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Receiver Types
| Receivers | |
|---|---|
| Title | Receiver Types |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
GNSS receivers can be categorized by their type in different ways, and under different criteria. Besides the professional-grade receivers (e.g. survey and precision), commercial Portable Navigation Devices (PND's) are very common inside vehicles today, and smartphones appear more and more equiped with integrated GNSS receivers. These receivers are implemented in a wide variety of platforms, from ASIC, DSP or FPGA, to general purpuse microprocessors. The choice of the target platform is often a trade-off of parameters such as receiver performance, manufacture and maintenence cost, expandability, power consumption, and autonomy. Some of the diferentiating applications and receiver implementations, differing in a number of design decisions and approaches to GNSS solution computation, are described in the following topics.
Multi-constellation
With the emergence of multiple satellite navigation systems (both regional and global), multi-constellation receivers are becoming widely available. This has been encouraged at system design level by working towards interoperability and compatibility among all systems, allowing for seamless combination of the different signal spectra and processing chains into a single, multi-constellation GNSS solution.This approach reflects on the four global GNSS receiver implementations:
From the receiver perspective, multi-constellation brings a key added value on solution availability, especially in urban canyon environments: with the increased number of constellations available, the number of satellites visible to the user is bound to increase. This allows several algorithm implementations to be further refined, and the final solution can be computed with higher accuracy and availability (for instance, see the improvements due to higher availability in Dilution of Precision (DOP)).
Multi-frequency
Several GNSS signals are allocated to different frequencies - for instance, the GPS L1 and L2 bands. Whether in single or multi-constellation approaches, receivers can benefit from multi-frequency signal processing for mitigation or removal of the frequency-dependent errors on the signals, hence improving receiver accuracy. The most important example is the correction for ionospheric delays, since these usually represent the main contributors to the overall measurement error.
Multi-frequency receivers, however, bring forth a new challenge, since there is a need for increasing RF hardware sections. Typical antennas, front ends, and filtering/sampling circuits are centered around one of the desired frequencies, and in most cases the same ammount of RF hardware is replicated for the other frequency (or frequencies) to process. For this fact, there is also trade-offs implied between cost, size, power consumption, performance, signal and band filtering, and analog circuitry quality.
Augmentation
GNSS receivers can also benefit from corrections or measurements provided by the available augmentation systems to improve their accuracy and performance. As the name implies, such systems aim at providing augmentation information to the GNSS users, consisting of corrections and/or auxiliary measurements that increase precision and accuracy in the calculated solution. As examples of receivers that use augmentation information, see:
Differential
Differential techniques enable improved receiver accuracy by providing the receiver with additional information, such as measurements from other receivers in the vicinities, or corrections computed independently. Such external information is then used within a receiver in a differential way, ainding in . Some of the most widely used differential techniques used in current receiver technology are:
Assistance
The definition of assisted-GNSS[1] (A-GNSS) gathers many different concepts, but can be split into two main cattegories:
- GNSS assistance information is used to improve acquisition speed: an assistance network, comprised of servers and information relays, transmits almanac and/or ephemeris data to the receiver, so that the initial searh for satellites can be performed faster. This allow the receiver to start tracking visible satellites quicker, thus providing a navigation solution in less start-up time.
- Data processing and solution computation is performed in the server: in this case, the receiver can send measurements like visible satellites, pseudoranges or phase information to the servers, where the heavier computational load on generation an accurate sollution is performed, and the results are sent back to receiver.
The assistance information can be accessed by the receiver beforehand (e.g. via Internet), or received on request (usually through wireless comunications). So, assisting information can be provided by different technologies, such as WiFi, GPRS/UMTS, or the internet. Depending on the solution envisaged, this might have an impact at several levels, such as availability, continuity, and power consumption.
Tracking and solution computation
This distinction on GNSS receiver types indicates whether the solution is computed from the pseudorange measurements (which are noisier), or from the carrier measurements. Although the latter are less noisy, they present a natural ambiguity related to phase measurements, that needs to be solved at receiver level - see some of the available techniques for fixing the carrier phase ambiguity here. The trade-off, in this case, is complexity and processing power versus accuracy[2].
Services
GNSS receivers may also support multiple services provided by the current and future GNSS generations. As an example, Galileo will provide four services: Open Service (OS), Commercial Service (CS), Public Regulated Service (PRS) and Safety of Life (SoL) services. Receiver architectures can also reflect the different services they are bound to use, as different services may be available on different frequencies, modulations, as well as carrying different messages.
Related articles
For a generic description on GNSS receivers, please visit the following link:
References
- ^ For a detailed description of different approaches in the A-GPS case, see Kaplan, E.D. et al, "Understanding GPS: Principles and Applications", second edition, chapter 9, section 9.4.
- ^ For details on different approaches in the signal tracking loops, see the signal processing articles.