quartz/content/notes/09-location-sensors-3.md

title	tags
09-location-sensors-3	lecture info305

Location Sensors - GPS (cont.)

GPS - Receiver

• Uses messages received from satellites (n≥4) to determine the satellite positions and time sent
• Gives roughly distance to satellite
• Applies Trilateration for computing location
• The receiver has four unknowns, the three components of GPS receiver position and the clock bias [x, y, z, b]
• Using four (or more) satellites, we can set up 4 linear equations to solve for x, y, z, b
• In some cases we know z or b we need less satellites! Urban Canyon
• Urban environment similar to a natural canyon
• Can impact radio reception of GPS receivers
• Buildings reflect and occlude satellite signals
• Reducing precision of positioning in urban environments
• Makes positioning impossible

[!INFO] Add a fourth satellite so we can find the clock bias there are timer networks where you can get time from an atomic clock (still not accurate but better)

Urban Canyon

• Urban environment similar to a natural canyon
• Can impact radio reception of GPS receivers
• Buildings reflect and occlude satellite signals
• Reducing precision of positioning in urban environments
• Makes positioning impossible www.hci.otago.ac.nz The end!

[!INFO] buildings block some satellites reflected signals take more time to arrive and disrupt accuracy also happens with lower building that are closer together happens also with trees

GPS - Problems

• Urban environment similar to a natural canyon
  • Can impact radio reception of GPS receivers
• GPS will not work indoors
• GPS quickly kills your battery
• Accuracy strongly depends on antenna design and device form factor
  • “Position tests by the mobile phone revealed a median error between 5.0 and 8.5m”
  • “Increase by a factor of 2 to 3 compared to standalone GPS sensors.”
  • “Very large errors are uncommon and rarely exceed 30m”
• Long startup time ( 30s - 12.5min needed for sending almanac)
• Several proposed improvements: AGPS, DGPS, RTKGPS

[!INFO] ^Depends on many parameters bad weather also affects travel time hence accuracy long start up time. almanac (satellite position and future positions) is a large amount of data that each device needs to recieve with data/wifi can download almanac from phone provider

Location Sensors - AGPS, DGPS, RTK GPS

A(ssisted) GPS

• Introduced by Qualcomm in 2004, used extensively in mobile phones
• Idea:
  • Traditional GPS only uses radio signals from satellites
  • Assisted GPS (A-GPS) uses network information (transmission of almanac)
  • Increase position accuracy by also incorporating Wi-Fi Positioning System and cell-site multilateration
• Advantages:
  • Faster location acquisition
  • Higher precision (with WiFi Positioning & cell-site multilateration)
• Disadvantages:
  • Requires Internet connection

[!INFO] most relevant to us. used in mobile phones. trad gps uses only satellite. AGPS used that and network information also incorporates wifi and cell location

D(ifferential) GPS

• Enhancement to GPS to increase location accuracy and integrity
• DGPS correct errors using a stationary receiver station with a known location
  • Stationary receiver knows own location and can calculate and Delta between known position and GPS position
  • Stationary receiver broadcasts signal correction information (Delta)
• Standalone GPS provides ~15m accuracy
• DGPS can provide ~3-5m (and max. 10-15cm) accuracy

[!INFO] a lot of towers around farming areas

• Various DGPS networks implemented worldwide
• iBase VRS (New Zealand) (see also Trimble)
• DGPS & RTK GPS within NZ
• http://www.geosystems.co.nz/solutions/ ibase-vrs/

Real Time Kinematic (RTK)

• Technique to enhance location precision of satellite-based navigation systems
  • Provides centimetre-level accuracy (approx. 5cm)
  • Similar to DGPS, as RTK GPS also uses a base station with a stationary known location
  • Uses measurements of phase of the signal carrier wave (whereas GPS only uses signal content information)
    • L1 receiver measures only L1 signal (19cm)

[!INFO] built on top of DGPS assumes you use diff GPS with custom rover which is close to you