Positioning systems for active safety applications Flashcards
Position critical applications (examples)
Position critical applications (examples)
- Navigation system
- Cooperative systems (many different)
- Automated vehicles • Road curvature estimation (e.g. overtaking support)
- Intersection support
- Test-track experiments (high price OK)
Performance of GPS
GPS
- 24 (+6) satellites
- ~20 000km orbit
- 3 ns time accuracy
- Exact positioning of satellites
- Hand held units – ”triangulates” position with satellites.
- Several sources of error
- Differential GPS – increased accuracy
What data is used? (GPS)
What data is used?
- Satellite Almanac Data - Orbital data to calculate approximate GPS satellites location. Valid days.
- Satellite Ephemeris data - Orbital data enables better satellite position for ranging. Valid for hours
- Signal Timing Data: Time tagging in GPS data stream. To determine satellite-to-user propagation delay for ranging.
- Ionospheric Delay Data: estimates of ionospheric delay
- Satellite Health Message: Information regarding the current health of the satellite
Explain Satellite Almanac Data.
• Satellite Almanac Data - Orbital data to calculate approximate GPS satellites location. Valid days.
Explain Satellite Ephemeris data
Satellite Ephemeris data - Orbital data enables better satellite position for ranging. Valid for hours
Explain Signal Timing Data
Signal Timing Data: Time tagging in GPS data stream. To determine satellite-to-user propagation delay for ranging.
Explain Ionospheric Delay Data.
Ionospheric Delay Data: estimates of ionospheric delay
The ionosphere (/aɪˈɒnəˌsfɪər/[1][2]) is the ionized part of Earth’s upper atmosphere, from about 60 km (37 mi) to 1,000 km (620 mi) altitude, a region that includes the thermosphereand parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth.[3]
Explain Satellite Health Message
Satellite Health Message: Information regarding the current health of the satellite.
What are GPS – Errors?
GPS – Errors
- Ionospheric Propagation Errors: Wave propagation. Error:1 m at night to 5-15 m daytime
- Tropospheric Propagation Errors: Error: 2.5 m in the zenith direction to 10-15 m at low satellite elevation angles.
- The Multipath Problem: Dominant source of error in differential positioning. E.g. “Bouncing of buildings”
- Ephemeris Data Errors: Error <1m. Removed with DGPS
- Onboard Clock Errors: Error <1m. Removed with DGPS
Explain Ionospheric Propagation Errors
Ionospheric Propagation Errors: Wave propagation. Error:1 m at night to 5-15 m daytime
Explain Tropospheric Propagation Errors
Tropospheric Propagation Errors: Error: 2.5 m in the zenith direction to 10-15 m at low satellite elevation angles.
Explain The Multipath Problem
The Multipath Problem: Dominant source of error in differential positioning. E.g. “Bouncing of buildings”
- GPS signals bounce of e.g. buildings
- Create false (typically larger) distance information, and consequently incorrect positioning
Explain Ephemeris Data Errors
Ephemeris Data Errors: Error <1m. Removed with DGPS
Explain Onboard Clock Errors
Onboard Clock Errors: Error <1m. Removed with DGPS
Explain GPS – Dilution of Precision
GPS – Dilution of Precision
- A measure of error due to satellite configuration
- Configuration of the satellites in the sky (if same direction – not easy to do good triangulation)
- Different Dilution of Precision:
- VDOP – Vertical Dilution of Precision
- HDOP – Horizontal Dilution of Precision
- Example: – Good when <4
Side track – What is a communication protocol?
Explain GPS protocol: NMEA messages
GPS protocol: NMEA messages
- National Marine Electronics Association
- Standard 4800bits/s over RS-232
- Build-up:
- Starts with ‘$‘, ends with a CR/LF
- Maximum 80 characters
- Single line with data items separated by commas
- Data in ASCII text
Explain Map projections
Map projections
- WGS84 – “Standard” worldwide longitude/latitude
- Pole-flattened (oblate) spheroid
- a = 6,378,137 m
- b = 6,356,752m
- Pole-flattened (oblate) spheroid
- Center in middle of earth
- Not very easy to convert to meters “on paper”
- RT90
- Rikets Triangelnät (Sverige)
- SWEREF 99.
- In meters
- X increases to north
- Y increases to east
- Example (Göteborg):
- X=6404510
- Y=1271140
Explain Differential GPS
Differential GPS
- Uses relative positioning
- “Local” (own?)
- By provider
- Somewhat expensive
- Significant error reduction
- Sub-meter precision
- Network RealTime Kinematic (RTK)
- Builds model
- Interpolates between stations
- Expensive (e.g. 500kSEK)
- Two way communication
Explain Map Matching
Map Matching
- Finding the “correct” road from positioning information and extract attributes
- Attribute examples:
- Road type
- Speed limit
- Number of lanes
- Distance to intersection
Explain Electronic horizon
Electronic horizon
- Building a local model of the future path of the vehicle
- Often uses map attributes fused with invehicle sensors
Explain Autonomous vehicle “mapping” using Google
Autonomous vehicle “mapping” Example: Google
- Use LIDAR to “map” roads
- Only mapped roads are used
- Two mapping passes, the second for verification
- Position accuracy ~2.5cm
Explain SLAM
SLAM - Simultaneous localization and mapping
- Building a map of the surrounding while keeping track of where the ”agent” is in the map.
- (Electronic horizon = sort-of SLAM)
Give example of other positiong than GPS
Other positioning: Cell phone
Is GPS good enough for active safety?
No.
- Positioning crucial for many applications
- Anyone can interface with a GPS, but GPS typically not good enough for active safety applications – but a complement
- Map matching and SLAM important
