Who is in control of the control and space segments? Military
How many times does satellite orbit in 24 hours? Twice
Can you see satellites without a telescope? No
Where is the master control station? Colorado Springs
Where are the four other control segment locations? Kwajalein, Ascension, Hawaii, Diego Garcia
What is the minimum number of satellites for the GPS system to work at full capacity? 24
How much does a global positioning satellite weigh? About 900 kg
How wide is a global positioning satellite? About 5 meters
What is the lifespan of a global positioning satellite? 7 and a half years
What is the orbital plane? 55 degrees to equatorial plane
How many orbits are there? 6
How many satellites are there in each orbit? 4
What altitude do satellites orbit at? 20,200 km
What type of system is GPS? Distance/Ranging
What principal does GPS operate on? Trilateration
How do receivers measure the time it takes for satellite signals to reach it? Receiver calculates time it takes to go from satellite to Earth and multiplies by constant to calculate how far
What is the velocity constant? 300,000 Km/Sec
Is GPS a one-way or two-way ranging system? One-way
Why it impossible to know where you are if you only know the distance to one satellite? Could be anywhere on sphere with that distance, millions of potential possibilities
Why is it impossible to know where you are if you only know the distance to two satellites? Could be anywhere on intersection of two spheres
Why is it impossible to know where you are if you only know the distance to three satellites? Could be at either of two intersections of the 3 spheres
How many satellites must receiver be able to compute distance to in order to get reading? Four satellites
How many seconds does it take for a satellite signal to reach the ground? 6/100 Sec.
What types of clocks do satellites have? Very precise atomic clocks
What types of clocks do receivers have? Inexpensive quartz clocks
How many frequencies does each satellite transmit on? Two
How is code specific information sent from satellite to receiver? As 0s and 1s superimposed on radio signals
What is an SV Ephemeris Error? Satellite vehicle not in predetermined orbit
Who is responsible for SV Ephemeris Errors? Military
What is a dilution of precision a measure of the geometry of the visible GPS constellation
What causes receiver noise? Electronics that aren't good at filtering radio signal from satellite
What kind of receivers are good at filtering noise? Expensive receivers
What errors are in the GPS Error Budget? Satellite clock error, SV Ephemeris Error, Ionosphere refraction, Troposphere refraction, receiver noise, multipath, selective availability, dilution of precision
How can you account for radio signal bouncing off of particles in the ionosphere and troposphere? By applying mask angle
What is the default mask angle? 15 degrees
If the mask angle is set at 15 degrees, will a satellite that is within the angle of the horizon and 15 degrees above the horizon be used to determine position? No because the liklihood of refraction is high because the signal travels a long distance
What is a multi-path error? An error that occurs in downtown areas because the signals bounce off of buildings and make the path to receiver longer
How can multi-path error be partially eliminated? By applying a mask angle
What is selective availability? The degradation of position accuracy by the Department of Defense
Why is selective availability applied? To deny hostiles high accuracy positioning
How does the DOD apply selective availability? By off-setting SV clocks and injecting ephemeris errors
What is the accuracy of GPS with selective availability on? about 100 meters
Where is selective availability an issue? Where US is involved in combat for non-military users
What happens if you have a large value for DOP? Will increase error because all errors are multiplied by DOP
What is the DOP factor that you could have? 1
What is the acceptable range of DOP values? 1-5
When planning mission, what 3 things do you need to enter into software? location where you want to collect data, time, and day
When planning a mission, what two graphics should you look at? DOP and number of satellites
What are almanacs? Information that satellites carry with them
What information is in almanacs? all the information for DOP and number of satellite graphics and predicted orbits for the next two months
Where can almanacs be downloaded from? directly from satellites
How long does it take to download an almanac? About 12.5 minutes
Who updates the data? Control Segment
What is another name for differential correction? relative positioning
What is differential correction? A method that allows you to correct position that has been collected and make it more accurate.
Where should you put the base receiver when applying differential correction? Over a known point (NGS Monument)
Where should you put the rover receiver? Over unknown point
What does the base receiver measure? Error Vector
What is the error vector? the difference between the true and GPS-Derived Position
Why is it important to gather base and rover receiver data at the same time? Because the error vector changes from second to second
What is applied to the rover position to get true position? The error vector calculated by the base
How can you get real-time correction? Use a beacon
What format do differential post-processing base data acquired from 'outside' sources use? RINEX
Why can't you do post-processing in real time? Because you must download data to software to make correction
Who uses post-processing? Geographers and Anthropologists
Who uses real-time differential correction? Military and aviators
Why do we need RINEX (Receiver Independent Exchange)? Because we have receivers from different providers
What are four advantages of post-processing? Highest accuracy, lower cost with free differential correction data, only requires one person if have free differential correction data, and less 'stuff; in field with free differential correction data
What are three cons of post-processing? Higher cost with 2nd receiver, 2 persons with 2 receivers, and more 'stuff' with second receiver
What are two advantages of real-time? Immediate correction and always one person
What are five disadvantages of real-time? not carrier, lower accuracy, higher cost with 2nd receiver, extra 'stuff' in field, and must maintain lock on two systems
What are four differential data sources? Your own second receiver, universities, federal government, and private vendors
What does C.O.R.S. stand for? Continuously Operating Reference Station
What does C.O.R.S. consist of? A group of GPS reference stations
Who coordinates C.O.R.S.? the NGS (National Geodetic Survey) and NOAA (Nat. Oceanic And Atmospheric Admin.)
What does C.O.R.S. provide? Post-processing data
How can you look C.O.R.S.? On Web site or using receiver
How close does your receiver need to be to the base station? within a few hundred miles; no more than 300-400 miles
A set of rules for specifying how coordinates are to be assigned to positions on the surface of the Earth Geodetic Coordinate Systems
How are geodetic coordinate systems defined? By X, Y, Z on an ellipsoid
What would we like to think the Earth is shaped like? Sphere
Equipotential Gravimetric Surface (Sea-Level) Geoid
Best fitting mathematical shape Ellipsoid
What is the best fit geoid? The world geodetic system of 1984 (WGS84)
How are GPS heights defined? With respect to the ellipsoid
height referenced to the surface of the Earth Orthometric height
Created by: aley1