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Receiver Operations
| Receivers | |
|---|---|
| Title | Receiver Operations |
| Author(s) | GMV |
| Level | Medium |
| Year of Publication | 2011 |
In the first years of GNSS, when only GPS was available for public use, a receiver was not expected to perform as the navigation devices we see today. In fact, a first-generation GPS receiver could be designed to only process 4 or 5 signals at any given time, and it was deemed suitable for positioning applications. Today, with the increased availability and potential of different GNSS signals and constellations, any receiver is expected to track at least 10 or 12 signals in parallel channels, and up to hundreds, resulting in a more accurate solution. However, at their core - and despite the many differences in target applications or implementation philosophy - most receivers still share common control and processing flow management properties, resulting in common operations performed. The following sections describe these common operation modes in which a receiver functions.
Background
A GNSS receiver main objective is to determine its distance to a set of visible satellites, by tracking their transmitted signals. These signals have and underlying periodic code modulation that is precisely time-tagged by each satellite. Since the satellite time reference is very accurate and the same for all satellites, a receiver can determine the time it took for each signal to arrive, thus determining its relative position to each satellite. This process translates into a pseudorange measurement, which is a rough estimate of the user-satellite distance (not compensated for signal transmission delays and clock offsets).
Besides the pseudorange, other measurements are extracted in successful tracking operations: the carrier phase, defined as the phase difference between the received and the internally generated carrier (codeless), is a measurement of the cycle turns (accumulated wavelengths) tracked by the receiver. This observation is very precise, but ambiguous, since the total number of integer wave cycles from transmission to reception is unknown. Also, the Doppler frequency seen by the receiver, caused by the dynamics of the satellite-user system, represents the difference between received and internally generated signals, and is a good representation of the relative velocity between satellite and receiver.
As a result, GNSS receivers operate in a way that these observables are continuously determined and refined in order to track the satellites, demodulate the navigation message and data, and compute the desired solution as accurately as possible.
Operations
One thing most GNSS receivers have in common is how they operate in terms of processing chain, from reception of signals to solution outputs. Although their types, architectures, and applications may vary, the operations (illustrated in Figure 1 from top left to bottom right) apply transversely.
Once the receiver is turned on, there is a start-up sequence needed to ensure that each channel is managed depending on the current operation status, and resources are allocated as needed. Before a receiver starts processing the samples from the RF stage, this initial operation mode sets up the necessary data and processes to ensure the best performance possible, also depending on the available a priori information (e.g. previous almanac or ephemeris information).
As start-up and configuration is concluded, the antenna and the front end blocks start providing digitized data continuously to the signal processing channels. This input data can be in the form of real IF samples, or baseband (real and complex representation of the signal in I and Q components), and the receiver starts operating in signal processing for acquisition and tracking, with no PVT solution available.
Each receiver channel acquires and tracks an input signal, by means of several correlations between the input signal and a local replica of the carrier modulated with the PRN code. These correlations, together with the tracking loops, provide sufficient information on the satellite's motion relative to the user, and code delay and carrier phase information is provided, yielding successful synchronization with the navigation message. The extraction of navigation data begins, and a degraded PVT solution can be provided as enough data is available: for instance, if only 3 satellites are tracked, a 2-D solution can be assessed, assuming a constant known height.
As the navigation messages are decoded from each satellite signal, almanac and ephemeris data is continuously gathered in the receiver, providing more and more information on the behavior of the GNSS constellation as a whole (at least until the full almanac is decoded). This information can be used to inform the acquisition process of which satellites are theoretically in view, enabling the search space to be refined. Also, from the navigation message, ephemerides for each satellite are continuously updated, and receivers can monitor and refine several correction computations to apply to the pseudoranges. When this process is successful for a large enough period of time, and for enough satellites, the extracted information is used to periodically compute the optimal PVT solution.