News Column

Patent Issued for System for Monitoring a Satellite Constellation of a Positioning System

May 21, 2014

By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventor Laine, Robert Andre (Paris, FR), filed on November 2, 2009, was published online on May 6, 2014.

The assignee for this patent, patent number 8717234, is Astrium SAS (Paris, FR).

Reporters obtained the following quote from the background information supplied by the inventors: "As known, satellite positioning systems comprise a constellation of satellites being arranged on average altitude orbits (of the order of 25,000 km) around the Earth. Such satellites and the orbits thereof are generally referred to in the art as 'MEO satellites' and 'MEO orbits', respectively (for 'Medium Earth Orbit'). The MEO satellites are evenly distributed in several orbit planes, so that, in any point of the Earth, a user is able to see several MEO satellites, that is being directly connected to them (at least three, but four if the user wants to know his altitude) and deduce therefrom his own earth coordinates. Each MEO satellite carries a high stability and accuracy atomic clock, as well as an electronic equipment sending to users positioning signals comprising the corrected time (brought back to a common reference) and the ephemerides of the satellite. From such positioning signals received from several MEO satellites, the user determines his distance to the various visible satellites and therefore deduces therefrom his position in earth coordinates. The present method, system and device aims at checking the integrity of such positioning signals.

"It is known that usual solutions uses earth stations being dedicated therefor. Such stationary stations permanently compare the information received from satellites (via positioning signals) their accurately known effective position, so as to optionally detect a cohesion matching error in the received signals. Possible local deviations are transmitted to one or more control centres processing the information and identifying the satellite(s) transmitting erroneous positioning signals. Such quality information from the received positioning signals are transmitted to users later on, so that they ignore the erroneous signals in their positioning calculation.

"The error causes in the positioning, signals being received can be ranked in two large categories, that is:

"common errors due to satellites. Such errors are mainly due to the drift of the on-board clock giving the time reference, to the drifts of orbit parameters of the satellites or to processing anomalies on-board the satellites; and

"local errors due to the propagation of signals through the atmosphere and/or to local multiple path phenomena. Such errors vary with time, simultaneously affect only a limited number of users, and are not of the direct responsibility of the operator who has no means for correcting them, as they depend on natural physical phenomena.

"As the satellite positioning systems all use the measurement of differences of arrival time for the positioning signals emitted by satellites for calculating the earth position, and as both above mentioned error categories have an identical effect, that is, a time error of the signals received by the user, it is easy to confuse a local propagation error with a time error of the signal sent by satellites. In order to try and suppress such an ambiguity, dedicated processing centres collect information coming from numerous stations on the ground, and, through an essentially statistic calculation, separate the common error part from the local error part. Indeed, only the common error part, that is that coming from one or more satellites, is worth being transmitted to all users. Such a complex processing is time consuming, as it involves having to wait for enough information in order to distinguish without any ambiguity that part being local to one or more stations (such as an ionospheric effect or a geometry of the constellation leading to multiple paths) from the part being common to all stations and the cause of which can thus only be located on-board one or more satellites.

"Until the positive identification of the erroneous satellite(s) is completed, all users will make an error in their positioning calculation, with potentially severe consequences, in particular, for mobile devices. It is easily understood that reducing the identification time of erroneous (satellite) positioning signals is a priority for any operator of such a positioning system, with a view to ensure the quality of his system to users."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventor's summary information for this patent: "The present method, system and device aims at solving the above mentioned drawbacks. It relates to a monitoring system being intended for monitoring positioning signals emitted by a satellite constellation belonging to a positioning system, for example of the GPS, GALILEO or GLONASS type, and allowing to remedy the above mentioned drawbacks.

"To this end, according to the present method, system and device, said monitoring system is remarkable in that it comprises: at least one monitoring satellite being positioned in orbit at an altitude lower than that of the satellites of said satellite constellation so as to receive the positioning signals emitted by such satellites to the Earth, and comprising at least the following on-board means: at least one receiver being able to directly receive positioning signals emitted by the satellites of said satellite constellation: at least one processing unit being formed so as to check the integrity of said received positioning signals, using a positioning information being independent from these positioning signals, and to determine the case being the satellite(s) having emitted erroneous positioning signals; and at least one emitter being able to emit to the Earth integrity data indicating, the case being, the satellite(s) of said satellite constellation with erroneous positioning signals; at least one control centre being arranged on the Earth, for controlling said monitoring satellite, determining the orbit thereof, and comprising means able to receive integrity data emitted by such a monitoring satellite; and at least one integrity message distributing centre being arranged on the Earth and comprising: means for receiving integrity data from the monitoring satellite, being transmitted by said control centre; means for determining, from such integrity data, integrity messages relating to satellites of said satellite constellation of the positioning system; and means for emitting such integrity messages to users.

"Thus, thanks to the present method, system and device, instead of using stationary stations provided on the ground for monitoring the constellation of satellites of a positioning system, the monitoring system uses at least one monitoring satellite being located at an altitude lower than those of satellites of said satellite constellation so as to be able to receive the positioning signals emitted by the latter (and intended for the users of the positioning system), but being placed in orbit so as to prevent local errors from occurring such as described herein above, due, more specifically, to local multiple path phenomena or to propagation delays.

"Consequently, a particularly reliable monitoring system is obtained, allowing, more specifically, to autonomously and rapidly detect, and without any ambiguity, the erroneous positioning signals emitted by one or more satellites of the positioning system being monitored, using, to this end, a positioning information being independent from said positioning signals.

"In a preferred embodiment; said monitoring system comprises a plurality of monitoring satellites such as mentioned herein above, being all placed at lower orbits than the satellites of the constellation and implementing the above mentioned functions.

"Advantageously, each monitoring satellite further comprises on-board auxiliary means, allowing to determine (with the help of information received from said control centre) a first position in the space of said monitoring satellite, without using said positioning signals emitted by satellites of the satellite constellation, and said processing unit comprises:

"first means for calculating at least one second position, with the help of said positioning signals received from satellites of the satellite constellation;

"second means for comparing therebetween said first and second positions; and

"third means for deducing therefrom, the case being, the existence of erroneous positioning signals.

"Furthermore, in a preferred embodiment:

"said first means are formed so as to calculate a plurality of second positions, each one of said second positions being calculated using the positioning signals received from a sub-group of satellites of said satellite constellation, each said sub-group comprising an identical first predetermined number of satellites (for example four satellites) and two any sub-groups having in common each time, at the most, one second predetermined number of satellites (for example two satellites);

"said second means are formed so as to compare each one of these second positions to said first position; and

"said third means are formed so as to determine, the case being, the satellite(s) having its or their positioning signals being erroneous, with the help of the results of the comparisons implemented by said second means and the composition of said sub-groups.

"Furthermore, each monitoring satellite additionally comprises a single receiving antenna being associated with said receiver, being positioned so as to avoid multiples paths, and being surrounded with one electromagnetic protective screen for removing unwanted propagation modes (through reflection or diffraction of the positioning signals on the structure of said monitoring satellite), allowing to remove the most frequent phenomena able to disturb the reception of signals by the antenna.

"In a preferred embodiment, said monitoring satellite(s) is or are placed at orbits:

"having higher altitudes than those of the dense layers of the ionosphere, allowing to avoid ionospheric delays from occurring resulting from positioning signals passing through the dense layers of the ionosphere; and/or

"having a plane different from the planes of the orbits of said satellites of the satellite constellation, so as to maximize the error detection accuracy.

"Furthermore, in a preferred embodiment, said control and distribution centres are located at the same place and belong to one single same management centre.

"Additionally, advantageously, said control centre further comprises:

"means for remotely controlling each one of said monitoring satellites; and

"means for calculating orbit parameters of said satellite(s) for monitoring and updating processing units on-board such monitoring satellite(s).

"Furthermore, advantageously, said distribution centre further comprises coding means for coding integrity messages before they are emitted so as to restrict using such information to particular customers (having for example subscribed a contract) provided with appropriate decoding means."

For more information, see this patent: Laine, Robert Andre. System for Monitoring a Satellite Constellation of a Positioning System. U.S. Patent Number 8717234, filed November 2, 2009, and published online on May 6, 2014. Patent URL:

Keywords for this news article include: Astrium SAS.

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Source: Journal of Engineering

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