There are those of us who have plugged in an inexpensive aftermarket Sat Nav only to end up driving past the turning before it tells us we need to turn.
Why is this?
The simple answer is accuracy. An inexpensive aftermarket Sat Nav will not have been designed with the highest performance GNSS antenna or receiver.
While high accuracy/precision isn’t required for some consumer products, there are some applications where greater cm or even decimetre accuracy is required. For all GNSS applications’ careful consideration of the component parts is the key to a successful product which utilises GNSS.
When GPS (USA) first launched, there was a single constellation available. With the introduction of other Satellite Constellations such as GLONASS (Russia), Galileo (EU), Beidou (China), QZSS (Japan) and IRNSS (India) there is now a far greater number of satellites available.
GNSS Signal Strength
If a single light bulb is switched on in space, it would take an incredibly powerful device to be able to detect it from the earth’s surface.
The same principle applies to Satellites and the signals being emitted by them. By the time they reach the earth’s surface they are often very weak, -125 Decibels or milliwatt (dBm) in an open field or even -150dBm in a forest.
When choosing a GNSS receiver, there are a lot of factors to consider. Firstly, what application is it intended for and what features are required? To derive it’s position a GNSS receiver will require uninterrupted view of at least 4 satellites, with more satellites from multiple constellations and using more than one band (e.g. L1, L2 and L5) the receiver has a greater chance of being more accurate.
Therefore, choosing a GNSS receiver which is capable of tracking more constellations can improve accuracy, other features to consider are how many satellites the receiver will be capable of tracking simultaneously, the environment, power consumption and lastly cost.
No matter how powerful and how many features a receiver has, it is only going to be as accurate as the signals being provided to it.
Choosing the right GNSS antenna to complement a carefully chosen GNSS receiver is a task that should be done wisely.
We should look at an antenna from the perspective of it being the GNSS Receivers ‘lens’ viewing the sky. With the correct deflection of unwanted signals, this lens can be incredibly powerful resulting in a complete system which is accurate to within metre, centimetres, millimetres or even decimetres.
Single Feed Antennas
Antennas which were ‘single feed’ tuned to a specific frequency (1575.42MHz for GPS) these antennas were pretty reliable. A well-tuned single feed patch will reject the LHCP signals at the centre frequency.
If we take the single feed antenna and attempt to use it to receive two different frequencies the separation between the RHCP and LHCP gain becomes very small. The result of this is the potential for more errors and in turn less precision.
What is a RHCP signal?
When a GNSS signal arrives from a satellite to the antenna directly this is referred to as RHCP or Right Hand Circular Polarised. These are the signals a GNSS receiver needs to be as ‘clear / clean’ as possible before it can derive its position accurately.
What is a LHCP signal?
When a GNSS signal arrives from the satellite and it has been reflected or delayed this is referred to as LHCP or Left Hand Circular Polarised. These LHCP signals will degrade the precision of the receiver.
Dual Feed Antennas
Antennas which are Dual Feed such as those from Tallysman’s Accutenna range, are designed to better reject LHCP signals. This is because each feed takes independent linear signals and combines them with a 90-degree phase shift to produce a circular signal. This technology is applied across the whole bandwidth of the antenna resulting in rejection of cross polarised signals.
Phase Centre Variation
Once the GNSS receiver has derived its position, it reports a location fix. As satellites move across the sky it is inevitable there will be a little movement on this position as they are not static. Dual feed antennas such as Tallysman’s Accutenna antennas are carefully designed to minimise the effects of this movement.
Does the quality of a GNSS antenna affect the results of your application?
The short answer is yes. The performance of a receiver is only as good as the performance of the antenna
GNSS Antennas and Receivers – How can we help?
We are proud to bring over 25 years of experience with supplying the best GNSS receivers to suit the best applications.
This expertise is supported by industry experts, some of whom have over 35 years of experience and multiple technology patents to their names.
The result of this expertise and collaboration is the provision of the best possible advice and support for customers looking to implement a GNSS system into their product.