Understanding Network Time Protocol (NTP), Atomic Clocks, and GPS: How Precise Timekeeping Powers the Modern World

In the digital age, accurate timekeeping is more critical than ever. From ensuring data consistency across global servers to enabling GPS navigation and military operations, the precision of our clocks underpins modern infrastructure. This post explores how Network Time Protocol (NTP), atomic clocks, and the Global Positioning System (GPS) work together to synchronize time around the world—and what stratum levels really mean in timekeeping hierarchy.

🕒 What Is Network Time Protocol (NTP)?

NTP (Network Time Protocol) is a protocol used to synchronize the clocks of computers and devices over a network. It allows systems to keep time within milliseconds of Coordinated Universal Time (UTC), which is the global time standard.

NTP works using a hierarchical structure of stratum levels, with each level representing the “distance” from the reference clock (usually an atomic clock or GPS-based source).

📶 How NTP Works

Here’s a simplified overview of the process:

1. A client computer sends a request to an NTP server.
2. The NTP server responds with the current time along with information about when the request was received and replied to.
3. The client calculates the round-trip delay and clock offset.
4. The local clock is adjusted accordingly, either gradually or immediately.

This process often repeats at regular intervals to maintain synchronization.

🧱 Understanding NTP Stratum Levels

NTP servers are organized into strata based on how close they are to the original time source:

• Stratum 0: These are the reference clocks—usually atomic clocks, GPS receivers, or radio clocks. They don’t connect directly to the internet.
• Stratum 1: Servers directly connected to stratum 0 devices. These are often publicly available NTP servers and offer highly accurate time.
• Stratum 2: Servers that sync with stratum 1 servers. Most users rely on these.
• Stratum 3–15: Each additional level syncs from the level above, with increasing latency and reduced accuracy.
• Stratum 16: Designated for unsynchronized servers or devices.

💡 Example:
If your Raspberry Pi is syncing from time.google.com (a stratum 1 server), your device becomes a stratum 2 client.

⏱️ What Is an Atomic Clock?

An atomic clock is the most precise timekeeping device available. It uses the natural oscillations of atoms—commonly cesium-133 or rubidium—as a reference.

Key Characteristics:

• Accuracy: Can measure time with precision better than 1 second in millions of years.
• Stability: Remains extremely consistent over long periods.
• Use Cases: GPS satellites, NTP stratum 0 devices, scientific labs, telecom networks.

The international definition of one second is based on the radiation cycles of cesium-133:
9,192,631,770 transitions = 1 second

🛰️ How GPS Provides Accurate Time

The Global Positioning System (GPS) is not just for navigation—it’s also a precise time distribution network. Each GPS satellite contains multiple atomic clocks. When your GPS receiver locks onto satellites, it can:

• Determine location using trilateration
• Calculate the exact time from satellite signals

GPS Time vs UTC:

• GPS time started in 1980 and does not account for leap seconds, unlike UTC.
• GPS receivers convert GPS time to UTC using data in the satellite’s almanac.

For time servers, GPS receivers act as Stratum 0 sources, making GPS-based NTP servers (Stratum 1) popular for time-critical systems.

🛠️ Real-World Applications

⛓️ When Accuracy Matters

A tiny time drift can cause:

• Log mismatch in security systems
• Transaction failures in banking
• Packet loss or desync in VoIP and online gaming
• Routing issues in network infrastructure

Thus, reliable NTP configuration, ideally syncing from multiple stratum 1 servers or running your own GPS-based server, is best practice for critical systems.

📍 Summary

Together, atomic clocks, GPS, and NTP form a robust global timekeeping system that powers everything from Google’s servers to battlefield operations and your APRS messages.

🔧 Build Your Own Stratum 1 NTP Server (Raspberry Pi + GPS + Chrony)

Creating your own Stratum 1 NTP server is a rewarding project, especially for ham radio operators, makers, and sysadmins who want reliable, low-latency timekeeping without relying on the internet.

🧰 What You Need:

• Raspberry Pi (any model with GPIO, preferably Pi 3/4/5)
• GPS module with PPS (Pulse Per Second) output (e.g. u-blox NEO-6M or NEO-M8N)
• Serial connection to GPS (UART)
• Internet for installation (optional afterward)
• Linux with chrony, gpsd, and PPS support

⚙️ Step 1: Hardware Setup

1. Connect the GPS module:
   • TX (GPS) to GPIO 15 (RXD) on the Pi
   • PPS pin to GPIO 18 (Pin 12)
   • VCC and GND appropriately
2. Enable UART and PPS on the Pi: sudo raspi-config
   • Interface Options → Enable Serial (disable console over serial, enable hardware UART)
   • Reboot when prompted.

📦 Step 2: Install Required Packages

sudo apt update
sudo apt install gpsd gpsd-clients chrony pps-tools

🧪 Step 3: Enable and Test PPS

1. Add the PPS overlay:

Edit /boot/config.txt and add:

dtoverlay=pps-gpio,gpiopin=18

2. Reboot:

sudo reboot

3. Check if /dev/pps0 appears:

ls -l /dev/pps*

4. Test PPS signal:

sudo ppstest /dev/pps0

You should see output like:

source 0 - assert 1716165796.000000000, sequence: 1234 - clear 1716165796.000001234

📡 Step 4: Configure GPSD

Edit /etc/default/gpsd:

START_DAEMON="true"
DEVICES="/dev/serial0"
GPSD_OPTIONS="-n"

Restart the service:

sudo systemctl restart gpsd

Verify:

cgps -s

You should see satellite data and GPS time.

⏱️ Step 5: Configure Chrony for GPS + PPS

Edit /etc/chrony/chrony.conf and add these lines at the top:

refclock SHM 0 offset 0.5 delay 0.2 refid GPS
refclock PPS /dev/pps0 refid PPS lock GPS

Also, comment out any existing pool or server lines if you want it to be completely standalone.

Restart Chrony:

sudo systemctl restart chrony

✅ Step 6: Verify Chrony Status

chronyc sources -v

You should see entries like:

#x  Name/IP address            Stratum  Poll Reach  LastRx Last sample
===============================================================================
#?  GPS                          0       4   377     10     -4ns[  +23ns] +/- 30us
#*  PPS                          0       4   377     9      -1ns[   -5ns] +/- 0.1us

The * next to PPS indicates it’s the preferred source.

🖧 Optional: Share NTP on Your Network

Edit /etc/chrony/chrony.conf to allow LAN clients:

allow 192.168.1.0/24

Replace with your local subnet. Restart Chrony to apply.

This allows you to sync your entire home lab or shack with millisecond precision without depending on the internet.