Patent Description:
The usual positioning system is for the terminal to determine its position thanks to GPS signal by measuring signals emitted by the Global Positioning System satellite network, then to report this information to the hub via any communication network (cellular, satellite.

Other systems exist, which use an hybrid approach, where the terminal measures some properties, e.g. the delay in the reception of pilot signals from two different satellites, and sends the results to the Network operating center (NOC) via a telecom network, e.g. satellite itself; in a second phase the Network operating center uses these measurements to calculate the terminal position.

Note that all methods described above require an active "collaboration" by the terminal, and rely on the exactness of the information provided by the terminal to the hub. The drawback in this scenario is that the terminal may report a false position or report false information so that the network operating center calculates a false position, and the network operating center has no way to certify the truth of the information. Within an application involving payments, or tracking dangerous goods, it is possible that "pirate" terminals are developed that intentionally report a false position so as to avoid or reduce payments; these pirate terminals would be undetectable by the network operating center.

Besides, in satellite environment, sophisticated methods exist to perform triangulation and locate, within ~<NUM>, the position of a source of interference or jamming. These methods are very expensive, as they require sophisticated material as well as access to different satellites in order to perform triangulation. Also, they are based on a long period of observation of the signal. They cannot be used for single signal or in the framework of a consumer service.

The document <CIT> proposes a solution based on measures of relative amplitudes. That invention requires the use of a multi-beam satellite, and the precision highly depends on the geometry of the beams operating on a same frequency band. This requirement restricts its applicability to few satellite systems in the world. Also, the problem of "impersonation", i.e. a terminal pretending to be another one, was not addressed.

<CIT> discloses the position determnation of an object transmitting signal to two satellites with known position. The position of the transmitter is refined by a cross Ambigiuity function to suppress the other possible solutions of positioning object.

<CIT> discloses a method for the position determnation of an object in communication with two low-orbit satellites with known position and velocities. The method includes the steps of determining a range parameter, a range difference parameter, and either or both of a range-rate parameter and a range-rate difference parameter. A range parameter represents a distance between one of the satellites and the user terminal. A range difference parameter represents the difference between the distance between a first one of the satellites and the user terminal and the distance between a second one of the satellites and the user terminal. A range-rate parameter represents a relative radial velocity between one of the satellites and the user terminal.

Today there are many satellites in GEO orbit conceived for telecommunication purposes. In order to exploit at the maximum the available spectrum, some frequency bands, notably C, Ku and Ka-band, are reused from different positions. A terminal wanting to communicate shall use a high-gain antenna, e.g. a dish, in order to send/receive signals to/from a single satellite, limiting interference to other satellites that are reusing the same frequency in the same region. In some other bands, e.g. S-band, frequency reuse is less likely. However, there could be different satellites operating on the same region and frequency for backup purposes, or some regions could be covered by a 'primary' satellite, also called first satellite <NUM> and with a much lower performance by a secondary satellite <NUM>, also called second satellite <NUM>.

The purpose of the invention is to exploit the "interference" signal generated by a terminal transmitting towards the first satellite <NUM>, but received also by other satellites: in that example by the second satellite <NUM> and the third satellite <NUM>. By using the techniques described hereafter, the signal is low enough not to harm communication services on the secondary satellites, but strong enough so that it can be found in the feeder links of the secondary satellites <NUM>, <NUM> and measured by network operating center <NUM>. Finally, by measuring the difference in time of arrival between signals in the primary satellite <NUM> and at least two secondary satellites, i.e. the second satellite <NUM> and the third satellite <NUM>, the network operating center <NUM> is able to approximately locate the transmitting terminal by simple triangulation.

This system is conceived for using existing geostationary (GEO) telecommunication satellites. However, nothing stops in the future to use "dedicated" satellites that are somehow optimized for this system. For example, these could be a constellation of low-Earth-orbit (LEO) satellites conceived to offer three return channels in the same frequency in the region of interest.

Note that the system relies on the use of the "return link" which is usually less expensive than the "forward link" of a satellite, where high EIRP (Effective Isotropic Radiated Power) is required from the satellite, thus high power consumption and high costs for the satellite. This means that in the future some "return-link only" payloads might be deployed on new satellites, at low marginal costs, in order to allow a larger deployment of this service.

The main principle of the method according to the first example of the invention is simple triangulation: to measure the exact times at which a single short signal or burst signal emitted from a terminal is received through at least three different satellites; the difference among the measured times of arrival, plus information on the exact position of the satellites, is then used to identify a small region on the Earth surface within which the signal was transmitted: in other words, it identifies a point but with a certain approximation.

Difference in time of arrivals is due to the fact that the paths from the terminal to the receiving station <NUM> via the three satellites <NUM>, <NUM>, <NUM> have different lengths. Measuring the difference in time of arrival corresponds to measuring the difference of path lengths. As the satellite positions are known, either from satellite control center, or using pilot signals from known terminals, a triangulation can be performed. The main difficulty to be solved is to measure with sufficient precision the time of arrival of signals received with a very low carrier-to-noise ratio (C/N). This problem is specifically addressed with the techniques exposed in this document.

Also, a terminal <NUM> may try to "impersonate" another one, i.e. sending a signal pretending to be another terminal. This is avoided by imposing that each terminal authenticates itself by sending, embedded in the message, a cryptographic sequence based on a secret seed, shared only by the terminal and the network operating center.

Besides, the key step is the possibility to determine a burst's signal Time of Arrival (ToA) with sufficient precision within an operating scenario not optimized for such measurements. In other words, the system is designed to work in presence of the satellite nominal communication signals, without creating interference and without being interfered by. To achieve such a goal, the use of Spread Spectrum Direct Sequence technique has been chosen.

Furthermore, an extension of the system could be towards the adoption of specialized satellite transponders, i.e. fully devoted to this service, so without any other communication signal, in which case the necessary evolution of the protocol goes towards existing techniques of Multi-Access-Interference rejection such as the asynchronous messaging protocol described in part <NUM> of the ETSI S-MIM standard (ETSI TS <NUM><NUM>-<NUM>).

The following aims to provide the really basic steps to achieve the geo-localization of a terminal <NUM>. For ease of discussion, three satellites <NUM>, <NUM>, <NUM> are considered, sufficiently away one each other along the orbital arc, their transponders being able to receive the burst signal within the same up-link frequency.

In order to design an optimized signal structure, it is worth to mention that the Chip rate shall be maximized with respect to the available transponder bandwidth, the data packet shall be limited to carry on the user identification only, i.e. generated by a cryptographic sequence based on a secret seed, and the pseudo-noise (PN) sequence length shall be as well maximized, since possibly its repetition cycle should correspond to the entire message duration so as to optimize auto-correlation properties. Such a signal structure, for which an example will be provided in the C-band use case, permits to optimize the performance in terms of accuracy leading to consider one chip precision of each satellite.

The steps involved in the execution of the process are:.

Note that, at step <NUM>, the exact time at which the message has been transmitted by the terminal is unknown in the general case, as this method is asynchronous and any synchronization would require 'collaboration' from the terminal, as well as much more complexity on it. So the absolute values of T1 or T2 cannot be used directly to determine a locus of points. However, in step <NUM>, the relative difference T1-T2 which is independent of the time of transmission, as it depends only on the different paths to the satellites can be used to identify a locus of points, i.e. hyperboloid that intersects the Earth surface. Similarly, T1-T3 and T2-T3 identify two other loci. All the loci intersect in roughly a point with an uncertainty that mainly depends on the accuracy of the measurements.

The most complex step in the process is Step <NUM>, i.e. finding the time of arrival on secondary signals coming from satellites <NUM>, <NUM>, which requires the use of a particular message structure as well as a careful link budget assessment.

About the satellites to be chosen, as long as they cover the same region with the same frequency and with a G/T compatible with the link budgets below, it is better to choose satellites as far apart as possible. In fact the accuracy increases with a shorter the chip duration thus larger occupied bandwidth and more spaced satellites. As per preliminary evaluations, using a <NUM> channel and a location around Paris, an orbital distance of more than <NUM> degrees corresponds to a localization accuracy of about <NUM> meters.

Use cases and possible implementations of the above method:.

In this scenario the terminal is mobile, equipped with an omnidirectional S-band transmit (TX) antenna.

In this scenario the terminal is fixed, equipped with a low gain omni C-band transmit antenna able to transmit to all the three satellites with approximately the same transmit gain (i.e. the C-band transmitting antenna has a radiation pattern able to send radio-frequency power over <NUM>° in elevation and <NUM>° in azimuth).

It is assumed the terminal is in a location where it is possible to consider a primary satellite (e.g. on main coverage, which means that the G/T with which it is received is quite good) and two secondary (e.g. on secondary coverage). Assuming typical C-band missions, G/T ranges from -<NUM> to -<NUM> dB/K.

The message is assumed to be composed by <NUM> data bits (containing the secure user identification), FEC (forward error correction algorithm) is Turbo Code with rate <NUM>/<NUM>, able to provide about <NUM> dB of gain, Preamble is made of <NUM> uncoded bits. The total is so <NUM> bits, the data transmission rate is assumed <NUM> bps.

Assuming typical C-band transponder size, <NUM> has been considered, which allow for a <NUM> Mchip/s chip rate. In order to maximize the auto-correlation peak, the spreading code length has been fixed in <NUM>^<NUM> -<NUM> chips (there is practically no repetition within the message duration).

In order to achieve the one chip time of arrival accuracy, it is important to maintain the Eb/No, the Eb/No being a different way to address the C/N ratio, representing the ratio of the Energy of a single bit against the Thermal Noise spectral density, for the primary satellite at about <NUM> dB (corresponding to BER=<NUM>-<NUM>), so as to exploit a perfect demodulation. The following Tables report a synthesis of the obtained Eb/No ranges by varying the User terminal EIRP.

As it can be seen from the above, a <NUM> dB range for the user terminal EIRP (Effective Isotropic Radiated Power) provides very good coverage to the Eb/No requirement. Assuming a 'small' C-band omni antenna, commercial type, a gain of <NUM> dB can be assumed, so that the necessary User terminal output power ranges between -<NUM> dBW and <NUM> dBW (<NUM> mW and <NUM> W).

This use case upgrades the present method to a multi-user scenario where MAI (Multiple Access Interference) becomes the driving factor to establish the performances.

As reported in the previous use case, the application provides adequate localization accuracy when the Eb/No values for primary and secondary satellites are within a given range. When MAI is considered, an obvious consideration would be to increase the relevant up-link levels to preserve the Eb/(No+lo) values needed for the application. The obvious increase in terms of user terminal output power / antenna TX gain can difficulty be accepted in a commercial environment. As such, the solution needs to be found in some other system-level aspects which could make this system work in a multiple-user scenario.

The necessary step is the introduction of a multi-user protocol which enables the simultaneous (or pseudo simultaneous) transmission of several users occupying the same channel bandwidth, giving at same time a mean to maintain the system level performance in terms of capacity and localization accuracy within adequate limits.

In order to upgrade the localization system to a multi-user environment, a protocol is proposedincorporating the following steps:.

The above points could be implemented in a first instance by adopting the ETSI - MIM asynchronous messaging protocol defined in ETSI TS <NUM><NUM>-<NUM>, although important upgrades could be added to the overall algorithm for the sake of maximizing the localization accuracy in the multi-user scenario.

The trade-offs relevant to the System level parameters in view of optimizing at same time localization accuracy and system capacity (number of simultaneous users) are specific to the deployed system and are not part of the present invention.

This use case extends the geolocalization method to fixed users who are already served by a Ku down-link (such as TV market) for the sake of permitting service providers to know their geographical localization.

This use case assumes the adoption of a small C transmitter in a fixed Ku-band VSAT (<NUM> receiving parabola or similar). The installation would require a minimal reworking consisting in installing a small C band TX section + feed on the mast supporting the Ku band RX feed, without degrading at all the receiving performance of the terminal.

The advantages of such an installation are several. Taking chance of the possibility to transmit in C band, the payload messages could be used for various needs:.

More in terms of technical details, the adoption of a small C band TX section, i.e. less than <NUM> W with a parabolic reflector <NUM> wide would permit to have comprehensive TX gain towards the three different satellites, provided they are in the visibility range of the antenna (main lobe + side lobes). As an example, the <FIG> shows the typical radiation pattern of an <NUM> dish used for C band transmission.

As it can be seen on <FIG>, the main, primary and secondary lobes are suitable for being used in the application. By assuming a transmit output power of 500mW (-<NUM> dBW) and an EIRP threshold of <NUM> dBW for the secondary satellite (as shown in first use case), it is enough to avoid that the secondary satellite lies into a radiation null. This occurrence can eventually be corrected by tilting the C-band TX feed towards the parabola, with a decrease of the boresight gain and a general depointing of the total radiation pattern, so permitting to avoid that a secondary satellite falls inside a null.

The exact parameters to be used in a real deployment depend on a multitude of factors. As a general reference, preliminary simulations have been carried out using the E-SSA waveform in a <NUM> channel. In this setting, the C/N required on the primary satellite is about -<NUM> dB, and on the secondary satellite is about -<NUM> dB. This difference of <NUM> dB shall compensate the difference in G/T of the satellites towards the user location, and of the user antenna gain towards the two satellites. In these conditions it is possible to determine the TOA with a single chip precision, and at the same time estimate the power within <NUM> dB (useful for combining results of this invention with results on the invention of the document <CIT>). If the C/N difference is more than <NUM> dB, a different waveform can be used in order to have a larger dynamic range.

We show a worked out example under the assumptions that the link budget satisfies the above requirement, and having three GEO satellites located at <NUM>° West, <NUM>° East and <NUM>° East.

<FIG> shows the distance in kilometers of various points on the Earth from a GEO satellite S1 located at <NUM>° East.

Assuming a satellite S2 is located at <NUM>° East, <FIG> shows, for each point of the Earth, the difference in kilometers between the distance from S1 and the distance from S2.

When a terminal transmits a signal, the network operating center can measure time of arrival via S1, via S2, via S3, i.e. TOA-S1, TOA-S2, TOA-S3. Their absolute values are not significant, because, not knowing the exact time when the signal was transmitted, they cannot be used directly to compute the distances from S1, S2 or S3.

However, (TOA-S1 - TOA-S2) is proportional to the difference of the distances of the terminal from S1 and from S2. So the NOC to calculate on which line the terminal is located.

It is possible to draw the following lines, and determine that the terminal must be located at (one of) their intersection(s) as shown on <FIG>.

Claim 1:
Method of localization of a terminal (<NUM>) sending a signal to a first telecommunication satellite (<NUM>) with a known position, the method using at least a second telecommunication satellite (<NUM>) with a known position, the method comprising the following steps:
- (a) Transmission of the signal by the terminal (<NUM>);
- (b) Reception of the signal by the first and the second telecommunication satellites (<NUM>, <NUM>);
- (c) Transmitting the signal from each telecommunication satellite (<NUM>, <NUM>) to a receiving station (<NUM>);
- (d) Demodulating only the signal received by the first telecommunication satellite (<NUM>) which is the signal with the highest power level to determine a signal content and the time of arrival of the signal at the receiving station (<NUM>) via the first telecommunication satellite (<NUM>) and computing the amplitude of the signal received via the first telecommunication satellite (<NUM>);
- (e) Determining the time of arrival of the signal at the receiving station (<NUM>) via the second telecommunication satellite (<NUM>) by using the signal content by means of a step of correlation between a clean reconstructed copy of the demodulated signal received via the first satellite (<NUM>) and the aggregate signal received via second satellite (<NUM>) and computing the amplitude of the signal received via the second telecommunication satellite (<NUM>);
- (f) Determining the position of the terminal (<NUM>) by triangulation by using the difference of time of arrival and the difference in amplitudes of the signal to be received by the receiving station (<NUM>) via the first and the second telecommunication satellites (<NUM>, <NUM>), knowing the coverage maps of the first telecommunication satellite (<NUM>) and of the second telecommunication satellite (<NUM>).