Systems and methods for receiving information at rover receivers in navigation satellite systems

Abstract

A navigation satellite system, one or more NSS reference station is set out for providing information to one or more rover receivers. The NSS reference station comprises a processing unit configured to determine one or more first range intervals representing first ambiguity windows based on estimated atmospheric effects, and determine one or more second range intervals representing a second ambiguity window smaller than each of the first ambiguity windows based on uncertainties of range measurements within a predetermined previous period. The NSS reference station further comprises a transmission unit configured to transmit, to the NSS rover receivers, first messages each comprising a modulo a first ambiguity window, and transmission of two first messages, to range measurement transmit between the NSS rover receiver, a plurality of second messages, each second message comprising a range measurement modulo one of the second ambiguity windows.

Claims

1. A navigation satellite system, NSS, rover receiver for receiving information from a NSS reference station, the NSS rover receiver comprising: a reception unit configured to: receive, from the NSS reference station, first messages each comprising a respective first range measurement modulo a first range interval, wherein a time interval between transmission of each first message is equal to or less than a time duration of a predetermined previous time period; and receive between reception of two first messages, from the NSS reference station, a plurality of second messages, each second message comprising a size of a second range interval, the time duration of the predetermined previous time period, and a respective second range measurement modulo the second range interval, the second range interval being smaller than the first range interval; and a processing unit configured to reconstruct the second range measurement associated with each second message.

2. The NSS rover receiver according to claim 1, wherein the first range interval is based on estimated atmospheric effects on the first range measurements determined from carrier phase observations.

3. The NSS rover receiver according to claim 1, wherein the second range interval is based on uncertainties of range measurements within the predetermined previous time period, the uncertainties of the range measurements being differences between first range measurements determined from carrier phase observations and second range measurements determined from satellite orbit parameters and a location of the NSS reference station.

4. The NSS rover receiver according to claim 1, wherein a time interval between reception of the two first messages is based on at least one of: data link reliability conditions between the NSS rover receiver and the NSS reference station; and a duration of a maximum acceptable interruption between the NSS rover receiver and the NSS reference station.

5. A system comprising: a NSS rover receiver according to claim 1.

6. Method for receiving information from a navigation satellite system, NSS, reference station at one or more NSS rover receivers, wherein the method comprises the steps of: receiving, at the NSS rover receiver, first messages each comprising a respective first range measurement modulo a first range interval, wherein a time interval between transmission of each first message is equal to or less than a time duration of a predetermined previous time period; receiving, at the NSS rover receiver, between reception of two first messages, from the GNSS reference station, a plurality of second messages, each second message comprising a size of a second range interval, the time duration of the predetermined previous time period, and a respective second range measurement modulo the second range interval, the second range interval being smaller than the first range interval; and reconstructing the second range measurement associated with each of the plurality of second messages.

7. The method according to claim 6, wherein the first range interval is based on estimated atmospheric effects on the first range measurements determined from carrier phase observations.

8. The method according to claim 6, wherein the second range interval is based on uncertainties of range measurements within the predetermined previous time period, the uncertainties of the range measurements being differences between first range measurements determined from carrier phase observations and second range measurements determined from satellite orbit parameters and a location of the NSS reference station.

9. The method according to claim 6, wherein a time interval between reception of the two first messages is based on at least one of: data link reliability conditions between the NSS rover receiver and the NSS reference station; and a duration of a maximum acceptable interruption between the NSS rover receiver and the NSS reference station.

10. The method according to claim 6, wherein the size of the second range interval for a second message is larger than the size of the second range interval for a preceding second message.

11. A nontransitory computer readable medium comprising a computer program loadable onto a processing unit of a navigation satellite system, NSS, rover receiver comprising code for executing a method comprises the steps of: receiving, at the NSS rover receiver, first messages each comprising a respective first range measurement modulo a first range interval, wherein a time interval between transmission of each first message is equal to or less than a time duration of a predetermined previous time period; receiving, at the NSS rover receiver, between reception of two first messages, from the GNSS reference station, a plurality of second messages, each second message comprising a size of a second range interval, the time duration of the predetermined previous time period, and a respective second range measurement modulo the second range interval, the second range interval being smaller than the first range interval; and reconstructing the second range measurement associated with each of the plurality of second messages.

12. A navigation satellite system, NSS, rover receiver for receiving information from a NSS reference station, the NSS rover receiver comprising: a reception unit configured to: receive, from the NSS reference station, first messages each comprising a respective first range measurement modulo a first range interval, wherein the first range interval is based on estimated atmospheric effects on the first range measurements determined from carrier phase observations, and a time interval between transmission of each first message is equal to or less than a time duration of a predetermined previous time period; and receive between reception of two first messages, from the NSS reference station, a plurality of second messages, each second message comprising a size of a second range interval, the time duration of the predetermined previous time period, and a respective second range measurement modulo the second range interval, the second range interval being smaller than the first range interval, wherein the second range interval is based on uncertainties of range measurements within the predetermined previous time period, the uncertainties of the range measurements being differences between first range measurements determined from carrier phase observations and second range measurements determined from satellite orbit parameters and a location of the NSS reference station; and a processing unit configured to reconstruct the second range measurement associated with each second message.

13. The NSS rover receiver according to claim 12, wherein a time interval between reception of the two first messages is based on at least one of: data link reliability conditions between the NSS rover receiver and the NSS reference station; and a duration of a maximum acceptable interruption between the NSS rover receiver and the NSS reference station.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a spatial compression of a user-satellite range measurement according to the prior art.

(2) FIG. 2 shows a temporal compression of a user-satellite range measurement according to the prior art.

(3) FIG. 3 shows the data packet size in a RTC transmission when using a temporal compression of a user-satellite range measurement according to the prior art.

(4) FIG. 4 is a schematic diagram of an exemplary hardware implementation of a NSS station that may carry out a method in accordance with embodiments of the invention.

(5) FIG. 5 shows an illustration of a method embodiment of the present invention.

(6) FIG. 6 shows an example of differences between observed and modelled carrier phase measurements over 10 s time intervals. The example is based on GPS, GLONASS, QZSS and BeiDou satellite measurements.

(7) FIG. 7 shows an example of a transmission schedule of the first and second message according to the present invention.

(8) FIG. 8A illustrate the size of data transmissions with the prior art CMRx standard for a given GNSS tracking example.

(9) FIG. 8B illustrate the size of data transmissions with the tiny CMRx (CMRxt) standard for the same GNSS tracking example as used in FIG. 8A.

(10) FIG. 9 shows an example of first and second range intervals and message sizes in accordance with some embodiments described herein.

DETAILED DESCRIPTION

(11) FIG. 4 is a schematic diagram of an exemplary hardware implementation of a NSS station 10 that may carry out a method in accordance with embodiments of the invention.

(12) The NSS station may for example be a reference station or a rover receiver. As illustrated, NSS station 10 may include a bus 15, a central processing unit (CPU) 13, a main memory 17, a ROM 18, a storage device 19, an input device 12, an output device 14, and a communication interface 16. Bus 15 may include a path that permits communication among the components of the NSS station 10.

(13) The CPU 13 may include a processor, a microprocessor, or processing logic that may interpret and execute instructions. Main memory 17 may include a RAM or another type of dynamic storage device that may store information and instructions for execution by CPU 13. ROM 18 may include a ROM device or another type of static storage device that may store static information and instructions for use by CPU 13. Storage device 19 may include a magnetic and/or optical recording medium and/or solid state medium (Flash memory) and its corresponding drive.

(14) Input device 12 may include a mechanism that permits an operator to input information to processing entity 10, such as a keypad, a keyboard, a touch-sensitive device, a mouse, a pen, voice recognition and/or biometric mechanisms, etc. Output device 14 may include a mechanism that outputs information to the operator, including a display, a printer, a speaker, etc. Communication interface 16 may include any transceiver-like mechanism that enables processing entity 10 to communicate with other devices and/or systems. For example, communication interface 16 may include mechanisms for communicating with another device or system via a network.

(15) The NSS station 10 may perform certain operations or processes described herein. These operations may be performed in response to CPU 13 executing software instructions contained in a computer-readable medium, such as main memory 17, ROM 18, and/or storage device 19. A computer-readable medium maybe be defined as a physical or a logical memory device. For example, a logical memory device may include memory space within a single physical memory device or distributed across multiple physical memory devices. Each of main memory 17, ROM 18 and storage device 19 may include computer-readable media. The magnetic and/or optical recording media (e.g., readable CDs or DVDs or Blu-ray/BDs) and/or solid state media of storage device 19 may also include computer-readable media. The software instructions may be read into main memory 17 from another computer-readable medium, such as storage device 19, or from another device via communication interface 16.

(16) The software instructions contained in main memory 19 may cause CPU 13 to perform operations or processes described herein. Alternatively, hardwired circuitry may be used in place of or in combination with software instructions to implement processes and/or operations described herein. Thus, implementations described herein are not limited to any specific combination of hardware and software.

(17) The NSS station 10 may be a NSS reference station. The navigation satellite system reference station may provide information to one or more NSS rover receiver(s) and comprise a processing unit 13 to determine one or more first range intervals representing first ambiguity windows based on estimated atmospheric effects, and to determine one or more second range intervals representing second ambiguity windows smaller than each of the first ambiguity windows based on uncertainties of range measurements within a predetermined previous period. Moreover, the NSS reference station may comprise a transmission unit 16 to transmit, to the NSS rover receiver(s), first messages each comprising a range measurement modulo a first ambiguity window, and to transmit between transmission of two first messages, to the NSS rover receiver(s), a plurality of second messages, each second message comprising a range measurement modulo one of the second ambiguity windows.

(18) The messages include range measurements modulo the ambiguity windows, and thus the size of the message can be reduced. Nevertheless, as it is assured that the respective ambiguity window (range interval) is larger than the difference between the predicted range and the measured range, the range can be reconstructed at the rover.

(19) The second ambiguity windows are smaller than each of the first ambiguity windows and the size of the second messages is thus reduced. A transmission bandwidth can be used more efficiently.

(20) Moreover, if the second ambiguity windows are determined based on uncertainties of range measurements within a predetermined previous period, it can be ensured that the second ambiguity windows are not too small. Thereby, the robustness of the transmission against poor data links is secured.

(21) The second messages advantageously each include all information needed for determining the position of the rover, i.e. they can be decoded even if the previous first message has not been received. In other words, the decoding of the second message does not rely on the reception of a recent first message providing that at least one first message has been received since the NSS rover receiver started to receive messages, i.e. the NSS reference station does not employ a pure major/delta approach. From the second message the full carrier phase measurement can be reconstructed. Therefore, the robustness of the data transmission is improved.

(22) A range measurement corresponds to a measurement performed by the NSS reference station to measure the distance between the NSS reference station and a satellite. The measurement may for example be a code-based pseudorange measurement or a carrier-phase-based measurement.

(23) Each of the first and second ambiguity windows corresponds to a range or distance that is measured in units of length such as meters. Other units such as degrees, cycles, or radians could be chosen provided that there is consistency between units when transmitting both code-based pseudorange measurements and carrier-phase-based measurements.

(24) In one embodiment, each of the second messages comprises the predetermined previous period.

(25) Since the size of the second ambiguity windows is determined based on uncertainties of range measurements the predetermined previous period, the predetermined previous period represents a period of how long the second messages can be reliable decoded without the reception of a recent first message. If the receiver fails to receive a first message within the predetermined previous period, the receiver can decide that the reconstructed carrier phase measurement is no longer reliable and suspend decoding until another first message is received.

(26) In one embodiment, the processing unit 13 is further configured to determine a time interval between transmissions of two first messages based on at least of data link reliability conditions between the NSS reference station and the NSS rover receivers, a duration of a maximum acceptable transmission interruption between the NSS reference station and the NSS rover receivers, and a rate of change of the relative distance of a satellite to the NSS reference station. The transmission unit transmit at least two first determined time interval.

(27) According to this embodiment, the time interval between two first messages is determined to improve the robustness of the data transmission. For example, if the data length reliability conditions worsen, the time interval between two first messages can be reduced. This is shown schematically in the example of FIG. 9.

(28) In one embodiment, the uncertainties are defined by differences between range measurements obtained from carrier phase observations and ranges calculated based on satellite orbit parameters and the NSS reference station location. In other words, the uncertainties are the differences between the measured ranges and the modelled ranges.

(29) In one embodiment, the size of a second ambiguity window used for a second message is larger than the size of a second ambiguity window used for a proceeding second message. Thus, the size of the second ambiguity window increases to account for degrading data link conditions and is therefore dynamically adaptable. Thus, the size of the second messages varies over time, and may increase and decrease. This is shown schematically in the example of FIG. 9. However, using this technique the reliability of the second messages can be further improved.

(30) In one embodiment, the range measurements used for the first message is a range measurement for a first satellite and the first message further comprises a range measurement for at least a second satellite modulo the second ambiguity window. In this embodiment, information with respect to two satellites is transmitted in a single message.

(31) In one embodiment, the range measurement used for the second message is a range measurement a first satellite and the second message further comprises a range measurement for at least a second satellite modulo the first ambiguity window. FIG. 5 shows an illustration of a method embodiment of the present invention.

(32) The method may be performed by a reference station. In step S20, one or more first ambiguity windows are determined based on estimated atmospheric effects. In step S21, one or more second range intervals representing first ambiguity windows are determined based on uncertainties of range measurements within a predetermined previous period. In step S22, first messages each comprising a range measurement modulo a first ambiguity window are transmitted from the NSS reference station to the NSS rover receivers. In step S23, a plurality of second messages are transmitted between transmission of two first messages from the NSS reference station to the NSS rover receiver. Each second message comprises a range measurement modulo one of the determined second ambiguity windows.

(33) The uncertainties used for determining the size of the second ambiguity windows may be defined by differences between range measurements obtained from carrier phase observations and ranges calculated based on satellite orbit parameters and the NSS reference station location. FIG. 6 shows an example of differences between observed and modelled carrier phase measurements over a 10 s period. Thus, in this case the predetermined previous period is set to 10 s. FIG. 6 illustrates that the largest difference in this period is approximately 3 m. Consequently, the size of the second ambiguity window may for example be conservatively set to 5 m or 10 m. It is advisable to set the second ambiguity window larger than the maximum expected size of the data content, otherwise there is a possibility that the ambiguity in the data may be incorrectly resolved during decoding.

(34) Furthermore, each of the second messages may comprise the predetermined previous period. In the case of FIG. 6 this value is 10 s. When the NSS receiver fails to receive a first message within the predetermined previous period, the NSS receiver can decide not to use the second messages any further until reception of a new first message. This is because the NSS receiver cannot be sure that the uncertainties cause the carrier phase measurements to exceed the size of the second ambiguity window, thereby rendering the range measurement within the second message ambiguous. The predetermined previous period may also be called a recovery period. Hence, the robustness of the transmission can be further improved by transmitting the predetermined previous period.

(35) A detailed example of the encoding and transmission process at the NSS reference station is described in the following with reference to the CMRx format.

(36) The NSS reference station (encoder) picks one NSS band as a reference. For example the L1 band may be chosen for GPS satellites, the B1 band for BeiDou, etc. Data for all other frequency bands are referenced to the reference band. Hence GPS L2, or L5 measurements are sent relative to the L1 reference band. The difference between L1 and L2 delta phase measurements over for example 10 s is normally on the order of a few millimeters due to changes in the ionospheric bias and multipath errors.

(37) In a next step, the NSS reference station selects a satellite for which the RTC data is sent in the full-form CMRx format. The CMRx format uses a first ambiguity window. The size of the first ambiguity window is determined based on atmospheric (ionospheric, tropospheric, etc.) effects. Although atmospheric errors are the main contributions of the uncertainty in the user-satellite range, there are further unmodelled effects that may be considered. In general, the unmodelled errors include: atmospheric, multipath, satellite clock/orbit errors and errors in the reference station coordinates. More than one satellite using the full CMRx format can be selected. The RTC data for the remaining satellites (and bands) is sent in a reduced form which may be called tiny or CMRxt.

(38) Then the NSS reference station looks backward over a predetermined period of data to find the largest difference between modelled and observed delta carrier phase data (i.e. find the satellite that produces the fastest change). This largest difference is then used to determine the size of the ambiguity window used for the tiny messages (the second ambiguity window). The NSS reference station selects an ambiguity window that ensures that it is large enough to cover the expected range of the data. An example of this determination is explained above with respect to FIG. 6 where the size of the second ambiguity window can be for example set to 10 m.

(39) The tiny message contains the following information: The size of the second ambiguity window as a table index (e.g. 0=256 cycles, or 1=64 cycles, or 2=32 cycles; 3=16 cycles etc) for all satellites in a constellation. However, the second ambiguity window may not be the same for all satellites; The observed carrier phase data is sent modulo the second ambiguity window size for each satellite (except the satellite(s) using the full CMRx format); The predetermined previous period (recovery period) value is sent for all satellites in message. The recovery period indicates the maximum time between Full-form CMRx transmissions to allow for unambiguous recovery of Tiny-form (e.g. maximum recovery period=60 seconds). The recovery period is also sent as a table index; Non-reference band observations (i.e. GPS L2 band) with respect to the reference band observation for respective satellites. The non-reference carrier phase are sent modulo the second ambiguity window size.

(40) The following example illustrates the calculation of the compressed code phase parameter (PCMRxt) that is sent in the tiny message:

(41) PCMRxt=PObs MOD Ambiguity_Size where:

(42) PCMRxt is the value sent in the message;

(43) PObs is the code phase observed by the NSS reference station sending the tiny message; and

(44) Ambiguity_Size is the second ambiguity.

(45) There are various phenomena that affect the NSS reference station observed code phase (PObs). These include the ionosphere, troposphere, multipath, receiver noise, broadcast orbit inaccuracies and other environmental noise. Therefore, the Ambiguity_Size is chosen to include the worst case effects that the system is expected to operate under as explained above. As an example, below we assume an Ambiguity_Size of 50 meters is large enough to account for the sum of the worst case effects under which the system is still expected to operate.

(46) Assume the NSS reference station observed a code phase value of 23192313.621 meters (range measurement) for a certain satellite. The NSS reference station computes PCMRxt as follows:

(47) PCMRxt=23192313.621 MOD 50.0=13.621 This value is transmitted as part of the tiny message to the rover receiver.

(48) Having transmitted a first message comprising the full message in the CMRx format for one satellite and the tiny message for the remaining satellites, the NSS reference station prepares a second message. The satellite for which the full message is sent may change in the second message. This rolling satellite transmission is shown in FIG. 7. More specifically, FIG. 7 shows an example of a transmission schedule of the messages according to the present invention. FIG. 7 illustrates the case in which RTC data for eight satellites are broadcasted by the NSS reference station.

(49) Data for satellite #1 is sent in the full CMRx format in the first message as indicated by F in FIG. 7. Data for satellites #2 to #8 are sent in the tiny format in the first message as indicated by T in FIG. 7.

(50) Data for satellite #2 is sent in the full CMRx format in the second message as indicated by F in FIG. 7. Data for satellites #1 and #3 to #8 are sent in the tiny format in the first message as indicated by T in FIG. 7.

(51) In other words, the range measurements used for the first message is a range measurement for a first satellite and the first message further comprises a range measurement for at least a second satellite modulo the second ambiguity window.

(52) In other words, the range measurement used for the second message is a range measurement to a first satellite and the second message further comprises a range measurement for at least a second satellite modulo the first ambiguity window.

(53) Data for satellite #3 is sent in the full CMRx format in the third message as indicated by F in FIG. 7. Data for satellites #1, #2 and #4 to #8 are sent in the tiny format in the first message as indicated by T in FIG. 7.

(54) The NSS reference station continues to create full CMRx messages for one satellite one after another.

(55) The NSS reference station changes the satellite using the full format in each message in the example shown in FIG. 7. Hence, the satellites are rolled such that data for each satellite is sent in the full format in a certain time period. The rolling scheme is not limited to the one shown in FIG. 7. The satellites using the full message can be chosen in any way, e.g. randomly or based on reception conditions.

(56) The rolling satellite approach ensures that full and unambiguous GNSS observations are sent for all satellites within a specified period of time (for example 20 seconds). By rolling the sending of the full messages, peaks in the overall message size are avoided. This is beneficial when using radio communications for sending the messages.

(57) A further embodiment relates to a mobile or rover receiver for receiving information from a NSS reference station. A schematic configuration of a rover receiver is shown in FIG. 4. The NSS rover receiver may comprise a reception unit configured to receive, from the NSS reference station, first messages each comprising a range measurement modulo a first range interval representing a first ambiguity window, and receive between reception of two first messages, from the NSS reference station, a plurality of second messages, each second message comprising a range measurement modulo a second range interval representing a second ambiguity window, the second ambiguity window being smaller than each of the first ambiguity windows. The NSS rover receiver may further comprise a processing unit configured to reconstruct a range measurement based on at least a received second message.

(58) A detailed example of the decoding and reception process at the NSS rover receiver is described in the following.

(59) In general, the NSS rover receiver receives first messages each comprising a range measurement modulo a first range interval representing a first ambiguity window. The NSS rover receiver receives, between reception of two first messages, from the GNSS reference station, a plurality of second messages, each second message comprising a range measurement modulo a second range interval representing a second ambiguity window, the second ambiguity window being smaller than each of the first ambiguity windows. Then the NSS rover receiver reconstructs a range measurement based on at least a received second messages.

(60) In one embodiment, the NSS rover receiver performs the following steps to decode the received messages: Obtaining the NSS reference station position (e.g. in WGS84 Cartesian coordinates) which is transmitted by the NSS reference station. Obtaining a valid orbit (broadcast or precise) for each satellitedecoded from GNSS satellites at recipient site. Using satellite orbit information and the NSS reference station position to compute the modelled delta phase between times tx and ty to resolve the ambiguity in the transmitted observation datai.e. determine the missing windows in each satellite observation. Using previous full carrier observation at time tx, based on previous Full-form CMRx message, or reconstructed Tiny-form CMRxt message for reformulating the full sender observations. Non-reference band carrier phase observations are reconstructed based on the reference-band carrier phase observations for each respective satellite.

(61) Consider the above example in which the NSS reference station transmits the value 13.621 in the second message. Assume that the NSS rover receiver computes (models) a range between the NSS reference station and the satellite to be 23192320.254 meters. The NSS rover receiver can do this because the NSS rover receiver knows the position of the antenna of the NSS reference station observing the data, the current time, and the NSS rover receiver has at least one of the broadcast orbits valid during this period.

(62) The NSS rover receiver reconstructs the original code phase as follows: Step 1: Predicted Range to satellite is 23192320.254. Step 2: Construct/compute ambiguous range as predicted range divided by ambiguity window, i.e.: 23192320.254/50.0=463846.40508 Truncate result to get an integer number: 463846.40508.fwdarw.463846 Compute ambiguous range as truncated result times ambiguity window, i.e.: 463846.0*50.0=23192300 Step 3: Add received value (PCMRxt) to reconstructed ambiguous range 23192300+13.621=23192313.621

(63) Thus, the NSS rover receiver has reconstructed the full measurement at the NSS reference station.

(64) For most applications of the data the NSS rover receiver only requires the observed minus computed range. However, by fully reconstructing the reference observations, the NSS rover station has the ability to process the reference data in many different ways, i.e. it can be ensured that the RTC scheme does not limit how the reference data is used by the NSS rover receiver.

(65) The CMRxt data format enables a very high level of data compression. FIG. 8A illustrate the size of data transmissions with prior art CMRx standard for a given GNSS tracking example. FIG. 8B illustrate the size of data transmissions with tiny CMRx (CMRxt) standard for the same GNSS tracking example as used in FIG. 8A. The CMRxt format leads to roughly a 40% reduction in the data throughput for the same GNSS observation data. Furthermore, the CMRxt approach provides low average and peak data throughput requirements which is beneficial when using radio communications.

(66) The present invention can be used for stationary reference station applications and applications where the reference station is moving.

(67) There is a vast spectrum of industrial high accuracy positioning applications that currently employ rovers using traditional GNSS positioning methods that will benefit from the high compression RTC methods and apparatus that the present invention will provide to GNSS high-precision, real-time NSS positioning applications. These include, but are not limited to: automatic positioning of agricultural machinery, civil construction machinery, and mining machinery; geodetic survey equipment; marine survey equipment; photogrammetry (including airborne platforms both manned and unmanned, the latter referred to as UAVs, unmanned aerial vehicles, or drones); GIS (geographic information system) equipment; and position monitoring systems (such as earthquake detection, bridge monitoring, and dam deformation).