CRPA Antennas Explained: Choosing and Testing the Best Anti-Jam Solutions for GPS/GNSS Resilience

Controlled reception pattern antennas (CRPAs) offer higher protection against GNSS navigation warfare (NAVWAR) threats. But how do you select and integrate the right CRPA for your systems?

As the only global position, navigation, and timing (PNT) service capable of providing an absolute position and autonomous time, global navigation satellite system (GNSS) signals are a key input to the multi-sensor positioning systems used by aerospace, defense, autonomous vehicles, advanced driver assist systems (ADAS), and other safety- and reliability-critical systems.

For developers, designers, and integrators of those systems, protecting the GNSS receiver against radio frequency (RF) signal interference is a key consideration. It’s an area where specialized anti-jam antennas can play an important role, but choosing which one and effectively integrating it presents a challenge.

In this blog we’ll explore the options for GNSS receiver integrators when selecting an anti-jam antenna and provide an introduction to testing and evaluating antenna performance.

Specialized GNSS antennas can help to counter interference threats

While natural forms of RF interference—like solar storms and ionospheric scintillation—are well known, artificial NAVWAR threats such as signal jamming and spoofing, as well as adjacent-band interference (ABI), are continually increasing in prevalence, diversity, and sophistication. Without mitigation, they can significantly affect a receiver’s ability to compute an accurate position.

One way of countering these threats is to use a specialized antenna capable of rejecting interference waveforms and focusing on genuine GNSS signals. Such anti-jamming and anti-spoofing antennas are designed to provide increased interference protection at the receiver’s RF front-end by maximizing antenna gain towards genuine GNSS signals and/or minimizing gain towards interfering signals.

There has been discussion and experimentation with regard to modifying fixed reception pattern antennas (FRPAs) to enhance anti-jam capabilities – including using choke rings and shaped ground planes – but our focus here will be on CRPAs.

CRPA: Adaptive multi-element antennas for advanced interference protection

A CRPA employs multiple physical antenna elements whose reception pattern can be dynamically adjusted by the receiver in response to interference signals detected in the environment. Because of this ability to detect and respond to interference, CRPAs are sometimes also called adaptive antennas.

CRPA designs vary, but typically include multiple individually controllable antenna elements arranged in a circular pattern around a central reference element. Signal processing algorithms detect interference signals and amplitude and phase from each antenna element are adjusted in real-time to either to form a null in the direction of the interference source (null steering), or to direct the peaks towards genuine signals (beam steering).

CRPAs can offer strong protection against interference, especially when paired with sophisticated interference detection and mitigation algorithms, but their large form factor can be prohibitive for smaller devices and devices where weight is a key consideration. They also consume more power than a FRPA and can be expensive, so making the right decision is critical.

Where are anti-jam antennas typically used?

Anti-jam antennas are typically used in GNSS receivers where either the equipment itself or a downstream application that relies on its PNT information requires high levels of protection against GNSS interference. Systems that would benefit from incorporating CRPAs include:

• Military vehicles and equipment: CRPAs originated in the military domain with the need to protect vehicles and equipment against adversarial jamming and spoofing, key tactics used in electronic warfare and NAVWAR. CRPAs can also protect military receivers against Blue Force Electronic Attack (BFEA) waveforms, a type of selective jamming used to disrupt enemy access to GPS signals.
• Critical National Infrastructure (CNI): CNI – including power and utility grids, mobile base stations, stock exchanges, and more – has an increasing and increasingly critical dependence on timing data derived from GNSS. Lower cost CRPAs have been proposed to protect these important functions from the growing threat of RF interference.
• Autonomous vehicles (AVs): Self-driving vehicles require advanced protection against harmful interference to GNSS receivers to ensure they can position themselves with a high degree of accuracy and navigate accurately and safely at all times. GNSS is frequently used as the ‘truth’ signal within a multi-sensor AV positioning system, both to determine an absolute position and to discipline inertial sensors prone to drifting, so protection against jamming and spoofing is essential. Whether or not CRPAs become commonplace in civil autonomous vehicles will likely be decided by cost and regulation as the technology matures.
• Commercial aircraft: Civil aviation is increasingly subjected to jamming and spoofing attacks. As more nation states bring these tactics into their defensive and offensive arsenals, this problem will only increase. Similarly to autonomous vehicles, whether or not CRPAs are leaned on to solve this problem will likely depend on regulatory aspects such as export controls, and physical aspects such as size, weight, power and cost.

What kinds of interference can CRPAs protect against?

CRPAs are designed to protect GNSS receivers against multiple types of RF interference. It should be noted that they can’t always be relied upon to provide 100% protection, and therefore should be used as one element in a layered approach to PNT hardening.

However, the types of interference they can help to protect against include:

• Jamming: Intentional GNSS frequency jamming is a widespread threat, often caused by illegal jammers used to override vehicle telematics or by nation states engaging in electronic warfare. It works by flooding the RF environment with waveforms that ‘drown out’ the faint signals from GNSS satellites. Unprotected receivers may lose lock or output an inaccurate position without warning.
• Adjacent-band interference (ABI): This refers to unintentional interference from radio transmissions in bands close to the GNSS frequencies. It can stem from faulty equipment emitting signals on the wrong frequency or from legitimate services operating in spectrum bands close to the GNSS bands. As with intentional jamming, ABI can cause loss of lock or degraded positioning accuracy.
• Spoofing: The intentional transmission of fake GNSS signals has become easier with the rise of software-defined radio (SDR). Spoofed signals can cause the GNSS receiver to compute an erroneous position and time and the equipment to behave erratically as a result. Spoofing attacks on military equipment or critical national infrastructure (CNI) are becoming a more frequent tactic in electronic warfare.
• Multipath: In built-up areas, GNSS signals tend to reflect off buildings and other objects in the environment. These signals have slightly further to travel, and so arrive at the receiver slightly later than line-of-sight signals. Without mitigation, multipath interference can cause the receiver to calculate an inaccurate range measurement, which translates into an inaccurate position output.
• Obscuration: GNSS signals work on a line-of-sight basis and are often blocked by buildings, hillsides, and dense foliage. An unprotected receiver can be especially vulnerable to jamming and spoofing attacks in areas of high signal obscuration or when exiting a GNSS-denied area like a tunnel or underground car park. As the receiver attempts to reacquire a signal, it can be subjected to a spoofing attack, causing it to lock on to the fake signal rather than a genuine one.

Evaluating CRPAs and other types of anti-jam GNSS antenna

Anti-jam antennas can vary significantly in cost, form factor, and functionality. To evaluate whether your device or system needs one, and if so, which one it needs, the key considerations are the function of the device or system and the type and amount of interference it’s expected to encounter.

Typically, anti-jam antennas are used in systems where continuous, accurate, and reliable GNSS-derived PNT is a non-negotiable requirement — that is, where a loss of GNSS availability or signal integrity could result in a serious safety issue, mission failure, or significant loss of business.

Due to their cost, size, and complexity, anti-jam antennas also tend to be limited to systems that are not only mission-critical, but are also likely to encounter frequent harmful interference, potentially of multiple different kinds. A thorough risk assessment should form part of the decision-making process when evaluating what type of antenna and receiver system to incorporate.

For the majority of safety- and mission-critical systems, a CRPA antenna will be the clear choice. The level of robustness added and the ability to enhance performance even when interference is not present make it the most powerful enhancement to GNSS.

How to test the performance of CRPAs

CRPAs must be thoroughly tested to ensure they perform as expected in different types of interference conditions.

While testing outdoors in a live sky environment is possible, it can only be done on certified test ranges with the right permissions and equipment. The logistics of setting up the test range and obtaining authorization to transmit interference signals make outdoor testing very expensive and something that typically only happens in the final verification stages of product development.

At earlier stages of receiver R&D and integration testing, lab-based simulation equipment is used to transmit GNSS signals and interference waveforms to the device under test (DUT). Lab testing of receivers that use CRPAs can be carried out in two ways:

Conducted testing: This involves playing simulated GNSS signals and interference to the receiver (or antenna electronics unit for a CRPA) via coaxial cable, bypassing the physical antenna. This kind of testing is used to test the receiver system as a whole at the R&D stage. However, because the signals are conducted directly to the receiver, it doesn’t test the performance of the antenna hardware.

Radiative or over-the-air (OTA) testing: This involves transmitting real or simulated RF signals over the air to the DUT to evaluate the performance of the antenna and antenna electronics. This kind of testing is typically carried out later in the product cycle to validate the performance of the whole system including the physical antenna subsystem.

Because of the need to isolate simulated GNSS signals and interference waveforms, lab-based OTA testing must take place in a closed (anechoic) chamber. The chamber can be configured in different ways, from using traditional fixed broadcast antennas in static scenarios, to using a more sophisticated ‘zoned chamber’ setup to simulate satellite movement in orbit.

Spirent solutions for CRPA testing

Spirent offers a full suite of solutions to support both lab-based and field-based testing of CRPAs and other types of anti-jam GNSS antenna. Our solutions include:

• GNSS & wavefront simulation: Spirent PNT X produces a comprehensive range of emulated multi-GNSS, multi-frequency RF signals, including classified signals such as GPS M-CODE and Galileo PRS, with class-leading flexibility, coherence, fidelity, performance, accuracy, and reliability. PNT X functions as an integrated wavefront simulation system designed for the testing of CRPAs and other adaptive antennas. Highly configurable capabilities, including embedded spoofing and high-powered jamming across 16+ elements, provide the flexibility and control needed for demanding applications. Unrivalled precision and phase alignment make this the established choice for CRPA integrators. Building on the groundbreaking capabilities of the GSS9000 platform, PNT X introduces intuitive CRPA configuration, enhanced realism for non-standard signals, continuous dynamic range, and more, combining unrivalled features with uncompromising performance.
• Zoned chamber support: Spirent test and measurement engineers have significant experience in configuring our patented zoned chamber environment for realistic OTA testing of adaptive antennas. We can advise and support on any aspect of setting up and configuring a zoned chamber.
• In-field testing: The Spirent Portable Simulator enables the introduction of interference threats to authorized range testing. From over-the-air interference to GNSS-synchronized spoofing testing, the Portable Simulator adds another dimension to CRPA testing.

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