Why Adaptive Null-Steering Anti-Jamming Antennas Cannot Be Used for RTK Differential Positioning?

The primary objective of adaptive null-steering anti-jamming antennas is interference suppression. However, the technical approach they adopt is fundamentally in conflict with the requirement of RTK antennas for a highly stable phase center.

Working Principle of Anti-Jamming Antennas

Such antennas are typically designed as antenna arrays, consisting of multiple GNSS antenna elements—two, four, seven, or even more. At the core of the system is a powerful digital signal processor working together with a set of adaptive algorithms.

Its processing flow can be simplified as follows:

1. Signal Reception:

All antenna elements in the array simultaneously receive both the GNSS satellite signals from space and the interference signals from the ground.

2. Interference Detection:

The processor continuously analyzes the characteristics of all incoming signals in real time. Since satellite signals originate from high altitude, the path differences to the various antenna elements result in phase variations that are regular and predictable. In contrast, interference signals—such as narrowband or wideband interference—typically originate from ground-based or low-altitude sources, and their phase relationships across the antenna elements are completely different from those of the satellite signals.

3. Weight Calculation:

The adaptive algorithm—such as the well-known LMS (Least Mean Squares) algorithm—computes an optimal set of “weights.” This set of weights essentially “tunes” the signals from each antenna element by adjusting their amplitude and phase.

4. Beam forming and Beam Nulling:

The signals from all antenna elements, after being “tuned” (weighted), are then combined.

• Beam forming: In the direction of the incoming satellite signals, the phases are adjusted so that the signals from all antenna elements add coherently, effectively forming a high-gain beam towards the satellites and enhancing the useful signals.
• Beam Nulling: In the direction of the incoming interference signals, the phases are adjusted so that the signals from the antenna elements cancel each other out in opposite phase, effectively forming a deep null toward the interference sources and significantly suppressing the interference.

Why is this fundamentally incompatible with RTK carrier-phase positioning?

The key issue lies in Steps 3 and 4—the dynamic adjustment of the weighting coefficients.

1. Dynamically varying phase center:

In order to cope with varying interference, the adaptive algorithm continuously and dynamically updates the weighting coefficients of each antenna element, especially the phase weights. As a result, the phase center of the synthesized signal from the entire antenna array is no longer a stable point fixed to the mechanical structure, but instead shifts randomly over time as the interference environment changes.

2. Large PCV:

For an RTK surveying antenna, what we pursue is a phase center variation (PCV) controlled within the millimeter level. However, when coping with interference, the equivalent phase center may shift by several centimeters, or even tens of centimeters. This magnitude of variation is already far greater than the carrier wavelength (for example, about 19 cm on the B1 band), and is sufficient to completely disrupt the continuity of the carrier phase.

3. Uncalibratable System Errors:

The reason RTK technology can achieve centimeter-level accuracy is that one fundamental prerequisite is the ability to calibrate the antenna’s PCO and PCV in advance (for example, using antenna calibration models provided by the IGS) so that these effects can be compensated for during the positioning solution. However, the phase center variation of an adaptive antenna is real-time, random, and inherently unpredictable, meaning that no calibration model is capable of correcting it. This unremovable dynamic error is introduced into the carrier-phase observations, making it impossible to fix the integer ambiguities. As a result the RTK solution inevitably fails.

A simple analogy:

RTK surveying antenna:

It is like a fixed-focus telescope made of premium optical glass and firmly mounted on a stable tripod. Its optical (phase) center is exceptionally stable, enabling you to clearly observe and accurately measure distant targets.

Adaptive Anti-jamming antenna:

It is like a camera that is constantly panning and zooming to track a fast-moving bird. Although it is capable of keeping the bird in the frame even under motion and interference—in other words, it can maintain signal lock—the constantly moving lens means you simply cannot rely on it for any kind of high-precision surveying or measurement.

Conclusion: 

Adaptive null-steering anti-jamming antennas attain their powerful interference suppression capability by sacrificing the stability of the antenna’s phase center. And this directly strikes at the weakest point of RTK technology. Therefore, traditional anti-jamming antennas based on antenna arrays and adaptive processing cannot be used for RTK high-precision positioning.

So is there really an antenna that can provide strong anti-jamming capability while still supporting RTK differential positioning?

This is the ultimate “want both at the same time” question. The answer is: yes, such antennas do exist. However, they are significantly more challenging in terms of both technology and cost, and they are definitely not achieved through traditional adaptive nulling techniques. At present, the industry mainly follows several technological approaches to address this challenge:

Approach 1: A compromise solution combining a geodetic-grade antenna with array-based spatial filtering

This is a relatively “moderate” anti-jamming solution. It still relies on an antenna array, but its adaptive algorithm is strictly constrained:

• Constraint:When the algorithm computes the weighting coefficients for interference suppression, it must ensure that the impact on the carrier phase of all satellite signals is minimized. In other words, it has to keep the synthesized phase center of the antenna array stable within a very small range.
• Effect:This solution can suppress relatively broad and not extremely powerful interference sources, but its “strength” and “depth” of its interference suppression performance are considerably reduced, making it much less effective than traditional adaptive nulling antennas.

Approach 2: A geodetic-grade antenna+RF front-end filtering+tightly coupled integrated navigation

This is currently the more mainstream and practical solution for achieving highly reliable RTK positioning.

1. High-qualitygeodetic-grade antenna:

First, a geodetic-grade antenna or a choke-ring antenna with a highly stable phase center is selected to ensure RTK accuracy right from the signal source.

2. RF front-end filtering:

At the rear end of the antenna and the front end of the receiver, high-performance band-pass filters and low-noise amplifiers are used. This helps effectively remove out-of-band interference and offers a degree of suppression for typical in-band narrowband interference.

3. Tightly coupled integrated navigation:

This serves as the “intelligent brain” of the system, implementing a tightly-coupled integration between the GNSS receiver and the Inertial Measurement Unit (IMU). When encountering short-term and high-intensity interference that causes GNSS signal loss of lock or positioning jumps, the IMU can provide continuous, high-frequency information on the carrier’s attitude, velocity and position, which are then used to assist and correct the GNSS solution.

• During the interference period, the system relies on the IMU to maintain a high-accuracy navigation solution.
• After the interference disappears, the IMU information helps the GNSS receiver quickly reacquire the satellites, fix the integer ambiguities, and rapidly restore RTK-level positioning accuracy.

The advantage of this approach is that it does not compromise the inherent phase-center stability of the GNSS antenna, but instead tackles interference through RF filtering and inertial navigation assistance. In this way, this approach makes it possible to maintain continuous high-precision positioning performance even in complex and interference-rich environments.

Approach 3: Emerging technology explorations

AI-powered anti-jamming technologies:

By using AI algorithms to more intelligently identify interference and signal characteristics, it may be possible to achieve more precise interference suppression in the future, while minimizing the impact on phase.

Conclusion

• Traditional adaptive null-steering antennas:

To suppress interference, these antennas must dynamically adjust their radiation beams, which inevitably leads to an unstable phase center. As a result, they are fundamentally unsuitable for RTK carrier-phase positioning.

• Practical approach to simultaneously achieving “both anti-interference and RTK”:

High-end approach: phase-constrained adaptive array (spatial filtering), offering a high cost and a compromise in performance.

• Mainstream and practical approach:

“High-quality geodetic-grade antenna+RF front-end filtering+Tightly coupled integrated navigation”. This solution ensures the continuity and reliability of RTK accuracy under the vast majority of interference scenarios. Therefore, it has become the preferred choice in high-reliability applications such as UAVs, autonomous driving, and high-precision surveying and mapping.