Positioning systems for active safety applications

Question 1

Position critical applications (examples)

Answer 1

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)

Question 2

Performance of GPS

Answer 2

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

Question 3

What data is used? (GPS)

Answer 3

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

Question 4

Explain Satellite Almanac Data.

Answer 4

• Satellite Almanac Data - Orbital data to calculate approximate GPS satellites location. Valid days.

Question 5

Explain Satellite Ephemeris data

Answer 5

Satellite Ephemeris data - Orbital data enables better satellite position for ranging. Valid for hours

Question 6

Explain Signal Timing Data

Answer 6

Signal Timing Data: Time tagging in GPS data stream. To determine satellite-to-user propagation delay for ranging.

Question 7

Explain Ionospheric Delay Data.

Answer 7

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]

Question 8

Explain Satellite Health Message

Answer 8

Satellite Health Message: Information regarding the current health of the satellite.

Question 9

What are GPS – Errors?

Answer 9

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

Question 10

Explain Ionospheric Propagation Errors

Answer 10

Ionospheric Propagation Errors: Wave propagation. Error:1 m at night to 5-15 m daytime

Question 11

Explain Tropospheric Propagation Errors

Answer 11

Tropospheric Propagation Errors: Error: 2.5 m in the zenith direction to 10-15 m at low satellite elevation angles.

Question 12

Explain The Multipath Problem

Answer 12

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

Question 13

Explain Ephemeris Data Errors

Answer 13

Ephemeris Data Errors: Error <1m. Removed with DGPS

Question 14

Explain Onboard Clock Errors

Answer 14

Onboard Clock Errors: Error <1m. Removed with DGPS

Question 15

Explain GPS – Dilution of Precision

Answer 15

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

Question 16

Side track – What is a communication protocol?

Answer 16

Question 17

Explain GPS protocol: NMEA messages

Answer 17

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

Question 18

Explain Map projections

Answer 18

Map projections

• WGS84 – “Standard” worldwide longitude/latitude
  
  • Pole-flattened (oblate) spheroid
    
    • a = 6,378,137 m
    • b = 6,356,752m
• 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

Question 19

Explain Differential GPS

Answer 19

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

Question 20

Explain Map Matching

Answer 20

Map Matching

• Finding the “correct” road from positioning information and extract attributes
• Attribute examples:
  
  • Road type
  • Speed limit
  • Number of lanes
  • Distance to intersection

Question 21

Explain Electronic horizon

Answer 21

Electronic horizon

• Building a local model of the future path of the vehicle
• Often uses map attributes fused with invehicle sensors

Question 22

Explain Autonomous vehicle “mapping” using Google

Answer 22

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

Question 23

Explain SLAM

Answer 23

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)

Question 24

Give example of other positiong than GPS

Answer 24

Other positioning: Cell phone

Question 25

Is GPS good enough for active safety?

Answer 25

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

Question 26

Answer 26