TECHNICAL PAPER: Performance Analysis of Next-Generation GNSS/INS System from KVH and NovAtel

Transcription

1 TECHNICAL PAPER: Performance Analysis of Next-Generation GNSS/INS System from KVH and NovAtel KVH Industries, Inc. 50 Enterprise Center Middletown, RI USA KVH Contact Information Phone: Web:

2 TECHNICAL PAPER: Performance Analysis of Next-Generation GNSS/INS System from KVH and NovAtel January 2013 Abstract KVH Industries, Inc., and NovAtel Inc., have upgraded the CNS-5000, a commercial-grade, single enclosure GNSS system they jointly developed in The system features a KVH Inertial Measurement Unit (IMU) comprised of fiber optic gyros (FOGs) and MEMS accelerometers providing inertial data at 100 Hz, and NovAtel's OEM628 TM GNSS receiver. NovAtel s OEM628 receiver is the GNSS engine. The system features the tightly coupled architecture that is a key characteristic of NovAtel s Synchronized Position Attitude Navigation (SPAN ) technology. The GNSS receiver provides aiding information for the INS and is reciprocally aided by feedback from the INS to improve signal tracking. The feedback from the INS to the GNSS engine is the deeply coupled aspect of the system. GNSS measurements are used to update the INS filter, providing high quality aiding information whenever there are at least two satellites available. This paper compares the position and attitude performance of the existing CNS-5000 (KVH's IMU paired with NovAtel s legacy OEMV platform) versus the upgraded CNS-5000 (KVH s IMU and NovAtel s next-generation OEM628). Both systems were tested in an urban canyon environment. 1. Introduction KVH and NovAtel jointly developed the CNS-5000, a single enclosure GNSS/INS system, and have upgraded the system to create the next-generation solution. KVH s patented FOG technology offers stable performance meant for the commercial market. Onboard the OEM628, NovAtel s Synchronized Position Attitude Navigation (SPAN) technology offers superior GNSS tracking performance due to its unique tightly/deeply coupled integration of the GNSS receiver and the IMU. By working together, KVH and NovAtel can offer a very robust commercial-off-the-shelf (COTS) system with a level of coupling typically only available in military systems. Applications include airborne mapping, land vehicle mapping, robotics guidance, autonomous vehicles, motor sports tracking, and precision agriculture. This paper compares the position and attitude performance of the existing CNS-5000 (KVH's IMU and NovAtel s legacy OEMV3 platform) versus the upgraded CNS-5000 (KVH s IMU and NovAtel s OEM628)[1]. Both systems were tested in an urban canyon environment. The test was conducted in a land vehicle, under normal driving conditions with the goal of showing the signal tracking benefit provided by the SPAN technology onboard the OEM628, and the navigation performance possible with the KVH IMU. 2

3 2. System Description A single enclosure was designed to keep the system small and light, and make the installation as simple as possible, as shown in Figure 1. The system weighs 5.2 lbs. (2.34 kgs), with dimensions of 6.67" x 6.0" x 3. 5" (170.2 x x 89 mm). Communication with the system is through a 37-pin circular connector. Firmware can be loaded onto the OEM628 and the IMU through the 37-pin connector. Through the same connector, commands can be sent (for configuration, for example) and system output is logged. The smaller connector above the 37-pin connector is the antenna input. The entire system operates with an input voltage of 9-16 vdc and consumes less than 15 watts. Mounting and alignment holes are provided on the baseplate of the unit. The center of navigation is clearly marked on the exterior of the enclosure to allow precise measurement of the vector between the IMU and the GNSS antenna (also referred to as the lever arm). Figure 1 KVH-NovAtel GNSS/INS System CNS-5000 The enclosure also accepts odometer inputs from quadrature encoders as well as pulse and direction encoders. Accepted encoder input levels range from RS-422 signaling to battery voltage level pulses. An input pulse frequency in excess of 45 khz is easily handled by the unit. All system configuration is completed through the receiver s (OEM628) standard serial ports or USB port accessible through the 37-pin connector. Navigation computations are done onboard the receiver. The IMU data is integrated with the GNSS data and a continuous real-time position, velocity, and attitude solution is available to the user at up to 100 Hz. Raw data can be simultaneously logged for post processing. All previous SPAN features are supported in this upgraded KVH-NovAtel joint system. 3

4 3. SPAN on OEM628 With SPAN, the GNSS and INS technologies work synergistically. Through tight coupling, the INS improves the GNSS tracking and positioning performance. Stable, high precision GNSS measurements are used to update the INS to compensate for the inertial errors that accumulate with time. Improved GNSS availability and quality enhances the INS by reducing complete outages and producing higher quality measurements to constrain any inertial error drift. SPAN s rapid GNSS signal reacquisition performance is maintained during short blockages and improved during prolonged or rapid, intermittent GNSS outages. The increased processing capacity of OEM628 receivers allow all GNSS and GLONASS satellites in view to be tracked while running SPAN. With more GNSS signals to use, SPAN s delta phase updates are more powerful on OEM628. When at least two satellites (of the same constellation) are available, a delta phase update is applied to control inertial errors. The OEM628 platform features Receiver Autonomous Integrity Monitoring (RAIM), an algorithm that detects and excludes faulty signals that cause erroneous positions to be produced by the pseudorange positioning filter. This delivers better quality position updates for the INS, especially in environments where a Real-Time Kinematic (RTK) solution is not possible. SPAN further strengthens the RAIM algorithm, allowing for intelligent measurement selection. Only the cleanest measurements are included in the solution. OEM628 allows GNSS logging rates up to 20 Hz simultaneously with raw IMU data and position, velocity, and attitude solutions up to 100 Hz. 4. KVH IMU The KVH IMU is based on the proven performance of the KVH DSP-3000 FOG coupled to MEMSbased accelerometers. The overall IMU performance is superior to existing MEMS solutions and is commercially available. For this specific pairing, the IMU has been configured to output data at 100 Hz. The KVH IMU data is compensated for temperature effects and tagged to GNSS time. All communication between the IMU and the OEM628 is handled within the enclosure, transparent to the user. KVH is the only FOG manufacturer in the world to fabricate 100% of the fiber used in its gyro products. More than 20 years of research into fiber design has resulted in the KVH E Core fiber, a proprietary class of D-shaped cross-section, non-stress induced fiber with an elliptical core. The temperature instability present in standard stress-induced fibers has historically precluded the use of FOGs in some applications. However, the elliptical core in E Core fiber acts as an optical waveguide for the FOG sensing light, resulting in stable performance over a wide temperature range that makes KVH FOGs extremely well-suited for navigation applications. KVH employs patented Digital Signal Processing (DSP) technology in its FOG-based systems. This breakthrough design virtually eliminates temperature-sensitive drift and rotation errors. As a result, the KVH IMU is well suited for 4

5 applications in which FOGs and accelerometers are combined to determine vehicle dynamic motions, as well as any application that requires precise measurement of rate or turning angle. Utilizing a mature, all-fiber circuit, the KVH IMU has no moving parts to maintain or replace so the KVH IMU lasts longer, functions better, and yields significant system life cycle savings. The specifications of the KVH IMU are provided in Tables 1 and 2. Table 1: KVH IMU Fiber Optic Gyro Specifications Bias Stability vs. Temperature (turn-on to turn-on, 1 σ) Bias Stability (constant temperature, 1 σ) Angle Random Walk (max) Scale Factor Error over Temperature Scale Factor Linearity Maximum Input Range Alignment Bias Offset 3 deg/hr 1 deg/hr 0.07 deg/ hr 500 ppm 1000 ppm ± 375 deg/s 8 mrad 20 deg/hr Table 2: KVH IMU Accelerometer Specifications Repeatability (turn-on to turn-on, max) Stability (over 48 hours, 1 σ ) Velocity Random Walk (max) Scale Factor Error Linearity Range Alignment Offset 0.75 mg 0.25 mg m/s/ hr 4000 ppm 3000 ppm 10 g 10 mrad 50 mg 5. Urban Canyon Test Procedure A test was conducted to compare the position and attitude performance of the existing CNS-5000 (KVH s IMU and NovAtel s legacy OEMV platform) versus the upgraded CNS-5000 (KVH s IMU and NovAtel s OEM628 receiver). Both systems were tested in an urban canyon environment. The test was conducted in a land vehicle, under normal driving conditions with the goal of showing the signal tracking benefit provided by the SPAN technology onboard the OEM628, and the navigation performance possible with the KVH IMU. The van was driven through real obstructions, namely downtown Calgary. Both receivers were in single point mode. The real-time trajectory computed onboard the SPAN-enabled OEMV628 was logged at 10 Hz. 5

6 Figure 2 Plot of the test route taken through downtown Calgary, overlaid on GoogleEarth imagery. The small camera icons in Figure 2 indicate the location that the photos shown in Figures 3 and 4 were taken. As the photos illustrate, the test was undertaken in fully urban conditions, with severe multipath effects and obstructions. Normal driving behaviors were followed, only stopping as directed by traffic control signals. Vehicle speed varied from 0-50 km/hr. Figure 3 Figure 4 Photo 1 of downtown test route, showing full urban conditions. Photo 2 of downtown test route, showing full urban conditions. 6. Downtown Test Results There were a total of 3000 seconds of the test in true downtown, urban canyon conditions. The results of the comparison test (Figures 5, 6, 7) show the improved performance of the upgraded CNS-5000 versus the existing CNS-5000 in an urban environment experiencing numerous signal blockages from tall buildings and pedestrian overpasses. Because the upgraded CNS-5000 with the OEM628 tracks GLONASS as well as all in-view GNSS satellites, the total number of satellites tracked was nearly doubled (Figure 7). 6

7 Figure 5 3D Position Error, upgraded CNS-5000 (blue) versus existing CNS-5000 (red) Figure 6 Azimuth Error, upgraded CNS-5000 (blue) versus existing CNS-5000 (red) 7

8 Figure 7 Satellite Tracking, upgraded CNS-5000 (blue) versus existing CNS-5000 (red) With additional signals tracked, the pseudorange positioning algorithm could be very selective about which measurements were used. The OEM628 uses a RAIM method to detect and reject multipath or direct reflected signals. The upgraded CNS-5000 further aided RAIM with inertial positioning, allowing better rejection of poor measurements, especially when fewer satellites were available. With the improved pseudorange positions, more positions were accepted as an update for the inertial filter, which increased the frequency of updates. Due to the increased number of satellites tracked, many more phase updates were available with the upgraded CNS Table 3 shows that the resulting INS position error was reduced by 50% horizontally and 64% vertically. The upgraded CNS-5000 trajectory was much smoother and repeated passes around the test route agreed more closely (Figures 8 and 9). Table 3: Position, Velocity, and Attitude Errors (RMS) 2D Position Error (m) Height Error (m) 2D Velocity Error (m/s) Up Velocity Error (m/s) Roll Error (degrees) Pitch Error (degrees) Azimuth Error (degrees) Upgraded CNS Existing CNS

9 Figure 8 Turn from 3rd Street to 9th Avenue in downtown Calgary (Existing CNS-5000 = red line. Upgraded CNS-5000 = green line. Note: Test vehicle is turning to the right in this image.) Figure 9 Turn from 9th Avenue to 1st Street in downtown Calgary (Existing CNS-5000 = red line. Upgraded CNS-5000 = green line. Note: Test vehicle is traveling toward vehicle in this image.) 7. Discussion The upgraded KVH CNS-5000 system provides excellent signal reacquisition performance with the integrated NovAtel OEM628 receiver. Although not detailed in this paper, in 95% of the samples, the KVH CNS-5000 with the OEM628 receiver was tracking all available L1 signals in 0.6 seconds. This is compared with nearly 11 seconds for the GNSS-only receiver, as with competitive products. With the upgraded KVH CNS-5000 design, there is very little delay in reacquiring the L2 signal after L1 is reacquired. On average, the upgraded KVH CNS-5000 reacquired all L2 signals within 0.7 seconds of L1 recovery, while the GNSS-only receiver took 5.1 seconds to acquire the L2 after L1. RTK ambiguity resolution can begin much faster with the SPAN receiver, since the L2 signals are available much faster. As a result, the SPAN system has many more accurate GNSS measurements available to produce 9

10 high-quality fixed integer position solutions. This keeps the INS error to a minimum by keeping the GNSS outages shorter. Many GNSS/INS systems may use a NovAtel receiver to time stamp the IMU data. However, with the deeply coupled integration of the CNS-5000 and the SPAN enabled OEM628 receiver, substantially more GNSS signals are available during conditions of intermittent blockages. With more GNSS signals, the INS filter is more frequently updated, resulting in a much stronger navigation solution since free navigation periods are minimized. A commercial-grade IMU in a SPAN system can perform as well as other high-quality, more expensive inertial solutions loosely coupled with a GNSS receiver. In the CNS5000, the GNSS outages experienced by the system are much shorter. Using direct GNSS carrier phase updates in the GNSS/INS navigation solution is also another key feature of the CNS-5000 system. Differenced carrier phase measurements can provide a very accurate measurement of position displacement. This process is sometimes referred to as satellite odometry [2]. As long as a minimum of two satellites remain in view, the SPAN filter has access to update measurements with just a few millimeters of variance. The CNS-5000 system performs phase updates frequently, exploiting the highquality carrier phase measurements even when RTK is not being utilized. Especially in high multipath environments, GNSS-based updates to the INS filter must pass quality checks to ensure that erroneous updates are not applied. The SPAN firmware on the OEM628 automatically detects and rejects poor quality position and delta phase updates. The SPAN firmware also continuously monitors the IMU data to automatically detect moments of zero velocity, and applies zero velocity updates whenever possible. 8. Summary The upgraded CNS-5000 with NovAtel s OEM628 receiver affords the end user added capability and improved navigation performance. The upgraded CNS-5000 will allow the ability to leverage multiple constellations, including GLONASS, Galileo, Beidou, and the U.S.-based GNSS satellites. The upgraded CNS-5000 can deliver attitude accuracy that is slightly under degrees in roll and pitch, and 0.06 degrees in azimuth. In addition to having the ability to track multiple constellations, the OEM628 platform features RAIM, an algorithm that detects and excludes faulty signals that cause erroneous positions to be produced by pseudorange positioning filters. This delivers better quality position updates for the INS, especially in environments where an RTK solution is not possible. SPAN further strengthens the RAIM algorithm, allowing for intelligent measurement selection. Only the cleanest measurements are included in the solution. The existing CNS-5000 with NovAtel's OEM3 is currently used in a wide variety of applications that range from aerial survey and ground mobile mapping to autonomous vehicle navigation; it has been in full-scale production since The upgraded CNS-5000 with the OEM628 receiver from NovAtel, deeply coupled to the KVH inertial solution within a single enclosure, will be ready for the market in April of

11 9. References [1] The initial basis for comparison in this paper is found in "Performance of a Deeply Coupled Commercial Grade GPS/INS System from KVH and NovAtel Inc.," by Sandy Kennedy, NovAtel Inc., and Jim Rossi, KVH Industries, Inc., presented at IEEE/ION Positioning Location and Navigation Symposium (PLANS) [2] B. Soon et al., Performance Analysis of an Integrated Navigation System in Urban Canyon Environment, Proceedings of the International Global Navigation Satellite Systems Society Symposium 2007, Dec 4-6, 2007, University of New South Wales, Sydney, Australia. For more information on the products discussed in this paper, visit: Contact KVH North America: Sean McCormack KVH FOG/OEM Sales Manager Direct Tel: smccormack@kvh.com EMEA & ASIAPAC: Christine Marion KVH International FOG/OEM Sales Manager Direct Tel: cmarion@kvh.com KVH Industries, Inc. 50 Enterprise Center Middletown, RI USA Tel: Fax: KVH and E Core are registered trademarks of KVH Industries, Inc. KVH Fiber Optic Gyros and Digital Signal Processing technology are covered by one or more patents; details available at All other trademarks are the property of their respective companies 11