Power sequencing

Summary¶

Connecting the battery on libdrone initiates a fixed power-up sequence. The ESC main bus charges immediately; the flight controller starts ~100 ms later; Betaflight initialises and runs gyro calibration for 3 seconds (the drone must not move during this window); the ELRS receiver connects; and GPS acquires a fix over 30–90 seconds. Only after GPS fix should the pilot arm. The sequence is not configurable — it is determined by the hardware and firmware startup behaviours. Understanding it prevents the most common first-flight errors.

Concept¶

The 3-second gyro calibration window¶

The gyroscope calibration at startup measures the sensor's zero-rate offset — the output the gyro produces when the drone is perfectly stationary. This offset is subtracted from all subsequent readings so that a stationary drone shows zero rotation rate.

If the drone moves during calibration, the calibration captures motion as zero — offset. Every subsequent gyro reading is wrong by that amount. The drone will drift continuously in the calibrated-wrong direction, requiring constant stick input to stay level. Worse, the PID loop sees a persistent non-zero rate and fights it, increasing motor temperature and reducing battery life.

The calibration window is 3 seconds from the moment Betaflight initialises (approximately 100–200 ms after battery connection). During this window, the drone must sit completely still on a flat surface.

GPS fix before arming¶

GPS position is recorded as the home point at the moment of arming. This home point is used for: - GPS position hold (holds the arming position) - GPS Rescue / Return to Home (navigates back to the home point if RC link is lost)

If the pilot arms before GPS fix, the home point is not set. GPS Rescue is unavailable. If the RC link fails, Betaflight falls back to attitude hold — the drone maintains its current attitude but does not navigate home. In open terrain this is manageable; near obstacles it is dangerous.

GPS cold start (no recent position data) takes 30–90 seconds for the M10Q-5883 to acquire a fix with 8+ satellites. Warm start (recent position data saved) takes 5–15 seconds. Galileo + GPS dual-constellation improves fix time by providing more visible satellites, particularly at high latitudes.

Betaflight shows GPS fix status in the OSD — the satellite count icon. Do not arm until the OSD shows the configured minimum satellite count (default: 8).

Full power-up sequence¶

t = 0 ms Battery connects t = 0 ms ESC main bus charges (XT60 → capacitor → ESC) t = 100 ms FC BEC powers on (internal LDO startup time) t = 100 ms H7A3-SLIM begins Betaflight initialisation t = 200 ms ELRS receiver powers on via BEC t = 200 ms GPS module powers on via BEC t = 300 ms Betaflight begins 3-second gyro calibration → DO NOT MOVE during t=300ms to t=3300ms t = 700 ms ELRS receiver acquires transmitter link t = 3300 ms Gyro calibration complete. FC shows "ready to arm" in OSD t = 30–90 s GPS acquires fix (cold start) t = 30–90 s Arm (after GPS fix confirmed)

Power-off sequence¶

There is no controlled power-off sequence. Removing the battery simultaneously cuts all power domains. Best practice: disarm first, wait 2 seconds (allows Betaflight to flush Blackbox buffer to flash memory), then remove battery. Removing battery while armed is not dangerous but may lose the last few seconds of Blackbox data.

Reference¶

Startup indicators (Betaflight OSD)¶

ELRS binding and first connection¶

On first use, the ELRS receiver requires binding to the transmitter. Binding is a one-time procedure that stores the transmitter's UID in the receiver. After binding, the receiver connects automatically within 500 ms of power-up whenever the transmitter is on and in range.

If the receiver does not connect (OSD shows no RSSI), check: transmitter is powered and in correct RF mode (2.4 GHz ELRS); receiver and transmitter are on the same ELRS version; receiver LED is blinking (searching), not off (no power).

Procedure¶

Standard pre-flight power-up¶

1. Place drone on flat, stable surface.
2. Ensure transmitter is powered and RC link is active.
3. Connect battery.
4. Do not touch drone for 5 seconds (3 s calibration + 2 s margin).
5. Verify OSD shows: battery voltage, "DISARMED", receiver RSSI, gyro calibration complete.
6. Wait for satellite count ≥ 8 in OSD.
7. Arm.

If gyro calibration is suspected to be wrong¶

Symptoms: drone drifts consistently in one direction in stabilised mode; requires persistent stick input to hover level; PID loop sounds laboured.

1. Disarm. Land.
2. Remove battery, wait 5 seconds.
3. Place drone on flat, stable surface.
4. Reconnect battery. Do not touch for 5 full seconds.
5. Re-arm and test hover.

If drift persists after confirmed clean calibration: check for mechanical asymmetry (bent arm, different prop sets), then check Betaflight accelerometer calibration.

Rationale¶

Why the gyro calibration cannot be extended¶

3 seconds is the window Betaflight allocates to average out sensor noise and measure the zero-rate offset. Longer calibration would improve precision marginally but would add to pre-flight time with diminishing returns — 3 seconds is sufficient for the sensor's noise floor to average to a stable value. The operator's cost of keeping the drone still for 3 seconds is zero if the pre-flight procedure is followed correctly.

Why GPS fix is a hard gate before arming¶

An armed drone without GPS Rescue enabled will fly normally until RC link is lost, then switch to attitude hold and drift with the wind until battery is exhausted or it hits something. GPS Rescue requires a valid home point. A valid home point requires GPS fix at arming time. The 30–90 second wait for GPS fix is the single biggest time cost in the pre-flight sequence — but it is not optional for safe operation in any area with obstacles.

Connections¶

requires: - lipo-batteries - power-rail-architecture related: - closed-loop-control leads_to: - piloting-operations