Basic Technology: "What is Timer Data in GPS?" | NEXTY Electronics, an electronic components and semiconductor trading company

Ephemeris data

GPS time timer data

The Global Positioning System (GPS), which is now installed in smartphones that we all use, is not just used for position measurement, but is also used for precise time synchronization and timing control. However, there is a key data that is used to calculate the precise position and time of GPS.
This technical column explains from a technical perspective the role and structure of time-delay timer data in GPS, and how it supports highly accurate positioning.

The role of GPS and ephemeris data

Basic structure of GNSS (Global Navigation Satellite System)

GNSS is an abbreviation for "Global Navigation Satellite System" and is a general term for positioning satellite systems operating worldwide.
The USA's GPS (Global Positioning System) is generally well known, but other countries also operate various satellites.

For more details, please refer to the following technical column: GNSS (Global Navigation Satellite System) Technical Column: What is GNSS? How is it different from GPS? Explanation of satellites operated by countries around the world

What is ephemeris data? (satellite orbital information and clock corrections)

Ephemeris data is important information that includes the precise positions of GPS satellites (orbital information) and satellite clock correction values (clock correction values).
As part of the navigation message received from GPS satellites, the receiver uses this ephemeris data to calculate its own position based on the signal arrival time from the satellite.

Orbit Parameters

It contains orbital elements and correction parameters to represent the position of each satellite as a function of time.
This allows the receiver to calculate the coordinates of the satellite at any time.
You can easily check the position and launch information of each satellite at your current location using the smartphone app "GNSS View."

Smartphone app "GNSS View"

Clock Correction

Although atomic clocks onboard satellites are highly accurate, small errors accumulate.
The ephemeris contains coefficients (bias, drift, etc.) to correct for this error.

Ephemeris Data: What the 2-Hour Expiry Means

How the receiver uses ephemeris to calculate position

There are three main points to the mechanism for calculating position.

Basic Principles: Trilateration for Positioning

GPS positioning is performed by measuring the time it takes for signals to arrive from multiple satellites and calculating distance.
This distance is called "pseudo range" and requires at least four satellites (3D positioning + clock correction).

The role of ephemeris data

Ephemeris data includes the precise position of each satellite (orbital information) and satellite clock corrections (clock corrections).
The receiver uses this to calculate the satellite's coordinates at the time the signal was transmitted.

Orbital information: Six orbital elements plus correction parameters determine the satellite's position as a function of time.
Clock correction: A coefficient used to correct for satellite clock errors.

Basic information flow and processing steps for position calculation

Receiving positioning signals from 4 or more GPS satellites.

Step 1: Measure the distance to each GPS satellite based on the receiver's built-in clock.
Step 2: Create navigation message and extract ephemeris data.
Step 3: Extract error information from the atomic clocks of GPS satellites.
Step 4: Calculate the measured distance, correct for satellite clock error, and calculate the position of each GPS satellite.
Step 5: Calculate and determine the position (latitude, longitude, altitude).
Step 6: Calculate and determine the time.

Ephemeris data: Impact of expiration

Impact of ephemeris data expiration

There are two major impacts.

Decreased location accuracy

Cause: Ephemeris data is a set of satellite orbit and clock corrections that are valid for a short period of time (usually 2 hours). Once it expires, errors occur in the satellite position calculations.

Impact:

An error of a few meters can grow to tens of meters or more.
This can be a fatal error, especially when traveling at high speeds or performing precise positioning (surveying, agriculture, drones).

Delay in reacquiring satellite constellation (increased TTFF)

Cause: The old ephemeris cannot predict the correct satellite position, so the receiver must receive the navigation message again to obtain the latest ephemeris.

Impact:

Calculating the first location (fix): Time To First Fix (TTFF) has increased significantly (from a few seconds to over 30 seconds).
If you are not using A-GPS or self-ephemeris, it may take more than a minute to reacquire the satellite signal.

Summary

Design constraints and innovations brought about by the two-hour expiration period

As explained above, positioning accuracy can be ensured by using the latest ephemeris data.
However, if the latest ephemeris data is not available to the receiver, its behavior varies from manufacturer to manufacturer.
For example, some receivers have the ability to calculate the latest ephemeris data from existing data.
Using this function increases the calculation accuracy and power consumption of the receiver, so it is necessary to carefully consider whether or not to use it in the customer's system.
Therefore, we recommend that you check in advance the design policies of each receiver manufacturer and the positioning specifications under various conditions.