Friday, June 20, 2008

Location Based Services for Mobiles II: GPS, Assisted GPS and Network Assisted GPS

[IEEE C802.16m-08/1105 Network Assisted GPS (N-GPS) Positioning in WiMAX/16m]
[Location Based Service for Mobiles I: Technologies and Standards]

GPS - Global Positioning System

GPS System
Figure 1. GPS System Archiecture: Space Segment, Control Segment and User Segment

GPS is a Global Navigation Satellite System for determining the positions of receivers using signals broadcast by satellites.  It was developed and operated by US government to enhance the effectiveness of allied and US military forces.  The first experimental Block-I GPS satellite was launched in 1978. Since 1983, GPS has become an aid to civilian navigation worldwide, and a useful tool for survey, commerce, and scientific uses.  As of September 2007, there are 31 actively broadcasting satellites in the GPS constellation.  Satellites orbit 20,163 kilometers above the earth at 3.87 km/s.  6 orbital planes, each with 4+ satellites. Typically 6 to 12 satellites are visible from any place on the earth.  GPS based positioning is playing a critical role in modern location based services.              
Figure 2. The Block Digrame of GPS Positioning

A GPS receiver measures approximate distance to 3 or more satellites. It measures the time required for signal to get from the satellite to the receiver. It then calculates the distance and obtains satellite positions from satellite broadcasts. Finally it calculates the position using trilateration. During the positioning, it corrects for errors to improve accuracy with  calibrating the clock bias or applying differential correction.  It also corrects deliberate noise, such as selective availability and caliberates variable ionospheric and tropospheric propagation delays. 
  1. All GPS satellites transmit on L1 and L2 frequencies.
    • Each satellite uses different ranging codes: C/A code and P-code
    • L1 band is for civilian use. 
  2. The C/A code (coarse/acquisition code) is modulated onto the L1 carrier only, while the P-code (precise code) is modulated onto both the L1 and L2 carriers. 
  3. The C/A code is less precise and less complex than the P-code and available to all users.
    • The P-code is intended for military uses and is added to both L1 and L2. 

GPS Message 
 Figure 3. GPS Frame Structure and Navigation Data

  • TLM – Telemetry: 30 bits, sent at the beginning of each frame.
    • It is used for data synchronization and satellite maintenance.
    • They are usually constant for any one satellite for a long period of time.
  • HOW – Handover Word: 30 bits, sent after TLM.
    • It indicates the time at the beginning of the next subframe.
    • It also contains a sub-frame ID, some flags and parity bits.
  • Ephemeris: It is sent in each frame by each satellite.
    • It may take the GPS receiver up to 30 seconds to acquire Ephemeris.
  • Almanac: It is spread out over all 25 frames of the message.
    • For receiving the complete Almanac, the GPS receiver may need about 12.5 minutes.

 GPS Error Sources
Figure 4. GPS Positioning Error Sources 

Assisted Global Positioning System

Assisted GPS (A-GPS) with assistance server were first come out by Bell Labs and later developed to enhance the positioning performance of a GPS receiver and satisfy US FCC's E911 mandate. 

Figure 5. The Block Diagram of A-GPS System

GPS assistance server can increase the capability of a stand-alone GPS receiver.  It can roughly locate mobiles along by itself. It can supply more GPS orbital data to the mobile. It has better knowledge of atmosphere conditions and other errors as well as better augmentation capability. With the GPS assistance server, A-GPS helps improve positioning in terms of

  • Location accuracy: the positioning error.
  • Yield: the positioning success rate.
  • Time to fix: the time for positioning.
  • Battery consumption: power consumption for positioning.
  • Mobile cost
Network Assisted Global Positioning System

Figure 6. The Concept of N-GPS
With N-GPS, key GPS assistance is provided through control plan instead of user plan. No additional data channel setup overhead required. No additional layer-3 authentication or access control required. No roaming issue. It is more reliable than layer-3 A-GPS approaches and more efficient since the assistance data is periodically broadcasted. It is fully compatiable with most existing A-GPS receivers.

Figure 7. An Application Scenario of N-GPS

Table 1:  A Comparison fo GPS, N-GPS and A-GPS

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