GNSS_UNAVAILABLE
Applicable alarms (1)
Likely outcomes — click to expand
Based on 907,061 missions where this alarm fired in the last 90 days as of 2026-10-13
Grounding vs discrepancy (of 6,536 ZERP issues filed on these missions)
Top work orders
- EV2: Inspect Aircraft after Droid Left Behind (DLB) · MAINTENANCE_ACTION 558
- Inspect Droid after Droid Left Behind (DLB) · MAINTENANCE_ACTION 337
- EV2/EV3 Diagnostics Test Without Motors · INSPECTION 278
- Reset Droid after DLB · REPAIR 241
- EV3: Inspect Aircraft after Droid Left Behind (DLB) · MAINTENANCE_ACTION 194
Frequently replaced parts
| Part | P/N | Removals |
|---|---|---|
| SERVO, ASSY, UNIVERSAL, P2 | 20398 | 1,721 |
| CW/CCW HOVER MOTOR ASSY, ZIP, P2 | 19607 | 1,364 |
| TAIL STBD, ZIP, P2 | 21840 | 1,048 |
| TAIL PORT DEPOP, ZIP, P2 | 31280 | 997 |
| WING STBD DEPOP, ZIP, P2 | 25266 | 948 |
Aggregated from fleet operations and refreshed daily. Individual outcomes may vary by site and configuration.
Background
Section titled “Background”The GNSS_UNAVAILABLE alarm fires when the aircraft has been unable to integrate GNSS measurements into its navigation solution for more than 1 second, meaning that the vehicle is effectively navigating on dead-reckoning alone.
Note that this does not necessarily mean the GNSS receivers/antennas are implicated directly – the quality of the input data is tracked by GNSS_A_DEGRADED and GNSS_B_DEGRADED. Instead, this alarm tracks the total data path required for using those measurements, also including IMU data availability, latency of data processing, etc.
This is a launch-blocking alarm. There is no in-flight response to this signal, but instead the vehicle will dead-reckon for as long as permitted before UNRECOVERABLE_FAILURE is declared.
Troubleshooting Steps
Section titled “Troubleshooting Steps”graph TD
is-gnss-a-degraded-or-gnss-b-degraded{"<small><b>Step 1</b></small><br/>Question: Is GNSS_A_DEGRADED or GNSS_B_DEGRADED..."}
see-gnss-a-degraded-and-gnss-b-degraded["<small><b>Step 2</b></small><br/>Action: See GNSS_A_DEGRADED and GNSS_B_DEGRA..."]
wait-5-minutes-for-the-condition-to-cle["<small><b>Step 3</b></small><br/>Action: Wait 5 minutes for the condition to c..."]
has-the-issue-resolved{"<small><b>Step 4</b></small><br/>Question: Has the issue resolved?"}
the-aircfraft-may-return-to-service["<small><b>Step 5</b></small><br/>Action: The Aircfraft may return to service."]
power-cycle-the-aircraft-for-steps-see["<small><b>Step 6</b></small><br/>Action: Power cycle the Aircraft. For steps, ..."]
has-the-issue-resolved-after-power-cycle{"<small><b>Step 7</b></small><br/>Question: Has the issue resolved?"}
the-aircraft-may-return-to-service("<small><b>Step 8</b></small><br/>Reference: The Aircraft may return to service.")
escalate-to-p2-navigation-on-call{{"<small><b>Step 9</b></small><br/>Escalation: Escalate to P2 Nav On Call."}}
is-gnss-a-degraded-or-gnss-b-degraded -->|Yes| see-gnss-a-degraded-and-gnss-b-degraded
is-gnss-a-degraded-or-gnss-b-degraded -->|No| wait-5-minutes-for-the-condition-to-cle
wait-5-minutes-for-the-condition-to-cle --> has-the-issue-resolved
has-the-issue-resolved -->|Yes| the-aircfraft-may-return-to-service
has-the-issue-resolved -->|No| power-cycle-the-aircraft-for-steps-see
power-cycle-the-aircraft-for-steps-see --> has-the-issue-resolved-after-power-cycle
has-the-issue-resolved-after-power-cycle -->|Yes| the-aircraft-may-return-to-service
has-the-issue-resolved-after-power-cycle -->|No| escalate-to-p2-navigation-on-call
style is-gnss-a-degraded-or-gnss-b-degraded fill:#dbeafe,stroke:#3b82f6,color:#1e3a5f
style see-gnss-a-degraded-and-gnss-b-degraded fill:#dcfce7,stroke:#22c55e,color:#14532d
style wait-5-minutes-for-the-condition-to-cle fill:#dcfce7,stroke:#22c55e,color:#14532d
style has-the-issue-resolved fill:#dbeafe,stroke:#3b82f6,color:#1e3a5f
style the-aircfraft-may-return-to-service fill:#dcfce7,stroke:#22c55e,color:#14532d
style power-cycle-the-aircraft-for-steps-see fill:#dcfce7,stroke:#22c55e,color:#14532d
style has-the-issue-resolved-after-power-cycle fill:#dbeafe,stroke:#3b82f6,color:#1e3a5f
style the-aircraft-may-return-to-service fill:#f3e8ff,stroke:#a855f7,color:#581c87
style escalate-to-p2-navigation-on-call fill:#fef3c7,stroke:#f59e0b,color:#78350f Hover or focus a block to preview it. Select the block to keep the preview open.
Resolution note
This plaintext block is ready to paste into the Resolution Note field on the ZERP issue you're working. The permalink below gives engineering review access to the full step-by-step timeline including any back-navigation; share it if the case needs a deeper look.
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