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GPS
Computer Cartography
Question | Answer |
---|---|
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 |