Final Report Summary - DISAP (Displacement Synthetic Aperture Antenna Advanced Technology Demonstrator)
Executive Summary:
Within the DISAP project two prototypes of synthetic aperture antennas/receivers for Global Satellite Navigation Systems (GNSS) have been designed, analyzed, build and tested.
The main application for these systems is to operate in environments which are prone to reflect GNSS signals multiple times and the reflections are received together with the line-of-sight signal. These reflections – or multipath – reduce the accuracy of the range measurements between the satellite and the receiver and worsen the positioning accuracy for a number of navigation applications even to level which renders GNSS impossible to use in a reasonable way. Typical applications are surveying in forest or surveying inside buildings. Also airports operate local GNSS stations to broadcast GNSS correction data to approaching aircrafts for safe landing. GNSS satellites near the horizon are typically excluded as the Earth’s surface also reflects the GNSS signal with the corresponding accuracy decrease.
The synthetic aperture antennas/receivers mitigate multipath and open the path for the use of GNSS in the aforementioned application areas.
Synthetic aperture GNSS antennas work like phased array antenna systems and exploit the principle of spatial diversity to reduce/identify multipath (or spoofing signals). The antenna is artificially moved along a predefined trajectory and signals received at different locations are coherently combined. This effectively increases the aperture of the antenna and one is able to control the gain pattern, to e.g. steer the maximum towards the direction of the satellite. One can also choose a zero gain in the direction of a multipath signal.
Two prototypes have been build and validated. A rotating antenna with a radius of 50 centimeters targets precision surveying inside forests or indoors. A vertical antenna with a maximum vertical displacement of 1.4 meters is designed for GNSS reference station use, where precise low elevation measurements are required, like for local area augmentations systems at airports. Both systems currently operate with a measurement rate of 0.5 Hz and support GPS L1, L2P, L2C, L5 and Galileo E1, E5a. The receiver itself is a modification of IFEN’s software receiver SX-NSR using the NavPort-4 RF front-end. All the signal processing takes place inside a PC in real-time. All-in-view capability is supported.
The antennas have been operated and tested in a dedicated multipath test bed on a roof top with good field of view. For comparison, a static antenna was present which was connected to an Ashtech uZ-12 receiver and the SX-NSR. In case the observations are corrupted by multipath the DISAP antennas suppress 70-80 % of the multipath. Generally the vertically moving antennas did show a slightly higher performance as most of the multipath was generated by the ground. The rotating antenna has been operated over many weeks in separate sessions. The vertically moving antenna was continuously operated over three weeks (then the project ended).
Both antennas are able to measure the elevation/azimuth dependence of the power for the received GNSS signals. Consequently they are able to identify and mitigate spoofing signals, as the direction arrival is generally different from the known satellite positions.
Project Context and Objectives:
IFEN GmbH produces a GNSS software receiver called SX-NSR capable of tracking all civil GNSS signals broadcast today, like GPS, Galileo, GLONASS or BeiDou. The receiver system consist of
• an GNSS antenna receiving the GNSS signal,
• a RF frontend responsible for amplifying, filtering and digitizing the GNSS signals
• and a PC/laptop running the software receiver, which tracks the signals and computes range measurements and positions.
The context of this project was to increase the ability of the receiver to reduce multipath errors by using the principle of synthetic aperture radar. The ability to form a synthetic aperture is achieved by artificially moving the GNSS antenna along a predefined path. A rotating and vertically moving antenna have been built.
Thus for the DISAP project the following objectives have been targeted and achieved:
• Pre-analysis of all important contributors to the process of GNSS synthetic array processing
• Full wave simulations of the electromagnetic field propagation around the antenna
• Formulation of requirements
• Sub-contracting the rotating and vertical antenna
• Extension of the NavPort-4 front-end to read in magnetic sensor data
• Extension of the SX-NSR software receiver for synthetic array processing
• Integration of all components (antenna, front-end, receiver) into a working system
• Laboratory and integration tests
• Outdoor and permanent operation tests, performance evaluations
• Re-analysis of electromagnetic properties based on build prototype
• Demonstration of scientific applications
• Formulation of conclusions and recommendations
Project Results:
Specification and Performance
The DISAP system generally consists of a PC/laptop, a NavPort-4 RF front-end, an OCXO module and either the vertical or rotating DISAP antenna. The specification and performance of those components are given in following sections after the possible applications are indicated.
Application Range
The DISAP system - in its current version - is multi-frequency GNSS-receiver which utilizes an artificial antenna motion to suppress multipath. It therefore provides code pseudorange, carrier pseudorange and C/N0 estimates of higher accuracy compared to a receiver operating on a static antenna. It also allows mapping the received signal power as a function of elevation and azimuth. The following applications can be envisaged:
• GNSS reference station (actually the main focus of the DISAP project) to be used for surveying, LAAS, sensor station, …
• Surveying in degraded environments like forests or indoor (still to be verified)
• Scientific multipath analysis via direction-of-arrival
• Measuring low elevation satellites for radio occultation and atmospheric sounding
• Detection and localization of spoofing signals
Core Receiver
The RF front-end connects via USB2.0 to the PC/laptop and needs a 6 V DC power supply. Usually the DISAP system is operated with an OCXO module – also developed in the DISAP project - which stabilizes GPS L2P cross-correlation tracking. Future software updates may decrease the need for an OXCO to track L2P and the internal oscillator of the NavPort-4 might be sufficient. The front-end connects to the DISAP antennas via the RF cable and the magnetic sensor cable (which is optional for the rotating antenna). Furthermore, the trigger input of the vertical DISAP antenna is connected to the PPS output of the NavPort-4.
The software receiver is generally capable of tracking all civil GNSS signals and the DISAP signal processing is prepared for that. Tests during the DISAP projects have been carried out on three frequency bands: L1/E1, L2, L5/E5a. The specifications of the software receiver are shown below:
• GPS frequencies: L1C/A, L2P (cross-correlation), L2CM, L5Q
• Galileo frequencies: E1C, E5aQ
• Sampling rate: 20.48 MHz
• RF bandwidth: 15 MHz (3 dB, dual-sided)
• Number of bits: 2
• Number of channels: All-in-view
• Correlator spacing: 2 samples
• RINEX obs. Output: V2.11 and V3.0
• Output rate: 0.5 Hz
• Other output files: Beamforming log files (ASCII files containing correlator values, clock and µ-trajectory estimates)
• Real-time visualization: Received signal power as a function of elevation and azimuth as overlay to sky-plot, purity of received carrier signal
Rotating Antenna Description
The rotating antenna requires a battery box which also contains the step motor driver. A transportation case is also available.
The rotating antenna was designed to be operated by a single surveyor. It is a task of around 5 minutes to set it up on a geodetic tripod. The rotation is controlled via an on/off switch of the battery box. The DISAP signal processing is able to detect if the antenna is rotating (even without magnetic sensor cable) or if it is static. The design idea of this system did foresee that the surveyor actually stands by while the antenna is operating (e.g. for surveying inside forests). The battery box is also able to power the NavPort-4 and the in principle also the laptop.
Whereas it is generally possible to operate the antenna also continuously – and this has been done during the DISAP project for more than one day – the system does currently not detect any failures of the rotation itself. We observed cases that very strong winds cause the step motor to become asynchronous and it is then not able to recover from this failure. Future software updates might allow monitoring the rotation via the magnetic sensor and to apply counter measures (restart the motor) in case the rotation gets distorted.
The rotation element is generally water proof (but not tested against any standard), the battery box is not. The latter one requires a separate housing (foil cover) to be protected against rain.
A specification of the rotating antenna system is listed below:
• Radius: 0.5 m
• Antenna element: NavXperiance 3G+C, RHCP with integrated LNA
• Rotation speed: 30 rotations per minute
• Beamforming interval and measurement rate: 2 s
• GPS frequencies: L1C/A, L2P (cross-correlation), L2CM, L5Q
• Galileo frequencies: E1C, E5aQ
• Number of channels: All-in-view
• Movement synchronized to GPS time: No
• Water proof: Antenna positioner against rain, battery box is not water-proof
• Antenna connectors: 2 x RF out , magnetic sensor output, step-motor input
• Power supply: 220 V (autonomous operation possible via battery box)
Rotating Antenna Performance
The rotating DISAP antenna mitigates multipath originating from all directions different to the line-of-sight direction and different to the specular ground reflection. Diffuse multipath from the ground (a very significant multipath source) is mitigated.
Extensive tests with GPS and comparisons with receivers using static antennas (Ashtech Z12 and SX-NSR) at exactly the same location (multipath test bed on the roof-top of AAS) show on average an improvement of 50 % in the multipath performance (determined from code-minus-carrier observations).
The values are mean values computed over the whole constellation. Many high elevation satellites are only little affected by multipath. Looking at a lower elevation satellite, the multipath mitigation potential becomes much more expressed.
Vertical Antenna Description
The vertical antenna target reference station operations and is intended to be mounted on a permanent basis.
A RF-cable, the trigger cable and the magnetic sensor cable connect to the NavPort-4 front-end. The housing of the antenna protects the mechanics against rain and dust. The mechanics itself is water tolerant (it may even operate under water). The seal between the antenna rod and the upper plate of the antenna is the most critical element regarding water protection. We observed that during heavy rain falls a few drops per day of water enter the housing, ran down the antenna rod and drop on the floor. So far this water entrance does not affect the antenna operation, but it needs to be monitored. The water can not get in touch with any electric part. The step driver controller and power supply unit are placed in a separate box, which is completely water proof.
The setup of the antenna needs a least two people. It is highly recommended that it is fixed to the ground using the foreseen screws. This ensures proper stability over time.
A coarse specification of the vertical antenna is shown below:
• Max. height variation: 1.4 m
• Antenna element: NavXperience 3G+C, RHCP with integrated LNA
• Movement rate: 2 s for up-movement and 2 s for down-movement
• Beamforming interval and measurement rate: 2 s
• GPS frequencies: L1C/A, L2P (cross-correlation), L2CM, L5Q
• Galileo frequencies: E1C, E5aQ
• Number of channels: All-in-view
• Movement synchronized to GPS time: Yes
• Water proof: Yes
• Antenna connectors: RF out, magnetic sensor output, step-motor input, 220 V, Trigger in-put
• Power supply: 220 V (autonomous operation possible via battery box)
Vertical Antenna Performance
As the vertical DISAP antenna was designed for permanent operation, it was much easier to collect a large amount of data. For comparisons to a static antenna, the antenna was put into the rest position and the Ashtech Z12 and the SX-NSR in static mode have been used to collect reference data. The reference data is different from the static reference data for the rotating antenna only by different antenna height.
The code-minus-carrier performance of the vertical antenna is better than the performance of the rotating antenna. Bearing in mind, that the code-minus-carrier analysis still contains thermal noise and residual ionospheric contributions, we conclude that at least 75 % of the multipath are mitigated on average.
General Drawbacks and Optimization
The major drawback of the DISAP system is that it moves mechanically. This serious disadvantage can only be compensated by the superior measurement performance. To which extend it pays off, is application dependent.
The following optimizations have been identified:
Signal Processing Drawbacks and Optimization
LP2 cross-correlation: The current cross-correlation scheme works with the predicted received clock over the next beamforming interval. This puts serious demands on the oscillator and in fact a stable OCXO is needed. A revision of this part of the signal processing code would allow to do the cross-correlation with the actual estimated receiver clock and then a lower quality oscillator can be used.
L5/E5a performance: Further investigations are necessary to figure out, why the code-minus-carrier performance is less on L5 compared to L1 (see [D13]).
PPP: The results from precise-point-positioning need to be understood given the fact that DISAP data can be processed without problems in a double-difference way (see [D12]).
Other frequencies: More frequency bands as well as GLONASS or BeiDou could be supported.
More specific multipath mitigation: With more experience on the DISAP applications, the multipath mitigation can be tuned to Null or suppress signals from certain direction-of-arrivals.
Drawbacks and Optimization for Rotating Antenna
Battery box: The battery box should be water proof.
Feedback SX-NSR: The SX-NSR should receive feedback from the magnetic sensor and should control the step motor of the rotating antenna. This can be accomplished by a simple software update and would increase the long term stability of the rotating antenna.
More applications: Tests is degraded environments (the main application of the rotating DISAP antenna) are still to be done including forests and indoor.
Use of second antenna: The secondary antenna port can be equipped with a LHCP antenna which would be useful for multipath mapping.
Drawbacks and Optimization for Vertical Antenna
More applications: The use of the vertical antenna is to provide low-elevation signals with high accuracy. This includes reference stations (eventually used for LAAS) or tracking low elevation signals for atmospheric sounding. Here further experimentation work is required.
Seal: The seal between the housing and antenna rod needs to be more thoroughly tested and eventually improved.
Comparison with Other Antenna Systems
To compare the DISAP system with other antennas exploiting spatial diversity, two references were found:
The Triumph 4X receiver (http://www.javad.com/downloads/javadgnss/sheets/TRIUMPH-4X_Datasheet.pdf) connects to four antennas simultaneously. If two of those receivers are used to compute a geodetic baseline, then 16 combinations are possible. This increases the robustness and reliability of the solution – also in term of ambiguity fixing – and seems to improve the accuracy too. The Triumph 4X receiver is somehow comparable to the DISAP system as the principle of spa-tial diversity is used. It seems however, that the signal processing in the Triumph 4X treats every antenna individually and phased array processing is not used. Thus we would expect that the multipath performance of the Triumph 4X is not better as for a receiver with a single antenna.
The GAJT-700ML antenna from NovaTel (http://www.novatel.com/assets/Documents/
Papers/GAJT-700ML.pdf) contains 7 antenna elements to receive GPS L1 and L2 signals. It has a weight of 7.5 kg, a power consumption of 20 Watts and diameter of 30 centimeters. This antenna is designed to suppress interference by placing up to 6 Nulls in the direction of the interferer. It has a single RF output, where a GPS receiver can be connected. This is a true phased array system but not optimized to mitigate multipath. The antenna does not distinguish between the different GNSS satellites; a single RF signal is output. Its main application is in the military or to protect critical civil infrastructure.
We conclude, that currently no product is on the market, which explicitly uses spatial diversity to mitigate multipath in GNSS observations. Furthermore, we recall that the main disadvantage of the DISAP system to a multi-antenna arrays is the mechanical movement which intrinsically makes the DISAP systems prone to mechanical failures. The big advantage are the ease of realizing large apertures, the absence to calibrate the systems electrically and the less complex (less tracking channels needed) receiver structure.
Conclusions and Recommendations
During this project we have investigated, designed, build and tested a GNSS receiver and antennas using a synthetic aperture to mitigate multipath. The developed prototypes have been brought to a maturity level which allows their operational use. The performance of the prototypes has been verified in a well defined test bed.
The DISAP project was characterized by a high coherency of all activities focusing on the specific aspects of building a synthetic aperture. Naturally it included contributions from electromagnetic wave propagation, mechanics and signal processing. The time frame was appropriate to finish the prototypes and to understand their operational characteristics.
We consider to systems to be finished and will start to test them for dedicated applications to show their benefits. During those tests, we will also try to further optimize them, but keep the architecture. These further activities will be funded by follow up research projects and product sales.
Potential Impact:
The socio economic benefits come from application of these GNSS antenna/receiver systems in the professional markets. The rotating antenna may help to perform surveying activities in areas (forests, indoors) which currently measure using cumbersome terrestrial methods using theodolites. The vertical and the rotating antenna can be used to supply critical infrastructure with spoofing resistant GNSS information. The vertical antenna can be used in increase the GNSS availability by using low elevation observations in local area augmentation system / ground based augmentation system to increase safety in aviation.
List of Websites:
www.ifen.com
Within the DISAP project two prototypes of synthetic aperture antennas/receivers for Global Satellite Navigation Systems (GNSS) have been designed, analyzed, build and tested.
The main application for these systems is to operate in environments which are prone to reflect GNSS signals multiple times and the reflections are received together with the line-of-sight signal. These reflections – or multipath – reduce the accuracy of the range measurements between the satellite and the receiver and worsen the positioning accuracy for a number of navigation applications even to level which renders GNSS impossible to use in a reasonable way. Typical applications are surveying in forest or surveying inside buildings. Also airports operate local GNSS stations to broadcast GNSS correction data to approaching aircrafts for safe landing. GNSS satellites near the horizon are typically excluded as the Earth’s surface also reflects the GNSS signal with the corresponding accuracy decrease.
The synthetic aperture antennas/receivers mitigate multipath and open the path for the use of GNSS in the aforementioned application areas.
Synthetic aperture GNSS antennas work like phased array antenna systems and exploit the principle of spatial diversity to reduce/identify multipath (or spoofing signals). The antenna is artificially moved along a predefined trajectory and signals received at different locations are coherently combined. This effectively increases the aperture of the antenna and one is able to control the gain pattern, to e.g. steer the maximum towards the direction of the satellite. One can also choose a zero gain in the direction of a multipath signal.
Two prototypes have been build and validated. A rotating antenna with a radius of 50 centimeters targets precision surveying inside forests or indoors. A vertical antenna with a maximum vertical displacement of 1.4 meters is designed for GNSS reference station use, where precise low elevation measurements are required, like for local area augmentations systems at airports. Both systems currently operate with a measurement rate of 0.5 Hz and support GPS L1, L2P, L2C, L5 and Galileo E1, E5a. The receiver itself is a modification of IFEN’s software receiver SX-NSR using the NavPort-4 RF front-end. All the signal processing takes place inside a PC in real-time. All-in-view capability is supported.
The antennas have been operated and tested in a dedicated multipath test bed on a roof top with good field of view. For comparison, a static antenna was present which was connected to an Ashtech uZ-12 receiver and the SX-NSR. In case the observations are corrupted by multipath the DISAP antennas suppress 70-80 % of the multipath. Generally the vertically moving antennas did show a slightly higher performance as most of the multipath was generated by the ground. The rotating antenna has been operated over many weeks in separate sessions. The vertically moving antenna was continuously operated over three weeks (then the project ended).
Both antennas are able to measure the elevation/azimuth dependence of the power for the received GNSS signals. Consequently they are able to identify and mitigate spoofing signals, as the direction arrival is generally different from the known satellite positions.
Project Context and Objectives:
IFEN GmbH produces a GNSS software receiver called SX-NSR capable of tracking all civil GNSS signals broadcast today, like GPS, Galileo, GLONASS or BeiDou. The receiver system consist of
• an GNSS antenna receiving the GNSS signal,
• a RF frontend responsible for amplifying, filtering and digitizing the GNSS signals
• and a PC/laptop running the software receiver, which tracks the signals and computes range measurements and positions.
The context of this project was to increase the ability of the receiver to reduce multipath errors by using the principle of synthetic aperture radar. The ability to form a synthetic aperture is achieved by artificially moving the GNSS antenna along a predefined path. A rotating and vertically moving antenna have been built.
Thus for the DISAP project the following objectives have been targeted and achieved:
• Pre-analysis of all important contributors to the process of GNSS synthetic array processing
• Full wave simulations of the electromagnetic field propagation around the antenna
• Formulation of requirements
• Sub-contracting the rotating and vertical antenna
• Extension of the NavPort-4 front-end to read in magnetic sensor data
• Extension of the SX-NSR software receiver for synthetic array processing
• Integration of all components (antenna, front-end, receiver) into a working system
• Laboratory and integration tests
• Outdoor and permanent operation tests, performance evaluations
• Re-analysis of electromagnetic properties based on build prototype
• Demonstration of scientific applications
• Formulation of conclusions and recommendations
Project Results:
Specification and Performance
The DISAP system generally consists of a PC/laptop, a NavPort-4 RF front-end, an OCXO module and either the vertical or rotating DISAP antenna. The specification and performance of those components are given in following sections after the possible applications are indicated.
Application Range
The DISAP system - in its current version - is multi-frequency GNSS-receiver which utilizes an artificial antenna motion to suppress multipath. It therefore provides code pseudorange, carrier pseudorange and C/N0 estimates of higher accuracy compared to a receiver operating on a static antenna. It also allows mapping the received signal power as a function of elevation and azimuth. The following applications can be envisaged:
• GNSS reference station (actually the main focus of the DISAP project) to be used for surveying, LAAS, sensor station, …
• Surveying in degraded environments like forests or indoor (still to be verified)
• Scientific multipath analysis via direction-of-arrival
• Measuring low elevation satellites for radio occultation and atmospheric sounding
• Detection and localization of spoofing signals
Core Receiver
The RF front-end connects via USB2.0 to the PC/laptop and needs a 6 V DC power supply. Usually the DISAP system is operated with an OCXO module – also developed in the DISAP project - which stabilizes GPS L2P cross-correlation tracking. Future software updates may decrease the need for an OXCO to track L2P and the internal oscillator of the NavPort-4 might be sufficient. The front-end connects to the DISAP antennas via the RF cable and the magnetic sensor cable (which is optional for the rotating antenna). Furthermore, the trigger input of the vertical DISAP antenna is connected to the PPS output of the NavPort-4.
The software receiver is generally capable of tracking all civil GNSS signals and the DISAP signal processing is prepared for that. Tests during the DISAP projects have been carried out on three frequency bands: L1/E1, L2, L5/E5a. The specifications of the software receiver are shown below:
• GPS frequencies: L1C/A, L2P (cross-correlation), L2CM, L5Q
• Galileo frequencies: E1C, E5aQ
• Sampling rate: 20.48 MHz
• RF bandwidth: 15 MHz (3 dB, dual-sided)
• Number of bits: 2
• Number of channels: All-in-view
• Correlator spacing: 2 samples
• RINEX obs. Output: V2.11 and V3.0
• Output rate: 0.5 Hz
• Other output files: Beamforming log files (ASCII files containing correlator values, clock and µ-trajectory estimates)
• Real-time visualization: Received signal power as a function of elevation and azimuth as overlay to sky-plot, purity of received carrier signal
Rotating Antenna Description
The rotating antenna requires a battery box which also contains the step motor driver. A transportation case is also available.
The rotating antenna was designed to be operated by a single surveyor. It is a task of around 5 minutes to set it up on a geodetic tripod. The rotation is controlled via an on/off switch of the battery box. The DISAP signal processing is able to detect if the antenna is rotating (even without magnetic sensor cable) or if it is static. The design idea of this system did foresee that the surveyor actually stands by while the antenna is operating (e.g. for surveying inside forests). The battery box is also able to power the NavPort-4 and the in principle also the laptop.
Whereas it is generally possible to operate the antenna also continuously – and this has been done during the DISAP project for more than one day – the system does currently not detect any failures of the rotation itself. We observed cases that very strong winds cause the step motor to become asynchronous and it is then not able to recover from this failure. Future software updates might allow monitoring the rotation via the magnetic sensor and to apply counter measures (restart the motor) in case the rotation gets distorted.
The rotation element is generally water proof (but not tested against any standard), the battery box is not. The latter one requires a separate housing (foil cover) to be protected against rain.
A specification of the rotating antenna system is listed below:
• Radius: 0.5 m
• Antenna element: NavXperiance 3G+C, RHCP with integrated LNA
• Rotation speed: 30 rotations per minute
• Beamforming interval and measurement rate: 2 s
• GPS frequencies: L1C/A, L2P (cross-correlation), L2CM, L5Q
• Galileo frequencies: E1C, E5aQ
• Number of channels: All-in-view
• Movement synchronized to GPS time: No
• Water proof: Antenna positioner against rain, battery box is not water-proof
• Antenna connectors: 2 x RF out , magnetic sensor output, step-motor input
• Power supply: 220 V (autonomous operation possible via battery box)
Rotating Antenna Performance
The rotating DISAP antenna mitigates multipath originating from all directions different to the line-of-sight direction and different to the specular ground reflection. Diffuse multipath from the ground (a very significant multipath source) is mitigated.
Extensive tests with GPS and comparisons with receivers using static antennas (Ashtech Z12 and SX-NSR) at exactly the same location (multipath test bed on the roof-top of AAS) show on average an improvement of 50 % in the multipath performance (determined from code-minus-carrier observations).
The values are mean values computed over the whole constellation. Many high elevation satellites are only little affected by multipath. Looking at a lower elevation satellite, the multipath mitigation potential becomes much more expressed.
Vertical Antenna Description
The vertical antenna target reference station operations and is intended to be mounted on a permanent basis.
A RF-cable, the trigger cable and the magnetic sensor cable connect to the NavPort-4 front-end. The housing of the antenna protects the mechanics against rain and dust. The mechanics itself is water tolerant (it may even operate under water). The seal between the antenna rod and the upper plate of the antenna is the most critical element regarding water protection. We observed that during heavy rain falls a few drops per day of water enter the housing, ran down the antenna rod and drop on the floor. So far this water entrance does not affect the antenna operation, but it needs to be monitored. The water can not get in touch with any electric part. The step driver controller and power supply unit are placed in a separate box, which is completely water proof.
The setup of the antenna needs a least two people. It is highly recommended that it is fixed to the ground using the foreseen screws. This ensures proper stability over time.
A coarse specification of the vertical antenna is shown below:
• Max. height variation: 1.4 m
• Antenna element: NavXperience 3G+C, RHCP with integrated LNA
• Movement rate: 2 s for up-movement and 2 s for down-movement
• Beamforming interval and measurement rate: 2 s
• GPS frequencies: L1C/A, L2P (cross-correlation), L2CM, L5Q
• Galileo frequencies: E1C, E5aQ
• Number of channels: All-in-view
• Movement synchronized to GPS time: Yes
• Water proof: Yes
• Antenna connectors: RF out, magnetic sensor output, step-motor input, 220 V, Trigger in-put
• Power supply: 220 V (autonomous operation possible via battery box)
Vertical Antenna Performance
As the vertical DISAP antenna was designed for permanent operation, it was much easier to collect a large amount of data. For comparisons to a static antenna, the antenna was put into the rest position and the Ashtech Z12 and the SX-NSR in static mode have been used to collect reference data. The reference data is different from the static reference data for the rotating antenna only by different antenna height.
The code-minus-carrier performance of the vertical antenna is better than the performance of the rotating antenna. Bearing in mind, that the code-minus-carrier analysis still contains thermal noise and residual ionospheric contributions, we conclude that at least 75 % of the multipath are mitigated on average.
General Drawbacks and Optimization
The major drawback of the DISAP system is that it moves mechanically. This serious disadvantage can only be compensated by the superior measurement performance. To which extend it pays off, is application dependent.
The following optimizations have been identified:
Signal Processing Drawbacks and Optimization
LP2 cross-correlation: The current cross-correlation scheme works with the predicted received clock over the next beamforming interval. This puts serious demands on the oscillator and in fact a stable OCXO is needed. A revision of this part of the signal processing code would allow to do the cross-correlation with the actual estimated receiver clock and then a lower quality oscillator can be used.
L5/E5a performance: Further investigations are necessary to figure out, why the code-minus-carrier performance is less on L5 compared to L1 (see [D13]).
PPP: The results from precise-point-positioning need to be understood given the fact that DISAP data can be processed without problems in a double-difference way (see [D12]).
Other frequencies: More frequency bands as well as GLONASS or BeiDou could be supported.
More specific multipath mitigation: With more experience on the DISAP applications, the multipath mitigation can be tuned to Null or suppress signals from certain direction-of-arrivals.
Drawbacks and Optimization for Rotating Antenna
Battery box: The battery box should be water proof.
Feedback SX-NSR: The SX-NSR should receive feedback from the magnetic sensor and should control the step motor of the rotating antenna. This can be accomplished by a simple software update and would increase the long term stability of the rotating antenna.
More applications: Tests is degraded environments (the main application of the rotating DISAP antenna) are still to be done including forests and indoor.
Use of second antenna: The secondary antenna port can be equipped with a LHCP antenna which would be useful for multipath mapping.
Drawbacks and Optimization for Vertical Antenna
More applications: The use of the vertical antenna is to provide low-elevation signals with high accuracy. This includes reference stations (eventually used for LAAS) or tracking low elevation signals for atmospheric sounding. Here further experimentation work is required.
Seal: The seal between the housing and antenna rod needs to be more thoroughly tested and eventually improved.
Comparison with Other Antenna Systems
To compare the DISAP system with other antennas exploiting spatial diversity, two references were found:
The Triumph 4X receiver (http://www.javad.com/downloads/javadgnss/sheets/TRIUMPH-4X_Datasheet.pdf) connects to four antennas simultaneously. If two of those receivers are used to compute a geodetic baseline, then 16 combinations are possible. This increases the robustness and reliability of the solution – also in term of ambiguity fixing – and seems to improve the accuracy too. The Triumph 4X receiver is somehow comparable to the DISAP system as the principle of spa-tial diversity is used. It seems however, that the signal processing in the Triumph 4X treats every antenna individually and phased array processing is not used. Thus we would expect that the multipath performance of the Triumph 4X is not better as for a receiver with a single antenna.
The GAJT-700ML antenna from NovaTel (http://www.novatel.com/assets/Documents/
Papers/GAJT-700ML.pdf) contains 7 antenna elements to receive GPS L1 and L2 signals. It has a weight of 7.5 kg, a power consumption of 20 Watts and diameter of 30 centimeters. This antenna is designed to suppress interference by placing up to 6 Nulls in the direction of the interferer. It has a single RF output, where a GPS receiver can be connected. This is a true phased array system but not optimized to mitigate multipath. The antenna does not distinguish between the different GNSS satellites; a single RF signal is output. Its main application is in the military or to protect critical civil infrastructure.
We conclude, that currently no product is on the market, which explicitly uses spatial diversity to mitigate multipath in GNSS observations. Furthermore, we recall that the main disadvantage of the DISAP system to a multi-antenna arrays is the mechanical movement which intrinsically makes the DISAP systems prone to mechanical failures. The big advantage are the ease of realizing large apertures, the absence to calibrate the systems electrically and the less complex (less tracking channels needed) receiver structure.
Conclusions and Recommendations
During this project we have investigated, designed, build and tested a GNSS receiver and antennas using a synthetic aperture to mitigate multipath. The developed prototypes have been brought to a maturity level which allows their operational use. The performance of the prototypes has been verified in a well defined test bed.
The DISAP project was characterized by a high coherency of all activities focusing on the specific aspects of building a synthetic aperture. Naturally it included contributions from electromagnetic wave propagation, mechanics and signal processing. The time frame was appropriate to finish the prototypes and to understand their operational characteristics.
We consider to systems to be finished and will start to test them for dedicated applications to show their benefits. During those tests, we will also try to further optimize them, but keep the architecture. These further activities will be funded by follow up research projects and product sales.
Potential Impact:
The socio economic benefits come from application of these GNSS antenna/receiver systems in the professional markets. The rotating antenna may help to perform surveying activities in areas (forests, indoors) which currently measure using cumbersome terrestrial methods using theodolites. The vertical and the rotating antenna can be used to supply critical infrastructure with spoofing resistant GNSS information. The vertical antenna can be used in increase the GNSS availability by using low elevation observations in local area augmentation system / ground based augmentation system to increase safety in aviation.
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