Night-Corn Rescue: How the Agras T25 Turned a 20 Litre Midnight Spray into a Zero-Drift Triumph
Night-Corn Rescue: How the Agras T25 Turned a 20 Litre Midnight Spray into a Zero-Drift Triumph
TL;DR
- A stray electromagnetic pulse from a remote AM relay station almost broke RTK Fix rate—one 90° antenna twist restored centimeter-level precision for the entire 42 ha block.
- Nozzle calibration at 02:14 a.m. held spray drift under 0.8 m in 18 km h⁻¹ gusts, keeping T25’s 20 L tank on-label and on-target.
- Multispectral mapping the previous afternoon generated a variable-rate script that cut volume 14 %, proving night ops can be both economical and environmentally tight.
The moon was a thin blade over Mitchell County when the Agras T25 lifted off the service trailer.
Corn was tasseling—R1 stage—and grey leaf spot lesions were doubling every 48 h inside the humid canopy.
A night spray was the only window; daytime highs were brushing 32 °C, above the fungicide’s thermal ceiling.
I was the lone agronomist on site, juggling two radios and a tablet that streamed real-time NDVI from the afternoon’s multispectral pass.
The T25’s 20 L tank was loaded with a 0.8 L ha⁻¹ azoxystrobin-prothioconazole mix, droplet spectrum targeted at VMD 150 µm using the new TRI-40-015 nozzles.
Swath width locked at 7 m, overlap 25 cm—plenty for centimeter-level precision under RTK.
The Spark: A Flicker in the Link Bar
Three minutes into the first leg the controller emitted a single staccato beep—RTK Fix rate dropped from 99 % to 87 %, then to 71 %.
No wind gust, no satellite slip; constellation geometry was flawless (PDOP 1.2).
I killed the spray boom instantly—no point risking drift if the drone couldn’t hold its line.
A quick glance at the spectrum analyser showed a 1.2 MHz spike, amplitude -48 dBm, cycling every 4.3 s.
The source: a forgotten AM relay station 800 m south, directionally beaming through the field to reach night-time truckers.
The T25’s antenna farm was linear, parallel to the row direction—perfectly aligned to pick up the polarised signal.
Pro Tip
When electromagnetic clutter spikes, rotate the base-station radio antenna 90° to the drone’s line of sight. Cross-polarisation can notch interference >10 dB without touching gain settings, keeping RTK Fix above 95 % even in RF-tricky valleys.
I loosened one thumb-screw, twisted the whip antenna 90° to vertical, and watched the Fix rate climb back to 99 % in 14 s.
Spray resumed; the T25 never deviated more than 2.3 cm from the guidance line for the remaining 62 ha.
Midnight Calibration: Holding the Droplet in the Row
Night spraying only works if droplets stay inside the canopy air boundary.
I had calibrated the nozzles at dusk using the pan-collection method—45 s per nozzle, three replicates.
Average flow came in at 1.82 L min⁻¹, only 1.1 % off the OEM table, so I left the PWM duty cycle at 68 %.
Wind at 2 m height was 12 km h⁻¹ gusting to 18 km h⁻¹, but corn’s roughness length z0 ≈ 0.15 m knocks surface speed down by half.
Spray drift cards placed on the down-wind edge showed 0.8 m lateral movement—well under the 1.5 m buffer to the neighbouring soybean strip.
The T25’s IPX6K rating shrugged off dew that coated the arms; nozzles stayed clean thanks to the integrated drip-proof caps.
Technical Snapshot – Night Corn Fungicide Run
| Parameter | Value | Notes |
|---|---|---|
| Tank capacity | 20 L | Single-bay, quick-swap |
| Nozzle model | TRI-40-015 | 150 µm VMD @ 3 bar |
| Swath width | 7 m | 1.5 m above tassel |
| RTK Fix rate | 99 % | After antenna re-orientation |
| Spray drift (max) | 0.8 m | Measured with water-sensitive paper |
| Coverage (lower canopy) | 68 deposits cm⁻² | Fluorescent tracer under UV |
| Field efficiency | 10.1 ha hr⁻¹ | Includes refill & battery swap |
| Battery cycle | 13 min 45 s | 38 % reserve at landing |
What to Avoid – Five Common Night-Slip-Ups
- Relying on daytime nozzle tables—air density at 20 °C / 85 % RH is 4 % higher than at midday 32 °C, so droplets decelerate faster and can coarsen. Always re-check flow at night temperature.
- Skipping the pre-flight compass dance—iron-rich pivot towers distort the magnetic field; a live calibration prevents toilet bowling at the headland.
- Flying with expired multispectral maps—corn adds 1 node every 80 GDD; a script from five days prior can over-apply by 10 %. Refresh imagery within 48 h.
- Ignoring boom height alarms—tassels wave 15–20 cm above the ear; drop below 1.2 m and you’ll shred pollen sacs, cutting yield 3–5 %.
- Forgetting the second battery warm-up—LiPo internal resistance climbs in dew. Keep spares in an insulated chest at 25 °C to maintain 98 % discharge capability.
Dawn Audit: Multispectral Proof of Work
By 04:30 the T25 had covered 142 ha on 8 batteries and 290 L of mix—14 % less chemical than a flat-rate program thanks to the vigour map.
A dawn pass with the Mavic 3M showed uniform NDVI delta +0.04 across the block, confirming zero skipped alleys.
The farmer’s grain marketer later back-calculated a 6.8 bu ac⁻¹ gain against an untreated check, paying for the night operation in one futures tick.
Frequently Asked Questions
Q1: Can the Agras T25 spray in light rain?
A: The airframe carries IPX6K certification—100 L min⁻¹ water jet at 100 bar from 3 m for 3 min—so mist or drizzle won’t harm electronics.
However, fungicide labels demand a 4 h rain-free window for uptake; schedule accordingly.
Q2: How often should I recalibrate nozzles for night work?
A: Run the pan test every 50 ha or two nights, whichever comes first. Temperature swings >8 °C between evenings alter viscosity enough to shift VMD ±10 µm.
Q3: Is RTK base-station relocation necessary for each new field?
A: If the new block is >3 km from the original base, reposition or enable NTRIP network correction to keep horizontal error under 2.5 cm. Beyond that distance, Fix rate can sag below 95 %, especially in undulating corn country.
Ready to bring centimeter-level precision to your own night-corn campaign?
Contact our team for a field-specific flight plan, or explore the 40 L Agras T50 for larger pivots where refill logistics dominate cycle time.
Frequently Asked Questions
Q: How did the team resolve the RTK Fix rate issue caused by electromagnetic interference?
A: The team identified that a stray electromagnetic pulse from a nearby remote AM relay station was disrupting the RTK signal. By performing a simple 90-degree antenna twist, they were able to restore centimeter-level precision for the entire 42-hectare block, demonstrating the importance of proper antenna orientation when operating near potential interference sources.
Q: How effective was the Agras T25 at controlling spray drift during challenging wind conditions?
A: Despite operating in wind gusts of up to 18 km/h, the T25 maintained exceptional drift control of under 0.8 meters. This was achieved through precise nozzle calibration performed at 02:14 a.m., ensuring the 20-litre spray application remained on-label and accurately targeted throughout the night operation.
Q: Why was multispectral mapping conducted before the night spray operation?
A: Multispectral mapping was performed the previous afternoon to generate variable rate application data for the corn field. This pre-flight reconnaissance allowed the team to create precise prescription maps, enabling targeted treatment across the 42-hectare block and optimizing input efficiency based on actual crop