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Positioning navigation and timing

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Modern transportation (and a host of other applications) are reliant on accurate radio-based navigation systems. We are working to improve and guarantee this accuracy, by understanding, mapping and predicting the physical effects and technological failings which can disrupt navigation signals. There are also many technologies which rely upon ultra-precise radio-timing signals. We work to understand and to compensate for the phenomena which can affect these signals.

Global navigation satellite systems

(Key people: Cathryn Mitchell, Jenna Tong, Julian Rose, Nathan Smith, Christopher Benton, Robert Watson)

A Global Navigation Satellite System (GNSS) uses timing signals from multiple satellites in order to calculate position. They are of vital importance to aviation, shipping, earthquake monitoring, search-and-rescue, and a wide range of other applications. The most notable example of a GNSS is the Global Positioning System (GPS) network. Other examples are the Russian GLONASS system, and the upcoming Galileo system.


A GPS satellite (Courtesy NASA)

GNSS operates by each satellite transmitting a signal containing both a reading from an on-board atomic clock, and precise information about its orbit. By measuring the time delay of these signals, it is possible to determine the distance that the signal must have travelled. By determining the distance to multiple satellites, the position of the receiver can be calculated.

See the How does GPS work? video for more information.

Ionospheric delay and scintillation

The accuracy and reliability of GNSS navigation can be affected by the ionosphere (the region in the upper extremes of the atmosphere, where the gas is partially ionized). The free electrons in the ionosphere slightly delay the timing signals, making the distance to the satellite appear larger, and so distorting the measured ground position. (For certain applications, this effect can be beneficial, as by measuring the delay from a network of receivers of known position, the distribution of electrons within the ionosphere can be deduced. See the space weather page for more information.)

Another ionospheric effect is scintillation, whereby the signal is observed to vary rapidly in brightness or phase. (The effect is analogous to stars twinkling in the night sky.) In extreme cases, scintillation can render GNSS unusable. We are involved in efforts to both understand scintillation further (see the space weather page) and to harden receivers against its effects.

Multipath distortion

GNSS accuracy is reduced by multipath propagation, whereby signals are reflected by other objects, rather than travelling directly from the satellite to the receiver. This makes the distance to the satellite seem longer than it actually is, leading to an inaccurate calculation of global position.


Variations in the signal-to-noise ratio (SNR) of a GPS signal, due to waves reflecting from a nearby building, and interfering with the direct signal.

Interference and jamming

GNSS is susceptible to interference from damaged, badly designed, or improperly configured radio equipment, transmitting at the same frequencies. It is also susceptible to deliberate jamming. Invert is part of the GAARDIAN (GNSS Availability, Accuracy, Reliability anD Integrity Assessment for Navigation”) project, which aims to detect such interference.


Prototype equipment for GAARDIAN project.

Invert is starting work on the Sentinel project, which follows on from the GAARDIAN project, and aims to determine the location of interference sources.

eLORAN

(Key people: Ivan Astin, Caspar Lebekwe)

The LORAN (LOng RAnge Navigation) system was developed from the 1940s onwards, for marine navigation. It has now largely been superseded by GNSS, but there is interest in reviving it, to provide an extra layer of reliability. The proposed eLORAN (enhanced LORAN) system will be accurate to within 8 metres, making it competitive with GNSS. Like GNSS, the accuracy of eLORAN can be improved by understanding the physical processes which can distort the signals.


Predicted accuracy of an eLORAN system around the UK, assuming that a given set of transmitters are constructed.

LORAN and eLORAN use ground-based longwave transmitters. At these wavelengths, the signals travel as a groundwave, meaning that they travel along (and close to) to the surface of the earth, and follow its curvature. The conductivity of the underlying sea or terrain influences the speed of the waves, which affects the timing delay between a transmitter and receiver, and so distorts the measured position. The relationship between timing delay and surface conductivity is nonlinear, making this a difficult problem to analyse.

If the signal travels over land, it is subject to diffraction from hills and other geographical features. In the long term, Invert would like to model and map these distortions, in order to predict and correct for the resulting inaccuracies.

Timing signals

(Key people: Julian Rose, Capsar Lebekwe)

Many technologies require highly accurate timing signals. For example, electrical grids require close synchronisation of power measurements, in order to detect faults. Data networks require accurate time-stamping of traffic, in order to coordinate sequence-dependent events, such as electronic financial transactions. Telecommunication systems, such as the mobile phone base-stations require extremely timing to prevent signals from different locations clashing.

An example of a timing signal, is that from the longwave transmitter at Anthorn. (This provides the signal for most radio-controlled clocks and watches in the UK.) The timing signal travels (like eLORAN signals) as a groundwave, and so is affected by ground conductivity. By modelling this groundwave propagation, it is possible to improve the accuracy of the signal.


Calculation of the signal to noise ratio of the Anthorn timing signal.

It is also possible to obtain accurate timing signals from GNSS. These are subject to the same ionospheric delays affecting GNSS navigation. Invert is working to calculate these delays, thus allowing them to be compensated for.