gps

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+## cold, warm and hot start

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+> GPS modules are outdoor positioning systems, so the antenna must be placed **outdoors** to get a fix. The first time you power on the module it performs a **cold start** and needs to download satellite data. This usually takes about **1–10 minutes**, so please be patient.

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+### Cold Start (冷启动)

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+A **cold start** means the module has **no stored ephemeris or almanac data** internally. This is the very first startup, or any startup where all such data has been lost.

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+- Since there is no satellite orbit information available, the module must begin receiving signals and **search the entire sky** within the antenna’s view to find satellites and download ephemeris.

+- During this first operation, the **power consumption is high** and the **internal computation is complex**, because it has to keep downloading the current effective ephemeris of the visible satellites.

+- **Cold-start sensitivity is lower**. Without any prior ephemeris, the module cannot estimate its approximate position. It’s like arriving in a completely unfamiliar desert: you have no reference points, so you must scan the entire environment for landmarks to figure out where you are.

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+In other words, during a cold start, the GPS module has to do **a lot of “wasted work”**:

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+- It scans **every corner of the sky** like sifting signals through a filter.

+- This process consumes a **lot of power**.

+- Once it finds a satellite, it starts downloading ephemeris. If the signal condition changes (for example, you’re in a moving car and position changes constantly), the module may fail to download complete data and has to retry, which **extends the cold-start time**.

+- In weak-signal environments, downloading ephemeris takes even longer.

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+You can compare this to downloading a file from the internet:

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+- If the connection is good, the file downloads quickly.

+- If the connection is unstable, it may disconnect and need to re-establish the link, restarting or slowing the download.

+- Users of **CDMA / GPRS** connections, especially on a train, know that the link can be intermittent and significantly worse than being near a base station.

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+Once the GPS module has continuously tracked **three or more satellites**, the situation improves:

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+- Like sketching a rough map in the field, you can at least estimate your **approximate 2D position**.

+- At **three satellites**, 2D (horizontal) positioning becomes possible.

+- At **four satellites**, you get **3D positioning** (including altitude).

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+After the module has been running for a while and has finished downloading enough current information:

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+- It enters a relatively **stable signal reception state**.

+- Small movements or moderate signal changes no longer affect it as much, because it has **stored the current ephemeris data**.

+- Even with brief signal loss, it can rely on **memory and internal calculations** to quickly restore its position, just like someone who has become familiar with a new environment and can quickly find their way back.

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+Once stabilized:

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+- The ephemeris data continues to **refresh periodically**.

+- After stabilization, the **power consumption naturally decreases**.

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+An analogy:

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+- The first time you visit a large amusement park (or zoo), you see elephants, monkeys, tigers, artificial hills, dense woods, etc. It’s dazzling and disorienting; you easily lose sense of direction.

+- After wandering around all day, you become familiar with the terrain and realize the whole place isn’t that big.

+- If someone then briefly covers your eyes and releases you, you can quickly figure out east/west/north/south.

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+Similarly, once the GPS module is in this familiar state:

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+- Even if you move into a weak-signal area from time to time, it can **quickly readjust**.

+- Thanks to the stored ephemeris and internal calculations, the positioning performance is **much better than during a fresh cold start in the same place**.

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+You might wish that the module **never** had to go through cold start. In practice, we can **shorten cold-start time** by using **network-assisted data**, but some limitations remain:

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+- Through the internet, you can download **almanac/ephemeris-like data** (e.g. from u‑blox servers).

+- However, **ephemeris (星历)** and **almanac (历书)** are different:

+ - Almanac is generated by u‑blox using data from **about 182 ground receivers worldwide**, then computing satellite orbits with software.

+ - Almanac files can cover **1 day, 7 days, 2 weeks**, or even longer. In theory, with good almanac you can avoid a full cold start for **long periods**.

+- In reality, satellite orbits are **constantly corrected**:

+ - Satellites’ actual paths change.

+ - Ground stations periodically adjust and correct the trajectory.

+ - So a “perfectly” computed almanac still has limited accuracy for precise positioning.

+- Nonetheless, having good almanac/assistance **significantly shortens cold-start time**. Technologies like **u‑blox A‑GPS** or **AssistNow** are classic examples, but they **require network connectivity**, which is sometimes limited in real applications.

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+### Hot Start (热启动)

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+If a cold start is the difficult process of adapting to a completely new environment, then a **hot start** is like living in a **familiar** place:

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+- You watch familiar TV, listen to familiar music, sleep in a familiar bed, and enjoy a pleasant, sunny day.

+- We also hope that the module can “stay in a good mood”, i.e. **remain familiar with its environment**.

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+Practical meaning:

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+- Sometimes we need to **briefly power off** the GPS or the whole device.

+- To allow the module to **quickly return** to its previous state when powered on again, we must **save the current ephemeris and related data**.

+- This is done with a **backup battery** (后备电池), which can maintain the module’s data for **about 8–12 hours**.

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+If the main power is removed but the backup battery is present:

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+- And we **reapply power within about 2 hours**, the module can perform a **hot start**.

+- In this case, it can reach a normal fixed position in about **3 seconds**.

+- The reason is that the satellite configuration overhead has **not changed much**:

+ - The satellites above your head have only moved slightly.

+ - It’s like walking near your home; the landmarks barely change, so you can quickly find your way back.

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+That **small backup battery or backup supply** can save you **a lot of waiting time**.

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+### Warm Start (温启动)

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+Warm start is the case **between cold and hot start**:

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+- In practice, we don’t always restart within 2 hours.

+- Suppose the main power to the module has been off for **more than 2 hours**, but the **backup battery still has power**.

+- If we restart within **8 hours** (approximate range) of power-off, the module will perform a **warm start**.

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+Characteristics:

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+- Startup speed is **slower than hot start**, but **faster than cold start**.

+- Signal environment requirements are **less strict than cold start**, but stricter than hot start.

+- The actual time needed to fix a position varies:

+ - The closer the **time and satellite configuration** is to the moment when data was last saved, the **shorter** the time to get a fix.

+ - The longer the power-off duration, or the bigger the change in satellite geometry, the **longer** it takes.

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+Analogy:

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+- The shorter the time you are away from home, the **faster** you re-familiarize yourself with your surroundings when you return—assuming your memory is fine.

+- If you have been away **too long**, things change and your memory becomes less useful.

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+If enough time passes:

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+- For example, after **20 hours**, the satellite configuration has changed significantly.

+- The stored ephemeris is no longer very useful.

+- In this case, the module effectively has to perform a **cold start again**.

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+### Summary

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+- A computer is a machine that executes programs designed by humans; its “thinking” cannot exceed human logic.

+- Many artificial intelligence products such as GPS systems are implementations of **human-designed strategies** based on **natural rules**.

+- By understanding how our own thinking and memory work, we can design **better and more efficient GPS behavior**, especially around cold, warm and hot starts.

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