Introduction
Every GNSS receiver - from a smartphone chip to a survey-grade geodetic instrument - performs the same fundamental task: extracting timing information from satellite signals that arrive at power levels 20 dB below the thermal noise floor. Understanding how satellite signals are structured and modulated is essential for engineers who design, select, or troubleshoot GNSS equipment. This lesson examines the signal architecture that makes this extraordinary extraction possible.
Carrier, Code, and Navigation Message
Every GNSS signal consists of three layered components:
- Carrier wave: A continuous sinusoidal radio frequency carrier, typically in the L-band (1 to 2 GHz). The carrier frequency determines the wavelength, which sets the ultimate precision limit of carrier-phase measurements.
- PRN code: A binary pseudo-random noise sequence, chip-by-chip XOR-modulated onto the carrier. Different satellites use different PRN codes, selected for their cross-correlation properties - codes from different satellites produce nearly zero correlation when compared against each other, allowing a receiver to track multiple satellites simultaneously on the same frequency.
- Navigation message: A slow data stream (50 bps for GPS L1 C/A) modulated onto the code, carrying satellite ephemeris, clock corrections, health flags, and ionospheric parameters.
Modulation: BPSK and BOC
Two primary modulation schemes are used across GNSS signals:
| Modulation | Full Name | Used By | Characteristic |
|---|---|---|---|
| BPSK | Binary Phase Shift Keying | GPS L1 C/A, GLONASS L1 | Simple, single spectral peak, established heritage |
| BOC(1,1) | Binary Offset Carrier | Galileo E1 OS, GPS L1C | Split spectrum, sharper correlation peak, less multipath |
| MBOC | Multiplexed BOC | GPS L1C, Galileo E1 OS | Combination of BOC(1,1) and BOC(6,1) for improved tracking |
| BPSK(10) | BPSK at 10.23 Mcps | GPS L5, Galileo E5a/E5b | Wider bandwidth, sharper code correlator, lower multipath |
BOC modulation splits the signal spectrum into two lobes centred above and below the nominal carrier frequency. This creates a sharper correlation peak - narrowing the pseudorange ambiguity region and reducing the effect of reflected multipath signals, which tend to corrupt the correlation peak near its centre.
Key Signal Parameters by Constellation
| Signal | Frequency (MHz) | Code Rate (Mcps) | Modulation | Code Length (chips) |
|---|---|---|---|---|
| GPS L1 C/A | 1575.42 | 1.023 | BPSK(1) | 1,023 |
| GPS L5 | 1176.45 | 10.23 | BPSK(10) | 10,230 |
| Galileo E1 OS | 1575.42 | 1.023 | CBOC(6,1,1/11) | 4,092 |
| Galileo E5a | 1176.45 | 10.23 | BPSK(10) | 10,230 |
| GLONASS L1 (FDMA) | 1602 + k x 0.5625 | 0.511 | BPSK(0.5) | 511 |
| BeiDou B1C | 1575.42 | 1.023 | BOC(1,1) + BOC(6,1) | 10,230 |
The Navigation Message
The navigation message carries the data a receiver needs to compute satellite positions and correct for satellite clock errors:
- Ephemeris: Keplerian orbital elements and correction terms allowing computation of the satellite position to within centimetres at any moment during the validity period (typically 2 hours for GPS).
- Clock parameters: Second-order polynomial coefficients describing satellite clock offset, drift, and drift rate relative to GPS system time.
- Almanac: Coarse orbital data for all satellites in the constellation, used to predict which satellites are visible from a given location and speed up initial acquisition.
- Ionospheric model: Klobuchar model parameters (GPS) or NeQuick parameters (Galileo) for single-frequency ionospheric correction.
- Health flags: Status bits indicating whether a satellite signals should be considered for use.
Why Signal Structure Matters for Precision
The code chip width determines the fundamental resolution of pseudorange measurements. GPS L1 C/A chips are 977 ns wide, corresponding to a physical length of approximately 293 metres. Code tracking can resolve to approximately 1% of a chip width - about 3 metres. GPS L5 chips are 10 times narrower, enabling proportionally better code-phase resolution of roughly 30 cm. Carrier-phase measurements exploit the much shorter wavelength of the carrier itself: 19 cm for L1, 25 cm for L2, 19 cm for L5 - but require solving the integer ambiguity before this precision is accessible.
Summary
GNSS signal structure is a carefully engineered solution to the problem of measuring centimetre-level distances across 20,000 km using signals that arrive weaker than radio noise. PRN codes enable simultaneous multi-satellite access on shared frequencies. BOC modulation reduces multipath sensitivity. Wider-bandwidth signals on L5 provide better pseudorange resolution. Understanding these foundations is prerequisite for the acquisition, tracking, and filtering topics covered in the following lessons.