A satellite navigation receiver can obtain a navigation solution (i.e., positioning information) provided that the receiver has reliable signal reception from a number of GNSS satellites to which the receiver is in contact. Reliable signal reception it typically enabled only under certain operating conditions, that is, an open sky environment when there are no obstacles to radio signals propagating from selected navigation satellites to the receiver's antenna. Any antenna blockage by natural or artificial obstacles (e.g., high trees with dense foliage, vertical walls of buildings, bridges, urban canyons, and structural elements of moving vehicles with a mounted GNSS antenna, and so on) will deteriorate the quality of signal reception. As such, the accuracy of these navigation solutions may suffer greatly including a full loss of the ability to accurately provide GNSS positioning in any way. If a GNSS receiver is integrated in a vehicle control system, even short-term GNSS positioning accuracy loss can result in a complete failure of the vehicle's control system and be unacceptable from at least a vehicle and/or human operator safety perspective.
A GNSS receiver with a single antenna measures coordinates and velocity vector components (to the extent the antenna is in motion) only for a single conditional point, the so-called antenna phase center (PC). If the GNSS antenna is fixed onto a body of a movable object, the measured coordinates and PC velocity are taken as coordinates and velocity of this movable object. As such, given the singular nature of the data, any orientation measurements of the movable object using a single antenna GNSS receiver is impossible given the need for multiple coordinates (e.g., roll, yaw, pitch, etc.) to determine orientation.
To determine vehicle orientation using a GNSS system, two or more antennas should be placed at different points on the vehicle body to measure their coordinates (one relative to the other) in the vehicle's body coordinate frame. In this way, orientation parameters can be calculated from simultaneous PC coordinate measurements of using a single-antenna (or one multi-antenna arrangement) GNSS receiver. It should be noted that two antennas allow a further determination to be made with respect to the vehicle's attitude (i.e., up to one turn about the baseline—a line connecting the PC of both antennas). Further, full orientation of the vehicle in the GNSS system can be determined by three or more spaced antennas.
A well-known method of avoiding short-term interruptions in the quality of navigation solutions is the multiplexing of a GNSS receiver and an inertial navigation system (INS). As will be appreciated, INS is capable of measuring coordinates, velocity and orientation of a vehicle irrespective of the availability (or unavailability) of any external information. However, errors with INS-only measurements tend to continuously grow (e.g., increasing drift), and INS navigational data is typically used in a motion control system only during a limited time period. A joint use of GNSS and INS allows for the correction of INS accumulated errors over all measured parameters (i.e., coordinates, velocity vector components, and orientation parameters). In this GNSS-INS joint operation, GNSS measurements are used for the elimination of potentially continuously growing INS errors for those vehicle trajectory paths for which GNSS solutions are fully accessible. During these paths, the INS solution typically has acceptable errors and needs no additional correction. At the beginning of the trajectory paths where there exist poor GNSS conditions (i.e., the GNSS solution itself is unacceptable) or has short-time accuracy degradation, the INS solution will still have acceptable errors with these errors most likely increasing over time. If the GNSS interruption is short in relation to the INS error growth the INS-only solution may be used to continue the entire navigation solution over this short time GNSS degradation interval. In addition, a continuous correction of INS errors with GNSS measurements enables full vehicle orientation to be determined even in case of single-antenna GNSS receiver. When INS is multiplexed with a dual-antenna GNSS system, INS measurements make it possible to permanently determine a rotation angle about the baseline which is impossible for a single dual-antenna system.
A key element of INS is the use of a well-known inertial measuring module (IMU). An IMU consists of a set of inertial sensors (e.g., gyros and accelerometers) rigidly fixed to a common base. A three-dimensional vector basis called an IMU Measurement frame (MF) is associated with the common base and the sensitivity axes of the separate sensors have a constant orientation relative to the MF. As will be understood, the coordinates, velocity vector components, and orientation measured by the INS are MF origin coordinates, MF origin velocity components and MF axes orientation, respectively.
In order to successfully multiplex the INS and GNSS receiver, the antenna's PC coordinates relative to MF should be equal to the pre-set values over the whole period of servicing of the multiplexed system. Further, the origin of MF coordinates needs to be as close to the antenna element's PC as possible. For example, an arrangement having the IMU inside the antenna housing typically satisfies these requirements, and PC coordinates with respect to MF origin can be determined at the design stage of a combined housing for the IMU and antenna.
If any information about physical parameters in proximity to the PC is used in the algorithms of the receiver, sensors of these parameters may be also located inside of the antenna housing. An example of such sensors are magnetic field vector component sensors or atmospheric sensors which measure pressure, humidity level, and ambient temperature directly at the point of receiving a GNSS signal.
In a GNSS navigation system in which the GNSS antenna and GNSS receiver are single modules connected by a RF cable, the distance between the antenna and receiver is limited by signal fading within the RF cable itself and having a range in the area of a few tens of meters. To use measurements from antenna sensors in signal processing algorithms, they need to be sent from the antenna to the receiver, and the measurements taken from all the antenna sensors need to be related to the receiver's time scale (i.e., the same time scale as that of the operating algorithms). This typically requires the transmission of a synchronization signal from the GNSS receiver to the GNSS antenna which can bind the moments of sampling of sensors and receivers in terms of the applicable time scale.
Processing a navigational signal received by a GNSS antenna necessarily requires the consideration of various antenna characteristics. As such, a single GNSS receiver capable of connecting different type antennas needs to determine antenna type and obtain numerical values of antenna parameters. This may be facilitated by reading, just after the antenna has been connected, such numerical values from a memory (e.g., non-volatile memory) resident inside the antenna. In addition, a receiver's firmware must have the capability of adjusting/reprogramming sensors and/or the non-volatile memory module inside the antenna. This means that a two-way information transfer channel is required between the antenna and receiver.
One solution of addressing two-way data exchange between an antenna and receiver is employing an additional cable through which the information is transmitted according to any well-known digital communication protocol. In this case, the antenna and receiver are connected by two cables: one RF cable to transmit GNSS signals and another cable to transmit information, with additional connectors serving to connect the respective communication links to the antenna and receiver housings. If the antenna and receiver are used within an existing cable network (e.g., a built-in RF-cable network as installed by the original manufacturer of a particular transportation means, for example, a car, airplane or boat), such an approach will require extra cable work which is not always possible or practical.
Alternatively, two-way communication between the antenna and receiver is provided using signals whose spectrum is concentrated in the low-frequency portion (i.e., from 0 Hz up to 5 MHz), which is considerably lower than the GNSS spectrum in the range of 1.2 GHz-1.5 GHZ. Such a difference in characteristic frequencies allows for a simple division of GNSS signals and the signals from sensors with the help of well-known frequency-selective techniques in the field of radio engineering. In this way, the two-way data communication between antenna sensors and the navigation receiver can be made through the same RF cable as that of the transmission of the received GNSS signal from the antenna element to the receiver RF path. The use of a common RF cable allows for the use of the available cable network to connect the antenna and receiver and eliminates any possible complication in the mechanical design of the antenna housing due to additional cabling/connectors, for example.
There are a number of known GNSS antenna configurations utilizing additional sensors, non-volatile memory and other additional information sources/receivers inside the antenna. For example, U.S. Pat. No. 8,446,984 to T. Kelin et al. describes a method of transmitting configuration parameters from a GNSS antenna via a RF cable using additional amplitude modulation of the GNSS signal as transmitted from the antenna module to the receiver. Signal demodulation and extraction of configuration data are performed in the receiver in the background mode during routine automatic gain control (AGC) operations. This technique does not require additional hardware blocks or special cable transceivers to receive and demodulate the additional signal. However, a potential drawback of this solution is principally a low communication rate, and the communication channel operates in one direction (i.e., from GNSS antenna to GNSS receiver).
United States Patent Publication No. 2011/0285584 to H. Le Sage describes a measuring system comprising different sensors integrated into a telecommunication antenna. Information from the sensors is transmitted to external users through a single cable. A main disadvantage of this method is the availability of the additional cable resulting in complicating the cabling system and adding extra mechanical connectors to the antenna and receiver, respectively.
U.S. Pat. No. 7,212,921 to M. Jeerage describes a satellite navigation system wherein a measuring unit consisting of a combined GNSS antenna module and IMU is connected to a remote GNSS receiver via a single RF cable. The GNSS signal and IMU measurements are transmitted to the receiver via a single RF cable. The antenna's low noise amplifier (LNA) and IMU are powered from the receiver through the same cable, and the power voltage, GNSS signal, and IMU signal differ in frequency and can be separated by a system of filters.
Chinese Patent Application No. CN 101765787 describes an ultra-tightly coupled GNSS-IMU system comprising an integrated antenna with built-in IMU. One potential drawback of this system is the additional cable to transmit measurements from the IMU to the remote GNSS receiver.
Chinese Patent Application No. CN 102590842 describes an integrated antenna configuration with built-in IMU which includes a transmission of GNSS signal, IMU measurements and IMU power supply through the same RF cable. In this antenna configuration, the IMU signal is modulated for transmission together with the GNSS signal through the RF cable.
As mentioned above, the above-described techniques have certain inefficiencies, for example, with respect to a lack of a channel to transmit data from the GNSS receiver to the GNSS antenna. As such, this makes any synchronization of IMU measurements with the receiver's time scale, as well as, updating of firmware (FW) inside a digital controlling device (which collects data from the IMU sensors) nearly impossible to achieve in any efficient manner.
Therefore, a need exists for an improved GNSS antenna having an integrated antenna element in combination with a plurality of built-in sources and/or receivers of additional information for exchanging the information and transmission of GNSS signals from the antenna element to a receiver over a single RF cable.
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