An alternative position, navigation and timing system and method

Abstract

A system and method for providing an Alternative Position, Navigation and Timing system (APNT) providing improved robustness against interference and jamming. The method comprises transmitting underlaid time stamped spread spectrum Pseudo Range signals using shared frequencies, from multiple communications and broadcasting satellites (20). The transmitted signals are time stamped and received at several dispersed time synchronized reference stations (30) at known locations. From the Pseudo Ranges, satellite orbits and Navdata are determined. APNT timing signals, comprising both the spread spectrum Pseudo Range and Navdata messages, are uplinked from the earth stations to the multiple communications and broadcasting satellites (20). The APNT signals are received from the satellites in an APNT receiver (50) comprising a multi-channel, multi-frequency receiver- and navigation processor. Based on the received APNT signals, the APNT receiver determines its position.

Description

[0001] An alternative position, navigation and timing system and method

[0002] TECHNICAL FIELD

[0003] The invention relates to a positioning system and method, and more specifically to an Alternative Position, Navigation and Timing (APNT) system and method, using signals underlaying existing satellite communications and broadcast signals, that are robust against interference and jamming.

[0004] BACKGROUND

[0005] Present Global Navigation Satellite Systems (GNSS) provides Position, Navigation and Timing (PNT) services to GNSS devices by means of multiple satellite constellations orbiting the Earth. Each satellite is continuously transmitting time stamped signals.

[0006] The satellites comprise accurate atomic clocks to keep track of time with high precision. Ground Stations are used to continuously monitor and synchronize the clocks on the satellites, and any deviations are corrected by sending updates to the satellites.

[0007] To determine the position of a receiver, it is necessary to receive time stamped signals from at least three different satellites at all times. The satellites are used for determining the position and time on the Earth by trilateration. A fourth satellite is required if elevation at the receiver is not known. Signals broadcasted to earth from each GNSS satellite contains a timestamp of when the signal is transmitted, as well as information about identity, position, and various codes.

[0008] Once the signals reach a GNSS receiver on the ground, the receiver will generate a second timestamp for each signal, comprising the Time of Arrival (ToA) of the signals. Based on this, the time it takes for signals to travel from the satellites to the GNSS receiver relative to its clock is measured and the distance to each satellite is calculated by multiplying ToA by the speed of light. This distance is termed Pseudo Range (PR) which is the pseudo distance between a satellite and a navigation satellite receiver, such as for instance a Global Positioning System (GPS) receiver. As there are offsets in the times measured, the term Pseudo Range is used rather than ranges for such distances.Based on the satellite orbit, clock and ionospheric delay information, the position and time of the GNSS device is determined in three-dimensional space. Latitude, longitude, and altitude are determined using an accurate Geoid model of the earth. GNSS systems such as GPS operate at L-band and have been widely used for nearly 50 years. It has become an important infrastructure in daily life. Signals from GPS satellites are transmitted on carrier waves at specific frequencies known as LI (1575.42MHz) providing accurate positioning data to GPS devices, L2 (1227.60 MHz) passing through clouds, trees and other obstacles, and L5 (1176.45 MHz) that is transmitted with a higher power than for the LI and L2 signals for improving performance.

[0009] The use of static frequencies and lack of security for civilian systems makes the GPS system sensitive to accidental and intentional interference aimed at the frequencies used. This may be critical for achieving uninterrupted and reliable GNSS services making the systems vulnerable for important infrastructure in our modern society.

[0010] Further, launching and operating a GNSS with GPS satellites flying in Medium Earth Orbit (MEO), or Low Earth Orbit (LEO) constellations requires many satellites which is costly.

[0011] An object of the invention is to alleviate the aforementioned disadvantages of the prior art such as GNSS jamming and spoofing and to provide a more robust alternative than today’s GNSS. The alternative system presented herein is less sensitive to interference and difficult to jam and utilizes different types of satellites comprising both communications and broadcasting satellites.

[0012] SUMMARY OF THE INVENTION

[0013] The invention discloses a robust and cost-effective satellite based Alternative Position Navigation and Timing system (APNT) using other or additional satellites and frequencies than Global Navigation Satellites. By using existing satellite infrastructure comprising both communications and broadcasting satellites, and by using wide range of frequencies and encryption, the APNT provides a cost effective and efficient system that is more difficult to jam or spoof.

[0014] Use of Geostationary Orbit (GEO) or Highly Elliptical Orbit (HEO) satellites reduces the number of Low Earth Orbit (LEO) and Medium Earth Orbit (MEO) satellites required for full time coverage by 25 to 50%. HEO and GEO Spread Spectrum Pseudo Range signals are transmitted efficiently below existing broadband signals using a small fraction of the transponder Effective Isotropic Radiated Power (EIRP) without noticeable interference to either system. By using communications and broadcast bands, the frequency used for the Pseudo Rangesignals may be changed dynamically and broadcasted in the Navdata message to avoid interference. Example frequencies for the APNT system include WRC-23 proposed loT frequencies at around 2 GHz, C-band, X-band and Ku-band and Ka-band.

[0015] In general terms and in a first aspect, the invention comprises a system providing an APNT system with improved robustness against interference and jamming.

[0016] The system comprises multiple communications and broadcasting satellites configured to transmit time stamped signals with underlaying spread spectrum Pseudo Range signals using shared frequencies.

[0017] As mentioned, spread spectrum means that the signal’s bandwidth is spread over a wide bandwidth to reduce interference and improve security.

[0018] By transmitting Pseudo Ranges as time stamped spread spectrum signals underlaying stronger signals, multiple signals are allowed to be transmitted simultaneously. This will minimize interference and increase accuracy and reliability of the positioning information.

[0019] According to one embodiment, the multiple communications and broadcasting satellites comprises at least four satellites in LEO-, ME0-, HEO- and GEO- orbits, and wherein at least one satellite is in a different orbit than the others to improve geometry and thereby accuracy.

[0020] The system further comprises multiple time synchronized time distribution and reference stations configured to receive the transmitted time stamped signals and determining Pseudo Ranges to respective satellites.

[0021] According to one embodiment, at least one of the satellites has processing capabilities and an internal clock configured to generate Pseudo Range and Navdata messages onboard. The Navdata message also comprises uplinked information. The system further comprises a Central Processor configured to generate Navdata message content and possible encryption.

[0022] The Central Processor comprises a satellite orbit predictor processor configured to determine satellite ephemeris. This includes calculating and predicting positions and velocities of the satellites in their respective orbits used in the Navdata message. The system further comprises one or more satellite earth stations configured to uplink APNT signals, comprising both the spread spectrum Pseudo Range and Navdata messages, to the multiple communications and broadcasting satellites. The system further comprises an APNT receiver 50 comprising a multi-channel, multi-frequency receiver- and navigation processor. It is configured to receive alternative Position, Navigation and Timing signals underlaying the signals transmitted by multiple communication and broadcasting satellites. It is furtherconfigured to determine Time of Arrival (ToA) of received signals in the multichannel, multi -frequency receiver- and navigation processor as well as extracting the Navdata messages.

[0023] The APNT receiver and navigation processor is further configured to calculate internal clock error and its position based on the Pseudo Ranges, Navdata and an accurate Geoid model for determining latitude, longitude and surface elevation. In a second aspect of the invention, there is provided a method for providing an APNT system with improved robustness against interference and jamming.

[0024] The first step of the method is transmitting time stamped signals, with underlaying spread spectrum Pseudo Range signals, using shared frequencies from multiple communication and broadcasting satellites.

[0025] In one embodiment, the multiple communication and broadcasting satellites are selected among at least four satellites in LEO-, ME0-, HEO- and GEO- orbits, wherein at least one of the satellites used is not a GEO satellite.

[0026] In one embodiment, the transmitted APNT signals underlaying communication and broadcasting satellites are transmitted with less power than the communication and broadcasting signals.

[0027] The next step of the method is receiving the transmitted time stamped APNT signals at several dispersed time synchronized reference stations at known locations and determining Pseudo Ranges to the satellites.

[0028] The next step is generating satellite orbit parameters, determining timing errors and generating the Navdata message based on the measured Pseudo Ranges to the satellites.

[0029] The next step is transmitting time corrected and synchronized uplink APNT signals, comprising both the spread spectrum Pseudo Range and Navdata messages, to the multiple communications and broadcasting satellites.

[0030] The next step is receiving the uplinked APNT signals, underlaying the communication and broadcasting satellite signals in APNT receivers comprising a multi-channel, multi -frequency receiver- and navigation processor.

[0031] According to one embodiment, analogue or digital bent pipe satellites relay or retransmit received Pseudo Range and Navdata signals that are uplinked from the one or more earth stations and optionally satellites with suitable processing capabilities use internal clocks to generate Pseudo Range and Navdata internally before transmitting these signals to the time synchronized reference stations. In one embodiment, at least one of the satellites of the multiple communication and broadcasting satellites generates the Pseudo Range and Navdata signals internally.Based on the above steps, the processor determines Time of Arrival (ToA) of the received Pseudo Range signals and demodulates the Navdata.

[0032] By using four Time of Arrivals times, knowledge of each satellite’s x, y and z positions [XN, yx, ZN] in an Earth Centric Earth Fixed coordinate system at the time of transmission, both the user location in 3D [xo, yo, zo] and the user reference clock error (to) can be calculated by determining the four unknowns using four equations:

[0033] ToAi= Sqrt((xi-xo)2+(yi-yo)2+(zi-zo)2) / c+ to

[0034] TOA2= Sqrt((x2-xo)2+(y2-yo)2+(z2-zo)2) / c+ to

[0035] TOA3= Sqrt (x3-xo)2+(y3-yo)2+(z3-zo)2) / c+ to

[0036] TOA4= Sqrt((x4-xo)2+(y4-yo)2+(z4-zo)2) / c+ to

[0037] where c is the speed of light.

[0038] Knowledge of both satellite and user positions enables ionospheric paths delays to be calculated using Navdata parameters to correct the Pseudo Ranges for ionospheric delays. A Geoid model (e.g. WGS84) is used to calculate latitude, longitude, altitude from the 3D location. Changes in user position is used to derive speed and heading using suitable filtering.

[0039] SHORT DESCRIPTION OF THE FIGURES

[0040] The present invention will be explained in detail by reference to an example embodiment shown in the figures, in which:

[0041] Figure 1 illustrates the different interacting devices comprised in the APNT system. Figure 2 illustrates the data and signal flows in the multi frequency APNT system. Figure 3 is a flowchart of the steps performed by a Central Processor comprising a Navdata message generator.

[0042] Figure 4 is a flowchart of the steps performed by the reference stations.

[0043] Figure 5 is a flowchart of the steps performed by the APNT receiver.

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] As described above, the present invention addresses the need for providing an Alternative Position, Navigation and Timing (APNT) system that is less sensitive to interference and more difficult to jam or spoof.In addition to using Geostationary Orbit (GEO) satellites, Low Earth Orbit (LEO), Medium Earth Orbit (MEO) or Highly Elliptical Orbit (HEO) satellites are used to improve robustness, geometry and accuracy. The system uses spread spectrum signals underlaying satellite communications or broadcasting signals. Existing satellite bands may be used with insignificant interference to incumbent services. Pseudo Range and Navdata signals from existing communications and / or broadcasting satellites are used to reduce the number of dedicated satellites required to provide Position, Navigation and Timing to a wideband APNT receiver independent of GNSS.

[0046] When the communications and broadcasting satellites do not have robust and reliable onboard GNSS information, the satellite current positions and time errors can be determined on ground using ranging or pseudo ranging methods using multiple ground time synchronized reference stations. The clock errors, satellite position and orbit prediction parameters are derived in a Central Processor and contained in the Navdata message uplinked to the satellites.

[0047] Several satellite downlink APNT signals are used by the APNT receivers to determine Pseudo Ranges to each satellite. The transmitted signal’s Effective Isotropic Radiated Power (EIRP) is much lower than the broadcasting (DVB) or communications (VSAT) signals that they are underlaying and use a small fraction of the satellite EIRP. Further, the existing frequencies are re-used with minimal degradation.

[0048] VSAT or DVB receivers normally operate at Carrier to Noise Ratios lower than 15 dB and use antennas with much higher gain (35 dBi) than the APNT receiver comprised in the inventive APNT system, which uses wideband omnidirectional low gain (2 dBi) antennas. The VSAT and DVB interference into the APNT receiver will therefore be well below the noise floor, causing insignificant interference. Similar to GPS, a low-rate Navdata message is transmitted. It contains synchronization and data (packet content definition, sequence number, system status, satellite orbit parameters, Pseudo Range transmission times, spreading codes pattern, frequency hopping pattern, ionospheric data, clock errors, padding, cyclic redundancy check sum and flush bits). The message may be encrypted.

[0049] The Navdata message uses Forward Error Correction and phase modulates the Pseudo Range spread spectrum signals. The PR signals are a phase modulated binary sequence selected for good autocorrelation properties with frequency offsets and low cross correlation with other spreading codes used in the system.

[0050] Analogue and digital bent pipe satellites relays Pseudo Range and Navdata signals uplinked from ground where these signals are generated. Multiple ground reference stations are used to determine the satellites’ orbits and clock errors. The Navdatadata rate depends on the link budget. LEO, MEO and HEO satellites may support higher data rates and provide higher margin than GEOs and can therefore rapidly provide information about all satellites in the Navdata message. It is therefore sufficient to receive Navdata from one satellite, but Pseudo Ranges from at least four satellites is needed to determine its x, y, z position and time.

[0051] Satellites with on-board processing capabilities generate Pseudo Ranges and Navdata messages internally based on uplinked Navdata messages when an onboard GNSS receiver is not available or desirable. For the APNT system to work, Navdata needs to be available from at least one satellite, but more satellites increase robustness.

[0052] The APNT receivers determines the received Time of Arrival of multiple Pseudo Ranges using correlation and interpolation techniques. The APNT receiver also despreads and extracts the Navdata messages from at least one satellite providing satellite locations and timing information. Using the Pseudo Ranges, correction parameters, transmission time, satellite position at the time of transmission, a processor in the APNT receiver determines its location and time in a similar way to GNSS systems. An accurate Geoid model can be used to determine latitude, longitude and altitude.

[0053] Figure 1 illustrates the different interacting devices comprised in one embodiment of an APNT system according to the invention. The system comprises both communications and broadcasting satellites transmitting signals with underlaying spread spectrum Pseudo Ranges signals using shared frequencies.

[0054] Uplink earth stations 10 comprising an antenna system are used for generating APNT uplink signals to each satellite within a transmit / receive chain. In the example shown, four satellites 20A-20D and four uplink stations 10A-10D are used. The uplink stations 10 are synchronized to a precision time system, e.g. a rubidium clock system or a national time reference. One modulator per satellite is generating the APNT uplink signal comprising spread spectrum Pseudo Range and Navdata signals. The signals are synchronized to system time and adjusted to remove known delays. This ensures that the Pseudo Range signal epochs are close to equal at the satellite outputs. The downlink APNT signal includes both Pseudo Range spread spectrum and a low bitrate Navdata message.

[0055] At least four communications or broadcasting satellites are used for re -transmitting the uplinked APNT signals. In the example shown in Fig. 1, different types of satellites are used, including LEO 20A, GEO 20B and 20C, and HEO 20D satellites. If a satellite has suitable on-board processing capabilities, it may use its internal clock to generate the Pseudo Range and the Navdata message. In this case, LEO 20A has processing capabilities and is able to calculate the Pseudo Range.Several dispersed time synchronized reference stations 30 with precision clocks each comprises wideband multichannel front end Pseudo Range signal, Time of Arrival (ToA) estimators and forward these to a Central Processor 40.

[0056] The Central Processor 40 determines the satellite ephemeris, i.e. satellite orbit predictor. The latest orbit predictor parameters and calibration factors such as range and clock errors, as well as ionosphere corrections comprises the Navdata message uplinked to all the satellites using earth stations 10. The message may be encrypted. An APNT receiver 50 comprises a multi-channel and multifrequency receiver- and navigation processor. The receiver measures Pseudo Range signals used by the system and determines ToAs relative to its internal clock. The Navdata message is also extracted. The processor determines location and time error. Ionospheric corrections included in the Navdata message or from other sources may be used to improve Pseudo Range accuracy.

[0057] Figure 2 illustrates the data and signal flow in one embodiment of the multi -frequency APNT system, wherein reference stations 30 use two-way satellite ranging to obtain high accuracy time synchronization. Time synchronization by using optical fibers is another method for accurate time distribution.

[0058] The different modules described above are included in the illustration. These are the earth uplink stations 10, different types of satellites 20, Central Processor 40 with a Navdata message generator and the APNT receivers 50. The data flow between said modules and each reference station 30 is illustrated.

[0059] The multiple communications and broadcasting satellites 20 also transmit time stamped spread spectrum Pseudo Ranges signals using shared frequencies. These signals are received by the reference station 30. Based on time stamps of the received signals and ToA, each reference station 30 will calculate Pseudo Ranges to respective satellites.

[0060] The measured data of a reference station is transmitted to the Central Processor 40. It determines the satellite orbits and satellite transmission time offsets and Navdata message content. This module also controls and monitors the overall system, sets system time, sets frequency hopping pattern, sets spreading pattern, determines ionospheric corrections and encryption used in the Navdata message and set by the configuration input.

[0061] The Central Processor 40 transmits the Navdata message to the earth uplink stations 10 that uplink the received Navdata and Pseudo Ranging to all the communications and broadcasting satellites used in the system 20.

[0062] APNT receivers 50 are used for determining their positions. Each APNT receiver 50 receives Navdata from the multiple communications and bent pipe or processingbroadcasting satellites 20. A crypto key may be loaded into each APNT receiver 50 to enable decrypting of the Navdata message. ToA of all PR signals are measured for determining its position, i.e. latitude, longitude, and altitude. All measurements and calculations are synchronized with the national or system time.

[0063] The legends in figure 2 describe the signals flowing between the different modules comprised in the APNT system.

[0064] Figure 3 is a flowchart of the steps performed by the Central Processor comprising the Navdata message generator.

[0065] Upon power up, a system self-check is performed. Then calibration data is loaded as well as a configuration file. The internal clocks are set to national / system time and the crypto key used is loaded. Frequency hopping patterns and spreading codes be used for all satellites are generated. Updated ionospheric data (e.g. Total Electron Content) is retrieved from an external source, and earth stations for Pseudo Range and Navdata transmission are configured, as well as reference stations for satellite ranging measurements.

[0066] When the above steps have been performed, continuous two-way satellite ranging delay measurements start.

[0067] Uplink time offsets required to synchronize Pseudo Range transmissions at the satellite downlink antenna port are determined and transferred in a start-up Navdata message to all Earth stations.

[0068] Earth Station continuous Pseudo Range and Navdata transmissions are then instructed to start.

[0069] After said actions, it is periodically checked if powering off is to be done or reconfiguration is necessary. If reconfiguration is necessary, configuration settings are updated and the self-check starts again followed by the steps described above. Figure 4 is a flowchart illustrating the steps performed by the reference stations. Upon power up, a system self-check is performed. Then calibration data is loaded as well as a configuration file. The internal clocks are set to national / system time. It is then prepared for satellite ranging measurements, and continuous two-way satellite ranging delay measurements are performed when commanded. The ranging delays are then transferred to a Central Processor, and hopping patterns used is retrieved from the Central Processor.

[0070] Navdata message is continuously generated, and continuous Pseudo Range and Navdata uplink transmission starts. The transmissions are compensated for satellite delays.

[0071] The transmissions continue until the Central Processor issues a switch off command.Figure 5 is a flowchart of the steps performed by the APNT receiver. Upon power up, a system self-check is performed. Initial position and time are then set to aid acquisition, and calibration data, configuration file, encrypted start Navdata file (initial orbit parameters, frequencies, spreading codes) and crypto key are loaded. Waveform parameters and frequency -time hopping patterns for all satellites are determined, and parallel time and frequency search using non -coherent correlator banks starts to search for the PR signals.

[0072] The process will proceed by demodulating all Navdata messages when PR signal synchronization is achieved and ToA is measured. Navdata message is decrypted and the Navdata files are updated for all satellites. Raw Pseudo Ranges are measured relative to internal clock to all satellites in view.

[0073] Ionospheric corrections are applied to the Raw Pseudo Ranges, based on geometry and TEC data, and corrected for satellite timing errors.

[0074] Corrected Pseudo Range measurements are used, as well as orbit predictor and Geoid model to determine local position, time, speed and heading. The Navdata file is then updated. Based on this, the PNT accuracy is estimated.

[0075] Data output from the APNT receiver comprises PNT data with auxiliary information such as accuracy, satellites used, elevation, azimuth, and C / NO.

[0076] The following is an example of Pseudo Ranges, Navdata and TV broadcasted from a GEO satellite at the same frequency. A DVB-S2 TV signal from the Norwegian Thor-7 GEO satellite is transmitted at Ku -band with an EIRP of 52 dBW using 31 MHz bandwidth. For a symbol rate of 25 Msps, 8PSK modulation and % rate FEC a nominal C / N of around 14 dB is expected using a 60 cm dish. The APNT PR and Navadata signal is underlayed the DVB signal with an EIRP of 30 dBW spread over 31 MHz using Direct Sequence Spread Spectrum. This signal is 22 dB below the TV broadcasting signal and causes an insignificant reduction in DVB-S2 link margin. An APNT receiver in Northern Europe with an omnidirectional antenna may receive signals from more than 20 TV broadcasting satellites. At least one satellite in a non-GEO orbit should be included to improve geometry and thereby accuracy. For this example, the Norwegian ASBM HEO satellites are selected. We have assumed the same satellite APNT downlink EIRP (30 dBW) at X-band. The HEO satellites provide much higher elevations angles than GEOs in Northern Europe.

[0077] The mobile APNT receiver in this example uses omnidirectional antennas covering the bands and frequencies of interest typically 7-12.5 GHz. The Ku-band TV satellite uses both horizontal (H) and vertical (V) polarization with a skew depending on location. A circular polarized (CP) antenna is preferred to handle both polarizations and skew. This results in a 3 dB polarization loss. The HEO signals are circularly polarized and also received by a circularly polarized antenna with lowpolarization loss. The HEO X-band CP link will have 7 dB higher margin than the Ku band GEO H / V signals.

[0078] Table 1 below provides an example APNT GEO link calculation for an APNT signal from the Thor 7 TV broadcasting satellite.

[0079] Table 1.

[0080] Value Unit DVB-S2 link C / N with underlayed APNT signal

[0081] DVB-S2 satellite signal EIR.P 52 dBW Frequency 12 GHz Satellite range 42000 km Free space pathloss 206.5 dB Boltzman's constant -228.6 dBW / K Hz DVB-S2 receiver G / T (60 cm dish) 14.5 dB / K DVB-S2 C / No 88.6 dBHz DVB-S2 symbol rate 25 Msps Excess bandwidth (roll off) 0,25

[0082] DVB-S2 signal bandwidth (31,25 MHz) 74,9 dBHz DVB-S2 C / N 14 dB DVB-S2 interference

[0083] APNT signal satellite EIR.P 30 dBW DVB-S2 C / l 22 dB DVB-S2 C / (N + I) 13.4 dB DVB-S2 C / (N + I) threshold (8PSK, % rate) 8 dB DVB-S2 margin 5.4 dB

[0084] APNT receiver DVB-S2 interference

[0085] Number of DVB-S2 interferers (2 polarizations) 40 (16 dB) Total DVB-S2 interference satellite El RP (40 carriers) 68 dBW APNT receiver DVB signal polarization loss 3 dB APNT receiver G / T -23,7 dBK APNT receiver l / No 63,4 dBHz APNT receiver lo / No -11,5 dB APNT receiver (lo + No) / No 0,3 dB APNT link margin

[0086] Pseudo Range and Navdata satellite EIRP 30 dBW APNT received C / No 25,4 dBHz DVB-S2 interference increase in APNT rx noise floor 0,3 dB APNT received C / (No+lo) 25,1 dBHz Navdata info rate 100 bps

[0087]

[0088] Navdata Eb / No threshold (BPSK, 1 / 4 Rate Turbo FEC) 2 dB Navdata link margin dB

[0089]

[0090] 3,1

[0091] The Navdata bitrates may be optimized for different types of satellites. The system may operate with only one Navdata link providing information about all satellites, orbits and clocks.

Claims

CLAIMS1. An Alternative Position, Navigation and Timing system with improved robustness against interference and jamming, comprising:- multiple communications and broadcasting satellites (20) configured to transmit time stamped signals with underlaying spread spectrum Pseudo Ranges signals using shared frequencies,c h ar a c t e ri z e d i n- multiple time synchronized time distribution and reference stations (30) configured to receive the transmitted time stamped signals and determining Pseudo Ranges to respective satellites,- a Central Processor (40) comprising a Navdata message generator configured to generate satellite orbits and Navdata message content, - one or more earth stations (10) configured to uplink alternative Position, Navigation and Timing signals, comprising both the spread spectrum Pseudo Range (PR) and Navdata messages, to the multiple communications and broadcasting satellites (20),an Alternative Position, Navigation and Timing receiver (50) comprising a multi-channel, multi -frequency receiver- and navigation processor configured to:- receiving the Alternative Position, Navigation and Timing signals underlaying the signals from the multiple communication and broadcasting satellites (20),determining Time of Arrival of received signals in the multichannel, multi -frequency receiver- and navigation processor and extracting the Navdata,- calculating time and a position of the Alternative Position, Navigation and Timing receiver (50), based on the Pseudo Ranges, Navdata and an accurate Geoid model.

2. The system according to claim 1, wherein the multiple communications and broadcasting satellites (20) comprises at least four satellites in LEO-, ME0-, HEO- and GEO- orbits, and wherein at least one satellite is in a different orbit than the others.

3. The system according to claim 1 or 2, wherein at least one of the satellites has an internal clock configured to generate Pseudo Range and Navdata messages onboard.

4. A method for providing an Alternative Position, Navigation and Timing system providing improved robustness against interference and jamming, wherein the method comprises the following steps:- transmitting time stamped signals, with underlaying spread spectrum Pseudo Range signals, using shared frequencies from multiple communications and broadcasting satellites (20), ch ara cte ri z e d i nreceiving the transmitted time stamped signals at several dispersed time synchronized time distribution and reference stations (30) at known locations, and determining Pseudo Ranges to the satellites, generating satellite orbits and Navdata in a Central Processor (40) from the determined Pseudo Ranges to the satellites,transmitting time synchronized uplink Alternative Position, Navigation and Timing signals, comprising both the spread spectrum Pseudo Range and Navdata messages, to the multiple communications and broadcasting satellites (20),receiving the uplinked Alternative Position, Navigation and Timing signals underlaying the signals from the multiple communication and broadcasting satellites (20) in an Alternative Position, Navigation and Timing receiver (50) comprising a multi-channel, multi -frequency receiver- and navigation processor,determining Time of Arrival of received signals in the Alternative Position, Navigation and Timing receiver (50) and extracting the Navdata,calculating time and a position of the Alternative Position, Navigation and Timing receiver (50), based on the Pseudo Ranges, Navdata and an accurate Geoid model.

5. The method according to claim 4, wherein the multiple communication and broadcasting satellites (20) transmitting the time stamped signals comprises at least four satellites in LEO-, ME0-, HEO- and GEO- orbits, wherein at least one is in a different orbit than the others.

6. The method according to claim 4 or 5, wherein the transmitted alternative Position, Navigation and Timing signals underlaying the signals from the multiple communication and broadcasting satellites (20) have less power than the time stamped signals transmitted from the multiple communicationand broadcasting satellites (20).

7. The method according to any of the claims 4 to 6, wherein analogue or digital bent pipe satellites re-transmit received Pseudo Range and Navdata signals that are uplinked from the one or more earth stations, and satellites with suitable processing capabilities may optionally use internal clocks to generate Pseudo Range and Navdata internally before transmitting these signals to the synchronized reference stations.

8. The method according to claim 7, wherein at least one of the satellites generates Navdata internally.

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