05 Positioning systems

Question 1

List applications of GPS

Answer 1

• Navigation system
• Cooperative systems (many different)
• Road curvature estimation (e.g. overtaking support)
• Intersection support
• Test-track experiments (high price OK)
• Automated vehicles

Question 2

Describe the principles of GPS

Answer 2

• 24 (+6) satellites
• ~20 000km orbit
• 3 ns time accuracy
• Exact positioning of satellites
• GPS-units – triangulates position with satellites.
• Several sources of error
• Differential GPS –increased accuracy

Question 3

Describe and reflect on main GPS issues

Answer 3

• Ionospheric Propagation Errors: Wave propagation. Error:1 m at night to 5-15 m daytime
• Tropospheric Propagation Errors: 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. For example “Bouncing of buildings”
• Ephemeris Data Errors: Error <1m. Removed with DGPS
• Onboard Clock Errors: Error <1m. Removed with DGPS

Question 4

Understand how to “talk” to a GPS unit

Answer 4

• 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 5

Map projections

Answer 5

WGS84
– “Standard” worldwide longitude/latitude
– 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 6

Map Matching

Answer 6

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

Question 7

SLAM - Simultaneous localization
and mapping

Answer 7

• Building a map of the surrounding while keeping track of where the ”agent” is in the map.
• Building the map as the vehicle moves forward
  *Extension: using previous maps/information (e.g., static objects locations)