Degrees, Minutes, and a 120-Metre Tower on the Return Path
Your client sends the target coordinate for a bridge abutment as 45°18'36"N 075°40'12"W. Your planning app accepts decimal degrees; the site tablet shows a ±5 m GPS accuracy circle and defaults to NAD83. The abutment is 3 m wide. The route is 1.8 km from the launch point to the target, mission airspeed 12 m/s, wind 15 km/h on the nose outbound, 25-minute battery, company reserve 25%. A 120 m transmission tower stands 150 m from the launch point, and the aircraft's lost-link default is "return to home at 100 m AGL". During pre-flight near the steel structure the GPS position icon jumps about 8 m.
1. Convert the coordinate and identify what could go wrong between the client's format and your app.
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Conversion.
- Latitude: 45° + 18/60 (0.3000) + 36/3600 (0.0100) = 45.3100° N
- Longitude: 75° + 40/60 (0.66667) + 12/3600 (0.00333) = 75.6700° W, entered as −75.6700
So: 45.3100, −75.6700.
Failure modes to check before you trust those numbers:
- Format confusion. Decimal degrees (45.3100) versus degrees-decimal-minutes (45.18.60) versus DMS. A quick sanity check is that the decimal part of DD should roughly equal minutes/60 - 0.31 fits 18'36", so the entry is consistent.
- Dropped sign. Western longitudes are negative in decimal degrees. Omit the minus and you have moved the target to the other side of the prime meridian - a catastrophic, and entirely plausible, data-entry error.
- Datum. NAD83 and WGS84 differ by a metre or two in Canada - negligible against other errors, but you must know the app's default rather than assume.
- Accuracy stack. ±5 m GPS plus an 8 m jump near steel is larger than the 3 m abutment. A single point is not a target: confirm with a photo, a surveyed reference feature, and a site walk before you position the aircraft precisely.
2. Using the endurance numbers, show whether the flight fits. What does the arithmetic tell you about the plan?
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Wind. 15 km/h = 4.17 m/s.
- Outbound ground speed: 12 − 4.17 ≈ 7.83 m/s → 1800 / 7.83 ≈ 230 s
- Inbound ground speed: 12 + 4.17 ≈ 16.17 m/s → 1800 / 16.17 ≈ 111 s
- Transit total ≈ 341 s ≈ 5.7 min
- Add 4 min of work at the target ≈ 9.7 min
Usable energy. 25 min × 0.75 reserve = 18.75 min usable, leaving roughly 9 min of margin against an estimated 9.7 min flight. Comfortable - but the margin is doing all the work.
What the numbers warn about:
- Only the outbound leg is penalised; if the wind shifts or strengthens at 300 ft, the return leg is the one that eats into reserve, not the outbound.
- A lost-link or missed approach is an unplanned extra leg, not a pause. Budget one full outbound-length leg (about 4 min into wind) as a contingency and the margin shrinks fast.
- Hovering costs more than cruise, and hovering in gusty air costs more still. Set an abort point: if the pack is below the calculated figure at the target, you leave without the shot.
3. How should the lost-link behaviour be configured here, and why?
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The default is unsafe as configured. A "return to home at 100 m AGL" path runs at 100 m in the vicinity of a 120 m tower 150 m from the launch point - the aircraft can fly its autonomous sequence straight into a structure, with no pilot input in the loop, which is the worst possible moment to be near an obstacle.
Configuration decisions:
- Lost-link altitude must clear the tower plus margin (e.g., 135–140 m AGL) if the RTH path can pass near it, or the RTH route must be set to approach the home point from a sector clear of the tower.
- Check what the RTH path crosses. 135 m AGL over the surrounding area may mean flying over roads, buildings, or people - which may be a worse outcome than hovering.
- Consider hover-and-land-in-place if the operating area is confined, unpopulated, and within VLOS. Many lost-link events are transient; the aircraft holding position for 20–30 s often restores the link. A land-in-place must be somewhere a descending aircraft is harmless.
- Verify the GPS before launch. The 8 m jump next to steel means position hold and RTH are based on a degraded fix. Turn on, let it settle, check satellite count and accuracy, and do not launch until the displayed position is stable and matches the map.
Brief the selected behaviour out loud before launch, and confirm it on the app - never assume the default.