Scenario 5 - Interference Hunt
Duration: 20-25 minutes
Difficulty: Intermediate
Mission Type: Troubleshooting
Shift Context
Section titled “Shift Context”Date: Wednesday, 1430 Local
Station: Vermont (VT-01)
Operator on Duty: You
Supervisor on Shift: Charlie Brooks
Turnover from Morning Shift
Section titled “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
Section titled “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
Section titled “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
Section titled “TIDEMARK-1 Operations Reference”Frequency Allocation
Section titled “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
Section titled “Downlink Signal Parameters”| Parameter | Value |
|---|---|
| Bandwidth | 36 MHz |
| Modulation | QPSK |
| FEC Rate | 3/4 |
| Polarization | Horizontal (TP-1) |
| Nominal C/N | 14-16 dB |
| Demod lock threshold | Required Es/N0 5.0 dB: with Es/N0 about 0.8 dB above Raw C/N, the modem acquires lock from about 4.7 dB C/N (Required 5.0 plus a 0.5 dB acquisition margin, less the 0.8 dB Es/N0 offset) and holds it down to about 3.7 dB |
| Service minimum | 8 dB C/N |
Vermont Station Equipment Status
Section titled “Vermont Station Equipment Status”| Equipment | Status | Notes |
|---|---|---|
| Antenna | Program-track on TIDEMARK-1 | Az: 153.2°, El: 35.2° |
| LNB | 5,250 MHz LO, powered | Reference locked |
| GPSDO | Locked | 10 MHz reference nominal |
| Receiver Modem | 1,532 MHz IF, 36 MHz BW | Monitors our own hub carrier as it comes back down; check C/N display |
| TX Chain | Transmitting, HPA enabled | The SeaLink hub carrier (1,057 MHz IF, 5,943 MHz RF): leave it up, it is the customer’s traffic |
Spectrum Analyzer Configuration (Current)
Section titled “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 | 10 Hz (manual) | Set for the beacon; far too narrow for a wide span: put it back on Auto |
| Amplitude | -140 to -40 dBm | Beacon (about -58 dBm) and its floor on screen |
This configuration cannot show the 36 MHz main signal or wideband interference. You’ll need to reconfigure.
Interference Mitigation Equipment
Section titled “Interference Mitigation Equipment”Notch Filter Quick Reference
Section titled “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 | Set on the Notch Filter card (adjustable) |
| State | Enabled/Disabled | Filter bypassed when disabled |
When Notching Works
Section titled “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.
A notch in our receive chain cleans up only our own receiver. Interference inside the transponder is on the downlink every terminal in the beam receives, so SeaLink’s ships keep seeing it until its source is fixed.
C/N Degradation Quick Reference
Section titled “C/N Degradation Quick Reference”| Margin (Es/N0 - 5.0 dB) | C/N, clean carrier | Modem Reads | Service Impact |
|---|---|---|---|
| about 5.8 dB or more | about 10 dB or more | Locked, good margin | Normal operations, fade margin in hand |
| 1.0 to 5.8 dB | about 5.2 to 10 dB | Locked, good margin | Decoding cleanly, little fade margin left |
| -0.5 to 1.0 dB | about 3.7 to 5.2 dB | Locked, Degraded margin | Hub still decodes error-free; customers’ small terminals are already dropping packets |
| below -0.5 dB | below about 3.7 dB | No lock | Outage |
The modem works on Es/N0, not C/N: QPSK 3/4 needs Es/N0 5.0 dB, RX Analysis shows it as Required Es/N0, and the status bar shows the margin over it. The thresholds have hysteresis: once Degraded, the modem reads Good again only from 1.5 dB of margin; once lost, it re-acquires lock only from +0.5 dB. On a clean carrier Es/N0 reads about 0.8 dB above C/N, which is where the C/N column comes from. With interference in the band the two read closer together, so judge the link by the margin in the status bar, not by a C/N rule of thumb.
The table is for the 9 m hub at VT-01. SeaLink’s ships run 1-2 m terminals with 15-20 dB less G/T, so the same interference in the transponder puts them much closer to their threshold. TIDEMARK-1’s carrier is concatenated-coded (Viterbi K=7 rate 3/4 inside, Reed-Solomon 223/255 outside; the Payload Data Integrity card shows both). That pair decodes error-free while Reed-Solomon can still correct every block the Viterbi decoder hands it, then fails abruptly within a fraction of a dB: the hub’s Payload Data Integrity card can show BER around 1e-12 and 0 CRC errors while the remotes are losing packets. Judge the customer’s link by the hub’s margin, not its BER.
C/N drops when carrier power decreases OR when in-band noise/interference increases: an interferer inside our passband counts as noise to the demodulator (C/(N+I)), and one inside the transponder also takes a share of its output power. The receiver’s AGC responds to total power in the passband, so a spike makes its gain sit lower (the AGC card shows only the current GAIN and INPUT; a narrow spike is worth well under a dB) - but that gain applies to carrier and noise alike, so the AGC’s gain change is a symptom of the extra power, not the cause of the C/N loss.
Diagnostic Checklist
Section titled “Diagnostic Checklist”Initial Assessment
Section titled “Initial Assessment”- Check receiver modem C/N display
- Compare to nominal (14+ dB expected)
- Note current degradation severity
Spectrum Analysis
Section titled “Spectrum Analysis”- Reconfigure analyzer: widen span to 50+ MHz, RBW Auto, amplitude -100 to -30 dBm
- 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
Section titled “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
Section titled “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
Section titled “Service Verification”- Confirm C/N back above the 8 dB service minimum (expect about 9-10 dB with the notch: the spike still takes its share of transponder power, so the full 14 dB returns only when its source is fixed)
- Monitor for stability
- Update trouble ticket with resolution
Escalation Guidelines
Section titled “Escalation Guidelines”Handle Yourself
Section titled “Handle Yourself”- Spectrum analyzer configuration
- Interference characterization
- Notch filter configuration
- Standard troubleshooting with clear technical causes
Escalate to Charlie
Section titled “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
Section titled “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.