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 potencial 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, such as:

Channel management: each channel is managed depending on the opeartion mode and current status, and resources are allocated as needed. For every step of the computations, each channel is seamlessly controlled in the receiver.

Signal processing: from signal capture at the antenna up to successfull tracking and data demodulation, different processing tasks are performed for each receiver channel for baseband processing. Examples are the correlation operations or the tracking loops.

Navigation solution computation: with the observables from signal processing stages, together with almanac and ephemeris from the data messages, the receiver periodically computes the different outputs, such as the PVT solution.

The following sections detail several operation modes controlled and managed within a receiver, that fall under the above categories.

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.

The describel realm of measurements are the basics for which a GNSS signal receiver operates, and a receiver typicaly processes and refines these measurements in different stages of operation.

Operation modes

One thing most GNSS receivers have in common is how they operate in terms of processing chain, from reception of signals to solution outputs. Alhough their types, architectures, and applications may vary, the operations of receivers apply transversly:

RF stage: the first stage of a receiver comprises the antenna and front end, responsible for the RF processing chain.

Start-up: a receiver starts processing the samples from the RF stage in different ways, depending on the available a priori information.

Acquisition: the first step, in many cases, is the search for visible satellites that can be tracked and used in the computed solution.

Tracking: after the initial determination of the satellites to track, several signal processing algorithms are use to continuously track the satellite's motion relative to the user.

Lock detection: a receiver constantly performes decisions to assess the quality of the observables in terms of noise or uncertainty, and these decisions determine the probability of a given tracked signal being "lost". This decision can yield a loss of lock, and the receiver channel is free to process other signals (e.g. start acquisition for other satellites).

Solution computation: when tracking is successfull for a large enough period of time, and for enough satellites, the navigation message can be decoded from each signal, and the extracted information is used to compute the best solution needed - in most cases, the user's PVT solution.