A GNSS signal is not a single, simple broadcast. It’s a layered composition, meticulously designed to travel over 20,000 km from space, penetrate our atmosphere, and deliver precise timing information. At its core, every signal consists of three key components stacked on top of each other.

1. Carrier Wave: A pure, high-frequency radio wave. Think of it as the delivery truck. GPS uses specific frequencies in the L-band, such as L1 (1575.42 MHz), L2 (1227.60 MHz), and L5 (1176.45 MHz). The wave itself doesn't carry information, but it's the medium upon which everything else is encoded.
2. Ranging Code: A complex sequence of 1s and 0s called a Pseudo-Random Noise (PRN) code. Each satellite has its own unique PRN, allowing a receiver to distinguish which satellite it's listening to. This is how multiple satellites can broadcast on the same frequency without interfering with each other, a method called Code Division Multiple Access (CDMA).
3. Navigation Message: This is the actual data, a relatively slow 50 bits-per-second stream containing the satellite's health, orbital position (ephemeris), and timing information, including corrections for atmospheric delays. Your receiver needs this data to calculate its position.

For decades, the workhorse of GPS was the L1 C/A (Coarse/Acquisition) signal. It was revolutionary for its time but has limitations. Its structure makes it susceptible to multipath error, where the signal bounces off buildings or terrain before reaching the receiver, confusing the timing calculation. It also uses a relatively simple modulation scheme called Binary Phase-Shift Keying (BPSK), which isn't as robust as modern alternatives.

To address these issues, GPS has been modernized with new signals. The L2 frequency, once reserved for military use, now carries a civilian signal (L2C). The L5 band was added specifically for safety-of-life applications like aviation. These modern signals offer several key advantages:

• Higher Power: They are broadcast with more strength, making them easier to acquire, especially under tree cover or in urban canyons.
• More Complex Codes: The PRN codes on L2C and L5 are much longer and more complex than the L1 C/A code. This makes them far more resistant to multipath interference.
• Data-Free Channel: L2C includes a pilot channel that carries no navigation data, only the ranging code. This allows the receiver to lock onto the signal much more robustly, even if the data message is corrupted.

The most significant evolution, however, is in the modulation. Instead of the simple BPSK, modern signals like L1C (the successor to L1 C/A) and L5 use a scheme called Binary Offset Carrier (BOC).

The magic of CDMA is the PRN code. These aren't just random numbers; they are carefully constructed sequences with a special property: they look like random noise, but they are perfectly deterministic. Specifically, GPS uses Gold codes for its C/A signals.

While most GNSS constellations, including modern GPS, Galileo, and BeiDou, use CDMA, Russia's GLONASS system traditionally used a different approach: Frequency Division Multiple Access (FDMA). Instead of each satellite having a unique code, each GLONASS satellite broadcasted the same code but on a slightly different frequency. While effective, this required receivers to have a wider-band front end to listen to all possible frequencies. More recent GLONASS satellites have begun incorporating CDMA signals to improve interoperability with other GNSS systems.

By layering these components—a carrier wave, a unique PRN code, and a navigation message—and broadcasting them on multiple frequencies with advanced modulation, modern GNSS provides a signal that is robust, reliable, and incredibly precise.