SVY207: Lecture 16 GPS Field Procedures and Computations Aim of this lecture: To consider the steps required to plan and carry out a successful GPS survey Overview Requirements Planning Field work Computational procedures

Task Requirements What does the task require? real-time or post-processing? data rate? time available for field work and computations? solution quality? (precision, accuracy, reliability,...) what coordinate system? integration with other systems? budget finite resources

Planning The Method The Equipment point, differential, relative? static, kinematic, RTK, etc.? The Equipment receiver? observables? dual-frequency? radio link? antenna? portability versus accuracy (want a backplane?) method of mounting (tripod; bipod; spider) compatability e.g., antenna phase centres known? data from other sources? eg, global GPS network

Planning (cont) The Survey consider differences to classical survey point selection history, sky visibility, accessibility, safety, datum tie, stability, multipath, power, weather schedule observations look at DOP (account for elevation cutoff, etc.) Note: generally have less receivers than points! design “sessions”, accounting for travel time allow redundancy in network least-squares approach detect blunders tolerance to survey problems (e.g., equipment failure)

Planning (cont.) The Analysis select and obtain appropriate software and training any special accuracy requirements? e.g. tropospheric estimation for precise heights (cm) If required, arrange for additional data precise orbits? data from global network? other navigational or survey data? (gyro, odometer, inertial guidance, altimeter, classical survey data) reference frame coordinates (to map your solution onto a particular datum)

Field Work The Reconnaissance confirm site selection assess suitability of existing points install new points or eccentric points if necessary station descriptions write instructions on how to find point

Field Work The Crew Training required? Distribute schedule, station descriptions, booking forms Brief the crew: schedule, contingency, communication Communication? mobile phones, radios, phone box supervisor (central or roving) post survey Crew’s responsibility to check equipment is it all there? is it all working? (batteries charged, ...) ancilliary equipment (maps, tapes, compass, etc.)

Field Work The Survey Often in discrete “sessions” due to lack of resources redundancy and quality assessment Travelling to and from region of survey between sessions If problems arise, may have to extend survey (number of sessions, period of sessions) May include additional control work (e.g., levelling to an Ordnance Survey benchmark). Classical detail work may be required

Field work On Site On-site input of parameters Carefully centre antenna Measure antenna height before and after date collection Carefully fill out booking form: draw attention to anything unusual Check to see if all is as expected (screen, batteries, etc.)

Computational Procedures Data transfer and Formatting RINEX: Receiver INdependent EXchange format antenna types, heights + other relevant information Preprocessing point position (for provisional position + clocks) triple-difference solution (for cycle-slips & outliers) form double differences (linearly independent set) form ionosphere-free combination LC (if > 1020 km) Processing Model: Obs-Comp (b), Design (A), Weight (W) Weighted least-squares (standard, or Kalman filter) Ambiguity resolution, and iterate Investigate residuals, perform statistical tests Network Adjustment Perhaps redo analysis if blunders found Network transformation