DGPS + UWB

Seamless Outdoor-to-Indoor Positioning

Ubiquitous location and navigation represents an ideal that many in industry and science pursue. That user expectation, however, remains currently unattainable due the difficulty of detecting GPS satellite signals indoors. Once it has been achieved, we will soon arrive at seamless outdoor-to-indoor pedestrian positioning and navigation.

Under the best-case clear signal conditions, the nominal received signal-to-noise ratio (SNR) for L1 C/A code is –19 dB. Since the typical detection threshold for a satellite's signal is +14 dB, a processing gain of 33 dB is required in order to detect the signal. However, signal attenuation and fading indoors can require an additional 40+ dB of gain (on top of the 33-dB gain requirement for the nominal case) by the receiver to detect to the signal. This is normally not possible with a standard GPS receiver. Furthermore, indoor dilution of precision (DOP) and user equivalent range error can increase to 10–100 and tens of meters, respectively. An alternative method is crucially needed in order to ensure reliable outdoor-to-indoor location and position continuity. Combining ultra-wideband (UWB) with GPS offers a possible solution.

UWB radio frequency (RF) signals have several characteristics that make them superior to GPS signals in poor to limited signal environments. UWB ranging provides the capability to augment GPS through high accuracy ranges. Furthermore, UWB's ability for fine time resolution and its robust performance in high multipath environments could enable a code GPS navigation system, such as for pedestrian positioning, to remain operational when the user moves indoors.

UWB-GPS integration may enable many important applications, because of UWB's ability to make measurements in high-multipath environments. From fire rescue to tracking of personnel and equipment in companies, this can increase safety, productivity and surveillance in the workplace. A proper UWB network in outdoor applications could facilitate positioning in urban canyons, deep valleys, osbstructed construction or mining sites, and under heavy foliage.

Pedestrian Navigation

Even with microelectrical-mechanical inertial measurement unit (MEMS IMU) aiding, GPS performance largely depends on the high variability of the signal environment.

Pseudolites utilize the same frequency as GPS signals and hence require little modification in the processing software. However, pseudolites may potentially interfere with in-band GPS signals, and indoor pseudolite signals also risk of significant multipath errors.

Code DGPS. Local-Area DGPS is one of the simpler forms of DGPS, where a stand-alone GPS unit is placed on a well-known, surveyed point. A nearby rover can then correct or reduce the satellite clock error, orbit errors, and atmospheric errors. Although the errors are time and spatially correlated, the distance between the DGPS base station and rover can be hundreds of kilometers.

However, spatially uncorrelated errors such as multipath remain, and indoor multipath is much more complex and difficult to predict than outdoor multipath. Aside from a changing satellite geometry, indoor multipath is also affected by satellite elevation, the material of the building, location of the building, and the location of the receiver within the building itself. It is also difficult for a receiver to differentiate between tracking a weak, but correct signal, and a potential strong and incorrect multipath signal; with the latter possibly giving way to large ranging errors.

The ability to acquire and track signals constitutes another indoor problem. Attenuation is a function of the material that the signal passes through. If the building consists of concrete blocks, signal attenuation can be quite severe.

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