Hot, Cold, and Warm Starts

A cold start means starting from scratch—no info about approximate coordinates, satellite almanacs, date, or anything else. A warm start happens when all that data is available. A hot start goes one step further—it also has ephemeris data.

Startup time is measured from power-on to solution availability. A cold start usually takes around 30 seconds, a warm start takes 5–15 seconds, and a hot start just 1–2 seconds. That’s why minimizing startup time is a common goal.

Ideally, you'd always want to start hot—with ephemerides. But here’s the catch: ephemeris data is valid for about 4 hours, or maybe 24 with some tricks. So it doesn’t make sense to store them for long. Where do you get fresh ephemerides? From the internet, of course. That’s where AGPS and AGNSS systems come in—they download fresh ephemerides and precise time via the internet.

Since hot starts require internet access, what about warm starts? That’s possible if you store almanacs (which are valid for 1–2 months), along with approximate coordinates and the year (which don’t age as fast). But here’s the problem: when should the receiver save almanacs? If it’s during shutdown, you need to notify the receiver before cutting power—or it needs backup energy to save data. The alternative is saving data frequently, which requires special memory.

Frequent writing means you need battery-backed static memory, since most flash memory degrades with heavy rewrites. That’s why some receivers use flash and don’t need a backup battery, while others use static memory that does.

Cold Start Issues

There’s also a quirky issue with warm starts: if you power off the receiver and then move it more than 300 km before powering it on again, it may take a long time to start. Why? Because the saved position is way off. That’s why cold starts are recommended after relocation. Most receivers have commands to force a cold start.

Power Backup Types

Receivers use lithium batteries or supercapacitors (also called ultracapacitors) for backup power. If you see a board with neither, it either uses flash memory or only supports cold starts.

Lithium Batteries

Some boards have a holder for lithium coin cells like MC621, SC621, or V364. These cells come in rechargeable and non-rechargeable types. Non-rechargeables last 1–2 years depending on humidity, battery quality (leakage current), and the receiver’s power draw. Rechargeables get topped off while the receiver is powered on and typically last 5–10 years.

A major downside of lithium batteries: they can’t be shipped by air in large quantities. Lithium is flammable and can’t be extinguished with water. That’s why we ship receivers without batteries—users buy them locally.

The Alternative: Supercapacitors

Supercapacitors don’t need replacing, even after 10 years. They can be safely shipped by air—even in bulk—and are easier to solder. So we install them instead of batteries.

However, high-capacity supercapacitors are large—often bigger than the board itself. So we use compact, lower-capacity ones. On small boards, they hold enough power for 30–60 minutes—plenty for a quick reboot or power interruption. On larger boards, the capacitors are bigger and can last 1–2 days—enough to power down overnight.

So when buying a board, check what it uses: a battery holder or a supercapacitor.

© Eltehs SIA 2025