Challenges and Opportunities in Mass Market

RTK

Altti Jokinen, Ali Pirsiavash, Marco Mendonça, Allystar Technology Ltd., Canada

Ryan Yang, Hongtao Yu, Mingo Tsai, Gary Hau, Allystar Technology Co., Ltd., Hong Kong SAR

Yi-Fen Tseng, Allystar Technology Co., Ltd., China

BIOGRAPHY

Altti Jokinen is Senior Principal Designer at Allystar Canada. He has over 13 years of experience on GNSS and its applications

from industry and academia. He received his PhD degree from Imperial College London in 2015. His primary knowledge is on

RTK and PPP specializing to carrier-phase ambiguity resolution.

Ali Pirsiavash is a Senior Algorithm Designer at Allystar Canada, with more than 5 years of academic and industrial experience

in GNSS signal processing and receiver design. He received his PhD in Geomatics engineering from University of Calgary in

2019, with specialization in positioning, navigation, and wireless location. He also holds BSc and MSc degrees in electrical

engineering and communication systems, received from K. N. Toosi University of Technology in 2008 and 2011, respectively.

Marco Mendonça is a Senior Algorithm Designer at Allystar Canada. He has worked on multiple GNSS and sensor integration

R&D projects over the past 10 years. His primary knowledge is on sensor integration and movement constraint models,

specializing in filter tuning and process automation.

Yi-Fen Tseng is the Deputy General Manager at Allystar. He has 19 years of experience in GNSS with domain expertise in the

multi-band multi-system GNSS chip architecture and positioning algorithms. He holds a Master of Science and a Bachelor of

Science in Electrical Engineering from National Taiwan University.

Hongtao Yu is a Senior Algorithm Designer at Allystar Hong Kong, with more than 8 years of academic and industrial

experience in signal processing, GNSS baseband and receiver design. Hongtao has received his MPhil degree in Electronic and

Computer Engineering from Hong Kong University of Science and Technology in 2013.

Ryan Kai-Yuan Yang works at Allystar Technology in Hong Kong as high-precision algorithm team leader and high-precision

project manager concurrently. His responsibilities are architecture design and algorithm development for high-precision

products. He has over 6 years of professional experience on GNSS R&D.

Gary Hau is an R&D Manager at Allystar Star Hong Kong. Over 10 years of industrial experience in GNSS chip and algorithm

design. Developed the world's first commercial dual-frequency GNSS receiver that supported BeiDou-3 satellite system.

Mingo Tsai has lead INS product development in Allystar since 2019 and lead GNSS Chipset development since 2002. He has

Master of Computer Information Science degree from Syracuse University, NY.

ABSTRACT

Allystar’s GNSS SoC with built-in dual-frequency Real Time Kinematic (RTK) solution is presented in this paper. This is the

lowest power and smallest size RTK solution available on the GNSS market. The Allystar RTK solution supports multiple

signal configurations such as GPS L1/L2C, BeiDou B1I/B2I or GPS L1/L5, and Galileo E1/E5a. Allystar RTK engine also

provides highly accurate and stable performance. Its characteristics are analyzed and compared in this paper to other

competitors’ receivers. Test results obtained under different static and kinematic scenarios show promising performance with

high fixing rate and low level of position errors. The focus is on harsh multipath and signal blockages environments as is a

common use case for low-power low-cost receivers.

After discussing Allystar RTK solution, a general overview is presented about the challenges and opportunities in mass market

RTK solutions. The focus is on the availability of suitable corrections and hardware limitations the situation is expected to be

improved in the coming years due to the growing support for the L5 band in RTK networks and availability of next generation

mass market hardware such as the Allystar Cynosure IV architecture. This will result in the availability of RTK solutions for a

wider range of users who cannot afford the cost, size, and high-power consumption of traditional OEM boards.

INTRODUCTION

There has been great interest to bring Real Time Kinematic (RTK) to mass market applications with strict constraints on cost,

power consumption and size. Traditionally, using RTK requires expensive and large geodetic receivers which limits the

application of RTK solutions for professional usages such as surveying. Nowadays, the latest innovations have changed this

by running RTK on small chips making RTK possible for a wider range of applications and users.

Allystar HD9311 chip is a leading mass market RTK product. The chip supports running RTK using two simultaneous signal

bands. For example, Allystar RTK can use L1 and L5 or L1 and L2 observations and those two options can be changed by

firmware configurations. RTK can be run on single 3x3 mm size Wafer-Level Chip-Scale Packaging (WLCSP) chip. This is

significantly smaller than any system-on-chip (SoC) solution available on the market. Allystar HD9311 operates under 39 mA

@ 3.3V of power, enabling RTK solution for battery-powered solutions or other highly power sensitive applications such as

smart phones, wearable devices, and drones.

Allystar evaluation board, chip and module are shown in Figure 1, demonstrating the size-efficiency of the different hardware

products. In Figure 2 and Table 1, the power consumption of Allystar receiver is also compared with that of some competitors

where results show significantly better performance for the Allystar solution.

Figure 1 Allystar multi-band GNSS module and evaluation kit

Figure 2 Allystar RTK receiver power consumption compared to competitors

Table 1 Allystar receiver power consumption values compared to competitors

As shown in Figure 3. Allystar receiver supports all available major GNSS signal bands. The receiver can be configured to

support L1 band and either lower signal band option A, B or C. Users can select which signal bands to use by firmware

configuration and no hardware modifications are needed. Available signal band options for RTK are A (L5 band) or B (L2

band). Currently, many users prefer to use the signal band option B, because of the support by existing RTK networks. However,

in the future, more users will likely prefer using the signal band A, because of the improved noise and multipath properties of

GPS L5, BeiDou B2a and Galileo E5a. The benefit of using GPS L5 compared to GPS L2C is demonstrated using kinematic

data collected in Tainan. A magnitude of multipath and noise is estimated using the Code-Minus Carrier combination (CMC)

[1]. Figure 4 shows the magnitude of the multipath and noise for GPS L2C and Figure 5 shows that for the GPS L5 signal.

Figure 3 Supported frequency bands of Allystar receiver

Figure 4 GPS L2C noise and multipath (RMS 3.2 m) using kinematic data recorded in Tainan

Figure 5 GPS L5 noise and multipath (RMS 0.9m) using kinematic data recorded in Tainan

The principle of RTK is to cancel or mitigate errors from different sources such as satellite orbit, satellite clock and ionosphere

by single-differencing observations between the rover receiver and base-station [2]. Double-differenced observations are

obtained by doing between-satellite-differencing for the single-differenced observations to cancel receiver clock error and

receiver code and phase biases. If the baseline between the rover and base-station is short enough (in the order of a few

kilometers), almost all errors will be mitigated to a negligible value. The main use case for Allystar is RTK on short baselines

or Virtual Reference Station (VRS) [3] networks. Allystar RTK engine supports corrections in the Radio Technical Commission

for Maritime Services (RTCM) 10403.3 format [4].

Allystar RTK engine employs a state-of-the-art Extended Kalman Filter (EKF) [5]. Position, velocity, and double-differenced

carrier-phase ambiguities are estimated as filter states. The filter is updated each epoch with code and carrier-phase observations

weighted using signal to noise ratio, elevation angle and estimated multipath level. Carrier-phase ambiguities are fixed to

integers using the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method [6].

Allystar has high reliability and high availability RTK firmware options. The aim of the high reliability version is to provide

RTK fixed solution with horizontal error always below 30 cm and the aim of the high availability version is to provide the best

possible fix rate while keeping reasonable accuracy. The high reliability solution is recommended for sensor integration and

other applications that require high accuracy and the high availability version is recommend for general users.

ALLYSTAR RTK TEST AND RESULTS

Kinematic performance of Allystar RTK engine is demonstrated by driving testing in Shenzhen and Beijing and drone test done

in Shenzhen. Static performance of Allystar RTK engine is tested in Calgary and this test will also include comparison between

L1/L2 and L1/L5 signal band options.

Shenzhen road test

Figure 6 Shenzhen test route and a sample of data collection environments

(a) (b)

Figure 7 (a) Test rack with all receivers and SPAN system and (b) Antennas installed on a test car

Figure 6 shows the route of Shenzhen road test. In this route, the environment is mostly challenging with frequent full and

partial signal blockages. The used test rack is shown in Figure 7 (a). All tested receivers are installed to the rack and a NovAtel

SPAN system is used to the generate reference trajectory to verify RTK solutions. Tests antennas are installed to a car as shown

in Figure 7 (b).

Figure 8 Horizontal RTK fixed solution error in Allystar high availability mode

Competitor’s receivers from “U” and “M” companies are used. All receivers are using GPS L1/L2C and BeiDou B1I and B2I

signals since these signals have VRS corrections available from the Qianxun Spatial Intelligence Inc. (QXWZ) [7] streams.

RTK fix solution results in Allystar high availability mode are shown in Figure 8 and in Allystar high reliability mode in Figure

9. It is shown that Allystar solution is more accurate than competitors, since the solution does not have large outliers, indicated

in the green boxes in Figure 9 and 10. RTK fixing rate from Allystar and competitors is shown in Table 2. Allystar has higher

fixing rate than the competitors in the high availability mode. Correction age and used satellites during the test are shown in

Figure 10. This highlight that the test was done challenging environment with frequent signal and correction blockage.

Receiver Fix rate

Allystar (high availability) 66.32%

Allystar (high reliability) 57.52%

Competitor 1 64.60%

Competitor 2 47.66% Table 2 RTK fixing rate using different Allystar firmware options and from competitors

Figure 9 Horizontal RTK fixed solution error in Allystar high reliability mode

Figure 10 The number of valid satellites and correction age in Allystar RTK solution

Shenzhen drone test

Allystar and competitor receivers were installed to a drone. A NovAtel receiver without SPAN was used as a reference, because

of the drone payload weight limitations. A ground track of the drone test and the drone used is shown in Error! Reference

source not found..

Figure 11 Picture of the drone

Horizontal errors from the drone test are shown in Figure 12 and the number of valid satellite and correction age are shown in

Figure 13. It is shown that Allystar receiver can provide smaller error than the competitor most time of time. The challenge in

the test is frequent correction blockages caused by bad connection to the drone. However, Allystar RTK can keep fixed

ambiguity solution, even though correction age is large.

Figure 12 Horizontal error in Shenzhen drone test

Figure 13 Used satellite and RTK correction age in Shenzhen drone test

Single vs dual-frequency comparison

Allystar dual-frequency RTK solution in high-availability mode is also compared to single-frequency RTK solutions from the

“U” and “M” companies. This kinematic test is done using route in Beijing shown in Figure 14. GPS and BeiDou observations

are used by the receivers. Horizontal error from the test is shown in Figure 15 and fixing rate of Allystar dual-frequency solution

is compared to competitors in Table 3. Based on the results, dual-frequency RTK provides significantly higher fixing rate.

There are two main reasons why using dual-frequency measurements improves the performance, even when the baseline is

short or when VRS is used First of all, a dual-frequency receiver provides redundant observations that averages out the effect

of multipath and noise over two frequencies and provide back-up, if one frequency has cycle-slip. The second reason is that

using dual-frequency enables forming the wide-lane measurement [8] combination or doing wide-lane effectively by the

LAMBDA method [6]. Because the improvement is so large and fundamental, it is recommended for customers to use dual-

frequency receiver.

Figure 14 Test route in Beijing

Receiver Fix rate

Allystar (high availability) dual-frequency 60.25%

Competitor 1 (single-frequency) 17.35%

Competitor 2 (single-frequency) 35.31%

Table 3 RTK fixing rate in dual- vs. single-frequency test

Figure 15 Horizontal error in Beijing dual- vs single-frequency test

Calgary static test

Figure 16 Calgary office antenna

Allystar RTK fixing performance in static conditions is tested using data collected from the choke-ring antenna shown in Figure

16. The antenna is mounted on top of Allystar’s Calgary office building in a medium multipath environment. RTK corrections

from UCAL IGS [9] station is used, and the length of the baseline is less than 1 km. The Allystar receivers are reset every 5

minutes during the test to demonstrate time to first fix performance.

Position error results when using GPS L1/L2 and BeiDou B1I/B2I are shown in Figure 17 and when using GPS L1/L5 and

Galileo E1/E5a are shown Figure 18. Used satellites in both solutions are shown in Figure 19, green refers to GPS/BeiDou

fixed ambiguity solution and blue refers to GPS/Galileo fixed ambiguity solution. The L5 band solution provides faster

ambiguity fixing in most cases. There are two primary reasons for this. First, the L5 band signals such as GPS L5, Galileo E5

and BeiDou B2a have better noise and multipath properties compared to the L2 band signals as was already shown in Figures

4 and 5. Secondly, the combined availability of GPS and Galileo was better in Calgary when compared to GPS and BeiDou.

Building full BeiDou constellation has progressed well and there are 44 operational BeiDou satellites in September 2020 [10].

Nevertheless, the challenge with using BeiDou-3 satellites in RTK is that many base-stations do not support full PRN range of

BeiDou. In addition, the new BeiDou L5-band signal B2a is not support by most base-stations and neither by the RTCM

standard [4]. Due to these things, it is not possible to obtain full benefit of BeiDou in RTK yet.

Figure 17 Calgary static test error when using GPS L1/L2 and BeiDou B1I/B2I

Figure 18 Calgary static test used satellites when using GPS L1/L5 and Galileo E1/E5a

Figure 19 Compare the number of used satellites between GPS/BeiDou vs GPS/Galileo Solutions

CHALLENGES AND OPPORTUNITIES

A few years ago, RTK was limited to applications that could afford high cost, large size, and high-power consumption of OEM

boards. Allystar and other mass market RTK providers have changed this so that RTK is now available for applications with

size and cost constraints. In addition, mass market RTK performance is now sufficient for even more demanding applications,

because Allystar has brought dual-frequency RTK into market. It was shown in this paper that dual-frequency RTK can provide

significant benefit compared to single-frequency RTK.

The ability of tracking GPS L2P in semi-codeless way has been an important feature in conventional geodetic receivers.

However, the feature has become far less necessary as there are already 21 GPS satellites (in September 2020) that broadcast

the L2C signal [10]. Combination of L2C signals with the other L2 band signals such as BeiDou B2I or B2b and Galileo E5b

can provide sufficient coverage of dual-frequency satellites to have a good dual-frequency RTK solution even with mass-

market receiver hardware.

Completion of full BeiDou constellations and progress in building Galileo and GPS modernization will enable transition from

L2 to the L5 band as secondary frequency. This is an important opportunity for mass market RTK, because L5 signals have

improved noise and multipath properties and this makes the difference between mass market and geodetic receivers even less

important. Even currently, there are enough satellites on the L5 band, but the issue is that many base stations do not support

BeiDou-3 PRN range or the new B2a signal. Therefore, it would be important that the RTCM organization would finalize

adding BeiDou B2a signal to the standard and base-station operators to upgrade their receivers to support BeiDou phase-3 PRN

range and B2a.

Hardware limitation on CPU power and memory resources is another issue when using mass market GNSS receiver. However,

this can be addressed by using only a limited number of observations in RTK filter and by efficient software design practices

such as avoiding unnecessary matrix operations and calculations. Another challenge is that customers like to use RTK in harsh

environments with high multipath and frequent signal blockages. This can be addressed by making RTK observations weighting

and ambiguity validation adaptable to conditions such as multipath and satellite visibility.

Access to RTK corrections could be a challenge for mass market RTK users. Conventionally, licenses to access RTK

corrections have been expensive since RTK networks were originally built for professional applications such as land surveying.

However, this is now changing, since RTK corrections providers have become more focused on mass market applications. In

addition, RTK corrections are part of the 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) Release 15

standard that can lead 4G/5G networks to include RTK corrections as a part of GNSS assistance data [11]. Accessibility of

RTK corrections can also be improved in the future by using satellite-based delivery to provide Precise Point Positioning (PPP)-

RTK corrections such as Centimeter Level Augmentation Service (CLAS) over the QZSS L6 signal [12].

Next generation mass-market receivers such as Allystar Cynosure IV are coming to the market in near future. This next

generation of the receivers have more tracking channels, more memory and more powerful CPU and they are manufactured

using more advanced Integrated Circuit (IC) technology. The performance difference between geodetic and next generation

mass-market receivers will likely become even less pronounced for typical users.

CONCLUSIONS

Allystar GNSS SoC with built-in dual-frequency RTK solution was discussed in this paper. Depending on the firmware and

receiver model, Allystar RTK engine can support L1/L2, L1/L5 or L1-only signal bands. Allystar provides high availability

and reliability firmware options for customers, depending on that what kind of characteristics are preferred by the customer. In

general, high reliability solution is more suitable for sensor integration or other higher integrity application and high availability

version is suitable for general users.

Allystar RTK engine was tested in multiple locations such as Beijing, Shenzhen, and Calgary. It was shown that Allystar RTK

engine can provide high accuracy and reliability even in challenging environments and it is more reliable than the competitors.

High performance is obtained regardless of lower power consumption and size of the Allystar GNSS solution.

Results obtained in this paper highlight the trend that RTK is become available for wide range of users and it is not limited

anymore for professional users that can afford expensive OEM boards. There are still challenges such as availability of RTK

corrections. However, the general trend is that mass market RTK challenges can be addressed in longer term and there is much

interest build necessary infrastructure to support mass market RTK.

There are multiple factors that can improve mass market RTK performance in the short term. One is transitioning from L2 to

the L5 band that will enable improved observation quality with mass market hardware compared to L2. Another important

factor is the availability of next generation mass marker receiver such as Allystar Cynosure IV that can provide larger number

of tracking channels, more memory and more powerful CPU.

ACKNOWLEDGMENTS

This project is supported by National Key R&D Program of China (No. 2018YFC1506203)

REFERENCES

1. Braasch, M. Isolation of GPS multipath and receiver tracking errors.J. Inst. Navig.1994,41, 415–435.

2. Counselman C. and Gourevitch S., “Miniature Interferometer Terminals for Earth Surveying: Ambiguity and Multipath

with Global Positioning System”, EEE Transactions on Geoscience and Remote Sensing (Volume: GE-19, Issue: 4,

October. 1981 )

3. Vollath U., Buecherl A., Landau H., Pagels C. and Wagner B., "Multi-Base RTK Positioning Using Virtual Reference

Stations," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of

Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 123-131.

4. RTCM 10403.3, Differential GNSS (Global Navigation Satellite Systems) Services - Version 3 + Amendment 1 (April

28, 2020) - A more efficient alternative to RTCM 10402.3, https://www.rtcm.org/publications

5. Larson R., Dressler R. and Ratner R., “Application of the Extended Kalman filter to ballistic trajectory estimation”,

Menlo Park, CA: Stanford Res. Inst., 1967

6. Teunissen P., “Least-Squares Estimation of the Integer GPS Ambiguities”, The General Meeting of the International

Association of Geodesy. Beijing, China, 1993

7. Qianxun Spatial Intelligence Inc., https://www.qxwz.com/en, accessed in September 2020

8. Melbourne W. (1985) The case for ranging in GPS based geodetic systems. In: Proceedings of the First International

Symposium on Precise Positioning with the Global Positioning System, 15–19 April 1985, pp 373–386

9. UCAL IGS station, http://www.igs.org/igsnetwork/network_by_site.php?site=ucal, accessed in September 2020

10. Information and analysis center, https://www.glonass-iac.ru/en accessed in September 2020

11. Ericsson, https://www.ericsson.com/en/blog/2018/11/lte-positioning-and-rtk-precision-down-to-the-centimeter, accessed

in September 2020

12. QZSS Centimeter Level Augmentation Service (CLAS) https://qzss.go.jp/en/overview/services/sv06_clas.html, accessed

in September 2020