Antennas: The key to accuracy for high-precision applications

New developments in antenna technology empower the final positioning solution with better accuracy and reliability. Leading experts discuss the technology advances producing greater user benefits.

The increasing prevalence of both intentional and inadvertent jamming, new wider bandwidths, and the significance of antenna phase-center variation all bring changes to the dynamic and evolving antenna sector.

Javad Ashjaee (Photo: Javad GNSS)

Javad Ashjaee

President & CEO, JAVAD GNSS

Advanced filtering techniques enable our antennas to defend against jammers and spoofers and to inform users with the details of these intrusive actions when they are detected.

Near-Band Interference. The J-Shield is a robust filter in our antennas that blocks out-of-band interference, in particular such signals that are near the GNSS bands like the LightSquared/Ligado signals. The graph below shows the protection characteristics of the J-Shield filters. It has a sharp 10-dB/KHz skirt that provides up to 100 dB of protection. It makes the precious near-band spectrums available for other usages and protects GNSS bands now and in the future.

In-Band Interference. Our in-band protection digital filter protects against in-band interference like harmonics of TV and radio stations when you get close to them, or against illegitimate in-band transmissions. Our in-band interference protection is based on the 16 adaptive 80th-order filters. Advanced interference mitigation (AIM) filters can be combined in pairs for complex signal processing. This filter can simultaneously suppress several interference signals.

Graph: Javad GNSS

The 16 finite impulse response (FIR) AIM filters can be combined in any number in chain. Each filter is a 255-order FIR filter. It can be used to suppress the stationary interference signal in programmable area (compare with adaptive AIM-filter) or for spectrum shaping. To have more suppressing areas or more aggressive suppressing, one can combine FIR AIM serial.

Neil Gerein, Portfolio Manager, NovAtel. (Photo: NovAtel)

Neil Gerein

Director, Product Management, NovAtel

At NovAtel we often say, “accuracy is addictive,” and to meet increasingly demanding accuracy and reliability requirements it is vital to concentrate on the antenna. After all, the antenna is the first in a long chain of key technologies that the GNSS signals must pass through to create a position, navigation and timing solution.

All modern GNSS transmit on multiple frequencies, with wide bandwidth signals, requiring antenna elements and integrated low noise amplifiers (LNAs) that operate across these frequencies. The challenge is to design the antenna element and LNAs for symmetric radiation patterns across all frequencies while minimizing multipath, phase center offset (PCO) and phase center variation (PCV). The result is better carrier-phase measurements, and therefore more accurate solutions in real-time kinematic (RTK) and PPP applications.

Photo: NovAtel

Since 2016 the Radio Equipment Directive (RED) has been in effect, and all GNSS receiver systems sold into the European Union must be compliant to the standard, including adjacent-band compatibility and spurious emissions testing. RED compliance is an end-to-end system test, where the filtering within the antenna must be analyzed in concert with the filtering capabilities of the connected GNSS receiver to meet the requirements. The antenna performance therefore becomes critical to any GNSS receiver system that is intended to be sold within the EU.

Gyles Panther, president and CTO, Tallysman Wireless. (Photo: Tallysman)

Gyles Panther

President and Chief Technical Officer, Tallysman

A fact often not appreciated is that the performance of a GNSS antenna is commonly the limiting factor in system accuracy. Digital signal algorithms in the receiver are helpful, but if the signal delivered by an antenna is less than optimum, the receiver cannot compensate.

Precision GNSS systems typically rely upon resolved wavelength ambiguity measurements, combined with ephemeris and clock corrections to determine signal time of flight. In real-time kinematic (RTK) and precise point positioning (PPP) receivers, the basis for this measurement is phase locked tracking of received satellite signals. Thus an over-arching measure of antenna performance in the specific application conditions is the proportion of the time that phase lock is maintained by the receiver.

The VeraChoke GNSS antenna. (Photo: Tallysman)

All this provides for an unprecedented level of accuracy, with precision antennas now more akin to the ends of a tape measure than providing a simple GNSS “fix.” To this end, key parameters include a best possible G/T ratio, high multipath rejection, excellent axial ratio, high front-back ratio and minimal phase-center variation (PCV), all with high uniformity in the azimuth — altogether a very demanding design task.

Combining these parameters to provide exquisite accuracy, the Tallysman VC6100 choke ring antenna has less than 1 millimeter PCV when combined with absolute calibrated corrections data, whilst the lower cost VP6000, with its less complex installation, can be used without corrections data and still be within a millimeter or two of the truth compared to its more precise cousin.