N-GPS: Network Assisted GPS
Copyright 2008 by Shu Wang, toswang@gmail.com . All Rights Reserved     << THE PREVIOUS PAGE

GPS based positioning plays a critiical role in modern location based services. 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 1. GPS System Archiecture: Space Segment, Control Segment and User Segment


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.   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.  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.
 Figure 2. GPS Frame Structure and Navigation Data
1) 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.
2) 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.
3) Ephemeris: It is sent in each frame by each satellite.  It may take the GPS receiver up to 30 seconds to acquire Ephemeris.
4) 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.


Figure 3. The Block Digrame of GPS Positioning


A GPS receiver measures approximate distance to 3 or more satellites.  The receiver measures the time required for signal to get from the satellite to the receiver. It calculates the distance and obtains satellite positions from satellite broadcasts. It calculates the position using trilateration.  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.



Figure 4. GPS Positioning Error Sources


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 device cost.


    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 if 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



     [Contribution: IEEE 802.16m Mobile WiMAX September 2008 meeting, Kobe, Japan]