Assisted GNSS

In a conventional, standalone GNSS receiver the main drawback is the long time to demodulate the navigation data message (ephemeris, clock corrections etc). If a GPS receiver could acquire the navigation signals instantly, an additional at least 18 seconds of continuous tracking would be required to demodulate the 50bps navigation data message for each satellite to extract the orbital parameters (ephemeris) and clock corrections. For Galileo, dependent on the signal acquired, up to 30 seconds of continuous tracking may be required to extract ephemeris and clock correction parameters.

Network assisted GNSS methods allow the increase in sensitivity and decrease in the acquisition time of the GNSS receiver. Acquisition time is reduced because the Doppler and code phase search space is much smaller than in conventional GNSS as the search space has been computed by the cellular network. This allows for rapid search speed and much narrower signal bandwidth, which enables enhanced sensitivity by allowing the receiver to dwell longer in each reduced code/Doppler search bin.

The cellular network sends the receiver a copy of the navigation message and hence the receiver does not need to extract the navigation data from the satellite signal. Time-to-First-Fix can be reduced to below 18 seconds.

In adverse signal conditions an assisted GNSS might be the only option for navigation. This is because the drop in signal levels makes it impossible for the GNSS receiver to obtain a copy of navigation message through the satellites. However, when the navigation message is provided to the receiver from the communications network, navigation is again possible. This feature is important in indoor conditions as well as in urban areas, where signal levels may vary significantly due to buildings and other obstacles.

The Assisted GNSS work package of GREAT project  addresses the three key areas of the development work which are the Assistance data generation and communications system simulation, receiver aiding information generation and GNSS signal acquisition, data fusion using assistance data and additional cellular measurements.

The main activity in this work package is the development of receiver algorithms to exploit the assistance data for TTFF reduction and sensitivity enhancement. This primarily affects the acquisition functionality of the software receiver. Areas to investigate in this work package include the following:

• Optimal BOC (1, 1) signal acquisition techniques in weak signal scenarios (BOC (1, 1) acquisition strategies).
• Coherent double side-band acquisition / Incoherent double side-band acquisition / Single side-band acquisition
• Pilot only acquisition or combined pilot/data channel acquisition
• Estimation of probability of false peak acquisition in weak signal environments
• Handling of cross correlation effects
• Impact of assistance data quality on performance
• Variable dwell time (combination of coherent and incoherent integration)

Improved TTFF (small search space) is crucial to the user, for instance, when positioning an emergency call. Assisted GNSS can also be of great help as it increases sensitivity of the searching process, which in turn improves the accuracy and yield. The use of cell phones with position location functionality enables a fast array of location based services enabling more possible applications with a better accuracy. With the help of assistance data, acquisition/tracking is possible at a very low SNR without the need to demodulate the navigation message and in general, where the demodulation is not possible. In addition to all these, as the continuous tracking of the satellite signal can be avoided with the help of assistance data, power consumption could be reduced, which is essential in mass-market type of GNSS receivers in a mobile phone environment.

Figure 1: Assisted GNSS positioning in Cellular Network

Figure 2: An example of Acquisition search space