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 10–15
W). 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