NZ Police's radio network withstands cyclone-strength winds and rain

The New Zealand Police’s Mimomax voice linking network on the East Coast of the North Island was put to the test (and emerged unscathed) in February when Cyclone Gabrielle struck, causing the most expensive clean-up on record from a Southern Hemisphere storm.

With at least three links operational in the region where the heaviest rains from the cyclone made landfall, and another three links in areas of extremely high winds, the Mimomax network was subject to extreme weather that downed trees, created landslides and washed out roads and bridges. Both the power and the communications networks were severely impacted across the whole region.

Deployed alongside roads, the fibre network was severed in multiple locations and, with the cyclone knocking out the power network, cell sites which can only typically run on battery power for a matter of hours were also taken out. Furthermore, those cell sites which did still have power lost connectivity when the fibre backhaul was wiped out.

“These failures with phone connectivity, data circuits and the cellular networks meant that communications-wise, the Eastern Districts were entirely cut off,” said Richard Hutchinson, Infrastructure Engineer, NZ Police. “In terms of NZ Police operations, the Gisborne and Hawke’s Bay areas had to fall back to regionalised dispatch.”

At the peak of the communications blackout, there were 272 cell sites down and the NZ Police radio repeater sites were running on backup battery and generator power. With roads destroyed in places, some sites could only be accessed by helicopter — a challenging prospect with cyclone-force winds blowing during the three-day event.

With some areas of Hawke’s Bay receiving 600% of their normal February rainfall over the course of 55 hours, the Mimomax radio network was rigorously stress-tested for rain-induced fading. Hutchinson noted, “The radio standards organisations, ITU and TIA, both indicate that links below 1 GHz can reasonably ignore rain events unless the links are very long or the rain is extremely heavy. NZ Police now have some local practical experience to back those standards up.”

Communications backhaul was established after approximately 60 hours using Starlink, enabling a limited form of centralised dispatch to be resumed. Hutchinson said, “The limited data bandwidth over Starlink prevented the collection of the Mimomax performance data at the time, but when full connectivity was restored the status pages showed that the Mimomax links had operated as designed with few, if any, errors.”

Despite winds registering 140–150 km/h in some locations, the antennas in the Mimomax network held firm. This was the result, according to Hutchinson, of a reasonable degree of effort going into the bracing during antenna installation with an aim to “design for the worst”. Furthermore, with power out for a number of days, the Mimomax radios were also used as a window into the status of the grid in that locality.

“The radio supply voltage indication on the status page for each radio helpfully allowed us to ascertain whether the mains supply was functional at these sites,” Hutchinson said.

Providing an excellent case study of what to expect from climate events, the impact of Cyclone Gabrielle on communications connectivity has highlighted three clear lessons:

1. The main issue for NZ Police communications sites was loss of mains power. All the sites were designed for outage periods based on typical restoration times; however, lack of site access made restoration much slower.
2. Robust installation techniques for antennas are crucial to promote resilience.
3. Where possible, have some level of connectivity available in frequency bands that perform well under precipitation events.

Image credit: iStock.com/Mykhaylo