United Kingdom

Alternate means of navigation are needed for when GNSS signals are unavailable. Satellite signals of opportunity have the advantages of global, 24/7 cover and are independent of any local outages that may disrupt terrestrial transmissions. This paper examines the use of Iridium, Globalstar and Orbcomm satellite signals for navigation on a moving platform. These signals are intended for use with small user terminals and so can be exploited with a relatively simple antenna array and receiver. The use of direction of arrival as well as Doppler and time of arrival measurements is examined. Data gathering trials are conducted using a vehicle platform. Measurements from all three constellations are combined with IMU data in an extended Kalman filter and the performance assessed compared with GNSS ground truth. It is found that the navigation accuracy achievable is predominantly dependent on the accuracy of the knowledge of the satellite positions and velocities. Using the SGP4 TLE information available from space-track.org, median radial errors around 400m are achieved when driving through a variety of land environments.

Navigation 2021

Using Satellite Signals of Opportunity for Alternate Navigation

### Welcome to Navigation 2021!

Our aim was to bring diverse disciplines together to stimulate insights and knowledge sharing towards a more navigable world. We hope we achieved this!

Navigation and timing are critical to the world we all inhabit. We believe that the work of the wider navigation community enables societal benefit including support to national grand challenges and also the United Nations’ Sustainable Development Goals. To achieve this requires collaboration and understanding across science, technology and practice.

Navigation 2021 is about much more than technology innovation, it also showcases science and research from across the wider community, including animal navigation, cognition and human factors.


Our aim was to bring diverse disciplines together to stimulate insights and knowledge sharing towards a more navigable world. We hope we achieved this!

technical paper

Timing has become a critical resource for various public and defense infrastructure. GNSS systems have been the defacto standard for providing timing service for these infrastructure. Various studies have concluded that GNSS system can be spoofed; hence a need to have multiple systems providing timing information to maintain the resilience. NextNav has developed a TerraPoiNT system, a terrestrial-based positioning, navigation and timing system which can complement existing systems as well as serve as a backup to the GNSS. TerraPoiNT system (a.k.a Metropolitan Beacon System), a type of Terrestrial-Beacon system, is a Time-Of-Arrival multilateration system offering 3D positioning and timing through a dedicated network of synchronized transmitters using Code Division Multiple Access spread spectrum signals. TerraPoiNT beacons can synchronize between themselves without the need for an absolute time source like GNSS, thus proving to be an ideal alternative to the GPS system. Beacons have a timing estimation sub-system which has access to one or more stable time sources like a precise atomic clock (such as Cesium/Rb) allowing long holdover time to be maintained. Each beacon can listen to multiple beacons adding resiliency into the network. This self-synchronization capability is scalable to a large network with only a few beacons requiring absolute time source or atomic clocks due to multi-hop beacon time transfer capability. TerraPoiNT System capabilities were assessed by the U.S. Department of Transportation at NASA-LaRC as part of the alternate navigation technology assessment for positioning, navigation and timing. TerraPoiNT timing accuracy in static indoor test over 72-hours was within 25 ns (95%) and within 30 ns (95%) in a basement location indicating an accurate and robust performance in a challenging locations as well as long-term stability. TerraPoiNT system demonstrated accurate, reliable and robust performance and it was rated #1 among all the technologies evaluated in position and timing trials

TerraPoiNT: An Advanced Terrestrial Timing System

Session 1.6.

Cryptochromes are blue light receptors found in plants and animals which have been linked to magnetosensitivity in multiple organisms. Evidence from migratory birds has shown that the avian Cry1a is localized to the eye in structures consistent with a role in magnetosensing (1). Furthermore, immunohistochemical staining of the avian retina showed that Cry1a undergoes conformational change in response to light at the wavelengths (UV, blue, green) which are active in mediating magnetosensitivy. Significantly, birds are able to orient to the earth’s magnetic field in near-UV and green (560nm) light in addition to blue light, and also are oriented under light pulse conditions wherein the magnetic field is applied only in the dark intervals between light pulses. These behavioural, obtained in multiple studies over 30 years from different labs, data exclude any possiblity that the avian Cry4 could function as magnetoreceptor via formation Trp/Flavin radical pairs, as has been recently suggested (2), since Cry4 does not occur in structures that can absorb UV light, and since the Trp/Flavin radical pair is not formed in green (565nm) light. This leaves Cry1a as the only possible candidate cryptochrome magnetosensor; however due to failure of prior efforts to purify rCry1a with flavin bound, the photochemical properties of the Cry1a have remained elusive. Here we provide evidence for two key properties of the avian Cry1a protein that support its role as a possible magnetosensor; namely its light wavelength sensitivity and its ability to undergo biologically relevant conformational change. The avian Cry1a was over-expressed in a recombinant mammalian HEK cell expression system. The expressing cells were passaged in the dark, then illuminated at differing wavelengths of light. Conformational change in the Cry1a protein was detected using a C-terminal antibody that specifically recognizes (binds) the light-activated form of the protein, but not the inactive state. Under UV, blue, turquoise, and green light, a significantly higher concentration of rCRY1a was immunoprecipitated with this antibody than in red light (650) or darkness. This indicated that the rC1A underwent C-terminal conformational change in response to light in this heterologous system. As a more direct probe for conformational change in rC1A, a small 13aa. epitope from the luciferase gene was introduced into various locations within the C-terminal domain of the rC1a gene, and the resulting construct expressed in HEK cells. When this epitope is exposed at the rC1A protein surface, it results in a strong bioluminescence signal (Promega inc. HIBIT lytic system). Illumination with UV, blue, or green light induced a strong bioluminescence signal change whereas red light or darkness did not. In this way, direct evidence of a light-dependent conformational change at the C-terminal of the rC1A was obtained, which moreover corresponded to the wavelength sensitivity of navigation. Finally, we discuss our results from in vivo spectroscopic analyses supporting that the Cry1a protein is flavin bound in vivo but loses the flavin upon storage, cell lysis and/or purification. 
In conclusion, our data support a mechanism wherein flavin in the avian Cry1a occurs as a mixture of oxidized/radical redox forms in vivo which, upon photoreduction, undergo conformational change to the biologically active reduced (FADH-) form. These findings fit both the wavelength spectrum for avian magnetosensing and the histochemical data for activation of Cry1a in the bird retina, and thereby provide additional support for Cry1a as an avian magnetosensor. 


Cry1a Cryptochrome has the Characteristics of a Receptor for Avian Magnetosensing

Many animals make direct paths back to their homes after tortuous foraging trips using a navigational strategy called path integration. During path integration, sensory information for direction and distance measurements are used to estimate the straightest path back to home. Decades of work with honeybees have established that they are excellent path integrators that rely primarily on celestial cues for orientation and optic flow for measuring speed/distance while in flight. While path integration has been well studied behaviorally, the neural basis of vector memory formation is unknown in any animal. A detailed 3D electron-microscopical map of the navigation center of the bumblebee’s brain, the central complex, and a comprehensive anatomically-grounded computational circuit model suited to carry out path integration from these data predicts concrete cellular substrates for all computations required for path integration in the central complex. However, the ability to test the hypotheses the model raises in bumblebees is lacking. Working towards this goal, we have developed circular navigation arenas with a hidden nest entrance and feeder to demonstrate that bumblebees (Bombus terrestris) use path integration over relatively short distances while walking. By manipulating artificial celestial cues in the arena, we demonstrate that bumblebees orient to overhead polarization patterns and artificial sunspots when navigating back to their nest while walking. Also, by displacing actively foraging bumblebees to an identical arena lacking a nest, we show that bumblebees accurately estimate the distances they must travel while walking home to find their nests. These result, when paired with neuroanatomical data of the bumblebee central complex and hypotheses generated by computational models, offer the potential to investigate the neural basis of path integration in bumblebees in the near future.

Path integration in walking bumblebees

Low anthropogenic electromagnetic radiation, also known as “electromagnetic noise” or “electrosmog”, disturbs magnetic compass orientation in migrant songbirds under laboratory conditions. Since electric devices continuously emit equally low electromagnetic noise wherever humans live, it is a biologically highly relevant debated, yet still unsolved question of worldwide importance, whether electromagnetic noise affects the migratory behaviour of free-flying migrant birds in the wild. If so, ecological consequences were global and had wide-ranging implications from animal conservation to human health issues. Therefore, we conducted field experiments to assess the potential after-effects of electromagnetic noise on wild night-migratory songbirds. Similar to the laboratory experiments, we exposed European Robins (Erithacus rubecula) and Northern Wheatears (Oenanthe oenanthe) to electromagnetic radiation, but released the birds afterwards to observe a potential after-effect in their migratory behaviour via radio-tracking. Additionally, we measured birds’ migratory motivation in another experiment. We found no biological relevant effects of electromagnetic radiation on the migratory motivation, and no after-effects on the probability to resume migration, the departure timing within the night, their initial and continued (~50 km) flight direction. However, Wheatears in the electromagnetic noise-exposed group departed significantly later than the controls. As the difference in timing was only 30 seconds in median, in comparison to several hours of nocturnal migration time, we consider this effect as biologically irrelevant. Nevertheless, it shows that these birds put more weight on accuracy in their migratory decision than on speed, when sensing is disturbed or no information form the Earth’s magnetic field available, which is in line with evidence-accumulation-theory. Birds were either able to detect the magnetic field directly after electromagnetic noise exposure or switched to alternative compass systems. We want to highlight that finding no after-effect in our study design does not mean that this man-made factor might not affect migrating birds in other circumstances.

No apparent effect of anthropogenic electromagnetic radiation on the migratory behaviour of wild songbirds

GALILEO Global Navigation Satellite System: from Reality into the future of Navigation on Earth and beyond

Session 2.1.

AIMGNSS – A tool and framework for investigating the application of Machine Learning techniques to GNSS anomaly detection

Lost in spatial translation: The relevance of memory for real-world navigation and the effect of Alzheimer’s disease on allocentric and egocentric processing

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