Testing Positioning and Navigation Performance – Getting it Right

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Raghavendra Rangarao
Raghavendra Rangarao, Solution Head – APAC South – Positioning Technology, Spirent Communications

Positioning and navigation is driving the world’s most advanced applications and with the proliferation of GPS applications, coupled with the advent of other global navigation satellite systems (GNSS) such as; Galileo, Glonass and BeiDou, GPS and GNSS testing has become more mainstream.

With increasing reliance on GNSS technology, it is important for designers, manufacturers and consumers to understand what to expect from such systems. This includes formulating an understanding of the limitations and challenges of PNT (positioning, navigation and timing) technologies and how to test them.

Is GNSS testing critical? The answer is yes. Multiple factors influence the performance of a GNSS – enabled system and only accurate testing can characterize the performance of the system to ensure, that it conforms to the manufacturers quality standards and meets the expectations of the end-user. GNSS receivers are complex and adopt a series of mathematical models to derive accurate location and time information, therefore performance characterization is important and highly challenging. However, there aren’t any global standards available for GNSS testing except in case of few applications and hence most of the GNSS and GNSS based applications testing are ad hoc in nature.

Understanding GNSS

Global Navigation Satellite System (GNSS) refers to a constellation of satellites providing signals from space that transmit positioning and timing data to GNSS receivers. The receivers then use this data to determine location coordinates (latitude, longitude and altitude) with high precision, along with speed and time information. It is important to note that GNSS is the only reliable source available to derive exact location and time information.  

Trends and Overview:

It wasn’t so long ago that GPS was the only commercially available satellite positioning service. With a single satellite constellation and positioning, accuracy and signal availability were limited. As the rest of the world began to catch up, adding more and more satellites to our skies demand for accurate, available positioning—especially in urban environments—began to grow. Today it’s the age of widespread, multi-frequency, multi- GNSS positioning.

There has been significant development in the availability of new GNSS services: GLONASS is Russia’s fully operational system, BeiDou / Compass is China’s growing satellite constellation, BeiDou-3, the third phase constellation covers the Asia-Pacific region, Galileo, the forthcoming system from the European Union, set to provide open access services and restricted services and is developing apace. These global networks are supplemented by an array of further, regionally-focused augmentation systems, like Japan’s QZSS and India’s IRNSS.

The Need for Test

As satellite-based navigation and positioning becomes must-have features of space, defense, aviation, automotive, autonomous driving / ADAS systems, communication networks and many more, including consumer electronic devices, each user segment has its own set of requirement with respect to GNSS performance.  Some applications need to find a position quickly or to work with low signal levels, while for others; the users care more about absolute timing and accuracy of position. As many factors influence the performance of a GNSS enabled system, defining a set of testing guidelines or standards to meet all requirements is challenging.

To ensure that the product performs as it should, it is important to test the device’s positioning performance at a number of stages from development to production and an accurate test plan for any given GNSS application is imperative. Evaluating and selecting the right GNSS receiver for a given application is crucial, as GNSS signals are highly vulnerable to atmospheric conditions, multipath effects, RF interference and sometimes intentional threats, like; spoofing and signal jamming, which may cause significant impact on mission critical applications.

With increasing GNSS demands in a wide range of applications, modern GNSS satellites broadcast on multiple frequencies. Multi-frequency provides signal diversity for robustness to radio interference and improved atmospheric correction. For example: L2C and L5 in GPS, E5 and E6 in Galileo and NavIC L5 for civilian usage. E911 and ERA‑GLONASS are initiatives that combine mobile communications and satellite positioning to provide rapid assistance to users in case of emergencies.

Selecting the Right Test Tool

Do I have the right testing tool to validate the performance? Laboratory testing of any GNSS receiver design requires a range of tests in order to determine the functionality under both normal and special operating conditions. GNSS appears to be the panacea for all navigation and timing requirements. It is brilliant, but is it sufficient to meet all positioning needs on its own. A combination of complementary / hybrid technologies would appear to hold the answer.

The most common and well accepted basic tests where the GNSS solutions are validated include:

  1. Time to First Fix(TTFF)
  2. Static and Dynamic Position, Navigation and Time Accuracy
  3. Acquisition and Tracking Sensitivity
  4. Reacquisition Time
  5. Susceptibility to Radio Frequency Interference
  6. Antenna characteristics
  7. Multipath and obscurations
  8. Robustness against GNSS vulnerabilities: GNSS impairments, Jamming, Spoofing.

The Test Setup

The scope of testing may vary from application to application. GNSS users expect accurate and continuous PNT (Position, Navigation and Time) information from their device. With accurate, rapid and versatile test technologies, it is important to benchmark performance when subjected to various errors, test its future readiness, ensure that it is fit for deployment, check with high dynamic scenarios, check vulnerabilities & resilience in the presence of possible threats and validate GNSS performance with co-existence of other RFs.

As such there is no standard specification in order to test GNSS receiver or GNSS enabled solutions. Since GNSS is an evolving technology and the receiver manufactures define their own test specification, they do follow few basic test cases as stated above. However, experts say, this is not good enough to qualify the GNSS solution. Key factors to be considered in GNSS testing include; environmental effects which cause the delay in receiving signals or any ramp in the pseudo range calculation, causing failures in Receiver Autonomous Integrity Monitoring (RAIM).

Product designers, manufacturers and system integrators involved in GNSS development should have the right GNSS test solutions for R&D, integration, verification and production testing. GNSS test approaches may vary depending on applications, therefore, choosing the right test instruments/tools is important to check the GNSS performance and benchmark their solution in-line with respective market demands.

One of the major test equipment for testing the GNSS solution is Multi Channel RF constellation simulator. A multichannel GNSS constellation simulator is capable of handling all the tests necessary for designing, developing and integrating GNSS receivers in the laboratory. The simulator is capable of running all the above-mentioned standard tests of GNSS receiver performance, together with their individual variations and offers unique ability to control test conditions, simulate new & future signals and repeats test precisely. To sum up, accelerating development, dedicated test solutions must be one step ahead in delivering maximum performance, while assuring; accuracy, speed, availability, integrity, continuity, reliability and high-quality user experience that customer’s demand.