100 lines
4.0 KiB
Markdown
100 lines
4.0 KiB
Markdown
---
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title: Interference
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tags: interference, jamming, spoofing, agc, cn0
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summary: The difference between noise, which degrades a measurement gradually, and a coherent source, which replaces the signal with a false one.
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---
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# Interference
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Interference is anything in the band that is not the wanted signal. The
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engineering consequences depend entirely on whether it is uncorrelated noise,
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in which case the measurement degrades smoothly, or a coherent source, in
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which case the receiver may be reporting something that was never sent.
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## Definition
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- **Noise** is uncorrelated with the wanted signal. It raises the noise floor
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and reduces the carrier-to-noise density ratio, which shortens the usable
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arc and increases the ranging error.
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- **Jamming** is a high-power source that drives the front end out of its
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linear range, so that the wanted signal is no longer recoverable at all.
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- **Spoofing** is a coherent source structured to look like the wanted
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signal, at a level below the jamming threshold, so that the receiver
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tracks the wrong position and the link appears healthy.
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The distinction is not about power. It is about coherence, and it is what
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separates a system that fails loudly from one that fails quietly.
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## Mathematics
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### Carrier to noise density
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The usable quantity is the carrier to noise density ratio, written as
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`C/N0`, which is the carrier power in a one hertz bandwidth over the noise
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power in the same bandwidth.
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```text
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C/N0 = P_r * G / (k * T_system * F)
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```
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A pointing loss, a rain fade or a jammer each reduces it, and the range error
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scales with the square root of it. A jammer that costs six decibels of
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`C/N0` does considerably more than six decibels of ranging accuracy.
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### The threshold
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A receiver has a usable range of input power, between a floor set by its
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noise figure and a ceiling set by where the analogue chain saturates. Above
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the ceiling, gain compression makes the output a function of the total power
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rather than of the wanted signal, and the receiver is blind to whether the
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wanted signal is present.
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This is why jamming at a moderate level can be more effective than jamming
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at a high level: a strong jammer is detected and a moderate one is not.
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### Coherence
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A spread spectrum receiver correlates the incoming signal against a known
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code. Uncorrelated noise does not correlate, so it is rejected in
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proportion to the processing gain. A coherent jammer that knows the code, or
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that replays it, is not rejected, because it satisfies the correlation. This
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is the property that makes the Doppler observable attractive in this
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environment: a jammer must reproduce the motion to corrupt it.
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## Unit conventions
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- Power in decibels relative to one milliwatt; `C/N0` in decibels hertz or
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dBHz.
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- Noise figure dimensionless, in decibels.
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- Signal strength on an unknown source cannot be converted without a
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reference, which is why interference reporting is inherently relative.
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## Applicability
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The mitigation available determines the risk. Frequency agility and adaptive
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coding help against noise and against a wideband jammer, and do nothing
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against a jammer that follows the agility. Direction finding and nulling
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help against a jammer whose position is not co-located with the wanted
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source, and do nothing against one that is.
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An automatic gain control is a liability in this context: it is what makes a
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receiver report a constant output power, which is exactly the behaviour that
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lets a jammer hide in a nominal-looking reading.
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Reporting interference is hard in the absence of a reference, which is why
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practically it is done by observing that the expected signal is absent rather
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than by characterising what replaced it.
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## Standards
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- Interference reporting: the Radio Regulations of the International
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Telecommunication Union
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- Signal quality reporting: the CCSDS recommendations on tracking data
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messages
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## See also
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- [[link-budget]] for the margin a jammer consumes
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- [[doppler-and-rate]] for why the measurement can be corrupted rather than
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degraded
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- [[operations-planning]] for the operational response |