GNSS (Global Navigation Satellite System) Location Accuracy and What to Consider When Using it For Worker Location Tracking

Background 

GPS is often used in place of the more technically correct term GNSS. GNSS stands for Global Navigation Satellite System, but unless you are involved in the satellite industry you probably never heard of that term. GPS is in fact the US satellite system and does not represent the rest of the global satellites in operation for global positioning. Europe, China, and Russia all have their own satellite systems for global positioning, and they are known collectively as Global Navigation Satellite System or GNSS. 

Introduction  

Location tracking of industrial workers is vital for safety and efficiency. GNSS technologies like GPS are a popular choice, providing outdoor location data. However, accuracy can vary based on factors like satellite connectivity. Choosing the right GNSS solution and being aware of techniques to improve precision are important for success. 

Location Accuracy vs Reality 

Often end users over specify the level of accuracy they need for tracking the location of their workers or assets. “We need sub 1 metre accuracy,” is a common request. In response, the question should be “Does your use case require that level of accuracy?”  

The use case will ultimately drive how accurate you need your location system to be. Generally, the more accurate you require your location tracking to be, the more infrastructure you will need to support it.  

GNSS location is often a good option in outdoor open areas as it requires less infrastructure to determine a tracking devices location. For example, a worker tracking tag. 

If you are on a large lay down yard and there is an asset somewhere within 10 metres, you will probably spot it. But when tracking personnel, accuracy becomes more important. If someone is tracked to within 5-10 metres, that is sufficient. But in an emergency when you need to find that person, accuracy of 2-3 metres is preferable. With 10 metre accuracy, the person could be several floors away in another building, which is not helpful.  

So, 2-3 metres will give a good indication of where a person is. Getting down to 10-centimetre accuracy probably won’t help find somebody any easier in an emergency versus 2-3 metres. Therefore, it is important to assess the use case in its entirety as this will help reduce the overall cost of the system. 

Calculating a Pseudo Range 

When calculating position using GNSS, you are calculating a pseudo range – an arc on the Earth’s surface based on the satellite signal. With more arcs from more satellites intersecting, the position accuracy improves. This is what the chipset does – using sophisticated maths to intersect the arcs and determine an accurate location. The more arcs, the better the accuracy – ideally 7 or 8 satellites. 

For further information on this topic, here is a good resource – How are pseudo ranges obtained? (physicsforums.com)  

Improving GNSS Accuracy  

There is a number of ways GNSS positioning can be improved. One of the most common is what is known as assisted GPS. Assisted GPS uses location data and corrections from a cell tower to provide faster time to first fix and improve accuracy, as your mobile phone does. Cell towers have permanent fixed position on the planet. Assisted GPS makes use of this fixed location to correct the inaccuracy from the satellite positioning, resulting in a more accurate position of your device. We see this when we use our mobile phones on maps, the time to first location fix is very quick and fairly accurate because your phone is a cellular device making use of assisted GPS. However, in process industries most location tracking uses devices without cellular connections, so do not benefit from assisted GPS. This can sometime lead to misconception of GNSS accuracy for location tracking and why there can sometimes be inconsistencies in location accuracy over time.  

If increased accuracy is what you require, solutions such as the Extronics iTAG XM Android app could be a good option. The iTAG XM provides assisted GPS by using the mobile device’s internal GNSS and cellular connectivity. This improves accuracy for use cases needing greater precision without requiring additional devices for workers to carry. 

For further information on this topic, here is a good explainer – What is Assisted GPS or A-GPS? (everythingrf.com) 

When Greater Accuracy is Important 

Greater GNSS accuracy is important in some, but not all uses cases as we previously mentioned. Here are a couple of examples of where improved accuracy may be more beneficial: 

• Event zones – restricted areas only certain personnel can enter. Sub 20 metre accuracy could erroneously put someone in an event zone. Assisted GPS solutions like iTAG XM are preferable here. 
• Emergency situations – like evacuations or man-down events, probably require accuracy to within 2 metres. This helps first responders quickly identify a worker’s location to provide assistance. Again, solutions like iTAG XM are ideal when GNSS is the location source. 

In summary, carefully consider the use case to determine what level of GNSS accuracy is truly needed. Solutions that provide assisted GPS can offer improved accuracy when high precision is required. Being aware of these factors will ensure you select and deploy the optimal GNSS solution to track workers safely and efficiently.