The Tech Blog

Yesterday many of you in India may have received the test alert with your phone going off in vibration and sound mode from the Govt. It got me to look up the tech stack that enables this. I wanted to explain it because it looks like an SMS, but it really isn’t.

Fundamental tech is called Cell Broadcast. It’s a one-to-many setup. A tower broadcasts an alert to compatible phones connected to that cell. Your phone does not need mobile data. It only needs to be in coverage area. Whereas SMS is one-to-one. The network sends a message to each phone number. That is fine for normal updates, but millions of messages can slow down during a cyclone, earthquake, gas leak or war.

Indian stack seems built around SACHET, the National Disaster Management Authority’s CAP-based (Common Alerting Protocol) Alert System, developed by C-DOT. Visualise it as an alert form that carries the event, area, severity, language, action and expiry time.

I think the flow might be something like a Govt agency like IMD creating a warning. SACHET validates and routes it. C-DOT’s Cell Broadcast system connects it to telecom operators. The operator maps the area to towers, and then selected towers broadcast the alert directly to phones. 

Once the tower broadcasts it, the phone’s modem listens for that alert channel and passes it to the OS as an emergency alert, not as a normal inbox message. That is why it can pop up with a loud tone and vibration without needing mobile data. In SMS, the network addresses your number and waits to deliver a message to you individually.

It can work on basic phones too, but only if that phone supports Cell Broadcast. If the phone is off, in airplane mode or out of coverage, it will not receive it.

I read that SACHET is not only for mobiles. The same platform can push warnings through SMS, Cell Broadcast, apps, TV, radio, social media, railway station boards, and coastal sirens. Its scale is impressive from a tech POV.

BTW: A Cell Broadcast alert does not need your phone number to reach you, just being in coverage is fine.

Recently I was in a discussion where folks were comparing Bluetooth(BLE) and Ultra WideBand(UWB) for a ranging application. Some were saying the latest BLE 6 will replace UWB for ranging. I do not think it is that simple. Let’s discuss.

I have explained both techs in great detail in older posts, so please refer to it if needed. In short, BLE 6 gets true ranging through Channel Sounding. Instead of guessing distance from RSSI, it compares phase across many 2.4GHz channels, using round-trip time as a secondary check. UWB takes a different path by sending very short pulses across a very wideband(>500 MHz) and measures time of flight very precisely.

Theoretically, Bluetooth Channel Sounding is impressive. The Bluetooth SIG says it is designed for around ±20cm accuracy and can measure out to 150m in the right conditions. In real world, things are slightly different. Silicon Labs testing showed about 0.5m error in clear line of sight, but this can grow to around 5m without line of sight, and even close to 10m with fewer channels.

UWB is still the cleaner tool when distance is the only consideration. UWB typically delivers around 10cm accuracy in line of sight and often maintains sub‑meter accuracy in non line of sight, with practical line of sight ranges up to about 200m. That is why UWB is used in digital car keys and phone-based precision finding. It holds up better when reflections, body blocking, and multipath get ugly.

So my practical advice is: If you already have BLE in wearables, locks, or accessories, BLE6 ranging can be good enough for presence, room entry without adding another radio. If you need consistent sub-meter accuracy in cluttered environments, or you are making decisions based purely on distance, UWB remains the more reliable choice.

BLE 6 chips like Nordic nRF54 and NXP KW47 are just emerging, while UWB parts like Qorvo DW3000 are mature. BLE 6 is new, so combining both, like Apple’s Airtag approach, gives low-power discovery plus precise UWB ranging, if BOM costs allow it. So do consider this next time.