lecture 1 Flashcards
what is a GPS
- satellite based navigation and positioning system
- based on collection of satellites orbiting the earth
- electromagnetic signals transmitted from satellites allow receivers to calculate their position anywhere on Earth to a high degree of accuracy
how was latitude determined in 1402 (Columbus’s voyage)
determined using the angle of the sun or a known star relative to the horizon
quadrant
used to measure angles on horizon
how was longitude dtermined in 1492
dtermined using the difference between a fixed time and local time while at sea and then converting to degrees of longitude
- split the earth into 15 degrees equal
- reference time is Greenwich time
how did they determine Greenwich time
Longitude act of 1714: designed the first chronometers to keep accurate time -
what is radio-based positioning
wireless radio technology emerged allowing transmission of time signals
- didnt require chronometers anymore
could determine bearings from them
Long Range Navigation
how does LORAN system work (radio based positioning)
used a chain of large ground transmitting stations sending out radio signals at regular intervals
receivers measure the time difference from stations to determine position
around 100-500m accuracy
SPutnik
Russias first artificial satellite that was succesfully launched into space
- tested satellite viability
what is the doppler shift
frequency of the radio signal broadcast by Sputnik varied based on its position
what was significant about Sputnik
knowing the location of the observation point and determining the Doppler shift meant researchers could determine SPutniks orbit
- therefore it was reversed and wondered if we could infer where we are on Earth based on satellite signal
what is the transit system
originally developed for missile guidance
based on DOppler shift and satelite orbits
launched in 1959 - sent out 15 in 5 years
improved availability, global coverage and accuracy
what are the main limitations of transit system
- users had to be stationary
- limited coverage and took around 100 mins
- only allowed for 2D positioning (x,y)
- accuracy was 200-500m
what were the goals for the next generation of satellite positioning system
available 24 hours a day 365 days
accuracy, precise positioning
3D location
global coverage
summary of history
celestial navigation (pre 1700) - chronometers (1700)- radio based positioning (1900) - early satelitte based system (1950)
NAVSTAR system
navigation system with timing and ranging
- replaced transit
- continuous positioning system that could be used anywhere in the world under any conditions
- provide estimates of position, velocity and time
3 parts of GPS system
space segment
control segment
user segment
space segment of GPS
first launched in 1978, have 24 satellites now
4-8 are visible at any moment
send out radio signals every minute that contain satellite ID, position, date and time, orbit and satellite status
control segment
tracks and communicates with GPS, moniters health and ensures that all satellite clocks are synchronized
- consists of master control station
- antennae, monitering and tracking stations are all around the world
can correct orbits, clocks and other information
user segment
receivers that pick up satellite signals
- have anntenaes tuned to frequency of GPS satellite
trilateration
distances measured according to time travelled from SPACE satellites to reach USER receiver
- GPS must be connected to a minimum of 3 satellites to calculate 2D position and 4 to calculate 3D
how do you calculate distance
speed x time
what is selective availability
- accuracy was restricted to military use
- degraded accuracy for civilians use but was removed in 2000
what factors affect GPS accuracy
blocked signals
multipath/reflected signals
satellite geometry
ionospheric interference
blocked signals
occur in dense foliage, ravines or canyons
satellite signals are physically blocked from receiver
- can result in degraded position accuracy
reflected multipath error
signal is reflected by terrain or building and cannot reach receiver on a direct line
- impacts accuracy of receivers determined position
ionospheric interference
electromagnetic waves are slowed when they move through the ionosphere
satellite geometry
satelite configuration in the sky
- must be widely spread
how do we quantify error
measurement = truth + bias + random error
random error can come from multiple sources
biases can come from ionospheric effects
accuracy
measure of nearness to correct value
- close
precision
measure of variation in recordings
- spread
GPS local refinement: Real time differential GPS (DGPS)
used primarly prior to remove selective availability
- enhances the accuracy of signal
GPS location refinement: wide area augmentation system (WAAS)
ground stations that calculate the error and correct it
GPS receiver calculates its position from the GPS and uses WAAS to correct its calculation
- doesnt help to determine location but refines
GPS accuracy
without WAAS: 5-15m
with WAAS: 3-7m
other GNSS (global navigation satellite systems)
united states: navstar
russia: glonass
europe: galileo
