Decoding the CRPA: An Intuitive Look at Jammer-to-Signal Performance

Controlled Reception Pattern Antenna (CRPA) systems are designed to resist jamming by steering antenna nulls toward interference sources while maintaining gain toward GPS satellites. The J/S specification—the jammer-to-signal ratio a CRPA receiver can tolerate—is a key figure of merit.

The rigorous math behind J/S, spatial filtering, and signal processing can be intimidating. But there’s a simple, intuitive way to grasp it by thinking in terms of ADC bits and quantization.

From 1-bit GPS to Modern Multi-bit Receivers

Early day GPS receivers used 1-bit ADC quantization—enough to detect the presence or absence of the signal after correlation. The satellite signal is extremely weak (~–130 dBm at the antenna), so it was common to digitize after the front-end IF amplifier with minimal dynamic range.

Today’s high-performance GPS/CRPA receivers use N-bit ADCs at the IF or baseband stage. For example, the AJAS-2 CRPA receiver uses a 12-bit ADC, which—ideally—has an SNR given by:

$$\text{SNR} \approx 6.02 \times N + 1.76$$

For $N=12$, this is about 74 dB SNR for a full-scale sinusoid.

Adding Filtering: More Than N Bits Out

After digitization, GPS receivers apply digital filtering with high-precision coefficients. A hardware filter with M bits in its coefficients can produce outputs with more than N bits. An intuitive way to think about it is N integer bits + M fractional bits, giving something we’ll call “N.M bits” in the output word.

How CRPA “Uses” the Bits to Remove Jammers

Now comes the analogy:
When a CRPA system receives both the satellite signal and a strong jammer, it first “allocates” part of the ADC’s dynamic range to represent the jammer. The jammer is much stronger than the GPS signal—often by tens of dB—so it takes up most of the integer bit range.

If the CRPA null-steering works perfectly, it removes all N integer bits worth of jammer energy, leaving only the weaker GPS signal in the M fractional bits region of the output.

Since GPS can still operate with the equivalent of ~1-bit ADC resolution after correlation, those M fractional bits are enough for navigation.

The “6 × N” Rule of Thumb for J/S

From this mental model, each ADC bit represents roughly 6 dB of SNR. If one has N integer ADC bits “dedicated” to removing the jammer, the CRPA could, in theory, tolerate:

$$J/S \approx 6 \times N \ \text{dB}$$

For a 12-bit ADC:

$$J/S \approx 6 \times 12 = 72 \ \text{dB}$$

This is exactly the simple rule that makes the AJAS-2 CRPA’s 72 dB J/S easy to remember.

Proof-of-Concept Lab Demo

Below demonstrates this with a simple test setup using an early stage ASAJ-2 prototype:

1. Signal Source – Spirent GSS7000 GNSS simulator outputs GPS L1 C/A at –115 dBm initially.

2. Jammer Source – A PlutoSDR generates FM noise. It’s initially off.

3. Procedure –

   • Start with –115 dBm GPS signal, no jammer.

   • Turn on PlutoSDR jammer at –50 dBm.

   • Reduce GNSS simulator signal to –122 dBm.

4. Result – The J/S is:

   $$J/S=(-50)-(-122)=72\text{dB}$$

   The receiver continues to function at the operational edge—matching our “6 × N” prediction.

You can see the behavior in this short video:

Why This Model is Intuitive (But Not Exact)

In reality, CRPA performance depends on:

• ADC noise floor and ENOB (effective number of bits)

• Jamming waveform bandwidth vs GPS pre-correlation bandwidth

• Spatial filter convergence and null depth

• Residual phase and amplitude mismatch

But as a first-order mental model, thinking in “ADC bits used for the jammer” gives a quick, memorable link between the ADC specs and the CRPA’s J/S capability.