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GNSS Reconfigurable Antenna Based Enhanced Localization

Final ReportSummary - GRABEL (GNSS Reconfigurable Antenna Based Enhanced Localization)

Beam forming is widely used in military GNSS receivers mainly as a means to achieve resilience against jamming. The purpose of the GRABEL project is to exploit beam forming in commercial receivers to achieve improved reliability, availability and accuracy.

A fully working prototype using several antenna arrays (triangular, square, hexagonal) and including an AHRS for basic antenna attitude determination has been constructed and verified. Before putting it into operation several calibration procedures are needed, i.e.

- AHRS calibration;
- beam former calibration.

The field tests have shown that GRABEL is a useful enhancement to GNSS and that the objectives of the project are achieved, although beam forming alone will hardly justify the increased cost with respect to a classical single antenna system. The conditions where beam forming is of good advantage - i.e. interference or attenuated LOS signal - are in fact rather rare in the real world (outside military). If other enhancements to complement beam forming are developed however, GRABEL will become a good and interesting enhancement at least for higher-end navigators that need to guarantee a certain availability and quality of the computed navigation solution.

The GRABEL system is currently at a prototype stage and would need to be further developed into a commercial product. The results obtained so far allowed to identify the following priorities:

1. improve sensitivity of the GNSS base-band processor (SY1031);
2. integrate GNSS navigation with IMU/INS, possibly adding also odometers / absolute position sensors (wheels of the vehicle) to the navigation solution;
3. push map-enhanced navigation forward, i.e. to navigation solution level (the navigation filter should actively use the information from the map).

User-friendliness needs also to be improved. The whole calibration procedure - indispendable for operation - needs to be done in background, in a transparent way and without user intervention to be acceptable. The application software currently is also not particularly practical and easy to use and requires improvements.

Project context and objectives:

GNSS signal propagation is often characterised by the absence of a strong line of sight (LOS), many non LOS (NLOS, multipath) components, possible interference, noise and other degradations of the already weak GNSS signal received from the satellites, such as may be encountered e.g. in dense urban environments. Under good signal propagation conditions most GNSS receivers are able to deliver reliable navigation with good accuracies. However, when the GNSS signal is weak or absent (e.g. inside tunnels) navigation may become inaccurate or unavailable.

At the same time, the demand for accurate and reliable localisation is increasing. The requirements for location aware services are becoming more demanding, while the user community is growing.

The GRABEL research activities investigated and developed improved GNSS receiver techniques that make better use of the weak GNSS signals while mitigating the adverse effects of multipath signals, fading, shadowing and interference.

The key idea in GRABEL is the use of jointly optimised reconfigurable antenna arrays and baseband beamforming algorithms to improve the performance and reliability of GNSS receivers and wireless receivers more broadly.

Project results:

As a result of the GRABEL prototype and the various tests performed, it can be concluded that an antenna beam forming system can be a useful enhancement also in commercial GNSS applications, in particular as a complement to other common enhancement methods such as inertial navigation or aiding. Its characteristics can be summarised as follows.

Advantages of beam forming

Spatial filtering:
The purpose of spatial filtering is to attenuate NLOS, i.e. multi-path signals. In this case the interest is set to the attenuation at angles different than the direction of arrival of the LOS signal (satellite) not on the LOS gain. The larger the array, the higher the spatial selectivity will be. Spatial filtering is the main advantage that can be obtained in a commercial GNSS application. As an example, the navigation close to large reflecting walls (e.g. streets close to steep mountains or to tall buildings). In more difficult signal propagation environments - such as indoor where the signal mainly consists of NLOS components - beam forming will be of no advantage, or rather will be a nuisance since the already weak NLOS signals would be attenuated further or even lost.

LOS gain:
By pointing the beam towards the signal source the LOS gain increases ideally by 3 dB for every doubling of the number of antennas. In the real world this is however no great advantage, as LOS attenuation is a rare phenomena (e.g. navigation under foliage or wood). In cases where a great LOS attenuation can be expected (e.g. indoors, where the LOS signal is far below 30 dB of attenuation), beam forming cannot recover it unless a very large number of antennas is deployed and is therefore of no use. High sensitivity receivers are able to compute the position thanks to the NLOS reflections, although somewhat reduced accuracy has to be conceded. A near-isotropic antenna rather than a directive one has to be used in such indoor cases.

Interference rejection:
By steering radiation pattern nulls towards the source of interferences, jamming can effectively be reduced. This is the most common use of beam forming in the military world where GNSS systems have to handle a great deal of intentional jamming. Outside military strong GNSS interference is quite rare and comes mainly from second or third harmonics of TV broadcasting transmitters or RF generators for industrial processes. Occasionally intentional GNSS jammers are placed near ports or other 'sensitive' sites. Due to the small reach of such jammers they rarely represent a problem. Providing interference rejection, at least to higher end GNSS systems, will however be an added value.

Heading determination:
If a beam forming array with sufficient size is used - i.e. with a spacing of a couple between two antennas at the extremes of the array - heading could be determined by carrier phase measurements between the signals received by these two well spaced antennas. Although this has nothing to do with beam forming itself (apart from reusing for a different purpose the same antennas of the array), it is a quite useful function to find the travel direction while the vehicle is still standing in the park.

Disadvantages of beam forming

Large size of the antenna array:
In order to be effective an antenna array for beam forming has to have a two inter-antenna spacing. This is one of the most limiting factors which makes beam forming impractical for most applications. Even considering minimal arrays, such as a three or four antenna array, the resulting size is still rather large (about 12 cm or so per side) and unacceptable for all portable and most vehicle-mounted applications. Attempts at reducing its size by optical means (high dielectric in front of the antenna) will likely fail due to the large thickness that the array would assume (easily > 6 - 8 cm extra thickness).

Stability of array phase centre:
The position of the phase centre of the array depends on the direction of the synthesised radiation lobes and moves when the lobes are moved. Although ideally the position of the phase centre could be calculated, accuracy is limited by the non-idealities of the array. Beam forming will therefore unlikely be usable in high precision - e.g. geodetic - applications.

High gain and phase stability:
Beam forming is very sensitive to variations in the antenna and RF front-end parameters. In particular the relative phase offset between the signals coming from different array elements has to remain stable. If commercial narrow-band antennas and RF front-ends are used, sufficient stability is difficult to achieve. Wide-band (20 MHz) antennas and front-ends would much simplify achievement of sufficient parameter stability, but - at least currently - such devices are too expensive even for a high quality commercial GNSS navigation system (outside military, wide-band GNSS systems are used mainly in geodetic or avionics applications costing at least an order of magnitude more than professional navigation systems). Some improvement can be achieved by thermally isolating the antenna / front-end system, such that the temperature differential inside the system remains small.

High cost of the beam forming system:
A beam forming system requires an array of antennas and RF front-ends that obviously has a cost directly proportional to the number of antennas used. Although the price of the antenna / front-end system is somewhat diluted into the price of the whole GNSS system, it still has a relevant impact, making beam forming practical only for the highest end systems.

Calibration:
The antenna and RF subsystem requires calibration. Considering the great variability of the RF and antenna system parameters there is no evidence that this calibration could be avoided. This calibration currently requires very long times (10 - 20 min), an echo free signal to be used as calibration source and fully static conditions. The last two conditions are most likely requiring the user to guarantee for them (difficult, unacceptable user / system interaction).

Unsuitable to indoor environments:
Beam forming is fundamentally unsuitable to indoor or urban canyon environments where the GNSS signal mostly consists of NLOS components. High sensitivity receivers are able to produce a fix also thanks to a huge number of NLOS reflected or diffracted signals. An efficient beam forming system attenuates all NLOS echoes which will therefore not be 'seen' any more by the receiver. In typical NLOS situations, this results in a degradation of performances due to a significantly reduced visibility and therefore reduced redundancy and poor constellation geometry.

Attitude determination system:
In all considered cases, the beam forming functionality requires a very accurate (and expensive) attitude determination system. This system includes typically an accurate low drift IMU. The IMU would however be used also by its own to enhance the position solution while the GNSS system incorporating it would be able to navigate with the inertial system for a certain time while the GNSS signal is obscured, thus its cost in relation to beam forming would be relative.

Open issues

The GRABEL prototype has been useful to determine performances and limits of the beam forming concept in GNSS and to define a way forward. From the hardware point of view, these issues will need to be attacked before (or during) the continuation of the project, i.e. the development of a commercial version of GRABEL:

Size and performance of the antenna system:
Since the antenna array size is one of the critical points of the beam forming, it is necessary to verify which are the best performances which can be obtained using a three or four element antenna array with the best possible phase and amplitude calibration, especially in terms of NLOS and jamming attenuation. The missing amplitude control of the current GRABEL implementation and the phase variability is preventing us from using such array in a optimal condition. Amplitude control could be implemented with signal blanking techniques without impacting on data-path width.

Stability of antenna system:
Amplitude and phase differences between the antennas in the array are critical to beam forming. This is important over the whole radiation pattern - not only at the Zenith - and is especially critical for interference rejection (it is a difference of like numbers). An implementation based on wider band (> 20 MHz) antennas sharing a common temperature-stable substrate is perhaps the best approximation that can be done.

Stability of the RF front-end system:
RF parameter variation is critical to beam forming. In order to reduce such variability (and to reduce price and power consumption at the same time), the design of a multichannel RF front-end would be an optimal solution. This would probably also limit the need to calibrate the system each time the system is started. In the absence of a multichannel front-end a compact single board system of small dimensions would be the next best solution, however at a higher cost and lower stability than a multichannel front-end could provide.

Develop a compact GRABEL module:
A close-to-market system, based on a less discrete and distributed approach should be designed and assembled. The analysis of such system would tell us which and how many opportunities the beam forming enhanced positioning really has. Non-idealities and variability of the current prototype are preventing from getting a clear feeling about the real benefits of the beam forming approach.

Potential impact:

GRABEL is an small to medium-sized enterprise (SME) research initiative set up to investigate the joint use of reconfigurable antennas and beam forming algorithms for the purpose of improving the localisation capabilities of GNSS receivers (including GPS and Galileo) in outdoor and light-indoor difficult propagation environments.

GRABEL investigates and develops improved GNSS receiver techniques that make better use of weak GNSS signals while mitigating the adverse effects of multipath signals, fading, shadowing and interference. The use of jointly optimised reconfigurable antennas and beam forming enables GRABEL receivers to approach theoretical performance limits; offering robust, high performance localisation of moving vehicles in dynamically changing, difficult or interfered general outdoor light-indoor environments. Thus GRABEL opens the door to a new generation of robust outdoor-indoor location aware services.

Having the capability to better distinguish between accurate LOS and inaccurate multipath signals gives the opportunity for a further accuracy improvement. Rather than using all available signals to calculate a position, only the LOS signals are used if possible. These signals can be integrated with other sources, such as inertial sensors or a compass.

The GRABEL consortium has focused its efforts in the hardware and digital signal processing domains to define the performances and main characteristics of the system from a technology point of view. In parallel to the architecture studies design and justification activities, a preliminary market study was performed. This market study allowed identifying potential application domains and selecting the best promising ones, based on the characteristics of the final receiver (dimensions, power consumption, expected performances etc.).

GNSS satellites are scattered over the whole hemisphere. GNSS antennas need to have a wide cardioid radiation lobe in order to receive all possible satellites. The disadvantage of the above is that ground-based signals such as multipath or jammers are also received. By combining the signals of several antennas with the proper amplitude and phase, the radiation lobes of the array can be sharpened and pointed towards the various satellites. This technique increases antenna gain in the direction of the satellites while decreasing gain for signals coming from other directions. With a proper combination of gain and phase radiation zeros can be generated. These can be pointed towards interferers thus reducing their power.

Increased reliance and popularisation of GNSS systems has many weaknesses. Current navigation systems are more than suitable for normal applications, and the large majority of improvements could be done purely at the software level, thus maintaining a low price. Nevertheless, some navigation applications could directly benefit from improvements at the hardware level. A new generation of receivers, however, will have to be constructed which will naturally raise the budget level. Therefore, GRABEL project focuses on the institutional markets such as safety of life and search and rescue where the choice of a navigator is mainly driven by performances rather than price.

The first application sector identified is the ambulances and rescue services, but other institutional markets could easily be addressed as well.

Based on this strategy, it is possible to state that a future GRABEL receiver will have a social impact in terms of quality of the rescue services.

Also considering some articles in recent publications, the GNSS leading companies and user groups are increasingly considering the degradation deriving from interference sources and are moving toward the identification of a solution.

It is possible to expect a strong evolution in the GNSS receiver platforms, moving in this direction.

Project Website:
http://grabel.eclexys.com

Project Coordinator:
ECLEXYS SAGL
Via dell'Inglese 6
CH-6826 Riva San Vitale
Switzerland
Contact person : Angelo Consoli (angelo.consoli@eclexys.com via e-mail)
Company website: http://www.eclexys.com

Project partners:

Centre Suisse d'Electronique et de Microtechnique S.A. (CSEM)
Rue Jaquet-Droz 1
CH-2002 Neuchâtel
Switzerland
Contact person: Nicolae Chiurtu (nicolae.chiurtu@csem.ch via e-mail)
Company website: http://www.csem.ch


Saphyrion S.A.G.L
Via della Posta 10
CH-6934 Bioggio
Switzerland
Contact person: Francesco Piazza (francesco.piazza@saphyrion.ch via e-mail)
Company website: http://www.saphyrion.ch


Istituto Nazionale Di Astrofisica (INAF)
Viale del Parco Mellini 84
I-00136 Roma
Italy
Contact person: Stelio Montebugnoli (stelio@ira.inaf.it via e-mail)
Company website: http://www.ira.inaf.it


Pole Star SA
11 rue Paulin Talabot
31100 Toulouse
France
Contact person: Baptiste Godefroy (baptiste.godefroy@polestar.eu via e-mail)
Company website: http://www.polestar.eu


Space HELLAS S.A.
312, Messogion Ave. Ag. Paraskevi
153 41, Athens
Greece
Contact person: Ilias Andrikopoulos (iand@space.gr via e-mail)
Company website: http://www.space.gr

MOBICS LTD.
27, Kifisias
115 23, Athens
Greece
Contact person: Christos Anagnostopoulos (anagnostopoulos@mobics.gr via e-mail)
Company website: http://www.mobics.gr