Inertial navigation within robotics is notoriously susceptible to all kinds of interference and technical problems that can cause drift at best, and collision at worst. This article aims to help you be aware of some of the most problematic interference elements from various signals and some helpful approaches to preventing and solving them.

 

Interference Elements to Shield Your Rover From

Interference threats for navigating with inertial sensors generally involve GPS systems and components. With the rise of sensor fusion as an economic method of implementing precise navigation, however, other threats and weaknesses are emerging in this fast-paced industry.

 

Interference Element: GPS Issues

 

1. Multipath Errors

Multipath errors occur when radio waves, usually from the same source, reach a receiver at the same time. Satellite signals travel through the atmosphere and can take a direct path to the rover or can be reflected off objects close to the rover to reach sensors at different times, resulting in errors in positioning accuracy as radio frequency (RF) waves bounce off the ground and other reflective substances. Materials that absorb RF waves can reduce or prevent some multipath errors. Multi-band GPS systems are also more resistant to and capable of correcting for multipath problems for rovers and other autonomous devices.

With single-band GPS, only one frequency is used for transmitting GPS information. This does not allow the same degree of verification and cross-checking that multi-band systems can offer, which results in lower overall accuracy and increased risk of multipath errors. Multi-band systems can significantly increase the information provided to GPS sensors, which can weed out some of the incorrect data that can occur from reflections off surfaces of the ground and other nearby objects.

 

2. Physical Position

The location in which your rover will operate can also affect the amount of interference for the signals it receives. Ideally, your rover would only operate in areas with an unobstructed view of the open sky. In fact, however, temporary or persistent obstacles can have a real impact on the amount of interference that can affect positioning for your rover and other autonomous devices.

Buildings, walls, overpasses, and other large physical objects can reduce the ability of satellite signals to reach the GPS sensors in a rover. Mountains and hills can also play a role in creating geological obstacles for GPS signals. In forested areas, large trees and leaf canopies are common culprits in blocking or deflecting GPS signals to your autonomous device.

Water is one of the most common physical obstacles to clear transmission of signals. In most cases, it will be necessary to extend an antenna of some sort above the waterline to receive signals for GPS devices located below the surface.

 

3. Competing Harmonic Radio Frequencies

GPS systems that operate solely on one frequency can often be affected by competing harmonic RF signals. Harmonic signals do not transmit at the same precise frequency as the signal intended to reach the device. Instead, they transmit at integer multiples of the signal frequency. In music, harmonics are one or more octaves above or below a particular note and can have a real impact on the accuracy of single-band systems. For example, the L1 frequency is crowded with transmissions from marine vehicles, television signals, and other RF sources:

  • Television signals for channels 66 and 67 use the 781-794 megahertz frequency range and can sometimes interfere with signals received by your rover. It would never occur to most people that a broadcast television channel could make it more difficult to manage GPS signals. Your GPS-guided rover, however, could be taking instructions directly from Univision, Fox, and many other broadcast channels that operate on this frequency.
  • Aviation communications at the 12th and 13th harmonics are also located within the GPS L1 frequency band, which makes it much more likely for your rover to pick up interference from planes flying the friendly skies overhead.
  • Military signals can also jam GPS signals inadvertently or deliberately. Even a small amount of interference from these sources can add up to real issues with navigation and GPS positioning for your rover.
  • Marine very-high-frequency channels at the 10th harmonic can overlap signal frequencies to create issues with rover navigation.

As you can see, planes in the air, ships at sea, television signals, and other competing signals can have an impact on the reliability and accuracy of the GPS signals sent to your devices. Most of the time, these interference elements pose little or no problem for your rover. When they are powerful enough, however, competing harmonic RF signals can have a significant negative impact on the navigational capabilities of devices that depend on GPS.

 

4. Length of RF Coils

RF coils are used to receive and, in some cases, to send signals to assist in autonomous navigation. The configurations of the coils and their length and overall size will often have a significant effect on the amount of interference or noise that affects these transmitters and receivers. The Inertial Sense team can help you determine the ideal configuration for your specific set of needs.

 

Interference Element: Electrical Issues

 

5. High-voltage Power Lines

The effects of power lines on RF transmissions may be overblown. The American Radio Relay League, better known as ARRL, states that nearly all noise that emanates from power lines does so because of a fault, spark, or arc in the equipment. In general, high-voltage lines and cables have been shown to have a minimal effect on noise levels and interference. In fact, tree coverage in the area can create about the same amount of interference as these overhead lines. If precision is essential for your drone or rover, you may want to take the presence of high-voltage power lines into consideration when determining the best way to achieve rover protection in the field.

 

6. Power Issues

Electrical system issues and loss of connectivity can create issues with interference and less-accurate positioning for your system. Even a very short-term loss of power can prevent accurate navigation for minutes at a time. Making sure that reliable power is always available to your rover or other device is essential to maintain the right positioning for your operations. Additionally, shielding any USB 3.0 data points can prevent noise from affecting the accuracy of navigation. Noise generated by USB 3.0 connectors typically occurs in the 2.4 to 2.5 gigahertz range. Accounting for these issues by planning for reliable power and implementing shielding for data points can promote the most precise navigation for mobile devices and autonomous rovers in real-world settings.

 

7. Balancing Power and Noise

Boosting the power of the signal transmitted to your rover can also increase the amount of noise in the signal. Too much power can increase the risk of reflections and leakage for RF transmissions. Too little power, conversely, can make it much more difficult to get the signal directly to your rover or other autonomous devices. Scrambling devices in the location can also require more power to operate over the background noise of these devices. Electronics Notes has written an article explaining that electronic and RF noise is usually classified into three categories:

  • White noise is present in all frequency bands and affects all of them to an equal degree.
  • Pink noise affects frequency bands at the lower end of the spectrum.
  • Band-limited noise, as its name suggests, is limited by filters or by circuits.

Inertial Sense can help you balance power and noise to achieve better rover protection and reduce issues with navigation for your devices. This can help you reduce the effect of noise while ensuring adequate power for your GPS and RF signals.

 

Interference Element: IMU Problems

 

8. Interference in the Inertial Measurement Unit

Inertial measurement unit sensors may be subject to interference. This is generally corrected through Kalman filtering or processing units located inside the autonomous navigation system. Some of the most common issues with the inertial measurement unit include the following:

  • Ferromagnetic substances can sometimes cause some interference for the magnetometers in autonomous navigation systems.
  • Gyro drift worsens over time and requires correction to ensure accurate navigational readings. According to Analog attributes this gyroscopic drift to imperfections that are inherent in these devices and instability caused by noise.
  • Accelerometer vibrations can also affect the accuracy of readings. Machine Design notes that accelerometers are especially susceptible to transverse vibrations, which can sometimes cause nearly five percent variance for the device’s results.
  • If barometric equipment is exposed to direct airflow, errors and interference can result.

Putting It All Together

We have looked at the three major sources of interference, which include IMU issues, power problems, and GPS interference. Understanding these three primary issues can help you and your company to make the most proactive decisions regarding your approach to interference in the autonomous navigation field.

 

Awareness of Interference Elements Is Key

In a perfect world, interference with RF signals would never be a problem. Unfortunately, we live in the real world where these problems are quite likely to arise at one point or another. Quality hardware and innovative product design are essential elements in avoiding serious issues with interference in real-world situations. That’s where Inertial Sense comes in.

At Inertial Sense, we offer practical solutions for companies and manufacturing firms looking for precision sensors and autonomy platforms. Our sales team and on-staff engineers consult directly with our clients to develop the best device and rover protection against interference elements in real-world situations. Our goal is to get your device operational as quickly and reliably as possible. Give us a call today or visit us online to touch base with our team. We look forward to the opportunity to serve you.

 

Learn More:

Sensor FAQs

Navigating Dull, Dirty, Dangerous Autonomously

The World’s Smallest “Better Than GPS” Inertial Navigation System Now Available