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 many common control flow properties^[1], 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 receiver operation modes controlled and managed within a receiver, that fall under the above categories.

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.