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ASM Magazine: Dr. Gao – I understand that you teach at the University of Calgary in addition to acting as CEO for Nexteq Navigation. What is your area of interest? 

Dr. Gao: My research area is satellite-based positioning and multi-sensor navigation systems. My current interest focuses on the development of next generation high precision GNSS technologies, and the integration of location, geo-spatial and wireless communication systems for location-based services. 

ASM Magazine: How did Nexteq Navigation begin, what was the impetus that motivated you to initiate the company? 

Dr. Gao: Nexteq Navigation was founded four years ago. At that time, the concept of Precise Point Positioning (PPP) was beginning to make larger advances in an international market that was dominated by Real-time Kinematic (RTK) processing for Global Positioning System (GPS) data.  

RTK processing involves the use of two GPS receivers, one acting as a base station and remaining stationary at a pre-surveyed position, while another (the rover) moves about collecting data and providing location information. Corrections generated from the base station are then applied to the rover, usually through radio communication links. Errors at the rover are eliminated because of the correlation between both devices. Precise Point Positioning on the other hand relies on very accurate satellite orbit and clock corrections which frees the user from the restrictions of a base station.  

So there was a broad interest within the GNSS user community for new high precision GNSS technology and product that could be more cost-effective and also more efficient in the field work. Although PPP was found to be a promising approach for that, there were no such products available from the market. This is what led me to begin Nexteq Navigation. 

ASM Magazine: What levels of accuracy can be achieved with precise positioning services?

Dr. Gao: For dual-frequency users, it is possible to achieve centimetre level accuracy using precise positioning approaches. Sometimes accuracy will be in millimetres. For single frequency users decimeter to sub-metre level accuracy is obtainable. 

RTK is able to achieve its highest levels of accuracy due to carrier phase processing. It is the primary observable. Excellent results have been achieved through the use of RTK worldwide, and that continues to be the case. It has had a revolutionary impact on the surveying market and has been known about for two decades at least. 

ASM Magazine: Does this suggest that there are limitations to RTK? 

Dr. Gao: The main issues for RTK relate to the relatively short separation distance requirements between the base stations and the rovers in the field. Typically this will be a few kilometers, but surveyors have been able to extend this distance at times – usually due to high quality devices with better firmware, better software processing capability and suitable environmental factors that result in lower atmospheric effects.  

A lot of current research is oriented around extending the distance between base station and rovers. That work led to the development of approaches for RTK networks under which the base station formed part of a wider network.  

ASM Magazine: It is that work that led to the development of Reference Stations Networks?

Dr. Gao: Yes. And using the RTK network approach means that rovers can be much further away from the base station. Often this approach provides good accuracy with separation distances of up to 30-50 km. Some people have claimed longer baselines, sometimes approaching 100 km with success.  

ASM Magazine: Then aren’t most problems already solved with respect to GPS location accuracy, particularly for regions? 

Dr. Gao: The RTK network approach is still oriented toward smaller regions. For example, setting up base stations 50-100 km apart could result in rovers being used 30, 40 or 50 km away. Multiple base stations in a network will obviously provide longer baselines with good accuracy. In mining, for example in Australia or Canada, it is useful to apply RTK networking because of the need for high levels of accuracy, but within relatively short distances. In most cases the base stations for these networks will become permanent – thus supporting re-use and reliability.  

However, in some cases ground subsidence can cause these local permanent base stations to shift. If the user is relying directly on these base stations then they could receive erroneous corrections.

ASM Magazine: RTK network approaches are still the predominate approach used I assume. But you mentioned new approaches that caused you to begin Nexteq Navigation. Can you explain those? 

Dr. Gao: Network RTK continues to be widely used. But it is important to understand that they provide their best results over smaller areas. In addition, due to the need for base stations, they also involve higher implementation and maintenance costs. Thus, it is cost-prohibitive to install over large areas for that type of network.   

The concept of precise point positioning began about 1997. Like RTK it involved the use of carrier phase processing, but it does not require a base station. The rovers are no longer dependent on another nearby receiver but essentially operate in a stand-alone mode.  

ASM Magazine: Can you explain precise point positioning? 

Dr. Gao: Since a base station is not used in PPP, then the corrections provided by the base station in RTK are now not available. Therefore, a different approach must be used to gain those corrections that allow for the higher levels of accuracy. The PPP technique makes use of globally available corrections and applying them to the rovers in real-time.  

ASM Magazine: Let’s back up a bit. Can you explain the errors that impact PPP? 

Dr. Gao: There are many sources of GNSS errors, but let’s begin with orbital errors. These can be attributed to the orbits of the GPS satellites in space. Knowing precise orbital information directly impacts the calculation of distances to rovers on the ground. Secondly, the GPS clocks themselves can contribute significant errors. Since GPS is a timing system, any error in clocks also directly impacts calculations. Finally, atmospheric errors contribute to significant errors in positioning. Although these are also present in RTK approaches, they are normally eliminated via the base station corrections. However, once you begin to move away from the base station these errors lose correlation and therefore introduce errors into your position.  

There are many other errors, but these are the three main sources to contend with. The question now is, “how to remove the 3 errors for precise point positioning?” 

Modeling is one approach and orbital and clock information can be readily achieved by using a global observing network. The IGS provides global data for all countries in a standardized fashion and operates several hundred of stations worldwide that gather information to develop these global corrections for orbits and clocks.  

ASM Magazine: How does that enable single field rovers to achieve high accuracy levels?

Dr. Gao: High precision and globally valid orbit and clock corrections and techniques necessary to mitigate atmospheric delays and various other types of errors and bias are essential to achieve centimetre accurate positioning solutions. Nexteq has developed patented technologies to achieve that goal.   

ASM Magazine: What about the errors you mentioned with atmospherics?

Dr. Gao: This remains a major source of error so we do need to continue to manage it. At Nexteq we use two approaches. One approach is to implement dual frequency carrier phase processing through PPP but for single frequency users, we also have special algorithms which can mitigate atmospheric errors to provide high positioning accuracies. In fact, many of the errors we consider in PPP are not considered at all in RTK.  

We might say that PPP is more complex in this regard, but it really means that we cover more bases with respect to error management. And it was due to that fact that PPP only became suitable for wider use about a few years ago. It took a lot of research and attention to error management for PPP to be fully realized with higher accuracy and reliability needed for commercial applications. 

Previously, PPP applications were also limited because of the availability of corrections and the regions they could be used for.   

ASM Magazine: But OMNISTAR and NavCom have both provided global services of this type for many years, what makes this different? 

Dr. Gao: That is correct. But both of those systems are mostly oriented toward marine applications and are more costly to users. Our company aims to substantially reduce the cost of these PPP services and to provide them for terrestrial use.   

ASM Magazine: How does this relate to EGNOS in Europe? 

Dr. Gao: EGNOS is also a correction service, albeit European oriented. It is often referred to as an ‘Augmentation Service’ but serves a similar purpose. It and other systems we call Satellite-based Augmentation Systems (SBAS), such as WAAS in North America and MSAS in Japan and East Asia, are not developed with carrier phase processing in mind. Instead, they provide coarser accuracy improvements to achieve 1-3 meter levels of accuracy. Since PPP is carrier phase based, it can provide much higher accuracy. PPP can be considered as a system suitable for surveying grade applications as compared to EGNOS and WAAS that provide a useful but lower level of accuracy.   

ASM Magazine: How would you summarise this discussion so far? 

Dr. Gao: Essentially, RTK provides greater accuracy at high cost for small areas. Non-RTK based approaches like PPP potentially offer similar accuracy at much reduced cost and reduced logistics involved with surveying operations. With PPP one can move anywhere provided corrections are available. 

This is why we see companies like OMNISTAR and NavCom involved in marine applications where higher accuracy and reliability are needed. RTK approaches do not work globally, certainly not across oceans.   

ASM Magazine: Both of those systems require satellite connections for communications also. But it seems that PPP is currently limited also by communications globally? 

Dr. Gao: That is correct. Satellite communication systems can contribute toward the higher cost of PPP services. While larger companies in certain fields may be able to afford these services, smaller companies and individuals can be seen as left out of the market for PPP applications. We aim to overcome that. Internet access is becoming more and more accessible, and affordable, even in very remote corners of the Earth. This will only lead to wider adoption of Internet based corrections services such as our i-PPP Service.   

ASM Magazine: It sounds like this is where Nexteq Navigation comes in? 

Dr. Gao: Nexteq Navigation is oriented to the user, and we want to advance the use of PPP for all users and applications. Through our i-PPP Service, users are able to achieve nearly instantaneously high accuracies without worrying about available base station networks in their work area. This can be very helpful in disaster or emergency response scenarios. In some cases such as after earthquakes or tsunamis, permanent base stations located in a region may be destroyed or disabled and therefore not capable of providing high accuracy corrections. Since i-PPP utilizes a global network using numerous stations, these corrections will still be valid and therefore provide users with the accuracy they need. 

Our first goal was to develop a cost-effective product that utilized free corrections services to enable decimeter level of accuracy. We achieved this goal with our Freedom line of products that used freely available SBAS augmentations services like WAAS, EGNOS and MSAS. Freedom receivers can reach the decimeter level of accuracy with no additional cost on the part of the user in areas already covered by SBAS, such as North America, Western Europe, Japan. Additional SBAS are being planned to over greater coverage, such as in India. 

While many other competing receivers are SBAS-ready, they are not PPP-ready. Freedom receivers apply many PPP concepts and techniques with SBAS corrections to improve accuracy twofold without the need of a base station or a fee-based correction service.  

ASM Magazine: It appears as if PPP is now turning a corner and becoming more mainstream?

Dr. Gao: PPP is the future of GNSS and has made great strides in the last ten years. While RTK remains relevant, and will so for a long time, the next generation of positioning will rely upon the real-time global PPP approach because of its cost and logistics benefits. We are solidly behind PPP and our goal is to advance this technology, and provide these benefits to as many users as possible. 

Our ‘Freedom’ firmware is already embedded in many GPS receivers and provides a significant improvement in accuracy. For example, receivers using EGNOS or WAAS that normally achieve 1 meter accuracy will be able to achieve 50 cm accuracy with Freedom; while another receiver that previously had 2 m accuracy would now be capable of meter level positioning. We are talking about all receivers; from inexpensive consumer grade receivers to the most expensive. One field that we have noticed our PPP technologies having meaningful benefits is for GIS applications; real-time GIS data collection becomes much more of a possibility using existing inexpensive GPS handheld data collectors without the need for post-processing.  

ASM Magazine: Will Europe’s GALILEO, Japan’s QZSS and the Russian and Chinese GPS systems benefit from this? 

Dr. Gao: As orbit and clock errors will also affect the measurements of these new systems, Nexteq PPP technologies like Freedom and i-PPP will still offer the same benefits of improved accuracy over large regions or globally. In addition, the increased number of satellites visible to a user at any one time has also benefits and will further improve on the achievable accuracy and reliability of the PPP approach as compared to today.  

ASM Magazine: I’m still not clear on how the services for corrections will be delivered cheaper using your technology, please explain? 

Dr. Gao: We have developed our own correction service called i-PPP and its corrections are valid and available globally. Unlike other services, our service is delivered exclusively through the Internet. Therefore, communication costs are negligible as compared to services using satellite communication.

In fact, with many existing GPS receivers and devices already having wireless data capabilities built in, no additional hardware is needed. Any such receiver when embedded with i-PPP technology has access to the i-PPP Service and its corrections.  

ASM Magazine: What excites you the most about your work? 

Dr. Gao: I’m excited about the benefits for the users. PPP is not fully accessible to users now and we are trying to change that. I’m excited about accuracy improvements and know that this work will lead to many new applications. That is amazing and exciting.  


Dr. Yang Gao 

Yang Gao earned a PhD in Geomatics Engineering from The University of Calgary and MSc and BSc in Surveying Engineering from Wuhan Technical University of Surveying and Mapping. Dr. Gao is currently a Professor in the Department of Geomatics Engineering at the University of Calgary, CEO of Nexteq Navigation Corp and an Adjunct Professor at several universities. Dr. Gao’s research work is focused on both theoretical aspects and practical applications of satellite based positioning and multi-sensor navigation systems. His research results have been published in over 200 papers and he has received many awards and recognitions for his contributions to the field. He has served on a number of scientific and professional organizations. He is the President of the Sub-Commission “High Precision GNSS” at International Association of Geodesy (IAG), the founding President of the Chinese Professionals in Global Positioning Systems (CPGPS) and the editor of several journals.