GPS Receivers

GPS
Title	GPS Receivers
Edited by	GMV
Level	Basic
Year of Publication	2011

A GPS Receiver is a L-band radio processor capable of solving the navigation equations in order to determine the user position, velocity and precise time (PVT), by processing the signal broadcasted by GPS satellites.

The GNSS Market Report, Issue 3, provided by European GNSS Agency, has estimated that the number of GPS enabled devices in 2012 were about two billion units.

GPS Receivers

Any navigation solution provided by a GNSS Receiver is based on the computation of its distance to a set of satellites, by means of extracting the propagation time of the incoming signals traveling through space at the speed of light, according to the satellite and receiver local clocks.

Notice that satellites are always in motion, so previous to obtaining the navigation message, the satellite’s signal is detected and tracked. The receiver’s functional blocks that perform these tasks are the antenna, the front-end and the baseband signal processing (in charge of acquiring and tracking the signal).

Once the signal is acquired and tracked, the receiver application decodes the navigation message and estimates the user position. The Navigation Message includes:

• Ephemeris parameters, needed to compute the satellite’s coordinates.
• Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions.
• Service Parameters with satellite health information.
• Ionospheric parameters model needed for single frequency receivers.
• Almanacs, that allow computing the position of all satellites but with a lower accuracy than the ephemeris.

The ephemeris and clocks parameters are usually updated every two hours, while the almanac is updated at least every six days.

The GPS Signal In Space is specified in the following documents:

• IS-GPS-200E: Interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for radio frequency link 1 (L1) and link 2 (L2)
• IS-GPS-705A: interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for radio frequency link 5 (L5).
• IS-GPS-800: interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for signal L1 Civil (L1C) transmitted in the frequency band of L1.

Particularities

Each GNSS system uses a specific Reference Frame; although a multi-constellation receiver is able to convert all information to the same common frame, a GPS-only receiver uses the WGS-84 reference frame. In an analogous way, each system has its own time reference defined by the respective control segments; the time reference for GPS is called “GPS Time” (GPST).

Each GNSS System transmits its own navigation message, defined in the respective Signal In Space Interface Control Documents, SIS ICD. As an example, GNSS satellites transmit information that allows the receiver to compute their positions. For the case of GPS (in-line with Galileo but unlike GLONASS), the satellites transmit the orbit parameters as updated by the Ground Segment and refreshed every 2 hours. The GPS receiver then computes the satellite position based on these transmitted ephemeris parameters. Another distinction regarding the transmitted navigation message with impact on the receiver is the ionospheric parameters transmitted to support the single frequency receiver in computing the ionospheric error; GPS uses the Klobuchar Ionospheric Model.

GNSS signals modulation, structure, navigation message contents and formats are often different among signals from the same system and from different systems. Most of these characteristics are easily implemented at the receiver (i.e. requiring only “software modifications”, such as the use of different PRN codes or the ability to cope with different message structures). The main difference among GNSS receivers falls into the specific characteristics that have impact at RF level, such as the Multiple Access Techniques employed. GPS (as Galileo and BeiDou) uses a CDMA technique allowing a simpler RF module (than for example GLONASS), since all signals in the same frequency band have a common carrier.

It should be noted that the current trend consists on facilitating the access of each system to the receivers, i.e. fomenting multi-constellation receivers. Hence, most discussions and agreements among the systems’ responsibles are conducted in the sense of taking this effort out of the user segment, focusing on compatibility and interoperability aspects in the system design.

Types of GPS Receivers

Receivers can be categorized by their type in different ways, and under different criteria. For instance, receivers can be stand-alone, or may benefit from corrections or measurements provided by augmentation system or by receivers in the vicinities (DGPS). Moreover receivers might be generic all purpose receivers or can be built specifically having the application in mind: navigation, accurate positioning or timing, surveying, etc. In addition to position and velocity, GPS receivers also provide time. An important amount of economic activities, such wireless telephone, electrical power grids or financial networks rely on precision timing for synchronization and operational efficiency. GPS enables the users to determine the time with a high precision without needing to use expensive atomic clocks.

The initial purpose of the GPS system was military but with the free availability of GPS signals and the availability of cheap GNSS receivers, the GPS technology is having a pervasive use in civil, industrial, scientific areas. Currently the use of GPS in Civil Applications is generalized, and it is well known that GPS Receivers have been spread very fast as well as the manufacturers dedicated to this (e.g. CSR, BroadCom, Garmin,...).

Related articles

• GNSS Receivers General Introduction
• Generic Receiver Description
• System Design Details
• Receiver Characteristics
• Interfaces and Protocols