Introduction
When a GNSS satellite broadcasts its signal, it's not just sending a simple "I am here" message. The signal is a carefully constructed blend of different components, each serving a specific purpose.
The Three Layers of a GNSS Signal
1. The Carrier Wave
The radio frequency that "carries" the information:
| Band | Frequency | Used By | Wavelength |
|---|---|---|---|
| L1 | ~1575.42 MHz | All systems | 19 cm |
| L2 | ~1227.60 MHz | GPS, GLONASS, BeiDou | 24 cm |
| L5 | ~1176.45 MHz | GPS, Galileo, BeiDou | 25 cm |
Why multiple frequencies?
- Compare them to measure ionospheric delay
- Redundancy if one frequency is jammed
2. The Ranging Code
A pseudo-random noise pattern that:
- Identifies the satellite (each has unique code)
- Enables distance measurement (receiver aligns its copy with received code)
Code types:
- C/A Code: Civilian use, 1,023 bits, repeats every millisecond
- P(Y) Code: Military, encrypted
- L2C, L5: Modern civilian codes, better performance
3. The Navigation Message
The actual data containing:
- Ephemeris: Precise satellite position (updated every 2 hours)
- Almanac: Approximate positions of all satellites
- Clock corrections: Adjusting for satellite clock drift
- Health status: Is the satellite working?
- Ionospheric model: For correcting atmospheric delay
Data Rate: GPS transmits at 50 bits per second, downloading the full message takes 12–30 minutes.
Modernized Signals
| Signal | Improvement |
|---|---|
| L2C | Better civilian signal on L2 |
| L5 | Higher power, longer code, safety-of-life |
| L1C | Interoperable with Galileo |
| E6 | Commercial service with authentication |
Vital Points
- GNSS signals have three layers: Carrier, code, navigation message
- Ranging code enables satellite ID and distance measurement
- Multiple frequencies allow ionosphere correction
- Modern signals offer better performance through improved design