Post-processing GPS/GNSS Base Station Position
In spring 2020, the world was hit by a pandemic that spread globally by March, causing universities and most of the world to move to remote means. Summer field camps, long hailed as a rite of passage in the geosciences, were cancelled throughout the US. The community moved quickly, with NAGT developing remote learning tools and arranging for sharing and collaboration between instructors and institutions. As such, UNAVCO (GETSI) and University of Northern Colorado embarked on a data collection campaign for a summer field course entitled "Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes" – originally slated for in-person teaching. The team collected GNSS data, drone imagery for use in structure from motion, and terrestrial laser scanning from a site near Greeley, Colorado on the Poudre River. This assignment uses static GNSS methods to establish a base station used to establish a control point for a kinematic survey (establishing ground control points used in Sheep Draw Structure from motion activity).
Day 2 - This activity is part of the 2-week remote field course Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes
This exercise is intended for majors-level geoscience courses that have field or remote (online) field components.
Skills and concepts that students must have mastered
This exercise assumes students have some familiarity with Global Navigation Satellite System (GNSS) such as the Global Positioning System (GPS), map projections, and coordinate systems. It complements, and could follow as homework after students do elements of Unit 1: GPS/GNSS Fundamentals from the GETSI module High Precision Positioning with Static and Kinematic GPS/GNSS.
How the activity is situated in the course
This exercise can come just about anywhere in a course from very early to near the end; but should come before more advanced work with GPS/GNSS and should follow a lecture on GPS/GNSS systems. In addition to the lecture, I also included an activity on accuracy and precision from the GETSI's Unit 1: GPS/GNSS Fundamentals in the module High Precision Positioning with Static and Kinematic GPS/GNSS.
This activity is the first component of a virtual field campaign and associated activities outlined in the course Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes.
This activity should not take more than an hour to complete. It could be assigned as homework and then followed up with a discussion the following day.
Content/concepts goals for this activity
Post-processing a datafile collected from a GNSS receiver during a static GNSS campaign into a known x, y, z coordinate to establish a control point.
Higher order thinking skills goals for this activity
Interpreting a static GNSS report for coordinate solution accuracy and the appropriate use of a coordinate system, geoid model, and ellipsoid height.
Other skills goals for this activity
Properly reading a scale and converting units.
Description and Teaching Materials
This exercise substituted students having hands-on training setting up and collecting GPS/GNSS data in a field course. It could easily be used following an actual in-person data collection campaign. In this case, a Septentrio receiver was set up over a marked location for ~4 hours at the Poudre River at Sheep Draw site, established as a virtual field location.
The instructor starts out with a synchronous lecture (remote or in person) on GPS/GNSS methods (~45 mins), such as outlined in Unit 1: GPS/GNSS Fundamentals and Unit 2: Kinematic GPS/GNSS Methods. An exercise such as GETSI's GPS/GNSS Accuracy & Precision exercise is also potentially valuable.
Students are introduced to the Poudre at Sheep Draw field site and GNSS methods with videos shot during data collection. Students are then given the RINEX data file and images of the antenna slant height measured during receiver setup and are prompted to use OPUS (Online Positioning User Service) to post-process the file. Students then interpret the OPUS report for the x, y, z location and error of the point used as a known coordinate for a real-time kinematic base in a subsequent activity. A synchronous class discussion follows (remote or in person) to discuss results. As an alternative to OPUS, the Canadian Precise Point Positioning server can be used.
- GPS/GNSS presentation (PowerPoint 2007 (.pptx) 48.6MB Sep5 20) - adapted from GETSI's Unit 1-GPS/GNSS Fundamentals
- Sheep Draw Vignette (Microsoft Word 2007 (.docx) 2MB Sep5 20) – context for entire course, field campaign, and data collection, including background information about the site and a list of other activities created associated with the Sheep Draw site.
- YouTube: Video Introducing the Sheep Draw Field Site
- YouTube: Video about GNSS and Structure from Motion Methods at Sheep Draw
- The Worksheet for students for Postprocessing Base Station Activity (Microsoft Word 2007 (.docx) 2.5MB Sep5 20)
- Sheep Draw Rinex file ( 32.6MB Sep5 20) – Rinex file needed to upload to OPUS
A computer with internet access. No GNSS equipment is needed if the Poudre River at Sheep Draw data provided are used.
Teaching Notes and Tips
- Be prepared for students to read the antenna height measurement incorrectly and convert it incorrectly, which propagates to final x, y, z coordinates.
- Students struggle with the concept of orthometric vs ellipsoid height and that of a geoid model. Some students also struggle with different coordinate systems and may need a review.
Formative assessment should be done through discussion with students as a whole group or individually.
As the summative assessment, students write a paragraph explaining their procedure, interpreting the results, and the difference between ellipsoid height and orthometric height, and anything that was surprising or confusing about the results. A rubric is included with the assignment.
References and Resources
- This activity was adapted from the GETSI Field module High Precision Positioning with Static and Kinematic GPS/GNSS. In particular, Unit 1-GPS/GNSS Fundamentals and Unit 2-Kinematic GPS/GNSS Methods.
- Precise Point Positioning (PPP) portals
- The Shape of Earth and Reference Ellipsoids
- Elevation for Beginners: What to Know About GPS Receiver Elevation - includes information on orthometric vs ellipsoid height
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