How to Design Trackers for Unpressurized Cargo Holds

Your tracker works flawlessly on the bench at 25°C. It passes thermal chamber testing from –40°C to +85°C. Then it flies in a cargo hold from Dubai to Anchorage and goes silent somewhere over the Arctic.

The problem isn’t that you designed a bad tracker. It’s that cargo holds don’t care about your datasheet margins. They’ll swing 100°C in three hours, drop to pressures where standard lithium-ion batteries become expensive paperweights, and then create condensation on your PCB during descent. Industrial-grade components that handle a factory floor in Minnesota will fail spectacularly at 35,000 feet.

This guide covers the specific environmental factors of unpressurized cargo holds and the design decisions that address them. You already know how to build trackers. This is about building trackers that survive aviation temperature extremes and the other stressors that make cargo holds uniquely hostile.

Understanding the Cargo Hold Environment

Unpressurized cargo compartments (typically the lower holds on narrowbody aircraft and some bulk compartments on widebodies) don’t receive conditioned air from the cabin. They’re along for the ride thermally and atmospherically.

Temperature range is the headline spec: ground handling in summer can reach +50°C or higher, while cruise altitudes expose the hold to ambient temperatures of –55°C to –65°C. But the rate of change matters as much as the extremes. A flight climbing from a hot tarmac to cruise altitude can impose thermal slew rates of 1–2°C per minute sustained over 30+ minutes. That’s not a steady-state problem; it’s a cycling fatigue problem.

Pressure drops from sea level to roughly 25,000-foot equivalent in many unpressurized holds (some aircraft and holds vary). That’s approximately 38 kPa absolute at altitude versus 101 kPa at sea level. Rapid decompression scenarios during descent can reverse this pressure differential quickly.

Humidity plunges to near zero at altitude, but here’s the trap: as the aircraft descends into warmer, moister air, cold-soaked components become condensation magnets. Your –30°C electronics re-entering a +25°C, 80% humidity environment will accumulate moisture on every surface below the dew point.

Vibration and shock exist. Cargo handling isn’t gentle, and turbulence is real. But for electronics survival, they’re typically secondary to thermal and pressure stressors. Standard ruggedization practices generally suffice.

The engineering challenge isn’t any single factor. It’s the combination and relentless cycling: hot-cold-hot-cold, pressurize-depressurize, dry-wet-dry, repeated across hundreds of flight cycles over the device’s service life.

Component Selection for Aviation Temperature Extremes

Commercial-grade components (0°C to +70°C) are obviously out. Industrial-grade (–40°C to +85°C) looks promising until you realize you’re asking them to operate 15°C below their rated minimum, and that’s before considering derating.

Critical components to scrutinize:

• Microcontrollers: Most industrial MCUs specify –40°C minimum, but check startup behavior, oscillator stability, and flash write reliability at that boundary. Some won’t reliably boot below –30°C.
• GPS/GNSS modules: The module might be rated to –40°C, but the internal TCXO may have accuracy degradation that affects position fix times or accuracy.
• RF components (cellular, satellite, LoRa): Power amplifier efficiency drops at temperature extremes, and crystal references drift. Check transmit power and frequency stability specs across temperature.
• Passives: Electrolytic capacitors lose capacitance dramatically at low temperatures. A 100µF cap might provide only 20µF at –40°C. Ceramics are more stable but verify the temperature coefficient of your chosen dielectric (X7R behaves differently than Y5V).

Automotive-grade parts (AEC-Q100 for ICs, AEC-Q200 for passives) are a reasonable starting point. They’re designed for under-hood environments with similar thermal cycling demands. But don’t assume the automotive qualifier means cargo-hold ready. Pull the actual datasheet curves for the parameters you care about at –55°C, not just the headline operating range.

Practical guidance: Test at temperature extremes, not just verify against specs. A GPS module that technically operates at –45°C might take four minutes to achieve a fix instead of 30 seconds. That’s unacceptable for a tracker that wakes briefly to report position.

Battery Selection and Power Management

Battery chemistry is where cargo hold designs succeed or fail. Standard lithium-ion cells become problematic below –20°C: internal resistance spikes, capacity drops 50% or more, and charging below 0°C risks lithium plating that permanently damages the cell.

Lithium thionyl chloride (LTC) primary cells excel in this environment: wide temperature range, high energy density, excellent shelf life. The tradeoff is they’re non-rechargeable and handle high pulse currents poorly. For GPS/cellular trackers that transmit infrequently, LTC works well.

Hybrid approaches pair a primary cell with supercapacitors to buffer pulse loads. The supercapacitor handles the 2A transmit pulse while the LTC battery slowly recharges it between transmissions. This works, but supercapacitor performance also degrades at low temperatures. Verify your specific parts.

Self-heating batteries use a resistive element to warm the cell before discharge. This solves the chemistry problem but creates a power budget problem: you’re burning stored energy to enable using stored energy. For long-deployment trackers, the math often doesn’t work. For shorter missions or rechargeable scenarios, it’s viable.

Pressure effects: Pouch cells are more susceptible to swelling under reduced pressure than cylindrical cells with rigid casings. If you’re using pouch cells, verify behavior at 38 kPa and design mechanical accommodation for expansion.

Thermal Management Strategies

Here’s where cargo hold design inverts your instincts. You’re probably used to worrying about heat dissipation and keeping components cool under load. In a –55°C cargo hold, your problem is the opposite: retaining enough internal heat to keep electronics in their operating range.

Insulation strategy: Design the enclosure as a thermal cocoon. Minimize conductive paths to the exterior (standoffs, screws, and antenna connectors are all thermal bridges). Use insulating materials with low thermal conductivity. Add thermal mass inside the enclosure. Components and PCB retain heat better than empty air space.

The sleep state problem: Modern trackers achieve long battery life through aggressive sleep modes drawing microamps. But microamps don’t generate meaningful heat. A tracker sleeping at 10µA in a –55°C environment will reach thermal equilibrium with its surroundings. That means your components are at –55°C when they need to wake up.

Consider periodic “wake to warm” cycles: briefly power on internal components to generate enough heat to maintain a minimum internal temperature. This costs energy, so model the tradeoff against your power budget and mission duration.

Condensation management: As the aircraft descends and the cargo hold warms, your cold-soaked device will condense moisture from the air. Conformal coating helps but isn’t foolproof. Design for drainage if moisture intrusion is possible. Avoid board layouts that create pooling areas.

Enclosure and Mechanical Considerations

Pressure equalization: A sealed enclosure at sea level becomes a pressurized vessel at altitude. The internal pressure exceeds external pressure by potentially 63 kPa. That’s enough to stress seals, deform enclosures, or fatigue pouch cell batteries. Options:

• Controlled venting: Gore-Tex or similar hydrophobic/oleophobic vents allow pressure equalization while blocking liquid water and dust. They don’t block water vapor, so internal humidity equilibrates with external.
• Sealed with margin: Design the enclosure to handle the pressure differential. This means rigid construction, robust seals, and tested structural margins.

Material selection: Standard ABS becomes brittle at –40°C and may crack at –55°C. Polycarbonate, ABS/PC blends, or glass-filled nylons generally maintain ductility at cargo hold temperatures. Verify the specific grade, as additives and fillers affect low-temperature behavior.

Seals: Silicone O-rings maintain flexibility across the cargo hold temperature range. Standard nitrile (Buna-N) stiffens significantly below –30°C. EPDM is another option with good low-temperature performance, but verify chemical compatibility with any materials it contacts.

Conformal coating: If you’re using conformal coating (and you should), select one that remains flexible at –55°C. Acrylic coatings can crack at extreme cold. Silicone-based coatings generally perform better across wide temperature ranges.

Design Workflow Notes

Start thermal simulation early, before you’ve committed to an enclosure design or board layout. Model the thermal trajectory during a representative flight profile: ground soak, climb, cruise, descent. Identify when and where components fall outside their operating envelopes.

Test across the full environmental envelope. Thermal chambers are accessible; altitude chambers less so, but they’re worth finding if pressure effects are a concern for your design. Test flight cycles, not just static temperature extremes. Cycling reveals failures that steady-state testing misses.

Build in design margins. If your critical path component is rated to –40°C and you’re designing for a –55°C environment, you need either a better component, active heating, or enough insulation and internal heat generation to guarantee that component never sees –40°C.

Make It Survive the First Flight and the Five Hundredth

Unpressurized cargo holds are hostile but predictable. The environment follows physics: you can model it, test against it, and design around it. The engineering isn’t exotic. It’s deliberate.

Get the environmental characterization right. Select components with genuine margin against the temperature extremes. Choose battery chemistry that functions at –55°C or design active thermal management to avoid it. Build an enclosure that handles pressure cycling and keeps moisture out.

Then test it. Put the prototype in a thermal chamber, run it through simulated flight profiles, and verify your models match reality. The trackers that fail in the field are the ones that never saw realistic conditions before deployment.

Hubble Network’s satellite-connected Bluetooth enables cargo tracking through aircraft structures without cellular infrastructure. Learn more →