6.1 Phase Observable
Learning Objectives
On the successful completion of this chapter you should be able to:
- explain and describe baselines
- explain the general theory behind phase observable, including measurements and differencing
- explain the errors associated with GNSS surveying and how to minimise them
- explain the common methods of GNSS surveying that use phase observable
- discuss the ways that accuracy is determined in GNSS surveying
- explain the difference between point positioning, DGPS and GNSS surveying.
In the beginning…
Understanding the extents of land, and the location of the objects on them is a critical requirement for any country, as the security of land plays a significant role in how economies can develop and thrive.
This is why most explorers were also considered surveyors, and why Mt Everest is named after a surveyor (despite the fact he didn’t want it named after him because he didn’t want people to mispronounce his name. But they ignored him, and his nightmare came true!). Surveyors have been measuring the size, shape and location of land for what seems like forever – well before that guy that looked down a well.
At the national or state level, surveyors would have to survey huge areas that would take decades to cover, setting up trigonometric stations (known by surveyors as trig stations) on top of the highest land around so they could take meticulous measurements over and over again. In the beginning they used theodolites for triangulation, as they didn’t have a quick, reliable method of measuring distance. With the invention of Electronic Distance Measurement, trilateration became achievable as distances could be measured in tiny timeframes compared to previously. However the need for trig stations continued as a visual line of sight was still needed for observations.
But then GPS became available to the public.
While autonomous positioning through GPS was a revolution for navigation and lower level accuracy activities, the need for surveyors to achieve high levels of accuracy remained. So some smart electrical engineers teamed up with some smart surveyors to develop the GNSS surveying technique we now know as phase observable.
In the early days surveyors had to get up at all sorts of crazy hours to make sure they could get four satellites, and the planning of the locations that had the best chance of getting accurate positions was meticulously and thorough.
Thankfully, a lot’s changed since those days, including the development of some even smarter phase observable differential techniques that make life loads easier for surveyors.
Phase observable
If you asked a group of people what the biggest benefit of GNSS has been, it would be a fair bet that being able to know your position really quickly would make the list. But as you’ve learnt in the last two modules, people are constantly figuring out ways to get more accurate positions from GNSS measurements. Phase observable techniques are no different from the code