How a CRPA antenna keeps GNSS working under jamming
A standard GNSS antenna hears the whole sky equally, so a jammer of a few watts can bury satellites that arrive at around −130 dBm. A CRPA antenna is an array of elements: it works out where the interference comes from and stops listening in that direction, while the satellites stay in view.
Satellites whisper. Jammers shout.
GNSS satellites orbit about 20,000 km up. By the time their signals reach the ground they are at around −130 dBm — below the receiver's own thermal noise; the receiver only finds them by correlating against a known code. A jammer needs no such trick. One watt, ten kilometres away and in line of sight, arrives at about −86 dBm.
That jammer is about 44 dB — roughly 25,000 times — stronger than the satellite. A standard antenna cannot tell the two apart, so the receiver loses lock.
Don't out-shout the jammer. Stop listening to it.
A standard GNSS antenna — a fixed reception pattern antenna, or FRPA — has one pattern that never changes. It hears the jammer with the same gain as the satellites, so once the jammer is louder, the fix is gone.
A controlled reception pattern antenna — CRPA — combines several elements electronically and reshapes its pattern in real time. Gain toward the sky is kept; gain toward the jammer is driven down to almost nothing — a null.
GNSS satelliteJammerReception pattern (elevation cut)
From elements to a clean fix
-
01
Elements
4 to 16 elements each receive the satellites and the jammer — at slightly different times, because each sits at a slightly different place.
-
02
Weighting
The anti-jamming module measures those differences and gives each element a weight in amplitude and phase.
-
03
Null
Summed with those weights, the jammer cancels: a deep null points at it. The weights are recomputed continuously, so the null follows a moving jammer or a turning platform.
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04
Fix
Satellites arrive from many directions, so cancelling one bearing costs them little. The cleaned signal goes on to the receiver, and the position and time solution carries on.
Drag the jammers yourself
Move a jammer, add more, switch the array size — and watch how many nulls the array can place and how many satellites it keeps.
- Nulls / capacity
- 1 / 3
- Satellites held
- 7 / 7
A simplified model for illustration: the real pattern depends on the array geometry and the installation.
What the datasheet numbers mean
N elements, up to N−1 jammers
An array of N elements can place at most N−1 independent nulls. That is geometry, not firmware — it sets how many separate jammers a terminal can suppress at the same time.
dBc: how much stronger the jammer can be
dBc is the jammer-to-signal ratio a terminal absorbs while holding a usable fix. ≥120 dBc means interference 10¹² times stronger than the satellite signal. The figure falls as the jammer count rises, because every null spends some of the array's freedom — so always read it together with the jammer count.
Coverage and interference types
A suppression figure is only comparable if you know what it was measured against, and over what sector. infoX terminals are specified against five interference types across the full sphere:
Azimuth 0–360°, elevation -90° to +90°
- Band-limited Gaussian white noise
- Spread-spectrum matched-spectrum interference
- Pulse interference
- Swept-frequency interference
- Mixed interference
Spoofing: the quiet threat
A spoofer transmits counterfeit satellite signals — correct codes, plausible timing — describing a position of its choosing. The receiver raises no alarm; it confidently reports the wrong place.
But every fake satellite comes from the same transmitter, so the whole counterfeit constellation arrives from one bearing. Real satellites cannot do that. An array can read direction, so it attenuates that bearing as a group — the same way it nulls a jammer.
Five things that decide field performance
Ground plane
The array is specified on a conductive ground plane. Give it a continuous, flat plane at least as large as its own footprint.
Clear sky
Every element needs the same clear view of the sky. Keep masts, gimbals and your own radios away from the aperture.
Cable and level
The output is into 50 Ω. Keep the run short and count every connector; if your receiver needs a different level, tell us before ordering.
Power
From ≤6 W at four elements to ≤40 W at sixteen. Check it against your continuous budget, not your peak, and give the unit somewhere to shed heat.
Test on the airframe
Verify with the terminal installed, the aircraft powered and every radio transmitting. If C/N₀ is already low before any jamming, the installation is eating your margin.
Not sure — ask
Send the platform, the weight and power budget and the interference you expect — we will name one model.
Not sure which array you need?
Send the platform, the weight and power budget, and the interference you expect. We name one model and reply within one business day.