Mastering True North: 5 Ways to Determine Your Absolute Heading
Have you ever turned on your GPS, but still been totally confused on wether to turn left or turn right? This happens because the GPS has not determined which which way is north yet! Prior to GPS, we relied on the Magnetic Compass more frequently, but it has its share of challenges too. This article reviews five fundamental ways to determine absolute Heading with respect to true North ordered by least accurate to most accurate.
1. Magnetic Compass
The magnetic compass utilizes the Horizontal Component of the Earth’s Magnetic field as a reference to determine North. Accuracy is limited by three factors: local magnetic field disturbances , accuracy of the roll/pitch “levelling” measurement, and the accuracy of the magnetic sensor itself.In practice, local magnetic field disturbances present a huge challenge to the magnetic compass. These fields maybe permanent, transient, or slow-changing and are classified as hard-iron and soft-iron effects. In a land vehicle or battery powered device, they can easily move the heading by several degrees.
Typical Achievable Accuracy: 2 Degrees or worse
Primary Limitation: Magnetic Disturbances
2. Single-Antennae Dynamic GPS
A stationary GPS receiver can not determine heading; however, once moving at greater than 1m/s a so called “Course over Ground” or “GPS Heading” is determined. For a wheeled vehicle that is not slipping GPS Heading and True North heading are both aligned. The accuracy achieved is a function of vehicle speed.
GPS Velocity accuracy (without RTK) is typically about 0.05m/s, therefore at 15m/s (30mph) the accuracy is limited to 0.2 degree. However at 3mph, the accuracy is 2 degrees — there are real challenges with low-speed initialization. Low-speed GPS-heading determination will be discussed in depth in a subsequent blog. GPS Heading also can not differentiate between forward and backward travel, and it is easy to get a 180 degree heading error if a system initializes when vehicle is in reverse. Finally, the solution is not good for aerial vehicles or tracked vehicles where the vehicle heading may be different from course over ground.
Typical Achievable Accuracy: 0.5 Degree
Primary Limitation: Low-Speed Accuracy, Doesn’t work on a lot of vehicle types
3. Dual-Antennae GPS Heading
An improvement on single-antennae GPS heading, is dual-antennae GPS heading. In this case, two GPS antennae are typically placed at one meter or more apart. Using RTK techniques, the vector between the two antennae is determined with about one centimeter precision. This leads to 0.01m/1m radian or 0.6 degree accuracy at for one meter antennae separation. Larger antennae spacing will further improve accuracy.
Dual antennae heading fixes a lot of the problems with single-antennae heading. It works at any speed and on any vehicle. The major downside is the distance between the two antennae should be relatively large, and it requires a very sensitive RTK fix between the two antennae.
Typical Achievable Accuracy: 0.6 Degree
Primary Limitation: Accuracy is dependent on distance between the Two Antennae, as well as maintaining very good GPS signal conditions.
4. Celestial Compass
Using the Sun, Moon and Stars to determine heading is probably the oldest heading determination technique of all. Today celestial calendars are incredibly accurate. Given a knowledge of local Latitude and Time, heading is quickly determined from the position of the Sun or Moon (single-body celestial).
Using sky polarization such as SkyPass from Polaris Sensor heading is determined on a partly cloudy day without direct line of site visibility. Star tracking is an even more accurate method. Accuracies below 0.05 Degrees are readily achievable with a good celestial compass. But there are some challenges too. Celestial Compass’s are specialized expensive solutions. Anything that prevents a sky view such as heavy clouds and dense trees will likely prevent heading determination. Day and night transitions also present challenges.
Typical Achievable Accuracy: 0.1 Degree
Primary Limitation: Accuracy is dependent on the weather / sky-conditions
5. North-Finding Gyro
A North-Finding Gyro or Gyro-Compass works like a magnetic compass except it measures the horizontal component of the Earth’s rotational rate. Unlike the magnetic field vector, the Earth’s rotation vector (what spins the Earth) is super stable and there is no localized way to perturbe this reference vector.
A high-accuracy (<0.1 deg/Hr) and low-noise gyro (<0.01 deg/rt(Hr)) can accurately detect the vector and precisely determine heading to high-accuracy. True North accuracy can be better than 0.1 Degree. The traditional challenge with North Finding gyros is the high-cost. Another challenge is sophisticated algorithms are required to manage any ambient motion during the North Finding phase. ANELLO is seeking to make North Finding gyros affordable within the next one to two years.
Typical Achievable Accuracy: 0.1 Degree
Primary Limitation: Cost (Today)
True North vs Magnetic North
Techniques 2–5 all determine true north where as a magnetic compass determines magnetic north. Magnetic north and true north are related by the magnetic declination. The world-magnetic model provides an accurate way to correlate the two measurements. https://www.ncei.noaa.gov/products/world-magnetic-model
ANELLO Solutions
ANELLO’s standard EVK implements techniques Single and Dual-Antennae based GPS Heading. ANELLO has a demo integration with the Celestial Compass from Polaris Sensor. ANELLO is working on next generation solutions that will enable a North Finding Gyro.