GNSS Fundamentals and Satellite Constellation Overview

What Is GNSS?

Global Navigation Satellite System (GNSS) is the umbrella term for any satellite-based positioning system. The United States GPS (Global Positioning System) is the oldest and most widely used, but three other fully operational constellations now serve the industry: Russia's GLONASS, the European Union's Galileo, and China's BeiDou (also called BDS). Modern survey-grade and drone-grade receivers track signals from multiple constellations simultaneously, which increases the number of visible satellites at any moment and improves both accuracy and reliability.

The Four Major Constellations

GPS is operated by the U.S. Department of Defense. The constellation is designed to maintain at least 24 operational satellites in medium Earth orbit (MEO) at roughly 20,200 km altitude, arranged in six orbital planes. GPS broadcasts on the L1 (1575.42 MHz), L2 (1227.60 MHz), and L5 (1176.45 MHz) frequency bands.

GLONASS is operated by the Russian Aerospace Defence Forces. It also uses MEO satellites at approximately 19,100 km altitude across three orbital planes. GLONASS uses a frequency-division multiple access (FDMA) scheme, meaning each satellite transmits on a slightly different frequency, unlike GPS which uses code-division multiple access (CDMA). GLONASS signals occupy bands near L1 and L2 frequencies.

Galileo is operated by the European Union Agency for the Space Programme (EUSPA). Designed from the start as a civilian system, Galileo broadcasts on E1, E5a, E5b, and E6 bands. The E5a band overlaps with GPS L5 (1176.45 MHz), allowing receivers to combine signals for improved ionospheric correction.

BeiDou (BDS) is operated by China and uses a mixed constellation of MEO, geostationary (GEO), and inclined geosynchronous (IGSO) satellites, giving it particularly strong coverage over the Asia-Pacific region. BeiDou signals are broadcast on B1, B2, and B3 frequency bands.

Why Multiple Constellations Matter

A receiver needs signals from at least four satellites to solve for three-dimensional position (latitude, longitude, and ellipsoidal height) plus receiver clock error. Tracking more constellations means more satellites in view at all times, which is critical in urban canyons, dense forest, or when working near tall structures where some sky is blocked. More visible satellites also improve satellite geometry.

Frequency Bands and Why They Matter

GNSS receivers that track only one frequency (single-frequency, L1-only) cannot separate ionospheric delay from position error. Dual-frequency receivers (L1/L2 or L1/L5) can mathematically model and remove most of the ionospheric delay by comparing how the two signals travel differently through the ionosphere. Triple-frequency receivers add a third measurement, further improving ambiguity resolution speed and accuracy. For survey-grade work and precision drone mapping, dual- or multi-frequency receivers are the professional standard.

Error Sources in GNSS

Four main error sources affect every GNSS measurement:

1. Ionospheric delay — The ionosphere (roughly 60–1,000 km altitude) slows the signal. The effect varies with solar activity and time of day. Dual-frequency receivers correct for most of this error.

2. Tropospheric delay — The lower atmosphere (below about 50 km) also slows signals. Models and pressure/temperature sensors help reduce this error.

3. Multipath — Signals reflecting off buildings, vehicles, water, or terrain arrive at the receiver via indirect paths, corrupting the measurement. Antenna placement and choke-ring antennas reduce multipath.

4. Satellite clock and ephemeris errors — Broadcast corrections and augmentation systems (WAAS, CORS networks, RTK) reduce these to centimeter or sub-centimeter levels.

Safety note: When setting up GNSS equipment in the field, assess the site for traffic hazards, unstable terrain, and overhead power lines before placing tripods or range poles. Never set up a tall tripod or range pole near overhead power lines. If lightning threatens, cease operations, lower all poles and antennas, and move to shelter immediately — metal tripods and poles are conductive. Follow your program's PPE requirements for outdoor fieldwork including high-visibility vests near roadways.

Satellite Geometry and DOP Values

Having many satellites visible is not enough on its own — their positions in the sky matter too. Dilution of Precision (DOP) is a dimensionless number that describes how satellite geometry amplifies or reduces ranging errors. A lower DOP means better geometry and smaller position errors; a higher DOP means poor geometry and larger errors.

Common DOP components:

• GDOP (Geometric DOP) — overall 3D position plus time
• PDOP (Position DOP) — three-dimensional position only; the most commonly cited value for surveyors
• HDOP (Horizontal DOP) — horizontal position
• VDOP (Vertical DOP) — vertical (height) accuracy
• TDOP (Time DOP) — receiver clock solution

As a practical rule of thumb widely cited in manufacturer documentation and NGS guidance, a PDOP of 4 or less is generally acceptable for survey work; PDOP above 6 is considered poor and observations should be delayed if possible. The relationship between DOP and position error is:

Position Error ≈ DOP × User Equivalent Range Error (UERE)

If your receiver's ranging error (UERE) is 1 m and PDOP = 2, your expected position error is roughly 2 m. If PDOP rises to 6, the expected error rises to roughly 6 m — using the same equipment under the same sky.

Planning GNSS Observations

Before collecting data, especially for static control surveys, professionals use GNSS planning software to predict satellite availability and DOP values for a specific location, date, and time window. You input the site's approximate coordinates and an elevation mask angle (the minimum sky angle above which satellites are tracked — commonly 10° to 15° in open terrain to exclude low-quality, high-delay signals). The software outputs a graph of PDOP over time and a sky plot showing satellite positions. You then select the observation window when PDOP is lowest and the most satellites are visible across all constellations. This planning step directly affects data quality and is a standard professional practice.