1. Technical Field
Embodiments of the present invention relate generally to global navigation satellite systems (GNSS) and, more particularly, but not exclusively, to a system and method that extends navigation capability to enclosed areas that are substantially blocked from receiving satellite signals, such as the indoor area of buildings.
2. Description of Related Art
A global navigation satellite system (GNSS) consists of a network or constellation of satellites in orbit around the Earth, in which the satellites transmit signals that are used by terrestrial receivers for purposes such as navigation. The most established and widely used GNSS system at this time is the GPS system operated by the United States of America. Other GNSS systems include the GLONASS system operated by Russia, and the forthcoming European Galileo and Chinese Compass navigation systems. The present invention is described herein in terms of the GPS system, but it will be clear to those skilled in the art that the principles of the present invention may be used in the context of other GNSS systems.
The GPS system includes between 24 and 32 satellites in medium Earth orbit. Each satellite continuously transmits a navigation message that is modulated by its own unique PseudoRandom binary Code (PRN) that is orthogonal to all the other PRN codes from the other satellites. The GPS ranging code that is available for public use is the “Clear Acquisition” (C/A) code, which is a PRN code of 1,023 bits. The navigation message includes such information as its time of transmission, an ephemeris (plural: ephemerides), which precisely describes the satellite's orbit, and an almanac, which contains coarse orbit and status information for all the satellites in the network. The ephemeris is updated every two hours and is generally valid for four hours.
At the receiving end, the GPS receiver generates its own copies of the C/A codes, nominally synchronized with the satellite transmissions. When the receiver receives the C/A codes of the currently visible satellites (typically 8-12 satellites in open terrain), the receiver cross-correlates its copies of the C/A code with the received C/A codes to identify the satellites whose transmissions have been received. The ranges to the satellites may be determined by subtracting the time of transmission from the time received (to get transit time), and dividing by the speed of light. Knowing the ephemerides, the receiver can compute the locations of the satellites at the times of the transmissions.
If the receiver's clock were perfectly synchronized with the satellites, then signals from three satellites would suffice to triangulate the position (horizontal coordinates x and y, and elevation z) of the receiver. Because clocks that could be synchronized that well with the atomic clocks used by the satellites are far too expensive for routine use, signals from four or more satellites are used to determine x, y, z and the time offset dt of the receiver's clock from the GPS clocks. If only the horizontal coordinates are needed for navigation then signals from three satellites suffice.
FIGS. 1A and 18 illustrate the above process by which conventional GNSS receivers obtain position information from GNSS satellite signals. In FIG. 1A four satellites, Sat-1 to Sat-4, transmit navigation messages at times t1 to t4. The four signals are received by a GNSS receiver, shown being held by a user. The receiver identifies the four satellites from each satellite's unique C/A code. The range ρ to each satellite is determined, as noted above, by subtracting transmitted time (t1 to t4) from the received time and dividing by the speed of light “C”. From the ephemeris, the coordinate position of each satellite at the time of transmission may be determined, i.e. position of Sat-1 (x1, y1, z1), Sat-2 (x2, y2, z2), etc. FIG. 1B shows how the range and position values may be plugged into the four equations to determine the coordinate position (X, Y, Z) of the GNSS receiver, and the time offset “dT” of the receiver relative to the satellite clocks.
An aspect of GNSS based navigation is that navigation generally cannot be performed indoors, since building walls and roofs act as barriers that block or diminish the satellite signals. Similar problems occur in some outdoor areas such as natural canyons, urban areas lined with tall buildings (“urban canyons”), and deep mountain valleys. In these environments the satellite transmissions are either unavailable or have power levels that are very low, such as below the background noise power level, so as to be unusable by a standard, unmodified GPS receiver.
This is a problem since an estimated 80% of daily activity occurs in indoor environments, such as homes, offices, shopping areas, and elevators. Further, continuity of service is disrupted when, for example, a person walks from the street into a shopping mall or building, or drives into an underground parking lot or through a tunnel.
A number of attempts have been made to resolve this problem. In one approach, networks of physical sensors, such as infrared, ultrasound, or pressure sensors, are distributed throughout a building or enclosed area. A problem with physical sensors however is the need for a wide deployment of infrastructure.
Another approach makes use of a different type of sensor that allows positioning at the user end using autonomous means. These include odometer, accelerometer, gyroscope, and magnetometer type sensors. The principle of this approach is to use GNSS signals when available, and to switch to the other sensor when GNSS signals are not available. This approach however requires very accurate modeling of the user's movements to be effective.
Other approaches use modern telecommunication technology rather than physical sensors. For example, one method uses mobile telecommunication networks such as GSM and UMTS, implemented by techniques such as TDOA, E-TDOA, and “Angle of Arrival”. This approach is considered impractical due to the requirement that for positioning, a minimum of three base stations have to be seen from the mobile terminal. The approach also suffers from positioning inaccuracy caused from signal multipath.
Another telecommunication approach makes use of free or publicly available infrastructure used for other purposes, such as mobile Internet access or WiFi. In this case, GNSS calculations are based on time measurements. However, time information supplied by non-GNSS sources is generally not accurate, causing a rough and jumpy effect on the resulting navigation outputs. For accurate implementation the time reference needs to be seriously upgraded as compared to current WLAN time capabilities.
Yet another location calculation technique is based on the Received Signal Strength Information (RSSI). This however requires the system to increase the number of Access Points to a level that is much greater than the number required for telecommunication purposes. These techniques also suffer from positioning inaccuracy caused from signal multipath.
Some approaches to the indoor navigation problem are GNSS-based. For example, one technique uses “pseudo satellites” or “pseudolites”, which creates a local terrestrial constellation of a few satellites (generators for instance). A problem with this approach however is in achieving synchronization between pseudolites. Additionally, this technique suffers from positioning inaccuracy caused from signal multipath.
Another GNSS-based technique involves placing GNSS repeaters in the indoor area. The repeaters however merely transmit information of an outside location. As a result, this technique does not really provide indoor navigation information.
Accordingly, current indoor coverage technologies generally provide only limited and inaccurate navigation in building indoors and other confined or blocked spaces. Further, many of these technologies require some type of modification of the GNSS receiver to enable it to recognize and use aided or augmented information. This is inconvenient, increases the cost of the receiver, and is useful only in areas that happen to have the particular technology installed.