What Is Satellite IoT Connectivity?

Detailed Explanation

Satellite IoT is the connectivity option of last resort — it applies when the device must communicate but is beyond the reach of cellular networks, LoRaWAN gateways, and Wi-Fi infrastructure. In Australia, this means deployments in the remote outback, offshore marine environments, high-altitude monitoring stations, and mobile assets that traverse regions with no cellular coverage. The key satellite IoT constellations differ substantially in orbit, latency, coverage pattern, and cost.

LEO satellite constellations for IoT

Iridium (Short Burst Data / Direct IP)

Iridium operates 66 LEO satellites in polar orbits at approximately 780 km altitude, providing true global coverage including the poles. Every point on Earth is always within view of at least one Iridium satellite. The standard IoT service is Short Burst Data (SBD): mobile-originated (MO) messages up to 340 bytes, mobile-terminated (MT) messages up to 270 bytes, with typical end-to-end latency of 10–30 seconds for the SBD service.

The Iridium 9603 module (and its development board, the RockBLOCK) is the standard hardware interface: UART-controlled with a simple AT command set for sending and receiving SBD messages. Typical transmit current: approximately 1.5 A at 3.3 V during a transmission burst — a significant current spike that requires a large decoupling capacitor (typically 1–10 F supercapacitor) if operating from a high-impedance battery source.

Swarm Technologies (SpaceX)

Swarm operates a constellation of 150+ small LEO satellites at approximately 550 km altitude, providing global coverage with pass intervals of 10–60 minutes depending on location. Data packets are up to 192 bytes. The Swarm M138 module uses a compact patch antenna and UART AT command interface. Unlike Iridium, Swarm does not provide real-time two-way communication — messages are stored and forwarded with latency matching the satellite pass interval.

Astrocast / Kinéis

Astrocast (now part of Kinéis) operates 25 nanosatellites in LEO, with a planned 25-satellite constellation providing coverage pass intervals of approximately 10–20 minutes. Targeted at low-data-rate uplink-heavy IoT applications (environmental sensors, asset tracking). The Astrocast module provides simple store-and-forward data with messages up to ~40 bytes.

Antenna requirements

LEO satellite IoT antennas face competing requirements: small enough to integrate into an embedded product, with sufficient gain to close the link budget over 500–2000 km. Common solutions:

• Patch antenna: a flat, low-profile directional antenna (approximately 50 × 50 mm for L-band) with approximately 3–5 dBi gain toward zenith. The standard choice for fixed outdoor installations with a clear sky view.
• Quadrifilar helix (QFH) antenna: a circularly polarised antenna providing near-hemispherical coverage — better for tracking applications where the satellite may be at any elevation angle.
• Chip antenna / PCB trace antenna: available for small Swarm modules but with lower gain; acceptable for applications in open environments with high satellite pass frequency.

Satellite link budgets are tight. Small antenna gain reductions (from nearby ground planes, nearby metal structures, or installation orientation) can prevent link closure entirely. Test the antenna performance in the actual installation configuration, not just in isolation.

Practical Examples

A remote cattle water trough monitoring system in outback Queensland is beyond any cellular coverage. A satellite IoT device (Iridium 9603 + STM32 MCU) measures water level via ultrasonic sensor and battery voltage, and transmits a 100-byte SBD message every 4 hours reporting current values and a 24-hour history buffer. Solar-charged LiFePO₄ battery powers the system; the 1.5 A Iridium transmit current spike is absorbed by a 5 F supercapacitor bank. Average daily data cost: approximately USD $0.24 at $0.04/message × 6 messages/day.

A marine buoy monitoring ocean temperature and wave height at a location 300 km offshore uses Iridium Direct IP for a higher-data-rate connection during scheduled 5-minute reporting windows. The buoy also receives command updates (setpoint changes, reconfiguration) via mobile-terminated SBD messages when a satellite pass occurs.

Design Considerations

• Satellite vs cellular in Australia: Australia's cellular coverage is concentrated along the coast and in population centres. Regions beyond cellular coverage (roughly 80% of Australia's land area) are satellite-only for conventional IoT deployments. Map the actual deployment area against Telstra/Optus LTE coverage before choosing between cellular IoT and satellite.
• Pass interval vs latency requirement: Swarm and Astrocast provide store-and-forward with multi-minute pass intervals — acceptable for environmental monitoring that reports every few hours, unacceptable for applications needing real-time remote control or alarm acknowledgements. Iridium SBD provides near-real-time (10–30 second) two-way communication.
• Power supply for transmit current spike: Iridium modules require 1.0–1.5 A during transmission. Li-SOCl₂ primary cells (the standard choice for very-long-life remote sensors) have high internal impedance that prevents delivering this current without voltage collapse. Use a supercapacitor buffer (1–10 F) or a LiFePO₄ cell capable of high pulse currents.
• RF design considerations: satellite antenna design at L-band (1.6 GHz for Iridium) follows the same principles as RF PCB layout: maintain the 50 Ω feed trace impedance, keepout zone around the antenna, and avoid ground copper in the antenna radiating region.
• Regulatory considerations: in Australia, satellite terminal operation requires ACMA licence unless the equipment is pre-approved. Iridium and Swarm modules used within their operating parameters with approved antennas fall under device-level licensing; verify with ACMA before deployment.

Common Mistakes

• Testing indoors and declaring success: satellite links require clear sky visibility. A module that appears to acquire a satellite signal in an office (partial sky view through a window) may fail entirely when deployed under a dense tree canopy or inside a vehicle enclosure.
• Underestimating transmit current spike: the 1–1.5 A Iridium transmit burst collapses the supply voltage if the power source cannot deliver it. Always include a supercapacitor buffer and calculate the energy required per transmission.
• Not accounting for pass interval variability: Swarm satellite passes vary from less than 10 minutes to over 60 minutes depending on location and constellation state. Design the application to tolerate the maximum expected pass interval, not just the average.
• Choosing satellite for applications better suited to LoRaWAN: if a LoRaWAN gateway can be installed at or near the site, LoRa delivers lower latency, lower per-message cost, and simpler integration. Satellite adds cost and complexity that is only justified when no terrestrial network option exists.