Why does it take so long to get a GPS fix?

Have you ever wondered why it sometimes takes your GPS module 10-20 minutes to get a GPS fix?   This post will explain why.

 

Each satellite sends a message every 30 seconds.  This message consists of two main components;

  • Ephemeris data, used to calculate the position of each satellite in orbit
  • Almanac , which is information about  the time and status of the entire satellite constellation.

Only a small portion of the Almanac is included in a GPS message. It takes 25 messages (12.5 minutes) to get the full Almanac. The full Almanac is needed before a GPS fix can be obtained.  This is Time To First Fix (TTFF).

TTFF is a measure of the time required for a GPS receiver to acquire satellite signals and navigation data, and calculate a position solution (called a fix).

The above happens during a cold start, this is when the GPS module has been off for some time and has no data in its memory. A full Almanac download is required to get TTFF. If the GPS module has clear line of sight to all satellites, the shortest time for TTFF is 12.5 minutes.

In a warm start scenario,  the GPS module has valid Almanac data,  is close to its last position (100km or so) and knows the time  within about 20 seconds. This approximate information helps the receiver estimate the range to satellites.  The TTFF for a warm start can be as short as 30 seconds, but is usually just a couple of minutes.

A receiver that has a current almanac, ephemeris data, time and position can have a hot start. A hot start can take from 0.5 to 20 seconds for TTFF.

 

Smarts phones use Assisted GPS (aGPS), this allows them to download the Ephemeris data and Almanac over the cell network which greatly reduces the TTFF.

BerryGPS Raspberry Pi GPS

 

 

 

4 thoughts on “Why does it take so long to get a GPS fix?”

    1. The BerryGPSv3 uses the ublox CAM-M8 GPS module. This module does support assisted GPS, however it can only be done with a uBlox GSM module.. they speak to each other via i2c.

      We will soon release a product which will have these two on the same PCB. Getting a GPS will only take a few seconds.

  1. Hi Mark,

    I hope you are still receiving comments. 🙂 So, I leave my Android smart phone on all the time and GPS is enabled all the time. So I guess my GPS starts would be warm or hot. That being said, over time, it seemed to take Google Maps MUCH longer to acquire a GPS signal than it did when I first bought the phone. I did not change my GPS setting in any way that I'm aware - the GPS acquisition simply got slower as time continued.

    My question is this: Would getting a new smart phone help in this matter? Does the age of a smart phone somehow factor into GPS acquisition performance?

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