It is difficult to acquire GNSS (Global Satellite Navigation System) signals in challenging environments such as urban canyons and indoors without any assistance information. The time-to-first-fix process is extremely long for positioning in such environments, especially for the case when the user is moving. In order to speed up the time-to-first-fix process, the receiver needs assistance information from a server for quickly acquiring and tracking the GNSS signals. This chapter introduces the assisted GNSS solution. It covers the topics of acquisition assistance, sensitivity assistance, and implementation for smart phones. The objective of the chapter is to provide the readers with a general overview of the Assisted-GNSS (A-GNSS) solution from the aspects of how the A-GNSS solution speeds up the acquisition process and improves the tracking sensitivity, what kinds of assistance data are needed, and how the assistance data is delivered to the mobile users. The chapter includes sections for an introduction to the topic, acquisition assistance, sensitivity assistance, A-GNSS implementation for smart phones, future directions, and brief conclusions.
TopIntroduction
GNSS (Global Navigation Satellite System) is the most common positioning technology used in smart phones. However, it typically takes a couple of minutes for the GNSS receiver to start providing locations after it has been turned on. This is especially true for the case when the receiver has no knowledge about the GNSS constellations. The time to obtain the initial position is called the Time-To-First-Fix (TTFF). It can take minutes in the signal challenging environments such as urban canyons and indoors. The main reason for a long TTFF is that the receiver needs time to
- •
Acquire the GNSS signal in order to obtain the ranging measurements, and
- •
Decode the navigation data, which include satellite clock corrections, ephemerides and other parameters.
A GNSS signal typically consists of three components: a carrier, a PRN (Pseudorandom Noise) ranging code and the navigation data as shown in Figure 1. For the GPS (Global Positioning System) L1 C/A (Coarse/Acquisition) code signal, the frequency of the carrier is 1575.42 MHz, the length of the PRN code is 1023 chips repeated for every millisecond and the navigation data rate is 50 bps (bit per second). The length of one navigation data bit is therefore 20 milliseconds.
Figure 1. The GPS L1 C/A code signal
Acquisition of a GNSS signal requires exploring a 2D frequency/code-delay search space. The search range for the frequency is about [-6, 6] kHz, while that for the code-delay is 1023 chips. The search range for frequency depends mainly on the Doppler shift and the frequency uncertainty of the receiver oscillator. The Doppler shift is caused by the relative velocity between the satellite and the receiver. Typical search step in the frequency dimension is 500Hz in the signal acquisition state. For weak signal acquisition and tracking, a smaller search step is required. In order to make the explanation more simple, a frequency bin size of 500Hz is adopted for this chapter. The search range of the code-delay depends mainly on the signal travelling delay (from satellite to receiver) and the clock error of the receiver. The code-delay search step is 0.5 chips in the acquisition state. This forms a frequency/code-delay search space of 24x2046 bins for each satellite.
For an unassisted solution, the acquisition process needs to explore the whole search space in order to identify the frequency/code-delay bin that has the maximum correlation. It takes a few tens of seconds to complete the search process depending on the number of correlators implemented in the receiver. The pseudorange measurement is determined by converting the code-delay to signal travelling time and then multiplying the signal travelling time with the speed of light.
Having completed the acquisition process, the receiver starts receiving the navigation data bits that include the satellite clock corrections and ephemeris. The transmission time of each navigation data frame is 30 seconds. Each frame consists of 5 sub-frames, and the transmission time for each sub-frame is therefore 6 seconds. The navigation data is transmitted in the first three sub-frames. Therefore, it takes 18 seconds, in the best case, to receive a complete navigation data set. Whenever there are bit errors that occur during this 18-second period, the receiver will have to wait for the next frame and try again. This process could take minutes or even tens of minutes in challenging signal environments such as urban canyons or indoors because the probability of the occurrence of bit errors is very high under such circumstances. Decoding navigation data bits require relative strong signals, for signals with strengths below the threshold, the receiver is not capable of decoding the navigation data bits. This is why the TTFF is typically very long in urban areas and indoors. The TTFF will be even longer if the receiver is moving around in such environments.
In order to speed up the TTFF, the following aspects have to be improved:
- •
Reduce the acquisition search space, and
- •
Obtain the satellite ephemeris and clock corrections from terrestrial sources.