AJAS-2: CRPA for Robust L1 GNSS Operation

1 | CRPA 101—Nulling a Jammer with Two Antennas

A controlled-reception-pattern antenna (CRPA) uses multiple GNSS elements whose complex outputs are combined with user-chosen weights (amplitude + phase). With two L1 antennas—spaced about λ⁄2 apart—you can:

1. Equalise their amplitudes so the jammer’s envelope is identical on both.
2. Add 180° of extra phase to one channel.

For a fixed-direction jammer this forces the two copies of the interference to cancel, creating a deep spatial “null” while leaving the low-level satellite signals (arriving from many directions) largely unaffected. Any residual amplitude or phase error limits the achievable null depth.

2 | RF-based CRPA: Architecture and Limitations

RF-based CRPAs implement their weights with discrete components in the RF analog domain:

Because the ideal amplitude/phase pair rarely lands exactly on one of these quantized settings, the cancellation is imperfect; field data for two-element analog nullers shows null depths that wander between roughly 20 dB and 40 dB.

3 | AJAS-2: Digital Signal Processing Removes the Step-Size Ceiling

AJAS-2 implements jammer rejection in digital domain digitizing each antenna element output at down-converted frequency using high-resolution analog-to-digital converter (ADC) and solves complex weights in much higher resolution:

In practice this yields 50–60 dB of null suppression—an order-of-magnitude improvement over stepped analog hardware.

Below is an antenna pattern example of AJAS-2 nullifying a jammer coming from 30 degree azimuth and 20 degree elevation direction.

4 | Where Two Antennas Excel—and Where They Don’t

A two-antenna CRPA can place one spatial null at a time.
If wide-band jammers arrive from 3 bearings simultaneously, you need ≥ 4 antennas (giving up to 3 nulls) to nullify 3 different direction wideband jammers. Some RF-based CRPA product advertise as four-element CRPA that offers “two nulls (L1 + L2)”; in reality that is the same one direction at L1/L2 two frequencies if encountering a L1/L2 jammer, unlike four antenna CRPA that could nullify wideband jammer in 3 directions. The AJAS-2 delivers one exceptionally deep, adaptive null in the L1 band—exactly what most applications need to survive a single wideband jammer.

5 | UAV GPS-Denied Navigation Application Example

GPS-denied navigation is a critical and rapidly evolving field. Below example demonstrates how the AJAS-2 can significantly help lower the overall cost of providing reliable 24/7 day and night precision guidance for defense UAV applications.

Because position drift in an unaided inertial navigation system grows with the cube of time (t³) when gyro characteristics dominates, we first measured real‑world drift on a lower speed platform cruising at about 100 km/h with a low-cost 0.5 °/h gyro‑bias‑instability IMU. The table below summarizes the positional error over ten 2 km runs. Using the same t³ scaling, we then project what these errors would be for a UAV flying at 200 km/h, as shown in the right‑most column. This is roughly in line with BAE Systems' recent white paper on APNT for UAVs requiring ≤ 10 m position error.