Scenario 5 - Interference Hunt

Duration: 15-20 minutes

Difficulty: Intermediate

Mission Type: Troubleshooting

Time Limit: 30 minutes

Mission Briefing

SeaLink Global Communications has filed a trouble ticket: their customer is reporting packet errors on TIDEMARK-1. The C/N ratio has dropped significantly, but the cause isn’t immediately obvious.

The spectrum analyzer is currently configured for beacon tracking with a narrow span. You’ll need to reconfigure it to investigate the main downlink signal and find what’s causing the degradation.

Mission Context:

Satellite: TIDEMARK-1
Ground Station: Vermont (VT-01)
Reported Problem: Degraded C/N ratio, packet errors
Your Task: Find the interference source and restore service

Who’s Guiding You:

Charlie Brooks: Will check in as you work through the diagnosis.

Background: The Interference Problem

Interference is one of the most common and challenging problems in satellite communications. Unlike equipment failures that produce obvious symptoms, interference often manifests subtly - degraded margins, increased error rates, or unexplained signal quality variations.

Why Interference Matters

Satellite transponders are shared resources. Multiple operators, sometimes dozens, share the same orbital slot and frequency bands. This sharing is managed through careful coordination:

Frequency planning: Assigning non-overlapping frequency slots
Polarization reuse: Using H and V polarizations to double capacity
Power limits: Preventing one user from overwhelming others
Antenna patterns: Minimizing spillover to adjacent satellites

When any of these coordination mechanisms fail, interference results.

The Economics of Interference

Satellite capacity is expensive. A single 36 MHz transponder on a commercial satellite might cost $1-3 million per year to lease. When interference degrades that capacity:

Service Level Agreements (SLAs) trigger penalties
Customers experience dropped connections, slow speeds, or complete outages
Reputation damage affects future business
Root cause investigation consumes engineering resources

Professional operators develop systematic approaches to interference hunting because the cost of prolonged interference far exceeds the cost of skilled troubleshooting.

Understanding Signal Quality: C/N Ratio

What is C/N?

Carrier-to-Noise ratio (C/N) measures how much stronger your desired signal is compared to the noise floor. Expressed in decibels (dB), it’s the fundamental metric of signal quality in satellite communications.

The Math:

C/N (dB) = Carrier Power (dBm) - Noise Power (dBm)

Typical Values:

C/N Range	Signal Quality	Service Impact
> 15 dB	Excellent	Full throughput, maximum modulation
10-15 dB	Good	Normal operations
6-10 dB	Marginal	Reduced modulation, increased errors
< 6 dB	Poor	Service degradation or loss

Why C/N Drops

C/N can degrade for many reasons:

Reduced Carrier Power:

Antenna mispointing (signal not captured efficiently)
Rain fade (atmospheric absorption)
Satellite power reduction (eclipse, load shedding)
Equipment degradation (LNB aging, cable losses)

Increased Noise:

Thermal noise (always present, temperature-dependent)
Interference (unwanted signals adding to noise floor)
Intermodulation (equipment generating spurious signals)
Solar noise (during equinox sun transit)

When troubleshooting C/N degradation, you must determine which factor changed - the carrier went down, or the noise went up.

Spectrum Analysis Fundamentals

Reading the Spectrum Analyzer

The spectrum analyzer displays signal power (vertical axis) versus frequency (horizontal axis). Learning to interpret this display is essential for RF troubleshooting.

Key Display Elements:

Element	What It Shows	What to Look For
Noise floor	Minimum detectable power	Should be flat and consistent
Signal peak	Carrier power	Height above noise floor = C/N
Signal shape	Modulation type	QPSK/8PSK: “haystack”, CW: spike
Bandwidth	Occupied spectrum	Should match expected value
Spurious signals	Unwanted emissions	Anything unexpected

Signal Shapes and What They Mean

Continuous Wave (CW) Beacon: A pure tone appears as a narrow spike on the spectrum. Beacons are intentionally simple - they exist only to provide a frequency and power reference for antenna tracking.

Modulated Carriers (QPSK, 8PSK, etc.): Digitally modulated signals spread their energy across bandwidth, appearing as a raised “haystack” or plateau. The width depends on symbol rate; the shape depends on filtering.

Noise: Random thermal noise appears as a relatively flat floor across all frequencies. Elevated noise in specific regions suggests interference.

Configuring for Different Tasks

Beacon Tracking Setup:

Narrow span (few kHz) to see the CW spike clearly
Low RBW for maximum sensitivity to weak beacons
Centered on beacon IF frequency

Wideband Signal Analysis:

Wide span (50-100 MHz) to see full signal and surrounding spectrum
Higher RBW for faster sweeps
Centered on signal of interest

Interference Hunting:

Wide enough span to see interference source
RBW narrow enough to resolve interference characteristics
May need to scan across the full IF band

The RF Signal Path

Frequency Conversion in the Receive Chain

Satellite signals arrive at RF (Radio Frequency) but are processed at IF (Intermediate Frequency). Understanding this conversion is essential for spectrum analysis.

The LNB’s Role:

The Low Noise Block downconverter (LNB) performs two functions:

Amplification: Boosts the weak satellite signal (typically 60 dB gain)
Frequency conversion: Translates RF to IF using a Local Oscillator (LO)

C-Band Conversion (High-Side LO):

For C-band with a 5,250 MHz LO:

IF = LO - RF

Signal Type	RF Frequency	IF Frequency
TIDEMARK-1 Beacon	4,175.5 MHz	1,074.5 MHz
Main Downlink	3,718 MHz	1,532 MHz

Why This Matters:

When you see a signal on the spectrum analyzer at a particular IF frequency, you must convert back to RF to understand what satellite signal you’re viewing. Interference that appears at one IF frequency corresponds to a specific RF frequency that may help identify its source.

Vermont Ground Station Equipment

Equipment	Key Parameters
Antenna	9-meter C-band, program-track mode
Position	Az: 161.8°, El: 34.2°
LNB	5,250 MHz LO, 60 dB gain
Receiver	1,532 MHz IF, 36 MHz BW, QPSK 3/4
GPSDO	Locked, providing 10 MHz reference

Types of Satellite Interference

Adjacent Channel Interference

Signals in neighboring frequency slots that “spill over” into your bandwidth due to imperfect filtering. Usually appears as elevated noise at the edges of your signal.

Mitigation: Tighter bandpass filtering, frequency coordination, guard bands.

Co-Channel Interference

Another signal on the exact same frequency, either from an adjacent satellite or a terrestrial source. Appears as a second signal overlaid on yours.

Mitigation: Antenna pointing optimization, polarization adjustment, regulatory action.

Cross-Polarization Interference

Signals on the orthogonal polarization that “leak” into your polarization due to imperfect isolation. This is particularly relevant for this scenario.

Mitigation: Polarization alignment, notch filtering, coordination with interfering party.

Intermodulation Interference

Non-linear behavior in amplifiers creates spurious signals at predictable frequencies (sum and difference of input frequencies). Often originates in the ground station’s own equipment.

Mitigation: Reduce drive levels, improve amplifier linearity, filtering.

Cross-Polarization: A Deeper Look

How Polarization Reuse Works

Electromagnetic waves oscillate in a plane. By using two orthogonal polarizations (Horizontal and Vertical, or Left-Hand and Right-Hand Circular), satellite operators can transmit two independent signals on the same frequency - effectively doubling capacity.

Ideal Isolation: In theory, perfectly orthogonal polarizations are completely independent - a receiver tuned to H polarization sees zero signal from a V-polarized transmission.

Real-World Isolation: In practice, isolation is limited by:

Factor	Typical Impact	Notes
Feed alignment	25-35 dB	Mechanical precision at antenna feed
Atmospheric effects	20-30 dB	Rain depolarization in heavy storms
Satellite antenna	30-35 dB	Spacecraft feed design
Cross-polar discrimination	25-30 dB	Overall system performance

When Cross-Pol Becomes a Problem

With 25-30 dB isolation, a signal that’s 10 dB stronger than yours on the orthogonal polarization will appear 15-20 dB below your signal - often visible but not problematic.

However, when:

The interfering signal is much stronger than expected
Your signal is weaker than normal
The interfering signal is narrowband (concentrated power)

…the cross-pol leakage can rise above your noise floor and cause measurable degradation.

The Scenario Setup:

Another operator is transmitting a relatively high-power narrowband signal on the orthogonal polarization. A small fraction leaks through into your polarization. Because it’s narrowband (concentrated in a small frequency range), it appears as a visible spike even though the total leaked power is small.

Automatic Gain Control (AGC)

Why Receivers Use AGC

Satellite signal levels vary significantly:

Rain fade: Can cause 10+ dB variation
Antenna pointing: Small errors reduce received power
Satellite variations: Eclipse seasons, load changes

The receiver must handle this dynamic range while maintaining optimal performance. AGC automatically adjusts gain to keep the signal at the ideal level for demodulation.

How AGC Works

1. Measure total power in the passband
2. Compare to target level
3. If too high → reduce gain
4. If too low → increase gain
5. Repeat continuously

AGC Parameters:

Parameter	Purpose	Typical Value
Target level	Optimal demodulator input	-30 dBm
Attack time	How fast to reduce gain	10 ms
Release time	How fast to increase gain	100 ms
Gain range	Min/max adjustment	-60 to +10 dB

The AGC Problem with In-Band Interference

AGC measures total power - it cannot distinguish between your wanted signal and interference. When a strong interference spike appears:

AGC sees increased total power
AGC reduces gain to prevent overload
Both your signal and the interference are attenuated equally
Your signal-to-noise ratio degrades
Demodulator performance suffers

The Counterintuitive Result:

A narrowband spike affecting only 3% of your bandwidth can degrade your entire signal because AGC responds to total power, not spectral distribution.

Notch Filters: Surgical Interference Removal

What is a Notch Filter?

A notch filter (band-stop or band-reject filter) attenuates a narrow frequency range while passing frequencies above and below with minimal loss.

Frequency Response:

     │ Pass    ┌──────┐    Pass
Power│ Band    │Notch │    Band
     │ ────────┘      └────────
     └──────────────────────────▶
            Frequency

Key Notch Filter Parameters

Parameter	Description	Considerations
Center Frequency	Where the notch is placed	Must match interference precisely
Bandwidth	Width of the attenuation region	Wide enough to cover interference, narrow enough to preserve signal
Depth	Amount of attenuation	40+ dB typically needed
Insertion Loss	Loss in passband	Should be minimal (< 1 dB)
Group Delay	Phase variation across frequency	Can cause signal distortion if excessive

When to Use Notch Filters

Good Candidates for Notching:

Narrowband interference (CW tones, narrowband carriers)
Stable interference (not frequency-hopping)
Interference at known, fixed frequency
Interference not centered on your carrier

Poor Candidates for Notching:

Broadband interference (spread across your signal)
Interference at your carrier center frequency
Intermittent or frequency-varying interference
Multiple interference sources at different frequencies

Filter Precision Requirements

Too Narrow:

Interference “shoulders” leak around the notch
Interference power still affects AGC
Incomplete mitigation

Too Wide:

Removes wanted signal along with interference
Increased insertion loss
Potential phase distortion from steep filter edges

Just Right:

Covers interference bandwidth with small margin
Minimal impact on wanted signal
AGC normalizes, C/N recovers

Systematic Troubleshooting Approach

The Interference Hunting Process

Professional operators follow a systematic approach:

1. Verify the Problem

Confirm customer complaints with measurements
Establish baseline expectations (what should C/N be?)
Document current conditions

2. Characterize the Symptom

What parameter is degraded? (C/N, BER, packet loss)
When did it start? (sudden or gradual)
Is it constant or intermittent?

3. Visualize the Spectrum

Configure analyzer for appropriate span and RBW
Compare to known-good baseline
Identify anomalies

4. Characterize the Interference

Frequency (convert IF to RF)
Bandwidth (narrowband spike vs. broadband elevation)
Power level (how far above noise floor)
Stability (constant, drifting, intermittent)

5. Identify the Source

Frequency suggests origin (satellite band, terrestrial, intermod)
Bandwidth suggests type (CW beacon, modulated carrier, noise)
Timing correlates with events (satellite maneuvers, weather, time of day)

6. Apply Mitigation

Notch filter for narrowband
Coordination for satellite sources
Regulatory action for illegal transmissions

7. Verify Restoration

Confirm C/N returned to expected level
Verify customer service restored
Document the solution

Equipment Quick Reference

Initial Spectrum Analyzer State

The spectrum analyzer starts configured for beacon observation:

Parameter	Initial Value	Purpose
Center Frequency	1,074.5 MHz	Beacon IF
Span	2 kHz	Narrow for CW beacon
RBW	1 kHz	High sensitivity

This configuration cannot show the main 36 MHz signal or wideband interference.

TIDEMARK-1 Signal Information

Signal	RF Frequency	IF Frequency	Bandwidth
Beacon	4,175.5 MHz	1,074.5 MHz	CW (< 1 kHz)
Main Downlink	3,718 MHz	1,532 MHz	36 MHz

Mission Phases Overview

Confirm Signal Degradation

Verify the customer complaint by checking the receiver modem’s C/N ratio display.
Widen Spectrum View

Reconfigure the spectrum analyzer to see the full signal bandwidth rather than just the beacon.
Center on Downlink Signal

Move the display to observe the main 36 MHz downlink signal.
Identify Interference

Look for anything within the signal bandwidth that shouldn’t be there.
Characterize the Interference

Determine the interference bandwidth and compare to your main signal.
Understand the Interference Source

Based on characteristics observed, determine the likely cause.
Understand the AGC Impact

Connect the interference to the C/N degradation mechanism.
Configure Notch Filter

Apply surgical filtering to remove the interference while preserving the signal.
Verify Service Restored

Confirm C/N has recovered and customer service is restored.

What You’ll Need to Know

This scenario draws on the following concepts covered in this read-ahead:

C/N Ratio: How to interpret receiver signal quality measurements
Spectrum Analysis: Configuring the analyzer span, RBW, and center frequency for different tasks
Frequency Conversion: Converting between RF and IF frequencies using the LNB LO
Interference Recognition: What different types of interference look like on the spectrum
Cross-Polarization: Why signals on the orthogonal polarization can leak into your receive path
AGC Behavior: How the receiver’s automatic gain control responds to total power
Notch Filtering: When and how surgical frequency removal can mitigate narrowband interference