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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 |
Created by:
aley1
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