Introduction to GPS for Water Industry

Hello, welcome to Section 2 on G-P-S Basics… <break time="2000ms"/>.

A long row of satellite dishes in the sunset. Basics and history of the Global Positioning System (GPS) Differences between GPS and Global Navigation Satellite Systems (GNSS) Key satellite generations and their features How GPS works including trilateration GPS accuracy Video lesson Class activity related to GPS Quiz.

Global Positioning System (GPS). A U.S. satellite navigation system Launched in 1978, operational by 1980 Has 31 active satellites (as of this writing) 12,500 miles above the earth Satellites orbit the earth twice daily i.e. pass over approximately the same world location every 12 hours 6 orbits (4 to 6 satellites per orbit) Diagram not true to scale.

Global Positioning System (GPS). Diagram true to scale GPS receivers read radio signals from orbiting satellites to calculate the exact spot of the receiver on the Earth by comparing the time taken by signals from 4 different GPS satellites to reach the receiver..

GPS vs. GNSS. Global Positioning System (GPS) devices use only the U.S. satellite system. Global Navigation Satellite System (GNSS) devices can use multiple worldwide satellite constellations for improved accuracy, reliability, and coverage. GPS (U.S.) – 31 satellites GLONASS (Russia) – not the same as GNSS – 24 satellites Galileo (EU) – 24 satellites BeiDou (China) – 35 satellites Total = 114 Misconception: Just like people often say “Xerox” for any photocopy or “Google” for any web search, “GPS” has become the default term for all satellite-based navigation, even when the device is technically using GNSS..

Block II-F Satellite. Bridge the gap between previous Navstar GPS generations until the GPS Block III satellites became operational. Built by Boeing for the US Air Force Features Operational = 11 Last launched = 2016 Design life = 12 years Altitude = 12,710 mile Orbital period = 12 hours height = 2.51 m width = 2.06 m depth = 1.8 m Weight = 1,633 kg Artist’s conception of a GPS Block II-F satellite in Earth orbit (public domain from NASA).

GPS III Satellite Series. [image]. Manufacturer: Lockheed Martin 10 satellites SV01 (2018) to SV10 Most recent SV07 (Dec 2024) 3X more accurate (5-10 m -> 1-3 m) Up to 8X improved anti-jamming capabilities.

Satellite Launch. Falcon 9 rocket launching the first GPS III satellite, December 2018 Photo courtesy of SpaceX (founded by Elon Musk in 2002).

GPS IIIF Satellite. Lockheed Martin GPS IIIF. Manufacturer: Lockheed Martin To be launched in 2026. 60X greater anti-jamming to ensure U.S. and allied forces cannot be denied access to GPS in hostile environments.

How GPS Works. A GPS receiver calculates its position using trilateration, comparing signals received from at least 4 satellites. It determines latitude, longitude, elevation, and time with increasing accuracy 1-5 meter horizontal on smartphones centimeter-level horizontal and vertical with survey-grade GPS equipment..

Class Activity. <break time="2000ms"/><break time="3000ms"/> Hello, This is your Teaching Assistant nora. Let’s take a short break for a quick class activity. Please open Google Chrome—or any other browser you prefer—and search for the difference between triangulation and trilateration. You’ll have 15 minutes for this activity. Once you're done, we’ll regroup and discuss what you found. Take your time and have fun exploring. <break time="2000ms"/><break time="3000ms"/>.

Class Activity: Results. Conventional (theodolite, total station) survey uses triangulation Measures angles GPS uses trilateration Measures distances Misconception: GPS uses triangulation..

The first satellite locates you somewhere on a sphere (top left). The 2nd satellite narrows your location to an area created by the intersection of the two satellite spheres (top right). The 3rd satellite reduces the choice to two possible points (bottom left). Finally, the 4th satellite helps calculate a timing and location correction and selects one of the remaining two points as your position (bottom right)..

How GPS Determines Position. [image] Graphical user interface AI generated content may be incorrect.

How GPS Determines Position. [image] Graphical user interface AI generated content may be incorrect.

How GPS Determines Position. [image] Graphical user interface AI generated content may be incorrect.

How GPS Determines Position. [image] Graphical user interface AI generated content may be incorrect.

How GPS Determines Position. [image] Graphical user interface AI generated content may be incorrect.

How GPS Determines Position. [image] Graphical user interface AI generated content may be incorrect.

Distance = speed x time = c x T where, c = speed of light = 299,792 km/second (670,616,629 miles/hour) T = signal travel time = t2-t1 t1 = time signal sent by satellite t2 = time signal received by GPS device.

Video Lesson 1. Title: How GPS Works Today (an animated introduction) Older Video (2019) Date published: June 17, 2019 Last played: April 30, 2025 Video link: https://www.youtube.com/watch?v=wCcARVbL_Dk or search YouTube by title (How GPS works today) Video content: An educational animated video of the history of GPS technology, how it works and is used today Video Producer: Bright Side on YouTube Video duration: 10:01 minutes.

Video Lesson 1. Presentation with media with solid fill.

Video Lesson 2. Title: How Does GPS Work? New Video (2025) Date published: Feb 22, 2025 (clipped from a 2022 BBC series Last played: April 30, 2025 Video link: https://www.youtube.com/watch?v=KItEcCJfHAs or search by title (How does GPS work?) Video content: An educational video of how GPS works. Video Producer: BBC Earth Science Video duration: 3:11 minutes.

Video Lesson 2. Presentation with media with solid fill.

GPS Limitations and Errors. GPS signals do not penetrate solid objects well → works best outdoors. Requires line-of-sight to multiple satellites. Signal obstructions: mountains, trees, buildings Error sources: Multipath errors (next slide). Atmospheric interference (e.g., ionospheric storms) Satellite Clock Errors: Small timing errors cause big position shifts. Orbital Errors: Slight inaccuracies in satellite path affect calculations. Receiver Noise: Internal device noise adds small errors. Ideal operating conditions: when above limitations and errors are minimum or absent. Rain, snow, clouds, and fog do not significantly affect GPS signals..

Multipath Error. Multipath is the interference caused when GPS signals reflect off nearby surfaces and arrive at the receiver with a time delay, creating position errors. Buildings, glass, metal, water A signal bounced off a building may arrive microseconds later→ creates false position shift by several meters. Affect both accuracy and precision of GPS data (next slide) Mitigation Techniques Use longer occupation time to average out errors High-quality GNSS receivers often include multipath mitigation technology..

Position Dilution of Precision (PDOP). A measure of best geometrical configuration of satellites Low PDOP = Satellites well spread out → High accuracy High PDOP = Satellites close together → Lower accuracy Use GNSS planning tools to identify times with low PDOP before field data collection..

Accuracy vs Precision. Misconception: often, the two words are mistaken as synonymous – but they are not. Accuracy How close a measured position is to the true location Example: Hitting the bullseye on a target Precision (reproducibility or repeatability) How consistent repeated measurements are Example: Hitting the same spot repeatedly, even if it's not the bullseye Key Differences Accuracy = correctness Precision = consistency You can have one without the other In GPS Surveys: High precision + low accuracy = consistently wrong location GPS corrections improve both (slide later) Note: Some GPS equipment specs incorrectly use precision for accuracy (e.g., precision 1-2 cm)..

GPS Accuracy. Raw GPS signal: Horizontal: 3-10 m (4 m 95% confidence interval) Vertical: generally, 2x horizontal Processed GPS signal: millimeters Varies with GPS equipment Occupation time (how long at each point?) Location: vegetation, obstructions (tall buildings), reflection Cost increases with accuracy Desired accuracy depends on the project-specific requirements.

Selective Availability (SA). SA was an intentional random signal distortion introduced by the US Department of Defense to prevent misuse of GPS by hostile forces. Midnight May 2, 2000: Selective Availability (SA) error was removed six years ahead of schedule by a Bill Clinton Presidential Order Corrections were available to hostile forces anyway (next slide). Increased GPS accuracy up to 10×overnight (from ~100 m to ~10 m). Inexpensive receivers became far more accurate. Autonomous GPS (no base station) became practical for many civilian uses. Survey-grade receivers saw little impact (they already used differential corrections). Why It Matters: Marked a turning point for civilian GPS adoption. Enabled widespread use in transportation, utilities, agriculture, mobile devices. Laid the foundation for modern GPS-enabled technologies we rely on today..

[image]. [image]. [image] Create a humorous meme style image that captures the feeling of sitting through a boring PowerPoint presentation specifically in the context of technical topics like GPS basics Show an audience of people in a classroom or meeting room some looking sleepy distracted or overwhelmed by complex slides filled with satellite diagrams equations and technical jargon Include a large overly detailed slide projected at the front with confusing GPS terms and satellite images while a presenter enthusiastically points at the screen Use a relatable lighthearted caption such as When the PowerPoint has more satellites than your attention span in a bold easy to read font The overall mood should be playful and relatable with a clean modern style and colors that match typical corporate or educational settings.

Utility GPS Accuracy Standards. Centimeter to sub-meter accuracy depending on application Centimeter-level Elevations are important manhole invert elevation sewer overflow weir elevation pipe slope PDOP ≤ 2 Survey-grade RTK GPS equipment Sub-meter Locations (coordinates) are important manhole, pump, valve and hydrant location asset inventory Mapping-grade GPS equipment PDOP ≤ 4.

Vertical Accuracy. In water and wastewater, elevations are important. Therefore, both horizontal and vertical accuracy is important. In sewer system H&H modeling (SWMM), node coordinates do not affect the results (depth, velocity, flow), but elevations do. Outfall overflows can change substantially, with overflow weir elevation difference of few inches. Measured or calculated from rim elevation (GPS) Higher the overflow elevation, lower the overflow peak, volume, duration, and frequency. Vertical accuracy is more important than horizontal accuracy. Need survey grade GPS equipment..

PCSWMM Model Example (Pittsburgh, 2025). A_18-0F 0.10 015 0.25 12PM 2 Thu.

GPS Corrections. Some GPS applications (e.g., surveying) require more precise and detailed location information. GPS corrections are needed because raw GPS signals are not perfectly accurate on their own. Why are GPS corrections needed? Non-ideal conditions (previous slide on GPS limitations) Without corrections, GPS accuracy can drift up to 10 meters or more. Corrections (code or carrier-based) reduce those errors, often down to centimeter-level accuracy..

GPS Correction Types. Aspect DGPS RTK Name Differential GPS Real-Time Kinematic Correction Type Code-based Carrier-based (wave phase) (much finer) Frequency Lower frequency, less precise High frequency, very precise Accuracy Sub-meter (0.5–1 m) Centimeter-level (1–2 cm) Base Station Yes (or public service like WAAS) Yes (or via network like NTRIP) Real-time Option Yes Yes Post-processing Common Available (as PPK) Applications Navigation, mapping, agriculture Surveying, engineering, construction Summary moderate accuracy, simpler setup, cheaper higher accuracy, more complex, more expensive.

Correction Methods for Real-Time GPS Accuracy. SBAS (Satellite-Based Augmentation System) 📡 NTRIP (Networked Transport of RTCM via Internet Protocol) 🌐 Free code-based corrections broadcasted by regional satellites directly to receiver. US: Wide Area Augmentation System (WAAS, FAA). EU: European Geostationary Navigation Overlay Service (EGNOS) Cellular delivery of RTK corrections No radio, no cellular Internet / cellular Rover connects to a correction service from a reference network (e.g., CORS) Replaces radio link, more flexible. Limitation: Must have cellular signals Accuracy (limited to horizontal): 1–2 m (WAAS) 1-3 m (EGNOS) Accuracy (H&V): 1–2 cm.

Continuously Operating Reference Station (CORS). It's a permanent GPS base station with a precisely known location that: Tracks satellite signals 24/7 Provides high-quality correction data for GPS users Supports post-processing and real-time (via NTRIP) applications Operated by government agencies, universities, and GPS manufacturers Provides cm-level accuracy without requiring a user base station. Public domain CORS services: Examples: NOAA’s National CORS Network (NCN), state DOTs Post-processed = Free and open Real-time = sometimes Free, depends on provider Commercial CORS services Free trial, monthly ($50-200) or annual ($600-2,000) subscriptions Stream RTK-quality corrections via NTRIP You don’t need to set up a base — just a rover with internet (usually cellular). Trimble: VRS Now Topcon: TopNETlive Leica: SmartNet.

Virtual Reference Station (VRS). A synthetic base station created near your GPS receiver using data from multiple real base stations in a CORS network. Eliminates need for a physical local user base station Provides RTK-level accuracy (1–2 cm) Reduces baseline errors by simulating a “nearby” base, even if you're miles away. Works via cellular connection (typically through NTRIP protocol) Applications: High-accuracy GPS work Utility RTK surveys Construction layout Engineering.

Quiz Time. 2. GPS Basics. Large group of pencils in a neat circle with quiz message in the centre.

Quiz: Section 2. 1) To function properly, a GPS receiver must receive signals from at least: A) 1 satellite B) 2 satellites C) 3 satellites D) 4 satellites.

Quiz: Section 2. 1) To function properly, a GPS receiver must receive signals from at least: A) 1 satellite B) 2 satellites C) 3 satellites D) 4 satellites.

Quiz: Section 2. 2) GPS measurements yield only a position, and neither speed nor direction. Is this: A) True B) False.

Quiz: Section 2. 2) GPS measurements yield only a position, and neither speed nor direction. Is this: A) True B) False.

Quiz: Section 2. 3) A GPS receiver calculates its distance from the satellite using this equation: A) D = c + T B) D = c - T C) D = c x T D) D = c / T.

Quiz: Section 2. 3) A GPS receiver calculates its distance from the satellite using this equation: A) D = c + T B) D = c - T C) D = c x T D) D = c / T.

H20 GIS. Falling drops creating ripples in liquid.