Increasingly dense urban environments pose a significant problem to navigation systems based on the reception of sometimes weak GPS satellite signals. As ever more systems (e.g. road pricing, fleet management, emergency services, etc.) depend on reliable, uninterrupted navigation, "dead reckoning” GPS is becoming increasingly important.
Dead reckoning aids traditional GPS navigation via intelligent algorithms based on a vehicle’s distance and directional changes during GPS signal interruption. This article describes two solutions for dead reckoning GPS based on individual wheel speed or gyroscopic information:
For OEM navigation and emergency call systems: Automotive dead reckoning (ADR) GPS receiver chips are designed for in-car navigation and telematics systems using a gyro and/or wheel tick information taken directly from the vehicle CAN bus.
For after-market add-on devices: Independent of the vehicle data bus, a GPS receiver module interfaces directly to the vehicle odometer and gyro. It is therefore suitable for after-market devices such as fleet and asset management, road-pricing, and insurance systems as well as automatic vehicle locators (AVLs).
Dead reckoning GPS extends coverage to areas without GPS reception, while boosting accuracy in areas with adverse signal conditions such as urban centers having heavy multipath effects.
As millions of people migrate to cities across the globe, higher and denser building construction has become the only way to accommodate the increasing populations. Add to that the requirement to accommodate more cars and increased underground traffic routes, and you have a significant challenge to GPS navigation in modern cities.
Because GPS satellites transmit their signals with the equivalent power of a 30W light bulb from a distance around 20,000 km (12,000 mi), they arrive with typical signal strength, in the best case, of 120 dBm (1 x 10–15W). This is millions of times weaker than a typical home WiFi signal! These signals can easily be degraded an additional 20-30 dBm in city conditions, or blocked completely, further impacting the accuracy of GPS navigation.
Challenges to urban navigation
For car navigation devices, at least 4 GPS satellites must be identified and their signals received and decoded before a position can be determined. Without this, GPS navigation is impossible.
Numerous barriers to already weak GPS signals include:
Tunnels and parking garages, the worst case scenario where GPS signals are completely blocked
Multi-level roads, overpasses and bridges which can confuse GPS receiver (which road am I on?)
Tall buildings which can reflect GPS signals (multipath propagation), fooling a GPS receiver into thinking it is somewhere else
The end result of these obstructions range from minor irritation to a major problem:
For drivers unfamiliar with the area, navigation can be intermittent or fail altogether, especially when exiting tunnels and parking garages, resulting in irritation, wasted time and fuel
For commuters who may already know their way, traffic-jam avoidance services can be rendered useless
For public transportation systems such as buses and trams, the loss of expected arrival times poses an inconvenience to thousands of commuters
For commercial transportation services such as taxis, freight, and logistics companies, the loss of location overview and security of transported goods can have major financial ramifications
Emergency vehicles such as police, fire and ambulance services are prevented from reaching the location of an incident quickly
Systems used for automatic road-pricing or pay-as-you-drive insurance have insufficient data to charge for road usage
Typically the Kalman Filter as used in an inertial navigation system is estimating the drift and bias of the gyros and accelerometers along with other modeled errors in the system. These values are what is being calibrated. The calibrated values are then used to correct the estimates made by integrating the accelerations.
It's not clear if the "constant calibration" is just positional calibration so that the nav system will know the last known good location, or if it is also calibrating for vehicle variables, such as tire size.
If the vehicle ends up with different size tires or after enough wear, the odometer / wheel sensors will not be giving accurate distance measurements. If the nav system is constantly reading those sensors and calculating the actual verses sensed speed and distance, the accuracy would be greater under a variety of real-world conditions.
Technology gets reinvented many times. In 1985 a Silicon Valley company, Etak, pioneered the consumer vehicle navigation technology. They invented and marketed a system that used differential wheel sensors, magnetic compass, and map matching for navigation.
Maps were stored on a cassette tape, heading was measured with a flux gate compass, and distance traveled was measured by a variable reluctance sensor mounted next to magnetic tape stuck to the inside of the wheel rim. The two non-driven wheels were instrumented so that relative turns could be measured. Maps were displayed on a CRT using vector graphics.
Several key patents were issued to them including map matching, absolute and relative heaing sensor fusion, heading up map display, and methods to label the streets. The popular vehicle navigation systems of today have their roots in the Etak Navigator.
The system suffered from the lack of an absolute position sensor (i.e. GPS). It would be another 15 years until GPS became economical.
It did work remarkably well for the limited sensors available. It would typically navigate without "getting lost" for about 200 miles in both urban and rural environments.
Etak went through several mergers and acquisition (News Corp, Sony, TeleAtlas and now TomTom).
Agree! If it were up to marketing types, we would have ridiculous words & phrases that may not have anything to do with the product.
In my dictionary, an alternate for 'ded'uced reckoning would be 'common sense!' It surprising how fast people let go of it these days, getting addicted to GPS's & mobile phones. Unlike most places in Asia, cities in USA are laid out in grid iron pattern and are quite easy to navigate without 'artificial' aides! Try doing that in a rural town in China or India, you will need the 'positioning' advice of a number of locals before you get your distination!
Which is why I'm glad marketing people aren't in charge of (re)naming things all the time... Taking 30 seconds to look up the name meaning came up with this: dead reckoning - a homophone of ded (for deduced) reckoning.
It's been for a while - all the gyros and odometer. If thinking seriously about autonomous driving, for increased safety, one need to provide network of transponders along motorways and process the signal similar way satellites are used. There are some technical issues, like precise timing of the network == huge cost, but balanced by better efficiency.
"...typical signal strength....of 120 dBm (1 x 10–15W)."
Shouldn't this be -120 dBm? 120dBm would be 1 x 10+9 watts - a gigawatt....
Nevertheless a great article. My GPS gets totally confused in Sydney's urban canyons (and so do I if I am going somewhere I am not familiar with). I'd thought about accelerometers myself to help the GPS.
As Erebus says, the old fasioned Map is also a good solution. I keep a Sydney CBD map in my glove box for these occasions.
Dead reckoning has been used for millenia for navigation the world wide. It is amazing to me how quickly people forget that we found places just fine with maps and street signs long before GPS became readily available.
Vehicles with installed inertial navigation systems and digital maps should be more than sufficient to augment the GPS driven navigation needed today.
That said, I cannot overstress the need for driver participation in the process. If you put all of your reliance on the computers, it will not end well.
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