The current NHWC-governed protocol for automated flood warning networks — faster, more reliable, and built for the next generation of hydrologic monitoring.
For more than four decades, the Automated Local Evaluation in Real Time (ALERT) protocol has been the backbone of flood warning networks across the United States and beyond. ALERT made it possible for remote rain gauges, stream sensors, and weather stations to transmit data to central base stations during storm events — enabling emergency managers to issue timely warnings and protect lives.
But as the demands on flood monitoring networks have grown — more sensors, denser networks, higher-stakes decisions — the limitations of legacy ALERT have become increasingly apparent. The protocol operates at just 300 baud, supports only 8,192 unique sensor IDs, and uses a collision-based transmission method that results in data loss precisely when reliable data matters most: during a major storm event.
In 2010, the National Hydrologic Warning Council (NHWC) — the governing body for flood warning standards in the United States — introduced ALERT2 as the official successor to legacy ALERT. Developed through the NHWC's ALERT2 Technical Working Group (TWG), ALERT2 was designed from the ground up to overcome the technical limitations of its predecessor while maintaining a clear path for agencies to upgrade gradually.
ALERT2 is an open standard. There are no licensing fees or proprietary restrictions, meaning any manufacturer — including HydroLynx — can build fully compliant equipment. This ensures long-term availability, competitive pricing, and true interoperability across networks.
| Feature | Legacy ALERT | ALERT2 |
|---|---|---|
| Transmission Speed | 300 bps | More than 10× faster |
| Unique Site IDs | 8,192 sensors | 65,000+ complete sites |
| Data Resolution | Integer only (0–2,047) | Full engineering values |
| Error Correction | None | Forward error correction (FEC) |
| Collision Handling | ALOHA (data loss risk) | TDMA (time-synchronized, collision-free) |
| Data Loss During Storms | Significant risk | Near-eliminated |
| Backward Compatibility | N/A | Compatible with legacy ALERT networks |
| Open Standard | Yes | Yes (NHWC governed) |
One of the most significant improvements in ALERT2 is its use of TDMA — a method of managing radio transmissions in which each station is assigned a precise time window in which to broadcast. Unlike the legacy ALERT approach, in which multiple sensors could transmit simultaneously and corrupt each other's data, TDMA eliminates contention entirely. GPS-synchronized clocks ensure each station knows exactly when to transmit.
ALERT2 incorporates forward error correction into every transmission. The receiving base station can detect and correct errors in a message without requesting a re-transmission — reducing data loss and improving reliability even in poor radio conditions.
Legacy ALERT networks in densely populated regions had nearly exhausted the available 8,192-sensor ID pool, making it impossible to expand networks without costly workarounds. ALERT2 expands this to support more than 65,000 complete monitoring sites — future-proofing networks for decades to come.
Where ALERT was limited to transmitting integer values between 0 and 2,047, ALERT2 supports full engineering-unit values with floating-point precision. This means more accurate rainfall totals, more granular water level readings, and richer meteorological data — all of which support better decision-making during flood events.
ALERT2 is built on a modern layered protocol stack architecture — consistent with how the internet and other modern communications systems are designed. This modular approach means individual layers can be updated or extended without overhauling the entire system, making ALERT2 more adaptable to future technology changes.
One of the most practical advantages of ALERT2 is that agencies do not need to replace their entire network at once. ALERT2 is designed with a phased upgrade path in mind. ALERT2-compliant encoders can capture legacy ALERT messages and retransmit them in ALERT2 format from existing repeater sites, allowing networks to transition gradually — station by station — according to budget and operational schedules.
HydroLynx Systems has built its entire current product line around the ALERT2 standard. Every packaged station, data transmitter, and receiver/decoder we manufacture is NHWC-compliant and interoperable with the broader ALERT2 ecosystem.
What is ALERT2?
ALERT2 is the modern open-standard protocol for real-time hydrologic telemetry, governed by the National Hydrologic Warning Council (NHWC). It is the successor to legacy ALERT and is the current standard for automated flood warning networks across the United States. ALERT2 supports more than 65,000 monitoring sites, uses TDMA to eliminate transmission collisions, and includes forward error correction for reliable data delivery during major storm events.
How is ALERT2 different from legacy ALERT?
ALERT2 transmits more than 10 times faster than legacy ALERT, supports 65,000+ monitoring sites versus ALERT's 8,192 sensor IDs, uses TDMA (time-synchronized transmission) to eliminate the data collisions that plague ALOHA-based ALERT networks, and includes forward error correction to recover from radio errors without re-transmission. ALERT2 also supports full engineering-unit data with floating-point precision, where ALERT was limited to integers from 0 to 2,047.
Do we have to replace our entire ALERT network at once to migrate to ALERT2?
No. ALERT2 is designed with a phased upgrade path. ALERT2-compliant encoders can capture legacy ALERT messages and retransmit them in ALERT2 format, allowing agencies to transition gradually — station by station — according to budget and operational schedules. HydroLynx offers ALERT2 firmware upgrade kits (Model 5096/A2) for compatible existing equipment, as well as complete replacement transmitters.
Is ALERT2 an open standard?
Yes. ALERT2 is an open standard governed by the National Hydrologic Warning Council (NHWC). There are no licensing fees or proprietary restrictions, meaning any manufacturer can build fully compliant, interoperable equipment. This ensures long-term availability, competitive pricing, and true cross-vendor interoperability across networks.
What is TDMA in the context of ALERT2?
TDMA (Time Division Multiple Access) is the transmission method ALERT2 uses to assign each monitoring station a precise time window in which to broadcast. GPS-synchronized clocks ensure each station knows exactly when to transmit, eliminating the data collisions that occur in legacy ALERT's ALOHA-based system when multiple sensors try to transmit at the same time. The result is dramatically improved data reliability, especially during major storm events when many sensors are reporting simultaneously.
Is HydroLynx equipment ALERT2 certified?
Yes. HydroLynx Systems has built its entire current product line around the ALERT2 standard. Every packaged station, data transmitter, and receiver/decoder we manufacture is NHWC-compliant and interoperable with the broader ALERT2 ecosystem.
Our team has supported flood warning agencies through phased ALERT-to-ALERT2 transitions for decades. We can help you plan a migration that fits your budget, timeline, and existing network.
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