GB2558289A

GB2558289A - Multi-GNSS method for measuring traceability in GNSS receivers

Info

Publication number: GB2558289A
Application number: GB1622202.8A
Authority: GB
Inventors: George Hoptroff Richard
Original assignee: Hoptroff London Ltd
Current assignee: Hoptroff London Ltd
Priority date: 2016-12-23
Filing date: 2016-12-23
Publication date: 2018-07-11
Also published as: GB201622202D0

Abstract

Determining the traceability or accuracy of Global Navigational Satellite System (GNSS) by receiving a first signal from a first GNSS 203, a second signal from a second GNSS 209. A first and second value is calculated in dependence on data in the first and second signals respectively. One or more of an average and a measure of traceability is then determined based on the first and second values. The first and second values are preferably location values and the average is calculated as a mean or median of the first and second values. Preferably an alert is issued if the traceability exceeds a traceability threshold which is determined based on a long term average of a measure of traceability.

Description

(71) Applicant(s):

Hoptroff London Limited

Trinity Street, London, SE1 1DB, United Kingdom (72) Inventor(s):

Richard George Hoptroff (56) Documents Cited:

US 20160216378 A1 US 20100328150 A1 (58) Field of Search:

INT CL G01S Other: WPI, EPODOC

G01S 19/07 (2010.01)

US 20150293230 A1 (74) Agent and/or Address for Service:

Richard George Hoptroff

Trinity Church Square, LONDON, SE1 4HT,

United Kingdom (54) Title of the Invention: Multi-GNSS method for measuring traceability in GNSS receivers Abstract Title: Multi-GNSS method for measuring traceability in GNSS receivers (57) Determining the traceability or accuracy of Global Navigational Satellite System (GNSS) by receiving a first signal from a first GNSS 203, a second signal from a second GNSS 209. A first and second value is calculated in dependence on data in the first and second signals respectively. One or more of an average and a measure of traceability is then determined based on the first and second values. The first and second values are preferably location values and the average is calculated as a mean or median of the first and second values. Preferably an alert is issued if the traceability exceeds a traceability threshold which is determined based on a long term average of a measure of traceability.

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Multi-GNSS Method for Measuring Traceability in GNSS Receivers

This invention relates to detecting when the location and timing data reported by Global Navigational Satellite System (GNSS) receivers are incorrect, for example due to signal jamming, spoofing (the deliberate transmission of an incorrect signal) or an inaccuracy in the data transmitted by the satellites, and for demonstrating that they are correct. It relies on the reception of signals from more than one GNSS system.

For clarity, the term traceable is used to mean there is an unbroken chain of comparisons relating a clock’s reported time to a known standard such as NPL (Teddington), NIST (Boulder, CO), or Rosstandart (Moscow).

The state of the art for determining location and time with reference to a highly accurate source are GNSSs. Referring to figure 1 and using GPS as an example, the correct time is intermittently transmitted 102 from standards institute NIST 101 to a constellation of satellites 103. The satellites 103 maintain on-board high-accuracy clocks which are ‘disciplined’ (their frequency is steered to ensure a near-exact relation to the definition of the second) and ‘traceable’ by reference to the primary source 101. At the same time their ephemerides (their positions over time) are computed using a combination of additional means, including the laws of gravitation, previous ephemeride measurements and the timing of propagation delays between the satellites and NIST.

Each satellite 103 then broadcasts 104 its location and the current time back to earth where it is received and decoded by receiver 105. Multi-satellite triangulation of the propagation delay is used to determine the receiver’s location L (106) and the current time t (107). While only two satellites are shown in figure 1, a minimum of four are required to complete the triangulation in both time and space.

A limitation of the approach is that it is a rather fragile path of traceablity to the reliable 5 source. Changes in the earth’s mass distribution, atmospheric effects, signal jamming and/or spoofing from a terrestrial source 108, software errors and clock physics can all potentially cause drift.

The state of the art in measuring such drift, and thus loss of traceability, is to measure any sudden drop in signal to noise ratio in the received signal, which only really covers matters relating to signal degradation.

An improvement would be to have multiple unbroken chains of comparisons relating a clock’s reported time to multiple known standards, and to quantify traceability as the variance between those multiple unbroken chains of comparisons. Such a measure would allow the degree of error in traceability to be measured.

Accordingly, and with reference to figures 2 and 3, the present invention is as follows:

1. As described above in the state of the art, a first source 201 transmits time to a satellite system 203 which then broadcasts the time to receiver 205 which then outputs a location value 206 and a time value 207 (figure 2)

2. At least one further independent source 208 transmits time to a different satellite system 209 which then broadcasts the time to a different satellite receiver 210 which outputs location value 211 and a time value 212.

3. Independent location values Li (206), L₂ (211), etc, and time values ti (207), t₂ (212), etc, are sent to computer 213.

4. Computer 213 computes a middle value of independent location values Li (206), L₂(211), etc, such as the mean or median, and outputs this value L (214).

5. Computer 213 computes a middle value of independent time values ti (207), t₂ (212), etc, such as the mean or median, and outputs this value t (215).

6. Computer 213 computes a variance of the independent location values Li (206), L₂ (211), etc, such as the standard deviation or maximum absolute difference, and outputs this value c_L (216).

7. Computer 213 computes a variance of the independent time values ti (207), t₂ (212), etc, such as the standard deviation or maximum absolute difference, and outputs this value c_t (217).

8. Optionally, computer 213 computes a long term average c_tL of c_t (217).

9. Optionally, as shown in figure 3, one or more highly accurate clocks (318) are disciplined with reference to time values ti (307) and/or t₂ (312), etc, and/or some middle value derived from them to provide additional time value(s) t_c (319) to computer 313 in addition to independent time values ti (307), t₂ (312), etc. Computer

313 computes middle value t (315) and variance value c_t (317) of the time values ti (307), t₂ (312), tc (319) etc.

10. Optionally, computer 313 raises an alarm if the variance value c_t (317) exceeds a threshold level c_tA.

11. Optionally, computer 313 determines threshold level a_tA relative to the long-term average a_tL of variance value c_t (317).

12. Optionally, as shown in figure 4, if the receiver’s location is known to be fixed, a long term moving average of location values Li (406), and/or L₂ (411), etc, and/or some middle value derived by averager (420) that computes additional location value(s) L_c(421) for computer 413 in addition to independent location values Li (406), L₂ (411), etc. Computer 413 computes middle value L (414) and variance c_L (416) of the location values Li (406), L₂ (411), L_c (421) etc.

13. Optionally, computer 413 computes a long term average c_Ll of c_L (416).

14. Optionally, computer 413 raises an alarm if the variance value c_L (416) exceeds a threshold level c_La.

15. Optionally, computer 413 determines threshold level c_La relative to the long-term average a_Li_ of variance value c_L (416).

Users may interpret t (215/315) as the correct time value.

Users may interpret c_t (217/317) and/or a_tL and/or c_L (216/416) and/or c_Ll as measures of the traceability of the time from the sources to the satellite receiver antenna(e), since they are computed from independently derived sources. It does not, of course, protect against any systematic error introduced after signal reception, for example antenna cable length.

Users may interpret L (214/414) as the correct location.

Users may interpret c_t (217/317) and c_L (216/416) as measures of the traceability of the location of the satellite receiver antenna(e), since they are computed from independently derived sources.

The introduction of a local clock 318 in step 8 provides for some measurement of traceability, which is independent in the short term, in the event of any rapid drift of any time values ti (206), t₂ (210), etc.

The use of location averager 320 in step 9 provides a short term measurement of traceability loss in the event of drift of location values Li (406), L₂ (411), etc, provided the receivers’ (205/210) location is fixed. The assumption that the location is fixed means that the measurement c_L (216/316) is not particularly useful in assessing the traceability of the location L (214/414), but it remains useful in assessing the traceability of the time t (215/315), and for triggering alarms indicating that the GNSS signal is unreliable. This step involves prior art disclosed in patent application GB1622184.8.

The computation of the long term moving average of variances c,_L and c_Ll provide longer term measures of traceability.

An embodiment of the invention is as follows, with reference to figures 5 and 6, which combined represent the embodiment but are separated for clarity:

1. Antenna 501/601 receives multiple GNSS signals.

2. Signal splitter 502/602 feeds the signal to multiple GNSS receivers.

3. First receiver 503/603 decodes the GPS signals to derive a first 1 Pulse Per Second (1PPS) time signal 507, and location data 606 in the form of NMEA serial messages output once per second.

4. Second receiver 509/609 decodes the Glonass signals to derive a second 1PPS time signal 512, and location data 611 in the form of NMEA serial messages output once per second.

5. The resulting 1PPS signals, being pulses at the start of each second, are captured by network interface card 522 with 1 PPS time capture capability 523, such as Intel i210 or Solarflare SFN7322F.

6. Captured time values 524 and 525 are sent to computer 513 that compares them and computes a mean value t (515) which it designates as the start of the second.

7. Computer 513 compares the captured time values 524 and 525 and computes their absolute difference o_t (517), which it designates as the measured degree of traceability of the time signal.

8. NMEA location data 606 and 611 are sent to computer 513/613 that compares the reported locations and computes mean values of latitude, longitude and altitude to determine a mean value L (614) which it designates as the antenna location.

9. Computer 513/613 compares NMEA location data 606 and 611 and computes their absolute difference o_L (616), in meters, which it designates as the measured degree of traceability of the received signal.

In an alternate embodiment, computer 513 calculates o_tL, the long term moving average of the absolute difference o_L, which it designates as the measured degree of traceability of the received signal.

In an alternate embodiment, computer 613 calculates o_Ll, the long term moving average of the absolute difference o_L, which it designates as the measured degree of traceability of the received signal.

In an alternate embodiment, if o_L exceeds a threshold level o_La, an SNMP alert is generated.

Ola is calculated as ten times the long term moving average of the absolute difference o_L(616).

In an alternate embodiment, if o_t exceeds a threshold level o_tA, an SNMP alert is generated. OtA is calculated as ten times the long term moving average of the absolute difference o_t(517).

In an alternate embodiment, with reference to figure 7, a second computer 726 computes t_m, the mean of ti and t₂, which is used to discipline atomic clock 727 to obtain a disciplined time signal t_c, which is additionally captured by network card 722 and sent to computer 713 and used to calculate t (715) and σ (717).

In an alternate embodiment, with reference to figure 8, computer 813 calculates a long-term moving averages of latitude, longitude and altitude to derive L_m (820), the mean of Li and L₂, which is additionally used to calculate L (814) and o_L (816).

An alternate embodiment might use multiple antennae, possibly from different manufacturers, rather than a signal splitter, in order to avoid any systematic error induced by the antenna.

An alternate embodiment might, in order to reduce cost, use a single receiver that sequentially decodes signals from successive satellite systems.

An alternate future embodiment might independently decode and compare three satellite systems such as GPS, Glonass and Galileo, and then designate the median values as the time and location, thus protecting against the failure of an individual satellite system.

An alternate future embodiment might independently decode and compare all four anticipated global satellite systems: GPS, Glonass, Galileo and BeiDou-2.

In an alternate embodiment, satellite receivers 503 and 509 might convey the time signals

507 and 512 via the PTP (IEEE 1588) or NTP protocols rather than 1PPS signals.

Claims

1. A method for determining traceability of signals received from satellites, the method comprising the steps of:

receiving a first signal from a first global navigational satellite system (GNSS);

receiving a second signal from a second GNSS;

determining a first value (ti, Li) in dependence upon data in the first signal from the first GNSS;

determining a second value (t2, L2) in dependence upon data in the second signal from the second GNSS; and determining one or more of an average (t, tm, L, Lm) and a measure (ct, ctL, cl, συ J of traceability, in dependence upon the first value (ti, Li) and the second value (t2, L2).

2. A method as claimed in claim 1, wherein one or more of the first value and the second value is a location value (Li, L2) including a longitude, a latitude and/or an altitude.

3. A method as claimed in claim 1 or 2, wherein the average (t, tm, L, Lm) comprises a mean or a median of at least the first value (ti, Li) and the second value (t2, L2).

4. A method as claimed in claim 1,2 or 3, wherein the average (t, tm, L, Lm) comprises a long-term moving average.

5. A method as claimed in any preceding claim, wherein the average (t, tm, L, Lm) is determined in dependence upon an additional value (tc, Lc) determined based upon the fact that a receiver is positioned in a fixed location.

6. A method as claimed in any preceding claim, wherein the measure (ct, ctL, aL, aLi_) of traceability comprises: a variance; a standard deviation; an absolute difference; a long-term average of variance; a long-term average of standard deviation; or a longterm average of absolute difference.

7. A method as claimed in any preceding claim, additionally comprising disciplining one or more clocks (318) in dependence upon the first value (ti, Li), the second value (t2, L2) and/or the average (t, tm, L, Lm).

8. A method as claimed in any preceding claim, additionally comprising initiating an alert if a measure (ot, otL, oL, cLl) of traceability exceeds a traceability threshold (Cia, Ola).

9. A method as claimed in any preceding claim, additionally comprising determining a traceability threshold (OtA, gla) in dependence upon a long-term average of a measure (ot, otL, oL, oLl) of traceability.

10. A method as claimed in any preceding claim, additionally comprising receiving a third signal from a third GNSS and determining a third value (t3, L3) in dependence upon data in the third signal from the third GNSS, wherein the average (t, tm, L, Lm) and/or the measure (ct, ctL, aL, cLl) of traceability is determined in dependence upon the third value (t3, L3).

11. A method as claimed in any preceding claim, wherein the first value is a first time value (ti), the second value is a second time value (t2) and the average is an average time value (t, tm) determined in dependence upon the first time value (ti) and the second time value (t2), the method additionally comprising the steps of:

determining a first location value (Li) in dependence upon data in a signal from the first GNSS;

determining a second location value (L2) in dependence upon data in a signal from the second GNSS; and determining an average (L, Lm) location value in dependence upon the first location value (Li) and the second location value (L2).

12. An apparatus for determining traceability of signals received from satellites, the apparatus including a processor arranged to:

process a representation of a first signal received from a first global navigational satellite system (GNSS);

process a representation of a second signal received from a second GNSS;

determine a first value (ti, Li) in dependence upon data in the representation of the first signal received from the first GNSS;

determine a second value (t2, L2) in dependence upon data in the representation of the second signal received from the second GNSS; and determine one or more of an average (t, tm, L, Lm) and a measure (ct, otL, oL, oLl) of traceability, in dependence upon the first value (ti, Li) and the second value (t2, L2).

13. An apparatus as claimed in claim 12, wherein one or more of the first value and the second value is a location value (Li, L2) including a longitude, a latitude and/or an altitude.

14. An apparatus as claimed in claim 12 or 13, wherein the average (t, tm, L, Lm) comprises a mean or a median of at least the first value (ti, Li) and the second value (t2, L2).

15. An apparatus as claimed in claim 12, 13 or 14, wherein the average (t, tm, L, Lm) comprises a long-term moving average.

16. An apparatus as claimed in any of claims 12 to 15, the processor additionally being arranged to determine the average (t, tm, L, Lm) in dependence upon an additional value (tc, Lc) determined based upon the fact that a receiver is positioned in a fixed location.

17. An apparatus as claimed in any of claims 12 to 16, wherein the measure (ct, ctL, aL, cLl) of traceability comprises: a variance; a standard deviation; an absolute difference; a long-term average of variance; a long-term average of standard deviation; or a long-term average of absolute difference.

18. An apparatus as claimed in any of claims 12 to 17, arranged to discipline one or more clocks (318) in dependence upon the first value (ti, Li), the second value (t2, L2) and/or the average (t, tm, L, Lm).

19. An apparatus as claimed in any of claims 12 to 18, arranged to initiate an alert if a measure (ct, ctL, aL, cLl) of traceability exceeds a traceability threshold (atA, Ola).

20. An apparatus as claimed in any of claims 12 to 19, the processor additionally being arranged to determine a traceability threshold (atA, cLa) in dependence upon a longterm average of a measure (ct, ctL, aL, cLl) of traceability.

21. An apparatus as claimed in any of claims 12 to 20, the processor additionally being arranged to process a representation of a third signal received from a third GNSS and to determine a third value (t3, L3) in dependence upon data in the representation of the third signal received from the third GNSS, wherein the processor is arranged to determine the average (t, tm, L, Lm) and/or the measure (ct, ctL, cL, cLl) of traceability in dependence upon the third value (t3, L3).

22. An apparatus as claimed in any of claims 12 to 21, wherein the first value is a first time value (ti), the second value is a second time value (t2) and the average is an average time value (t, tm) determined in dependence upon the first time value (ti) and

5 the second time value (t2), the processor additionally being arranged to:

determine a first location value (Li) in dependence upon data in a representation of a signal received from the first GNSS;

determine a second location value (L2) in dependence upon data in a representation of a signal received from the second GNSS; and

10 determine an average (L, Lm) location value in dependence upon the first location value (Li) and the second location value (L2).

Intellectual

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