GPS/GNSS Positioning – The Basics
GPS/GNSS Satellites have extremely accurate atomic clocks on board.
In essence, GPS/GNSS works by measuring the time it takes for a signal to travel between GPS/GNSS satellites and a receiver.
For this position to be as accurate as possible, the receiver needs to see this information from at least four satellites and here is how / why:
A GPS/GNSS Receiver uses the time a signal is received minus the time the signal is sent to determine the elapsed time. This elapsed time is then multiplied by the speed of the signal (speed of light) to estimate the distance.
One Satellite
Receiving information from a single satellite defines a sphere around that particular satellite. Based on the information provided by this satellite, the GNSS receiver could be located anywhere within this sphere.
Two Satellites
The Receiver uses information to define an individual sphere around each of the two satellites.
This is more useful than a single satellite. However, the receiver determines its position to be anywhere within the intersect point of both spheres.
Three Satellites
The Receiver uses information to define individual spheres around each of the three satellites. There are three possible intersect points between the three spheres. Working on the principle that only one of these intersect points is on the earth’s surface it is possible to use the information from three satellites to determine the receiver’s position.
Four (or more) Satellites
The Receiver uses the information to define individual spheres around each of the four satellites.
This provides a definitive location as this sphere is only likely to intersect the other three spheres at one location.
Inaccuracies (Errors)
In a perfect environment, the signals arriving from the satellite would be enough to calculate a position perfectly. However, the reality of GPS/GNSS is that there are quite a number of factors which lead to inaccuracies. There are the satellites themselves which have errors, there are atmospheric errors which are distortions caused by the satellite signal passing through the earth’s atmosphere and there are the environmental errors caused by the environment around the receiver (such as reflections from buildings).
Antenna Importance
At this point, it is important to note the role antennas play in receiving the ‘cleanest’ signals from a satellite. The world’s best receiver is only as good as the antenna attached to it.
A ‘that’ll do’ approach to the antenna will only lead to inaccuracies.
For advice on the best type of antennas, please refer to our earlier article: ‘How does GNSS antenna quality affect your application?‘ which explains in detail that using a $10 single band GPS antenna is not going to give you the same result as using a Triple Band, Dual Feed, eXtended Filter antenna which has been precision designed to deliver the best, cleanest signal to the GPS/GNSS receiver.
Stand Alone
GPS/GNSS receivers when operating by themselves are referred to as Stand Alone. This method of receiving information from satellites makes calculating a position accurate to within 1-2 meters. Acceptable when following a satellite navigation system in our cars or directing someone to a general location. This accuracy can be improved a little (to within 0.5m) by using a multi-band GPS/GNSS receiver, this is because a multi band receiver can see more of the available signals being broadcast by the sending satellite.
Corrections
Good quality antennas can be used to mitigate the surrounding environmental errors the GPS/GNSS receiver is exposed to.
GPS/GNSS receivers are, however, incapable of correcting the satellite and atmospheric errors mentioned above by themselves. Satellite errors are specific to individual satellites and will occur the same anywhere in the world. Atmospheric errors can vary widely depending on location. A receiver based in Inverness, Scotland will see very different errors to one based in Bournemouth, England. This is because the atmosphere that signal has had to travel through is very different in each location.
Base Station
A base station is a GPS/GNSS receiver which is positioned in a fixed location.
Real-time kinematic (RTK) positioning
Real-time kinematic RTK positioning works by the GPS/GNSS receiver (rover) receiving correction data from a single base station or a reference network (a series of interconnected base stations positioned in multiple locations). GPS/GNSS receivers then use this correction data to remove most of the GPS/GNSS satellite and atmospheric errors. For this to work, the GPS/GNSS receiver and the Base Station must be located close together. Ideally, this would be within 60km (37 miles) for a multi-band receiver. This would mean the GPS/GNSS receiver and GPS/GNSS base station are ‘seeing’ the same satellite and atmospheric errors allowing them to be cancelled out resulting in much higher accuracy.
Using this method, the ‘real time’ accuracy for a good GPS/GNSS system is likely to be within 1-2cm.
For RTK to be as accurate as possible the base station needs to be in a ‘known’ location. The best way to achieve this is to either set it up over a known point* or to ‘survey it in’ using a known point*.
*A known point is a precise, recorded location. Usually, a bench mark or triangulation point. In the UK these are no longer maintained, there is however a database of these including their condition for those who wish to survey in their own base stations.
RTK Corrections Service
A solution, when there is cellular/mobile phone coverage is to use a third party RTK corrections service such as RTKFnet. This uses a reference network of different base stations to provide the most accurate results without the need to set up an independent base station. Ideal when a receiver is being used across multiple locations and over a very wide area. A RTK correction service, when provided with the receivers’ location, can provide the receiver with the best correction data for the location.
In conclusion – Why do we use corrections?
In order to get an accurate GPS/GNSS location the receiver needs to be provided with accurate corrections data from a fixed base station or reference network.
For example, a typical field (12 hectares / 12000m2 according to a google search) assuming of a change every 5m there could be up to 1000 variations in soil quality within just one field. The addition of a precise location of each variation means the application of the fertiliser can be varied at very precise locations resulting in a reduction in fertilisers which is both good for costs and the environment.
