SAP vs. STAP: Understanding Multi-Tap CRPA Anti-Jam Performance
10th Apr 2026
CRPA (Controlled Reception Pattern Antenna) systems suppress jammers by steering spatial “nulls” toward interference sources. But not all CRPA implementations are the same. The two main architectures — SAP (Spatial Adaptive Processing) and STAP (Space-Time Adaptive Processing) — differ fundamentally in how many jammers they can handle at once.
This article explains the difference and presents measured results from the AJAS-2 CRPA anti-jam GNSS receiver.
SAP vs. STAP: Degrees of Freedom
Each null a CRPA places costs one degree of freedom. Think of degrees of freedom as the budget the adaptive processor has available for suppressing jammers.
A 1-tap SAP CRPA with N antenna elements has N−1 degrees of freedom. A 2-element array gets exactly one null — enough for one jammer from one direction.
A multi-tap STAP CRPA adds temporal processing taps to each antenna channel. Each tap creates additional degrees of freedom, so the same 2-element array can handle multiple jammers arriving from different directions simultaneously. This is the fundamental reason why fielded military CRPA systems universally employ STAP.
Test Setup
To illustrate the difference, we ran the AJAS-2 in both a 1-tap SAP configuration and multi-tap STAP configuration, using three progressively harder interference scenarios. A PlutoSDR tuned to GPS L1 (1575.42 MHz) served as the spectrum analyzer for all captures.
| Scenario | Interference Sources | Directions |
|---|---|---|
| 1. Single jammer | One 40 MHz wideband chirp | 1 |
| 2. Jammer + reflection | Same chirp + multipath reflection | 2 |
| 3. Jammer + 2 CW tones | 40 MHz chirp + two narrowband CW jammers | 3 |
How to read the spectrum traces
Each scenario shows three observation points:
- RF Input — the raw RF spectrum at the AJAS-2 antenna input port.
- CRPA Input — the digital baseband signal at the input of the CRPA adaptive processor, looped back out through the RF transmitter for spectrum capture.
- CRPA Output — the digital baseband signal after adaptive null-steering, captured the same way.
These three observation points sit at different stages of the receive chain — before and after conditioning, gain control, and loopback — so their absolute power levels are not directly comparable. Jammer suppression is measured as the change between the CRPA Input and CRPA Output traces.
The narrow spike at the center of every trace (1575.42 MHz) is the PlutoSDR’s own local-oscillator leakage — an artifact of the spectrum analyzer, not part of the jammer or the CRPA output.
Scenario 1: Single Jammer — One Direction
With one 40 MHz wideband chirp arriving from a single direction, both 1-tap SAP and multi-tap STAP achieved approximately 33 dB of jammer suppression.
RF Input — raw spectrum at the AJAS-2 antenna port
CRPA Input — digital baseband before adaptive processing (loopback capture)
1-tap SAP Output — 33 dB jammer suppression
Multi-tap STAP Output — 33 dB jammer suppression
One jammer, one direction, one null needed. Both architectures perform identically here. For the most common civilian jamming threat — a single wideband jammer — a 1-tap SAP design is sufficient.
Scenario 2: Jammer + Multipath Reflection — Two Directions
In practice, even a single physical jammer produces reflections off buildings, terrain, and vehicles. This turns one emitter into multiple virtual jammers arriving from different angles. We simulated this by adding a multipath copy of the same chirp arriving from a second direction at 6dB reduced power.
RF Input — wideband chirp plus multipath reflection
CRPA Input — digital baseband before adaptive processing (loopback capture)
1-tap SAP Output — 0 dB suppression
Multi-tap STAP Output — ~28 dB suppression
With two interference directions, a 1-tap SAP has exhausted its single null — it can suppress one path but not both. The multi-tap STAP, with its additional degrees of freedom, places nulls on both the direct and reflected paths and delivers approximately 28 dB of suppression.
This scenario is significant because multipath is the default condition in any environment with reflective surfaces — urban areas, mountainous terrain, or naval environments where sea-surface reflections are constant.
Scenario 3: Jammer + Two CW Tones — Three Directions
This scenario combines the wideband chirp with two narrowband CW jammers at different frequencies and different angles of arrival — three distinct interference sources from three directions.
RF Input — wideband chirp plus two narrowband CW jammers
CRPA Input — all three interference sources visible in the digital baseband (loopback capture)
1-tap SAP Output — 0 dB suppression
Multi-tap STAP Output — ~25 dB suppression
With three threat directions, the 1-tap SAP again cannot address more than one source with its single null. The multi-tap STAP suppresses all three sources simultaneously, delivering approximately 25 dB of suppression — each jammer attenuated by more than a factor of 300.
Summary
| Scenario | 1-Tap SAP | Multi-Tap STAP |
|---|---|---|
| Single jammer (1 direction) | 33 dB | 33 dB |
| Jammer + reflection (2 directions) | 0 dB | ~28 dB |
| Jammer + 2 CW tones (3 directions) | 0 dB | ~25 dB |
For a single jammer from one direction, both SAP and STAP perform equally well. The difference emerges when the interference environment becomes more realistic — multipath reflections, multiple jammers, or a combination of both. In these scenarios, the additional degrees of freedom provided by STAP’s temporal taps are essential.
AJAS-2 with multi-tap STAP architecture ensures robust anti-jam performance not only against the simple single-jammer case, but also in the more demanding real-world environments where reflections and multiple interference sources are present.
Combined with OSNMA anti-spoofing protection, the AJAS-2 addresses both sides of the GNSS vulnerability equation — jamming and spoofing — in a single receiver.
For more information on the AJAS-2 or to discuss your application requirements, please visit the AJAS-2 product page or leave us a message.