Apr 21, 2024 22:37:00

GPS signals are extremely weak

Satellite positioning systems such as GPS are used in a variety of fields, including map apps, car navigation systems, and photo location recording. Philip Tennen, creator of the GPS-based positioning app '

Gypsum ,' explains how GPS signals work.

Building a GPS Receiver, Part 1: Hearing Whispers | Phillip Tennen
https://axleos.com/building-a-gps-receiver-part-1-hearing-whispers/

GPS satellites transmit 'precise time' and 'satellite position' to Earth. There are more than 30 GPS satellites in operation , and accurate position information can be calculated by receiving signals from four of them.

GPS satellites emit extremely weak radio waves, with the signal strength reaching a receiver dropping to minus 130 dBm . Tennen likens the difficulty of receiving a GPS signal to 'watch a light bulb 20,000 km away go on and off a million times per second.'

In addition, there are many different types of radio waves flying around the Earth, and there is a lot of noise that is stronger than the radio waves emitted by GPS satellites. Commercially available smartphones and car navigation systems must find the GPS signal from the large amount of noise to calculate location information.

Each GPS satellite emits a unique signal called a 'PRN code' 1,000 times per second. A GPS receiver can identify which GPS the signal is coming from by reading the PRN code.

Because GPS satellites move at high speeds in orbit, the signals they transmit are subject to the

Doppler effect, so GPS receivers must take the Doppler effect into account when comparing the PRN code information stored in the receiver with the information in the received radio waves.

Using the information he has gathered about GPS, Tennen has developed an app called 'Gypsum' that enables software-defined radio receivers to perform positioning using GPS signals. Gypsum is available at the following link:

GitHub - codyd51/gypsum: GPS receiver from a raw antenna 🛰️
https://github.com/codyd51/gypsum