How do field teams communicate without coverage?

Utility crews, surveyors, foresters, inspectors and response teams work where cellular networks are thin or absent: remote roads, valleys, forests, plant rooms and basements. A phone with no tower in range has no route to the backend, and even where towers exist, a busy network can fail at the wrong moment. CISA’s guidance on public safety radio warns that commercial cellular voice and data networks used as backup “may be overwhelmed by congestion and capacity issues during emergencies”.

So field teams plan for several layers, each with a different job: voice, emergency alerts, and the data their apps produce.

For crew voice, the long-standing tool is land mobile radio (LMR). CISA’s LMR 101 describes LMR as terrestrial wireless systems used by emergency responders, public works companies and the military for voice and low-speed data. A typical system has:

• Handheld radios, carried by people, with a limited transmission range.
• Mobile radios in vehicles, using the vehicle’s power and a larger antenna for greater range.
• Base stations at fixed sites such as dispatch centres.
• Repeaters, which retransmit received signals to extend the range of handhelds and mobiles.
• A network connecting base stations into one system.

Conventional systems give each group dedicated channels; trunked systems are computer-controlled and assign channels from a shared pool, which CISA says increases capacity and reduces congestion at the cost of more infrastructure.

Two digital radio standards are relevant here. Project 25 (P25) is a suite of standards for interoperable digital two-way radio, developed in North America under the Telecommunications Industry Association. Digital Mobile Radio (DMR) is an ETSI standard; the DMR Association explains that it splits one 12.5 kHz channel into two timeslots so a single channel carries two independent calls, and it lists a direct mode of operation among its features, which lets radios talk to each other without going through a repeater.

Radio is built for voice and low-speed data. It is not designed to carry an app’s work orders, forms, photos or checklists.

Satellite fills some of the gaps. iPhone 14 and later models can text emergency services by satellite when there is no cellular or Wi-Fi coverage; Apple’s support page for Emergency SOS via satellite says you need to be outside with a clear view of the sky and horizon. That makes it a last-resort safety channel, not a link that a crew working under tree cover or inside a building can rely on for routine data.

For public safety agencies in the United States, FirstNet adds broadband on its own spectrum. Its deployables are mobile cell sites that link to FirstNet by satellite, do not rely on commercial power, and are sent to disasters, large planned events and remote areas. They help when a team can wait for one to arrive and is eligible to use it.

Much of what a crew records has to reach a person standing nearby before it reaches the cloud: a handover between shifts, a permit a supervisor must countersign, the latest checklist state for a job two people are sharing.

Apps built to work offline save this work on the device and sync it later. That keeps each person productive, but two phones that cannot reach the server cannot see each other’s changes either. The supervisor’s phone does not know the job is done until both phones find coverage.

Phones carry their own radios. Over Bluetooth LE, two phones within range can exchange data with no tower at all, and phones in a group can relay for each other to reach someone further away. Combined with store-and-forward delivery, a record waits in an outbox until a path to its recipient or to the backend appears, and any phone that later reaches coverage can pass it on.

Offline Protocol’s mesh SDK works this way: nearby phones connect over Bluetooth LE with no tower, unsent messages wait in an outbox, content is encrypted with MLS, and the backend stays the system of record.

A layered plan for a crew might look like this:

Each layer covers a failure the others do not, so the question is less which one to choose than which job each one owns.