Scenario 5 - Interference Hunt
Duration: 15-20 minutes
Difficulty: Intermediate
Mission Type: Troubleshooting
Shift Context
Date: Wednesday, 1430 Local
Station: Vermont (VT-01)
Operator on Duty: You
Supervisor on Shift: Charlie Brooks
Turnover from Morning Shift
Morning operator logged the following at 0800:
TIDEMARK-1 link nominal at handoff. C/N running 14.2 dB, well within margin. No weather concerns. SeaLink Global’s contract renewal meeting is scheduled for Friday - they’ve been happy with service quality this quarter.
Nothing unusual. You’ve been monitoring routine operations all afternoon when the trouble ticket comes in.
Trouble Ticket
Ticket #: NOC-2026-0612
Opened: 1425 Local
Source: SeaLink Global Communications (customer-initiated)
Priority: High
Customer Report:
Packet errors on TIDEMARK-1 service starting approximately 1400. End users reporting degraded throughput and intermittent connectivity. C/N appears lower than normal.
Charlie’s Note:
“Just saw this come in. The spectrum analyzer is still configured for beacon tracking from this morning’s pointing verification. You’ll need to widen the view to see what’s happening on the main signal. Start with the receiver C/N - see how bad it actually is.”
Active Services
| Satellite | Customer | Service Type | Status at Shift Start |
|---|---|---|---|
| TIDEMARK-1 | SeaLink Global | Maritime Data | Operational (nominal C/N: 14+ dB) |
SeaLink Global runs maritime data services for commercial shipping. Their contract renewal meeting is Friday. Service degradation this week would be poorly timed.
TIDEMARK-1 Operations Reference
Frequency Allocation
| Parameter | RF (MHz) | LO (MHz) | IF (MHz) |
|---|---|---|---|
| Beacon (Downlink) | 4,175.5 | 5,250 (LNB) | 1,074.5 |
| Main Carrier (Downlink) | 3,718 | 5,250 (LNB) | 1,532 |
Downlink Signal Parameters
| Parameter | Value |
|---|---|
| Bandwidth | 36 MHz |
| Modulation | QPSK |
| FEC Rate | 3/4 |
| Nominal C/N | 14-16 dB |
| Minimum C/N | 8 dB (threshold) |
Vermont Station Equipment Status
| Equipment | Status | Notes |
|---|---|---|
| Antenna | Program-track on TIDEMARK-1 | Az: 161.8°, El: 34.2° |
| LNB | 5,250 MHz LO, powered | Reference locked |
| GPSDO | Locked | 10 MHz reference nominal |
| Receiver Modem | 1,532 MHz IF, 36 MHz BW | Check C/N display |
Spectrum Analyzer Configuration (Current)
The spectrum analyzer is configured for beacon observation from this morning’s pointing check:
| Parameter | Current Value | Notes |
|---|---|---|
| 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 36 MHz main signal or wideband interference. You’ll need to reconfigure.
Interference Mitigation Equipment
Notch Filter Quick Reference
The receive chain includes a configurable notch filter for narrowband interference removal.
| Parameter | Range | Notes |
|---|---|---|
| Center Frequency | 950-2150 MHz | Set to interference IF frequency |
| Bandwidth | 100 kHz - 10 MHz | Match interference width |
| Depth | 40+ dB | Fixed attenuation in notch |
| State | Enabled/Disabled | Filter bypassed when disabled |
When Notching Works
Notch filters are effective against:
- Narrowband interference (CW tones, narrow carriers)
- Stable, non-hopping interference
- Interference not centered on your carrier
They cannot help with broadband interference or interference centered on your signal.
C/N Degradation Quick Reference
| C/N Range | Signal Quality | Service Impact |
|---|---|---|
| > 15 dB | Excellent | Full throughput |
| 10-15 dB | Good | Normal operations |
| 6-10 dB | Marginal | Reduced throughput, errors |
| < 6 dB | Poor | Service degradation or loss |
C/N drops when carrier power decreases OR when in-band noise/interference increases. The receiver’s AGC responds to total power in the passband - a strong interference spike can cause AGC to reduce gain on the entire signal.
Diagnostic Checklist
Initial Assessment
- Check receiver modem C/N display
- Compare to nominal (14+ dB expected)
- Note current degradation severity
Spectrum Analysis
- Reconfigure analyzer: widen span to 50+ MHz
- Center on main signal IF (1,532 MHz)
- Look for anomalies in or near the signal bandwidth
- Characterize any interference: frequency, bandwidth, power
Interference Identification
- Determine interference IF frequency
- Calculate RF frequency using LO (IF = LO - RF for high-side conversion)
- Assess whether narrowband or broadband
- Evaluate if notch filtering is appropriate
Mitigation
- Configure notch filter center frequency
- Set notch bandwidth (cover interference with minimal margin)
- Enable notch filter
- Verify C/N recovery on receiver modem
Service Verification
- Confirm C/N returned to nominal range
- Monitor for stability
- Update trouble ticket with resolution
Escalation Guidelines
Handle Yourself
- Spectrum analyzer configuration
- Interference characterization
- Notch filter configuration
- Standard troubleshooting with clear technical causes
Escalate to Charlie
- Interference source identification requiring coordination
- Equipment failures beyond filter configuration
- Customer communication requirements
- Situations requiring regulatory involvement
Charlie is on shift and available. This is a troubleshooting scenario - work the problem, but don’t hesitate to ask if you hit a wall.
End of Briefing
SeaLink Global is experiencing degraded service, and their contract renewal meeting is in two days. The cause isn’t immediately obvious from the current spectrum analyzer view - you’ll need to reconfigure to see the full picture.
Work methodically: verify the problem, visualize the spectrum, characterize what you find, and apply the appropriate mitigation. The tools are available; you just need to use them correctly.
Charlie’s watching, but this one’s yours.
Find it. Fix it.