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ALERT and ALERT2 — the protocols behind real-time flood warning
When a storm moves over a watershed, emergency managers need data now. ALERT and its modern successor ALERT2 are the radio telemetry protocols that make that possible — and HydroLynx builds its entire product line around them.
ALERT: event-driven reporting since the 1970s
ALERT (Automated Local Evaluation in Real Time) is the radio protocol that flood-warning networks across North America have relied on for decades. Field stations transmit a short data burst the moment something happens — a rain-gauge bucket tips, a water level changes — so operators see conditions as they unfold rather than on a fixed schedule.
It is simple, robust, and proven. Its limitations are structural: it runs at 300 baud, offers only 8,192 sensor IDs network-wide, two stations can transmit at the same instant and collide, and there is no error correction to recover a burst damaged by a weak or noisy radio path — so data is most at risk exactly when the network is busiest, during a major storm.
ALERT2: the current standard
ALERT2 is the modern successor, governed by the National Hydrologic Warning Council (NHWC) and the standard new networks are built around. It keeps the event-driven philosophy and solves the old limitations:
What ALERT2 adds
GPS-synchronized TDMA
Time-Division Multiple Access gives every station its own time slot, synchronized by GPS — so two stations never talk over each other, even on a crowded network.
Forward error correction
Reed-Solomon and convolutional coding let the receiver reconstruct data damaged on a weak or noisy path — good data or no data, never silently wrong data. The 2025 AirLink v1.2 spec makes FEC modes configurable.
16× the data rate
ALERT2 transmits at 4800 bps versus legacy ALERT's 300 baud. After error-correction overhead, that nets roughly 4–5× more usable data throughput — shorter time on air, richer engineering-unit messages, and more sites sharing one channel.
65,535 sites × 255 sensors
Up to 65,535 uniquely addressed sites, each reporting up to 255 sensors — over 16 million sensor identifiers, where legacy networks in dense regions had exhausted their 8,192-ID pool.
Inside the protocol: four layers
ALERT2 is layered like modern network protocols, so each part can evolve independently. Knowing the four layers helps when reading spec sheets and configuring equipment:
Application layer
Encodes the measurements themselves — a control byte, a timestamp, then one or more sensor reports in a compact binary format.
MANT layer
The envelope around the data: source address, hop limit, and routing metadata that tell the network how a message travels from station to base.
AirLink layer
How a message physically goes over FM radio — GMSK modulation, forward error correction, and ALOHA or TDMA media access.
IND API
How ALERT2 devices talk to everything outside the radio network — dataloggers, base-station software, SCADA integrations.
Configuring stations well
ALERT2 gives you choices legacy ALERT never did. A few of them matter for network health — repeater airtime is a shared, finite resource, and shorter messages survive marginal radio paths better.
| Report type | Best for | Worth knowing |
|---|---|---|
| Multi-Sensor Report (MSR3 English / MSR4 metric) | Most stations, most of the time | Very efficient, sensible preconfigured defaults — usually the right answer |
| Tipping-Bucket Report (TBRG) | Rain stations where every tip matters | Message length varies with rainfall; small buckets plus intense rain on a busy repeater can congest a network |
| General Sensor Report (GSR) | Unusual sensors and special cases | Most flexible but least efficient, and easy to misconfigure — e.g. sending signed data with an unsigned type |
| Status Report (MSR5) | Battery, health and diagnostics | Efficient and preconfigured where supported |
Frames, slots and channel capacity
TDMA divides time into repeating frames, typically one to three minutes long, and each frame into slots from 250 milliseconds to a few seconds. Every station owns a slot and transmits only then. The trade-offs are simple: a longer frame fits more stations on the channel but data arrives less often; a longer slot carries more data per station but supports fewer stations. Repeaters should always run TDMA — repeater airtime is the scarcest resource on the network.
The 2025 AirLink v1.2 specification adds configurable forward error correction and 250 ms slots: sites with clean radio paths can send roughly twice the data in the same airtime — which can be spent doubling network capacity or halving the reporting cycle. Use the TDMA slot capacity calculator to plan how many stations fit on your channel.
Securing two-way networks
One-way reporting is hard to abuse; a command path is another matter. The ALERT2 protocol includes encryption and authentication for exactly this reason, and HydroLynx two-way SCADA transmitters 50388 combine data encryption with FEC and TDMA — so gate, pump or siren control commands can't be spoofed by anyone with a radio. If you're planning command-and-control, talk to us about securing the return path from day one.
HydroLynx stations arrive configured for your network, and our engineers review report types, transmit offsets and repeater loading as part of every system design. Planning capacity yourself? The ALERT2 Technical Working Group's specifications and tools are public — see the resources below.
Standards & further reading
ALERT2 is an open standard governed by the National Hydrologic Warning Council through its ALERT2 Technical Working Group — HydroLynx builds to these public specifications:
- AirLink Protocol Specification v1.2 (2025) — radio layer, modulation, FEC, TDMA
- MANT Protocol Specification v1.2 — addressing and routing
- Application Layer Specification v1.3 — sensor report encoding
- IND API Specification v2.1 — device integration interface
- TDMA slot capacity calculator — plan how many stations fit on a channel
Specifications hosted by Blue Water Design, a fellow ALERT2 TWG participant.
How a network fits together
A complete flood-warning network has three layers, and HydroLynx builds equipment for each:
Field stations measure
- Rain, level, weather, and water-quality stations transmit on events and on timed schedules. One-way 53xx stations report; two-way 58xx stations add a return path for polling and control. See packaged stations →
A radio path gets data home
- Stations report directly, or through one or more hilltop repeaters 50386P/A2 where terrain blocks a direct path. Cellular, satellite, Wi-Fi, and IP options cover sites where radio licensing is not practical. See telemetry equipment →
A base station decodes and displays
- A receiver/decoder 5052RD-K-H feeds software such as NovaStar5 or your SCADA system, turning raw bursts into the maps, alarms, and dashboards operators act on. See software →
Upgrading legacy ALERT
You do not have to replace a working network to modernize it. Field-installable upgrade kits add ALERT2 encoding and GPS to existing 50386 transmitters, and networks can run mixed during a migration — legacy stations keep reporting to your existing decoder while new ALERT2 stations come online. See the upgrade kit →