1. 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

2. 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

3. 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

4. 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)

5. 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)

6. 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

7. 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.)

8. 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

9. 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.)

10. 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