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 observable techniques in this regard – our pursuit to remove (or at least minimise!) errors to squeeze the last bits of accuracy into our position is relentless.
Surveyors are no exception; when you make a living off knowing how to measure things incredibly accurately, it’s pretty much your job to find better ways to do things – to remove as much of those pesky errors as possible!
So while the majority of the population is incredibly happy with being able to get an autonomous positon accurate to around five metres in a matter of second using the code observable, surveyors’ need for millimetre accuracy means that they are willing to put out huge numbers of GNSS receivers over permanent survey marks for hours, days or even years at a time. Then some of them will spend more hours, days, years and even entire careers, pouring over the relative positions of these marks, at which point we still might not have a final position of the marks! Seems insane right?!
Well, in some ways it is, but in other ways phase observable is actually an incredibly smart and accurate way to measure huge distances and small movements in equal measure, simply by having a whole bunch of permanent survey marks.
In phase observable we are able to measure the differences in position of two or more of these survey marks, relative to each other, which you may recall are call baselines. If one or more of these marks has an accurate position, we can then use the baselines to ‘transfer’ those coordinates to other marks. This is done using essentially the same differential technique that was discussed in Chapter 5, but with receivers and antennae that can also read the carrier wave parts of GNSS signals, to within a couple of millimetres.