Draft:SuperGPS

• Comment: There are large sections of unsourced text in Performance and the lead section. Lewcm Talk to me! 08:41, 18 December 2023 (UTC)

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SuperGPS is a Dutch research project that targets the development of network-based timing and positioning systems that can be more accurate and reliable than Global Navigation Satellite Systems (GNSS). The system, colloquially known as 'SuperGPS' after the name of the project, actually represents a local positioning system that operates independently from GNSS. In 2022, results of a proof-of-principle demonstration were published showing that the system can achieve positioning with centimeter-to-decimeter uncertainty in an urban environment, along with wireless time distribution with residual time errors of below one nanosecond.

Concept

The system consists of two subsystems: a fiber-optic network used for time synchronization of network nodes, and a constellation of base stations that are located in the nodes, and which synchronously transmit radio signals for positioning, navigation and timing (PNT) of a mobile receiver via pseudo-range multilateration.. Conceptually, this architecture bears resemblance to that of mobile telecommunication networks, and one of the aims of the SuperGPS project has been to make the system compatible with existing (mobile) telecommunication networks. To this end, the system makes use of generic components and modulation formats for optical and wireless networking, such as universal software-defined radio peripherals that employ orthogonal frequency-division mutiplexing transmission, and White Rabbit Ethernet equipment for sub-nanosecond time distribution. Because of its joint positioning and telecommunication functionality, the system has also been called a terrestrial networked positioning system (TNPS)

Performance

The White Rabbit subsystem enables time synchronization of Ethernet network nodes and base stations with an uncertainty of about one tenth of a nanosecond. To this end, the White Rabbit system continuously estimates and corrects for the propagation delay of the data symbols travelling through the fiber-optic connections. This synchronization scheme allows base stations, located in the network nodes, to synchronously transmit radio messages for PNT, which propagate at nearly the speed of light of 299,792,458 meters per second. A residual timing error of 0.1 nanosecond therefore translates to an intrinsic ranging error of about 3 centimeters, which forms an indication of the hypothetical positioning performance of such a system.

The positioning accuracy, however, is also dependent on the occurence of multipath propagation; that is, the presence of delayed copies of the radio signal caused by reflections off buildings and other large objects. Multipath signals can be more easily resolved from the direct line-of-sight signal if a larger radio bandwidths is used, and for this reason the initial SuperGPS system employed a bandwidth of 160 MHz, significantly larger than the 20 MHz bandwidth associated with GNSS radio signals. Proof-of-principle measurements carried out in the SuperGPS project in an urban outdoor envrionment yielded positioning errors due to multipath propagation of 1 to 2 decimeters.. Likewise, multipath effects degrade the accuracy with which clock of the receiver unit can be synchronised, leading to time errors of several tenths of a nanosecond

Other potential sources of positioning degradation are reduced visibility of base stations and geometric dilution of precision.

Compatibility with telecommunication networks

The implementation of the SuperGPS system would benefit significantly from the re-use of existing telecommunication infrastructure. The system design therefore incorporates equipment and techniques that enhance the compatibility with existing optical and wireless networks, with examples given below.

White Rabbit time synchronization

The wavelength channels allocated for the White Rabbit system can be selected by the network operator, so that White Rabbit signals can be merged with other flows of optical data that make use of the same fiber-optic cable using cost-effective wavelength-division multiplexing. It has also been shown that White Rabbit can operate over free-space millimeter-wave communication links.

Radio signal structure and modulation format

The PNT radio signals are transmitted using OFDM modulation, which is the same modulation format as used in 4G, 5G and Wi-Fi wireless networks. It has been argued that the PNT signals may be designed such that more than 99% of the wireless capacity remains available for telecommunication.

Radio bandwidth considerations

Radio spectrum is scarce, and spectra licensed to mobile network operators typically consist of several bands with widths of 5 to 20 MHz, spread out over the range 400 to 2600 MHz. Achieving a spectral bandwidth of approximately 200 MHz, as required for decimeter-level positioning in multipath-rich environments, may therefore be considered a challenge. To address this, the SuperGPS prototype system made use of sparse virtual bandwidths that occupy only a fraction of the total bandwidth. Such virtual bandwidths might be tailored so as to match the footprint of licensed mobile spectra, which themselves resemble sparse virtual wideband spectra.