For many of us, satellite navigation is almost indispensable in our day-to-day lives. It’s a technology that is continually improving. But paradoxically, as navigation technology gets better, we rely on it more – and so when it does fail us, we notice the lack of it even more keenly.
For example, you drive your car out of a parking garage in an unfamiliar city, and check your map screen to get your bearings. Unfortunately, you see the location jump around wildly as your navigation device tries to figure out your location – or maybe it shows no clear position at all. You have to decide which way to turn as you leave the building, so you need to know where you are and which way you’re facing. The navigation system lets you down at just the moment when you most depend on it.
It’s frustrating. But, before we blame the manufacturer, let’s consider what a challenging task satellite navigation is. We can then take a look at dead reckoning technologies, which fill in the blind spots of GPS and other GNSS (Global Navigation Satellite Systems) to make a complete, reliable navigation system.
Weak and far away
The first, and greatest, problem is that GNSS satellites are far away, 12,500 miles (20,000 km), and have surprisingly little power. Receiving a GPS signal is like switching on a light bulb in Paris, and seeing it in London.
GPS satellites often have to work under meagre power budgets. The solar panels of a modern navigation satellite only generate around 3,000 watts, and most of that is allocated to running atomic clocks, gyros, battery chargers and other systems. Less than 100 watts is available for transmitters. Compare that to TV or radio stations, which have transmitter power measured in tens of thousands of watts.
By the time it reaches your phone or car navigation system, the power of that GPS signal is tiny: only about 10-16 watts. But it gets worse because the signals are also blocked or garbled by terrain, buildings, the thinnest layers of metal, foliage and rainfall, radio interference and multipath reflections.
Lost in the Urban Canyon
So, when you drive out of a building or a tunnel, your vehicle’s navigation system faces a host of challenges. In a city, the sky may be mostly blocked by buildings – the so-called ‘urban canyon’ effect. Then, few satellites are visible, and all in the same region of the sky, which makes triangulation less accurate. Before the system can acquire a good navigational fix, it must wait for enough satellites to come into view.
For systems without internet this ‘Time to First Fix’ (TTFF) can be longer. They can’t use mobile phone and WiFi signals to augment the satellite fix, and may also have to wait for the satellites to slowly transmit updated orbital position data.
This is why your car navigation system can take tens of seconds to pinpoint your location – or even minutes.
Dead reckoning to the rescue
The key premise of dead reckoning technology is to augment radio signals with a completely independent navigational method. Dead reckoning complements GNSS and other radio frequency systems with physical acceleration sensors which aren’t affected by the problems that degrade radio signals.
In a city test, the u-blox EVA-M8E GNSS module with dead reckoning was at least two to three times more accurate than a GNSS module. With the modules under the vehicle dash, the EVA-M8E achieved 2x better positional accuracy, 2x better height accuracy, and 13x better heading accuracy.
In that challenging scenario, the raw GNSS track resembles a plate of spaghetti, suggesting the vehicle is wandering on nearby buildings – not on the road. Navigation software hides these errors by adjusting the position to the nearest road (and often augments the GNSS data with other sources like cell towers and WiFi access points). Sometimes, however, the software guesses wrong, and GNSS will confusingly claim you are on another nearby road, not at your actual location. So we see our GNSS location scoot from one road to another as the software changes its guess. Dead reckoning is far more accurate, producing a track that matches the road, not a plate of spaghetti.
Moreover, when radio is totally blocked, such as inside a building or tunnel, dead reckoning still functions. Tested in a 4km tunnel, the EVA-M8E maintained a reasonable accurate location, which of course immediately corrected once the vehicle emerged into a view of the sky once again.
Untethered dead reckoning’s advantages
The EVA-M8E is an untethered dead reckoning (UDR) module. Unlike a tethered dead reckoning device, it does not need to receive wheel speed information or have any connection with a vehicle besides power. Instead, it relies solely on its own gyroscopic sensors and accelerometers. This self-contained design makes it easy to integrate anywhere, including as an aftermarket add on. There are fewer potential maintenance or compatibility issues with different vehicle models.
Industry’s smallest UDR product, with dev kit support
With its 7x7x1.1 mm LGA 43 package, the EVA-M8E is the industry’s smallest UDR product, with a low cost of ownership making it ideal for high volume, and is tuned for making it ideal for automotive applications.
For evaluation of u-blox’s UDR technology, the C93-M8E kit is built around the EVA-M8E module. It includes antenna, real time clock and peripheral components, in a small case ready for mounting in a test vehicle.