Executive Summary
Overview
Controlled Reception Pattern Antennas (CRPAs) — also referred to as anti-jam antennas — are a critical frontline technology for keeping positioning systems accurate and reliable in hostile NAVWAR (navigation warfare) environments. Whether you’re defending a vessel, guiding an aircraft, or hardening land navigation systems, CRPAs enable receivers to acquire and track authentic signals while suppressing or rejecting jamming and spoofing signals.
This page will take you from the big picture down to an approachable explanation of the mechanics:
- Why CRPAs matter
- How CRPAs work
- Signal deep dive
- Why testing is mission-critical
- How to test
Introductory Principles
Figure 1. Modeled GPS satellite orbits
What is GNSS?
Global Navigation Satellite Systems (GNSS) are satellite-based networks that provide positioning, navigation, and timing (PNT) services worldwide.
One of the most well-known GNSS systems is GPS, which powers everything from smartphones and vehicles to advanced military platforms. Modern receivers can detect and use GPS signals to pinpoint positions within meters, or even centimeters with advanced techniques.
Each GNSS satellite continuously broadcasts a radio signal containing precise location and timing information. Remarkably, by the time these signals reach Earth, they are millions of times weaker than a typical Wi-Fi signal — which makes them easy to block (jam) or fake (spoof).
What is a CRPA and why use it?
For systems that rely on GNSS in critical missions, resilience under all types of signal interference and environmental challenges is essential. That is where CRPAs come in. These advanced, adaptive antennas filter out jamming, detect fake spoofed signals, and focus receiver tracking towards the real GNSS “truth” signals — all in real-time. In GPS-contested situations, known as navigation warfare (NAVWAR) environments, CRPAs are an essential anti-jamming, anti-spoofing technology that can be the difference between mission success or failure.
A CRPA is made up of multiple small antennas called antenna elements, which work together as an antenna array. These elements are spaced evenly and combine incoming radio signals to function like one single, steerable antenna. The signals from each element are sent to the antenna electronics (AE) unit, which processes them. This AE unit can either be built into the same housing as the antenna array or placed separately, depending on the use case.
The number of elements typically varies from 4 (minimum for effective anti-jam protection) to 16, but large specialized CRPAs with 20+ elements are being developed. One common CRPA implementation is a 7-element design, with one center antenna and 6 surrounding it. For GNSS applications, the antenna array would typically be located on top of a vehicle with the field of view pointing to the sky.
Top: A representative array with 7 antenna elements
Bottom: A conformal CRPA designed to fit aircraft fuselage
Top: A 4-element antenna and separate antenna electronics (AE) unit
Bottom: An integrated CRPA with antenna and AE in one
How does a CRPA work?
A CRPA improves GNSS receiver performance by using an adaptive array of antenna elements.
Single antenna vs. array
- A single-element antenna has a fixed reception pattern at a given frequency.
- A CRPA, by contrast, dynamically adjusts its reception patterns by combining signals from multiple elements.
How the array adapts
- The antenna electronics apply weights—adjustable values that modify each signal’s amplitude (signal strength) and phase (timing or wave alignment)—to the RF signals received at each element..
- These weighted signals are combined to form a new reception pattern.
- The pattern is optimized to:
- Amplify desired GNSS signals: beamforming
- Suppress interference/jammers: null steering
Result
- The CRPA dynamically “points” beams at satellites and “steers” nulls toward interference sources.
- This preserves GNSS signal tracking, even in hostile NAVWAR environments.
Figure 4. An example of combining the antenna elements of a 7-element CRPA to form a simple beam pointing directly overhead
The following plots illustrate CRPA array performance with the blue line representing a desired signal where the CRPA array provides a beam peak, and the red line(s) representing jammers where the CRPA array provides a null.
Each of the three plots show a different arrangement of desired and interfering paths. The beam peaks and nulls may be calculated separately for each incoming signal and updated rapidly to track the movement of the satellites, jammers, and the vehicle.
Figure 5. Illustrative beam patterns of a CRPA antenna in the presence of jamming (Source: Michael Jones, GPSWorld.com)
Technical Analysis
Digging deeper
To summarize what we have discussed so far, CRPA systems are used with GNSS receivers to improve overall system coverage and performance in NAVWAR environments. To do this, CRPAs combine signals from multiple antenna elements using carefully chosen weights—timing and power adjustments—that align the incoming signals from a desired direction. This process, known as beamforming, strengthens the desired signal. CRPA systems can also apply weights that cause destructive interference in the direction of unwanted signals, a technique known as null steering. Together, these methods improve the jamming-to-signal (J/S) ratio, making the navigation system more robust.
Advanced receivers may also use more sophisticated signal processing techniques—such as space-time adaptive processing (STAP) or space-frequency adaptive processing (SFAP)—to further enhance signal detection, tracking, and recovery in high-interference environments.
Because CRPA is an add-on to the GNSS receiver, it provides a flexible, system-level way to improve J/S performance. Its design and behavior can be tailored to the specific needs of the platform or mission it supports.
Now let’s take a closer look at the underlying mechanisms that enable CRPA systems.
An overview of GPS signal propagation
This section provides an overview of GPS signal propagation, focusing on the characteristics of right-hand circular polarized (RHCP) waves and the planewave model used to describe their behavior as they travel from satellites to receivers.
CRPA linear array modeling
Points having equal phase, e.g., the peaks of the planewave, are drawn as parallel lines (///) in the diagram below to represent a received planewave.
The uniform linear array is the simplest example to illustrate beamsteering and null forming principles. Figure 9 illustrates a 4-element linear array. Each of the array elements, A1-4, are fixed antennas, assumed to have an ideal omni-directional pattern; however, a unique pattern could be applied. A constant antenna spacing of dv is assumed to be ½ wavelength. In this example, a planewave is arriving with an angle of φ from the array boresight, the axis perpendicular to the array elements.
Figure 9. Illustration of a 4-element linear antenna array
Notice that there is an increasing delay to the arrival of the planewave at each of the 4 antennas with the given arrival angle. Each delay results from the distance traveled to encounter the array element. If the signal bandwidth is small as in the case of GNSS signals, then each delay can be represented by a unique phase at the center frequency, i.e., fc = C/λ.
Forming a beam in the direction of the incoming signal will maximize the received signal strength. This requires the weights to be set with a negative phase advance to that observed from the planewave, so there will be a phase alignment from each signal and they will sum to a peak.
Figure 10 depicts the behavior of an 8-element linear antenna array operating in the presence of jamming signals.
Figure 10. Animation of an 8-element linear array in the presence of jamming signals.
Interactive beamforming plot
The beam plot of a uniform linear array is achieved by scanning the weights to evaluate pointing the beam at a range of angles, i.e. where is varied φm is varied from -90 to +90 degrees, using this equation: Φm=exp(-j2π(m-1)dv/λ sin(φ)). As expected, when the beam pointing angle matches the incoming planewave angle, there is a peak in the response.
In Figure 11 below, use the sliders to vary the number and spacing of elements and the beam pointing angle to see the resulting changes in the linear array.
λ
°
Figure 11. Interactive beamforming plot for a uniform linear array
Figure 12. Linear CRPA nullsteering
Null steering plot
For a null steering example, the weights could be set in various ways to produce a null for the incoming planewave angle. In Figure 12, the weights were multiplied by [1, -1, 1, -1], so that the sum forms a null in the direction of the undesired signal at +35 deg when the desired signal is at -35 deg.
Figure 13. Linear arrays contrasted with planar arrays
Planar arrays
More complex arrays can be formed with a 2D grid of elements, called a planar array. This type of array can form a beam in azimuth (horizontal angle) and elevation (vertical angle) and is contrasted with a linear array in Figure 13.
The array response observed from a circular pattern is based on the location of each element in the array and the complex response of each antenna. As previously discussed, these weighted parameters produce an amplitude (signal strength) and phase shift (signal timing) of the signal received on each element, which will be processed in the receiver to optimize the performance of the desired signal.
The beam peaks and nulls may be calculated separately for each incoming signal and updated in real time to track the movement of the satellites, jammers, and the vehicle the CRPA is on.
How to Test a CRPA
Why test a CRPA system?
It is critical for developers and integrators to test a CRPA system before deployment for the following reasons:
-
Mission-critical applications
- CRPA systems are used in defense, aerospace, and high-stakes navigation.
- Reliable performance is essential for mission success and safety.
-
Need for resilience in NAVWAR
- Must operate under extreme conditions such as:
- High-power jamming
- Spoofing
- Rapid platform dynamics on munitions, aircraft, or vehicles
- Systems must be validated against corner cases to ensure robustness.
- Must operate under extreme conditions such as:
-
Consequences of insufficient testing
- Unverified systems can lead to:
- Mission failure
- Compromised navigation or targeting
- Loss of assets or lives
- Unverified systems can lead to:
-
Purpose of rigorous testing
- Confirms system reliability and accuracy under real-world and worst-case scenarios
- Builds operational confidence in fielded systems
- Ensures the system can be trusted when it matters most
CRPA test methodologies
There are several approaches to CRPA testing; each will be discussed below.
Figure 14. Live-sky testing at White Sands Missile Range in 2017 (Source : US Air Force, afmc.af.mil)
Outdoor live-sky field testing
Live-sky testing uses real satellite signals in outdoor environments to measure system performance in realistic conditions. While it offers valuable insight into real-world behavior, it comes with several major drawbacks. These tests are not repeatable or controllable—satellite positions, signal conditions, and environmental factors like multipath and interference constantly change. This makes it hard to isolate variables or compare results over time. Additionally, live-sky testing is often expensive, requires specialized permits, and can be logistically difficult to conduct consistently.
Figure 15. A wavefront impinging on a 4-element CRPA. Wavefront simulation requires precise phase alignment of all signals across all CRPA elements.
Conducted wavefront testing
Conducted wavefront testing—also known as direct injection—is a powerful way to test CRPA systems without ever radiating signals over the air. The CRPA’s intelligence lies in the antenna electronics that process and weight the received wavefronts, so a simulator can connect directly to the AE via cables, bypassing the antenna. Capable wavefront simulators such as the PNT X model how signals arrive from space, as well as simulating jamming and spoofing and other complementary PNT signals.
This enables controlled, repeatable, in-the-lab testing of even the most complex NAVWAR scenarios, making conducted wavefront testing a fast, scalable, and incredibly efficient way to validate CRPA performance under realistic conditions.
Figure 16. Two planewaves encroaching on a CRPA (left), and the identical effects created through simulation (right)
Conducted wavefront testing models each of the satellite vehicle signals at the proper azimuth and elevation observed and processed by the CRPA array. Figure 16 demonstrates how this process works on an array and how a simulator replicates it.
In Figure 16-left, two circularly polarized planewaves are traveling from satellites at different locations in space toward the 7-element CRPA antenna. The red and blue geometric planes define points of constant polarization for the propagating planewave, which defines the wavefront being observed as it intersects with the antenna array. The red planewave is shown to arrive first and contacts the array elements in order of arrival, followed by the blue planewave.
Each received planewave, which can be described as a flat wavefront having constant phase, propagates across the array and energizes each element at a specific time. Thus, there is a relative delay between the arriving signal when it is observed at each array element. These delays are proportional to a certain phase at a given frequency. For narrow band signals like GNSS it is sufficient to model each signal using the calculated phase offset to model the spatial characteristics.
When the antenna elements are energized by the red or blue wavefront, they are illuminated to illustrate the wavefront signal contacting with each element. Then a red or blue dot representing the relative arrival time differences follows the cables associated with each antenna element. Therefore, the direction of arrival information can be observed from the relative delays indicated by the red and blue dots traveling down the wires. (Delay differences can be emulated as phase differences when the bandwidth is small as with GNSS signals.)
In Figure 16-right, the PNT X simulator is shown reproducing the signals for a CRPA test modeling the same two satellite signals being received as shown by the red and blue planewave signals. As observed, the same spatial information is present in the test signals (illustrated by the red and blue dots) representing exactly what was observed by the CRPA antennas.
For more information on CRPA wavefront testing, watch the Spirent webinar, Testing, Simulation & CRPA Innovation, on-demand now.
Figure 17. Flexible / scalable wavefront CRPA testing for NAVWAR: GNSS signals, jamming, & spoofing (Click to view full scale)
Conducted Wavefront Testing Summary
- The PNT X supports conducted CRPA testing
- Precise phase alignment
- Thermal phase stability
- High J/S levels
- Low uncorrelated noise levels
- Interference and spoofing are also available –all in a perfectly carrier-phase-aligned wavefront
- Key capabilities
- Support for 4-16 elements and more
- Support for all GNSS signals including classified signals
- >130dB J/S, BFEA
- Combine with (multiple) instances of inertial simulation & alternative navigation
- Inject custom IQ waveforms
Figure 18. A F-16 under test in an anechoic chamber at Eglin Air Force Base, Florida (Source: US Air Force, eglin.af.mil)
Anechoic chamber testing
Anechoic Chamber testing uses simulated GNSS signals transmitted over-the-air using probes inside the chamber. This enables testing of the full receiver with its CRPA antennas included so that all aspects of the device-under-test is evaluated. Since the probes are at specific locations in the chamber, this produces a spatial model for the SV signal that includes and azimuth and elevation, so the probes can be used to represent satellite locations in the sky.
Fixed probes cannot replicate the movement of satellites across the sky, so a fixed probe arrangement is limited to a short simulation time before it would no longer accurately represent the SV locations.
Spirent’s patented zoned chamber
To overcome standard anechoic chamber limitations, Spirent developed a zoned chamber solution. Independent zones simulate real-world sky views, each tied to a probe that transmits one or more SV signals. As time progresses, SV signals can switch zones to reflect movement, enabling test scenarios to run for hours instead of minutes.
This extended realism makes the zoned chamber ideal for full CRPA system validation. Systems with inertial sensors can be tested in static or dynamic scenarios using a positioner. Interference sources like jammers and spoofers can be simulated alongside GNSS signals.
For more information on anechoic chamber testing for CRPA systems, watch the Spirent Federal webinar, Resilient PNT for NAVWAR & Civil Applications: Anechoic Chamber Test Innovations, on-demand now.
Figure 19. Spirent’s patented zoned anechoic chamber approach to CRPA testing provides day-long test durations and unmatched realism
Chamber Testing Summary
- PNT X supports anechoic chamber testing, i.e., testing the CRPA including the antenna
- Option 1 employs fixed antenna array
- Option 2 leverages Spirent’s patented zoned chamber
- The zoned chamber enables simulation of the motion of the satellites as they orbit—enabling long scenario durations and high degree of realism
- Spoofers, jammers, multipath can be simulated as well
- Spirent can simulate DUT motion through rate table or via SimINERTIAL
Test Considerations
Spirent CRPA Testing Solutions
To ensure the highest levels of reliability and integrity, CRPA systems must be thoroughly tested in a wide range of real-world scenarios. Spirent PNT X is uniquely engineered to exceed the complex requirements of current and future CRPA system testing:
- Architecture. PNT X is purpose designed using expertise honed by decades of experience and innovation. Ultra-low phase noise and unrivaled pseudorange accuracy provide the most realistic environment for testing CRPA systems, even in high-dynamic and high-jerk scenarios. Using a substandard architecture in CRPA testing can lead to inaccurate test results, false conclusions regarding resiliency and performance, and failure in the field.
- High-fidelity capability. In wavefront CRPA testing, signal carrier and code phase alignment are critical. High-quality phase alignment is particularly important for testing multi-element antennas like CRPAs.
- If a simulator needs continuous monitoring and run-time corrective adjustments to the signal to remain phase-aligned, it is a fundamentally flawed approach for applications such as CRPA testing that require aligned signals for precision wavefronts.
- When phase-alignment calibration occurs during the test, it introduces considerable phase noise that impacts the performance of the simulator and could skew test results.
- Spirent solutions are calibrated once before the start of a scenario. The wavefront is precisely calibrated, and performance is maintained for the entire test.
- Scalability. Using a scalable architecture like PNT X ensures an existing solution can easily grow to test additional antenna elements as systems evolve.
- Encrypted signals. For 40 years, Spirent has been the trusted test partner for testing restricted GNSS signals, including GPS MNSA M-Code and Galileo PRS.
- EGI/IMU signal simulation. The SimINERTIAL software module enables you to integrate simulated signals from inertial sensors into test scenarios, allowing you to characterize the performance of an integrated or embedded GNSS/Inertial solution.
To learn more, read our white paper, Characterizing CRPA and Other Adaptive Antennas, or contact us to schedule a demonstration.
