Collision Avoidance Maneuver
Understanding Collision Avoidance Maneuvers
Space surveillance networks, primarily the U.S. Space Force's 18th Space Defense Squadron, continuously track objects larger than 10 cm in LEO and 1 m in GEO using ground-based radar and optical sensors. When two tracked objects are predicted to pass within a screening volume (typically 1 km radial by 25 to 50 km along-track/cross-track for LEO), a conjunction data message (CDM) is issued to the satellite operators involved. The CDM contains the predicted miss distance, relative velocity, covariance matrices for both objects, and the calculated probability of collision.
If the probability exceeds the operator's action threshold and the miss distance is below a safe margin, the operator plans a CAM. The maneuver is designed to change the satellite's along-track position at the time of closest approach (TCA), usually by raising or lowering the orbit slightly. An in-track displacement of 1 km requires approximately 0.02 m/s delta-v for a typical LEO orbit at 550 km altitude. The maneuver is executed via RF command uplink through the TT&C subsystem, with real-time telemetry confirming thruster firing, attitude stability, and orbit change. Post-maneuver tracking verifies the new orbit and confirms the conjunction is resolved.
Maneuver Delta-V and Orbit Change
Δx ≈ 3π × Δv × t / vorb × R
Simplified LEO (550 km):
Δv ≈ 0.02 m/s per km of along-track shift
Probability of Collision:
Pc = ∫∫ exp(−½ rT C−1 r) / (2π|C|½) dA
Where Δv = velocity change (m/s), t = time between maneuver and TCA (s), vorb = orbital velocity (~7.6 km/s at 550 km), R = orbital radius (km), C = combined covariance matrix, r = relative position vector, dA = cross-section area element. A 0.1 m/s burn 12 hours before TCA shifts along-track position by approximately 5 km.
CAM Decision Matrix by Orbit Regime
| Parameter | LEO (<2000 km) | MEO (2000-35786 km) | GEO (35786 km) | Mega-Constellation |
|---|---|---|---|---|
| Screening volume | 1 km × 25 km | 5 km × 50 km | 10 km × 75 km | Operator-defined |
| Action threshold | Pc > 10−4 | Pc > 10−5 | Pc > 10−5 | Automated at 10−5 |
| Typical Δv | 0.01 to 1 m/s | 0.01 to 0.5 m/s | 0.01 to 0.05 m/s | 0.01 to 0.1 m/s |
| Planning time | 6 to 24 hours | 24 to 48 hours | 48 to 72 hours | Minutes (autonomous) |
| Command link | S/X-band TT&C | S/X-band TT&C | C/Ku-band TT&C | Ka-band + inter-sat |
Frequently Asked Questions
What triggers a collision avoidance maneuver?
A CAM is triggered when conjunction data messages (CDMs) from space surveillance indicate probability of collision exceeding the operator's threshold, typically 10−4. The 18th Space Defense Squadron issues CDMs when tracked objects pass within a screening volume. The operator refines the conjunction using their own ephemeris and decides based on collision probability, miss distance, and operational impact.
What RF systems are involved in executing a CAM?
The ground station transmits thruster commands via TT&C uplink (S-band 2.0 to 2.3 GHz or X-band 7.9 to 8.4 GHz), requiring positive acknowledgment before the burn. Telemetry downlinks carry real-time health, attitude, and propulsion data. Communications transponders may need reconfiguration during the burn. Post-maneuver, the tracking network acquires the satellite at its new position and updates ephemeris, with onboard GPS providing immediate verification.
How much delta-v does a typical CAM require?
Most LEO maneuvers need 0.01 to 1 m/s, shifting the orbit 0.5 to 5 km. A 0.1 m/s burn executed 12 hours before TCA shifts along-track position by approximately 5 km. Electric propulsion systems need 5 to 50 minutes of firing; chemical systems complete the same burn in seconds. The maneuver is ideally executed 6 to 24 hours before TCA to allow ephemeris verification.