GPS/GNSS Fundamentals

What are the three GPS carrier frequencies and their wavelengths?

L1: 1575.42 MHz (~19 cm) L2: 1227.60 MHz (~24 cm) L5: 1176.45 MHz (~25 cm)

What is pseudorange?

An approximate distance from satellite to receiver calculated by measuring signal travel time. Called 'pseudo' because it includes clock biases and atmospheric delays.

What is carrier phase measurement?

A GPS observable measuring the number of carrier wave cycles between satellite and receiver. Provides millimeter-level precision but requires integer ambiguity resolution.

What is the C/A code?

Coarse/Acquisition code modulated on L1. Chipping rate: 1.023 MHz. Code length: 1 millisecond. Publicly available for civilian use.

What is the P code?

Precision code modulated on L1 and L2. Chipping rate: 10.23 MHz. Now encrypted as Y-code for military use.

What are the three GPS segments?

1. Space Segment: Satellite constellation 2. Control Segment: Ground monitoring and upload stations 3. User Segment: All GPS receivers and applications

What is a PRN code?

Pseudorandom Noise code - a binary sequence that appears random but is deterministically generated. Each satellite has a unique PRN for identification.

What is the GPS navigation message?

Data broadcast at 50 bps containing: • Satellite ephemeris • Clock corrections • Ionospheric parameters • Almanac for all satellites

What is the GPS satellite constellation?

Nominally 24 satellites in 6 orbital planes at 20,200 km altitude with 55° inclination. Current constellation has 31+ operational satellites.

What is GPS Time?

Atomic time standard running at constant rate without leap seconds. Started January 6, 1980. Currently differs from UTC by 18 seconds.

What is the difference between broadcast and precise ephemeris?

Broadcast: Real-time, 2-4 hour validity, 1-2 meter accuracy Precise: Post-processed, from IGS, centimeter accuracy, available 12-18 days later

What is the IGS?

International GNSS Service - a voluntary federation providing high-quality GNSS data products including precise orbits, clocks, and atmospheric parameters.

What is GLONASS?

Russian Global Navigation Satellite System using FDMA (Frequency Division Multiple Access). Combined GPS/GLONASS increases satellite availability.

What is Galileo?

European Union's global navigation satellite system, designed for civilian control with multiple frequency signals compatible with GPS.

What is BeiDou?

Chinese navigation satellite system providing global coverage with multiple frequency signals.

What is Multi-GNSS?

Simultaneous use of multiple satellite systems (GPS, GLONASS, Galileo, BeiDou) for improved accuracy, reliability, and availability.

What is SBAS?

Satellite-Based Augmentation System using geostationary satellites to broadcast GPS corrections. Examples: WAAS, EGNOS, MSAS.

What is WAAS?

Wide Area Augmentation System - FAA's SBAS providing 1-3 meter accuracy for aviation navigation in North America.

What is L2C?

A modernized civil signal on L2 frequency providing improved tracking without requiring access to encrypted P(Y) code.

What is the difference between L5 and other frequencies?

L5 (1176.45 MHz) is designed for safety-of-life applications with improved signal structure for better accuracy and interference resistance.

What is a GPS Week?

Primary GPS time unit starting midnight UTC January 5-6, 1980. Contains 604,800 seconds. Week numbers roll over every 1024 weeks.

What information is in the almanac?

Reduced-accuracy orbital parameters for all GPS satellites, valid for months. Used for satellite acquisition and mission planning.

What is the code phase?

The timing offset of a received PRN code relative to a receiver-generated replica, used to calculate pseudorange.

What is wavelength in GPS context?

Distance traveled by one complete carrier wave cycle. L1 ≈ 19 cm, L2 ≈ 24 cm. Carrier phase counts wavelengths for precise positioning.

What are tracking channels in a GPS receiver?

Independent signal processing units. Modern receivers have 200+ channels to simultaneously track multiple satellites on multiple frequencies.

What is the function of a correlator in GPS?

Electronic component comparing received PRN code with receiver-generated replica to determine code phase and carrier phase measurements.

What is an elevation mask?

Minimum satellite elevation angle for data collection, typically 10-15°. Excludes low-elevation signals affected by atmospheric delay and multipath.

What is data rate in GPS?

Frequency of GPS observations. 1 Hz for static surveys, up to 20 Hz for kinematic. Higher rates capture motion but increase data volume.

What is a choke ring antenna?

GPS antenna with concentric conducting rings that reduce multipath by attenuating low-elevation signals. Preferred for precise applications.

What is a dual-frequency receiver?

GPS receiver tracking signals on two or more frequencies (L1, L2, L5), enabling ionospheric correction and faster ambiguity resolution.

What is Signal-to-Noise Ratio (SNR)?

Measure of GPS signal strength relative to background noise. Higher SNR enables more precise measurements and reduces cycle slips.

What is selective availability?

Intentional degradation of GPS accuracy (discontinued May 1, 2000) that previously limited civilian accuracy to ~100 meters.

What is Anti-Spoofing (A-S)?

Encryption of P code into Y code to prevent hostile generation of false GPS signals. Still active.

What is the purpose of triple-frequency observations?

Improved ambiguity resolution and atmospheric modeling. Available with L5 modernization on GPS and other GNSS.