Design a GPS Constellation in STK
Arrange satellites in space so no point on Earth loses navigation coverage.
Last reviewed: March 2026Overview
The Global Positioning System consists of 24 baseline satellites arranged in six orbital planes specifically chosen to guarantee that at least four satellites are visible from every point on Earth's surface at all times — a minimum needed for a 3D position fix. The geometry of visible satellites, quantified by the Position Dilution of Precision (PDOP), is as important as raw satellite count: a cluster of satellites close together in the sky provides a much worse geometry fix than the same number spread across the hemisphere. Designing a constellation that balances satellite count, orbital altitude, inclination, and phasing to achieve target PDOP everywhere is a classic systems engineering problem.
Ansys Systems Tool Kit (STK) is the industry standard for constellation design and coverage analysis. Its Coverage and Figure of Merit analysis tools compute global coverage statistics, PDOP maps, and revisit times over arbitrary time windows, accounting for Earth's rotation, orbital mechanics, and masking angles. In this project you will use STK to reproduce a simplified GPS-like constellation, analyse its coverage properties, and then explore modified designs — regional augmentation, reduced satellite counts, alternative altitudes — quantifying each design's performance with STK reports and 2D/3D visualisations.
The project teaches systematic space system trade studies and gives you hands-on experience with the tool used by satellite operators, government agencies, and commercial constellation designers worldwide. Skills developed here apply to GPS augmentation systems (SBAS, GBAS), LEO broadband constellations like Starlink, and next-generation PNT architectures under development at the US Space Force and allied agencies.
What You'll Learn
- ✓ Define Walker constellation geometries and understand how inclination, altitude, and phasing affect coverage
- ✓ Set up STK Coverage and Figure of Merit analyses for PDOP and minimum satellite visibility
- ✓ Interpret PDOP maps, coverage reports, and access interval statistics from STK outputs
- ✓ Conduct a documented constellation trade study comparing at least three design alternatives
- ✓ Connect constellation design decisions to real-world GPS system architecture and modernisation efforts
Step-by-Step Guide
Reproduce the baseline GPS Block II constellation
Create an STK scenario with a 24-satellite Walker 24/6/1 constellation at 20,200 km altitude and 55° inclination. Configure an Earth coverage grid with 0.5° resolution and compute the Figure of Merit as Maximum PDOP over 24 hours. Verify that global PDOP stays below 6 everywhere — the GPS performance standard — and visualise the result on the 3D globe.
Analyse coverage holes and satellite geometry
Identify the geographic regions with worst PDOP (typically near the poles for 55° inclination). Use the Access tool to compute how many satellites are simultaneously visible from five ground sites (equatorial, mid-latitude, Arctic) over 24 hours. Plot satellite count histograms and explain the correlation with the PDOP map.
Trade satellite count against coverage
Reduce the constellation to 18 and then 12 satellites (maintaining the Walker geometry proportions) and recompute the PDOP Figure of Merit. Create a table showing 90th-percentile global PDOP vs. satellite count. Identify the minimum number of satellites that still achieves PDOP < 6 globally for more than 99% of the time.
Explore altitude and inclination trade-offs
Fix 24 satellites and vary orbital altitude (MEO 19,100 km vs. 26,600 km) and inclination (55° vs. 63°). Run STK coverage analyses for each combination and compare polar coverage improvement vs. increased radiation environment exposure at higher altitudes. Summarise findings in a trade matrix.
Design a regional augmentation overlay
Add four geostationary augmentation satellites (like WAAS) positioned over specific regions. Compute the improvement in average PDOP and 99th-percentile PDOP for North America and Europe with the augmentation overlay active. Quantify the coverage benefit in terms of equivalent additional MEO satellites.
Produce a constellation design report
Export STK coverage reports, PDOP maps, and access statistics. Write a formal Systems Engineering trade study report that includes a requirements section (coverage and PDOP thresholds), a trade matrix with weighted scoring, a recommended design, and a risk section discussing launch constellation build-up and satellite failure contingencies.
Career Connection
See how this project connects to real aerospace careers.
Space Operations →
Satellite constellation operators use STK daily for mission planning, link analysis, and coverage verification — this project develops that core toolset.
Aerospace Engineer →
Space systems engineers performing constellation trade studies and coverage analyses use STK as their primary tool through all design phases.
Air Traffic Control →
ATC navigation systems depend on GPS constellation health; understanding PDOP and coverage geometry helps controllers interpret NOTAM outage impacts.
Pilot →
Instrument-rated pilots relying on GNSS approaches benefit from understanding how constellation geometry drives approach authorisation and backup requirements.
Go Further
- Model a LEO constellation (600 km, 500 satellites) and compare revisit time and latency against the MEO GPS design.
- Use STK's Comm module to add link budget analysis and identify where path loss and atmospheric delay most degrade positioning accuracy.
- Export constellation TLEs and propagate them in Python using the
skyfieldlibrary to reproduce the PDOP calculation independently. - Design a regional navigation system for a specific continent (e.g., a Galileo-like regional overlay) and compare performance against the full GPS constellation.