Topic 12: GNSS Flashcards
1
Q
How does ranging work?
A
- Satellites broadcast a time code
- GNSS receivers calculate range (distance) to satellites using travel time of signal
- Calculating successive ranges for two or three additional satellites allows us to use trilateration to fix position
- Usually need 4 satellites (fourth correct clock errors and is a minimum) (more satellites = better location)
2
Q
Satellite ‘Trilangulation’
A
- Measured distance to multiple satellites in space to determine locations on earth
- NOT triangulation, since there are no angles involved
- it is actually trilateration
3
Q
Measuring Distance from a satellite
A
- Velocity x Time = Distance (velocity is known (estimated)) ( Time = offset receiver clock and signal sent from satellite clock)
As signal travels from satellite, through atmosphere to device, it has slowed down - room for error - Navigation messgae includes information on orbital parameters, clock corrections and other system updates
4
Q
Measuring Time at “Light Speed”
A
- An error of 1/1000 of a second would translate into a 500 km mistake
- Speed of light (EMR) in a vacuum would be about 3 x 10^8 m/s or 300000 km/sec
- Satellites carry atomic clocks (Receivers don’t have atomic clock, reason for multiple satellites)
- The fourth satellite measure is a cross-check; Errors uncover inaccuracies in the receiver clock, which are then connected by the CPU on the device
5
Q
Types of errors: noise
A
- Code noise - enough satellites can filter out code noise
- noise in the electromagnetic signal
- noise in the instruments, multiple satellites usually corrects that
6
Q
Types of error: Bias
A
- Selective availability (discontinued May 2000) - intentionally scrambling of signals so that people could not get a good location
- atmospheric effects (eg., heavy clouds make it more difficult)
- Multipath interference - signals bounce off buildings, trees, and people
- Human blunders - the biggest one (user error - pick wrong geoid, ellipsoid, projection)
7
Q
Geometric Dilution of Precision (GDOP)
A
- Widely separated satellites can minimize error
- Receiver will choose best geometry of satellites
- Environments with trees or urban don’t get good geometry cause they approximately in the same spot
8
Q
Differential GNSS
A
- Accuracies in the order of centimeters (Phone is usually around 5 cm, with higher unit it is harder to get worse than that especially with RTK)
- Key is a second reference receiver the remains stationary - Because satellites are so high (20 000 km) you can assume that accuracy and timing errors affect both receivers the same
- Goal is to filter out atmospheric effects
9
Q
What is real time Kinematic (RTK)
A
- Analyzing nature of signal
- corrections for inconsistencies
- Start getting into millimeters
10
Q
Differential GNSS Operation
A
- Towers transmit errors to roving receivers vis radio waves
- Enabled receivers can collect this information and make corrections in real time
- Basis for ground-based augmentation systems (GBAS)
11
Q
Different satellite based augmentation systems (SBAS) and their functions?
A
WAAS
- Wide are augmentation system
- Combination of ground reference stations and also satellite (location and communication) signal to GNSS
- Difference between not having WAAS to having WAAS makes difference in cm and easier navigation
EGNOS
- European Geostationary Navigation Overlay Service
- referred to as Galileo
- Satellite system
12
Q
What is WAAS?
A
- Network of permanent base stations (WAAS - USA/ Canada, EGNOS - Europe)
- Plus geosynchronous satellites to broadcast correction code
- GPS accuracies 1-3 meters for common handheld GPS units with extra receiver built-in or separate
13
Q
How does your phone calculate location?
A
- GNSS (GLONASS, Galileo, GPS are definitely on phones
- Uses the location of known of cell towers
- WI-fi locations are known and can trilateration to get location on your phone
- Bluetooth can also be use for positioning
- RFID (radio frequency identification)
- Magnetic
