Patent Application
20150247928
The present disclosure relates generally to satellite-based navigation or location sensors for determining present location through location and tracking of orbital satellites.
Satellite positioning receivers (SPRs) or location sensors are in widespread usage for a variety of applications, such as portable or vehicle-based navigation systems, aircraft navigation systems, etc. A variety of different satellite-based positioning systems exist, such as the Global Positioning System (GPS), in which location sensors determine the current geolocation or position and current time information by tracking and receiving signals from multiple orbital satellites, using a localization algorithm to determine the current position. In the GPS system, for example, a group of orbital satellites each broadcast navigation messages in the same frequency band using different spreading sequences encoding information related to the satellite position. The GPS navigation messages are currently constructed as 25 frames, each including five subframes of 300 bits each, and the satellites broadcast the messages at a rate of 50 bits per second. The messaging from the GPS satellites, moreover, includes information relating to the time the message was transmitted and the position of the satellite at the time of transmission, as well as orbital information including ephemeris components specific to orbit of that satellite and almanac components with information and status related to all the satellites in the GPS satellite system. In operation, satellite-based location sensors (sometimes referred to as receivers) calculate their current position by determining the pseudorange to each tracked satellite based on the transmission and receipt times of a given message from a given satellite, and use computed pseudoranges for at least four satellites to compute the sensor location via a localization algorithm such as iterative least squares search based on linearization of the pseudorange equations. In order to perform a position determination computation, the location sensor tracks four or more visible satellites using the ephemeris and almanac information.
However, when the sensor initially powers up after a lengthy period of inactivity (cold start) neither the ephemeris, almanac nor the last position are known, and the sensor must acquire satellites and decode ephemeris information from the received navigation messages for a significant period of time in order to begin tracking and accurate position determination. In particular, the ephemeris is transmitted once every 30 seconds in a single navigation message, and the sensor must initially search for satellite messages before beginning to decode the orbital information obtained from the messages. A sensor may be “warm started” in a condition where almanac information is still current and the present time is known, but the sensor must still acquire a number of satellites and decode the ephemeris information to begin tracking. The initial operations to acquire satellites and decode ephemeris information take a significant amount of time and consume power to operate microprocessor and receiver circuitry of the location sensor. In certain applications, moreover, the amount of power required to begin tracking from a cold start condition is unavailable to the location sensor. Accordingly, a need remains for improved satellite-based location sensor apparatus and systems by which geolocation can be determined in an efficient manner, particularly where the sensor begins operation without current orbital, time and position information (a cold start).
The present disclosure provides new and improved apparatus, systems and methods by which low power sensor devices wirelessly exchange acquisition and tracking information with one another and share decoding and other computational results facilitating reduced power consumption by individual sensors to determine their positions in shorter time periods.
A location sensor apparatus is provided in accordance with one or more aspects of the present disclosure, which includes a processor and electronic memory, along with a wireless receiver to receive signaling from satellites, and a wireless transceiver to communicate with one or more other location sensors. The sensor processor is configured to identify or acquire a given satellite and to provide initially available information received from the satellite to the other sensors via the wireless transceiver. The processor decodes and locally stores orbital information related to the given satellite, provides the decoded orbital information to the other sensors, and selectively computes the sensor position at least partially according to the decoded orbital information in memory.
The sensor apparatus may cooperate with other sensors to share orbital information decoding tasks, such as decoding ephemeris information and almanac information. In certain embodiments, the sensor processor receives decoded orbital information related to one or more satellites from other sensors via the wireless transceiver, and stores this decoded orbital information in the electronic memory. In addition, the processor transmits the identity of a given satellite and a corresponding carrier-to-noise or signal-to-noise ratio to the other location sensors, and selectively decodes orbital information for that given satellite if the decoded information is not already in the electronic memory and if the other sensor or sensors are not decoding the orbital information related to that satellite with a sufficiently high carrier-to-noise or signal-to-noise ratio. In this manner, the cooperating sensor having appropriate sensitivity settings to first find the satellite signal will proceed to decode a given satellite's ephemeris, and other sensors that have identified or acquired that satellite can conserve time and energy by performing other tasks, with the decoding sensor eventually reporting the decoded orbital information when completed. In this manner, two or more sensors can work in parallel to decode ephemeris, almanac and/or other orbital information, with the group of sensors reaching steady state tracking and geolocation operation sooner than if each individual sensor performed all these tasks separately.
The sensor apparatus may exchange other important information with the other sensors via the wireless transceiver, for example by receiving and locally storing such information from other sensors, and computing, storing, and transmitting such information to the other sensors. In certain embodiments, the apparatus receives pseudorange information related to one or more of the satellites from at least one other sensor, and stores the received pseudorange information in the electronic memory. In addition, the apparatus in certain embodiments computes pseudorange information related to a given acquired satellite, stores the computed pseudorange information in the electronic memory, and provides this information to at least one other sensor via the wireless transceiver.
In certain embodiments, moreover, the sensor apparatus receives and stores soft demodulation information related to one or more satellites from another location sensor, and also provides soft demodulation information related to the given acquired satellite to the other sensors via the wireless transceiver. This sharing of soft information can advantageously facilitate the expeditious and accurate decoding of information by the group of sensors.
In various embodiments, the sensor apparatus receives and stores coarse and/or fine time of day information from one or more other sensors, and may determine time of day information based on signaling from the given acquired satellite and provide this to the other sensors via the wireless transceiver.
In certain embodiments, the sensor processor is configured to periodically provide time of day information to the other sensors via the wireless transceiver, thereby facilitating operation of other sensors in low-power or sleep mode, with the ability to enter normal operation and quickly receive a periodic time of day message.
In certain embodiments, moreover, the sensor apparatus receives position information from another sensor via the wireless transceiver, stores the received position information in the electronic memory, and provides its computed sensor position to the other sensors via the wireless transceiver.
The sensor apparatus may be further configured to send a help request message to one or more other location sensors via the wireless transceiver if it has not identified or acquired any satellites after a predetermined time period.
Certain embodiments of the sensor apparatus further facilitate expeditious time to first fix, with the sensor processor being configured to use available orbital information, such as ephemeris and/or almanac information in the electronic memory to initialize or update a satellite search when the sensor position or time of day becomes available. For example, the sensor in certain embodiments may use available almanac information and position or time of day information to selectively discontinue or stop searching for one or more of the satellites which are known to be not currently visible. In another example, the sensor apparatus may use available ephemeris information related to a given acquired satellite to narrow a search for the satellite around an expected Doppler frequency indicated in the ephemeris information.
In certain embodiments, moreover, the sensor apparatus wirelessly receives an indication that another sensor is searching for a particular satellite, and selectively refrains from searching for that satellite if the other sensor has been searching longest for that particular satellite.
In accordance with further aspects of the disclosure, systems are provided, including a plurality of location sensors which individually include a processor, memory, a wireless receiver to receive communications signaling from satellites, and a wireless transceiver to communicate with one or more other location sensors. The individual location sensors of the system store and cooperatively exchange information related to acquiring and tracking four or more satellites to facilitate determination of their positions through a localization algorithm, for example. The location sensors in certain embodiments, moreover, individually search for satellites by random or arbitrary selection of a satellite index and search beginning at a random circular rotation of a pseudorandom noise sequence, as well as wireless broadcasting of the selected satellite index to the other sensors. In this manner, the likelihood that a given satellite will be searched increases, and the sharing of satellite searching resources of the sensors is facilitated.
The individual location sensors in certain embodiments of the system intelligently share decoding tasks and information, with the individual sensors receiving and locally storing decoded orbital information from another sensor. In addition, the individual location sensors in these embodiments indicate the identity of the given acquired satellite and a corresponding carrier-to-noise or signal-to-noise ratio using the wireless transceiver, and selectively decode the orbital information if not already in memory and if another sensor is not decoding the same orbital information with a sufficiently high carrier-to-noise or signal-to-noise ratio. The individual sensor may thus refrain from decoding the orbital information if decoded orbital information is already available in the electronic memory or if another sensor with a sufficient carrier-to-noise or signal-to-noise ratio is already decoding that information.
In certain embodiments, moreover, two or more of the sensors use different sensitivity settings to search for satellites, so that some sensors look only for satellites with high signal-to-noise ratios while others will also identify or acquire satellites with low signal-to-noise ratios.
The individual sensors in certain embodiments may also provide initial information received from a located satellite to the other sensors via the wireless transceiver, including the located satellite index, a carrier-to-noise or signal-to-noise ratio of the signal received from the located satellite, a Doppler frequency associated with the located satellite, and a pseudorandom noise sequence location at which the signal was found.
In certain embodiments, the individual location sensors cooperate to compute the distance between themselves to improve performance and accuracy.
The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description when considered in conjunction with the drawings, in which:
One or more embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale. The present disclosure provides a location sensor apparatus for use in satellite-based navigation systems using cooperation among multiple sensors for satellite acquisition and tracking. The various concepts of the disclosure may be advantageously employed to facilitate deployment, use, and maintenance of a remote satellite-based navigation system that must accurately track its position and report information in a reduced power mode while facilitating short time to first fix, although the disclosed apparatus and techniques find utility in other usage scenarios and are not limited to the aforementioned applications.
Referring initially to
In particular, the processor 20 in this example is programmed to perform various geolocation processing for determining its current position based on data and information received from satellites 10 and from other sensors 2, and to share GPS processing related information with the other sensors 2 for load and information sharing purposes to facilitate reduction in power consumption of the individual sensors 2 while implementing GPS satellite location, acquisition and tracking functions. The processor 20 and apparatus 2, moreover, are operable to implement various tasks and functions associated with GPS processing, examples of which are set forth in U.S. Pat. No. 8,441,398 to Rao et al., issued May 14, 2013, the entirety of which is hereby incorporated by reference.
In addition, the sensor 2 is programmed or otherwise configured to implement load sharing with other sensors 2 via wireless communication using the transceiver 6 by which the GPS processing can be shared among multiple sensors 2, thereby reducing the power consumption requirements of the individual sensors 2 and the system 12 as a whole. The sensor 2 is thus operable to transmit or broadcast information to other sensors 2 regarding GPS satellite information it has acquired, and to receive such information transmissions from other sensors 2, and to improve positioning or navigation performance by sharing information once a satellite vehicle 10 has been identified or acquired, with the sensors 2 being further configured to cooperate to improve sensitivity for demodulating GPS data bits.
As seen in
An exemplary GPS navigation message frame 200 is illustrated in
Returning to
Referring to
Upon start-up, the location sensor 2 begins acquisition processing 101 in
After decoding information about the located satellite 10 and broadcasting this information to other sensors, the location sensor 2 checks its local information 50, 52, 54, 56, 58, 60, 62, 64, 66 stored in memory 30 and makes a determination at 114 as to whether enough satellite indexes have been located to begin tracking position information. The local list of information contained in the data structures of memory 30 in one embodiment includes a concatenated list of the satellite indexes that have been acquired and their strength, the time of day, a pseudorange, Doppler frequency and time stamp of measurements for each satellite index. This information can be used to improve performance of each location sensor 2 by isolating/mitigating a multipath. If enough satellites have been located (YES at 114), the location sensor 2 moves to the tracking phase at 116 and computes its position at 118. Otherwise (NO at 114), the location sensor 2 continues the acquisition processing 101 as described above.
If the searched satellite has been identified or acquired (YES at 128), the location sensor 2 stores and broadcasts the initially available information 52 to the other sensors at 132. In certain embodiments, the initially available information 52 can include the index of the located satellite 10, the carrier-to-noise ratio or the signal-to-noise ratio of the satellite signal, the Doppler frequency, and the location in the pseudorandom noise sequence where the satellite 10 was found. The navigation sensor 2 then proceeds to decode and store the ephemeris data 54 at 134 in
If the responses 58 from the other sensors indicate that another sensor 2 is decoding the ephemeris (YES at 152), the sensor 2 determines from the information 58 provided by the other sensor 2 whether the other decoding sensor 2 has a sufficiently high carrier-to-signal or signal-to-noise ratio. If not (NO at 154), the sensor proceeds to attempt to decode the ephemeris at 156, and broadcasts that the decoding has begun, where the broadcast message indicates the sensor's carrier-to-noise or signal-to-noise ratio. By this technique, the other sensor 2 will be informed that the current sensor 2 is decoding with a sufficiently high carrier-to-noise or signal-to-noise ratio, and may discontinue decoding that ephemeris by similar operation in that sensor 2. If the other location sensor 2 has begun decoding the ephemeris but has a sufficiently high carrier-to-noise or signal-to-noise ratio than the querying location sensor 2 (YES at 154), the location sensor 2 selectively refrains from decoding this ephemeris and may wait for the ephemeris to become available from the other sensor 2 and then proceed with decoding and broadcasting the time of day information at 138, with the process then returning to 140 in
Following the ephemeris decoding at 156, the sensor 2 performs a parity check and determines at 158 whether the parity check was successful (passed). If the parities match (YES at 158), the ephemeris was properly decoded and the location sensor 2 stores the decoded ephemeris 54 in the memory 30 and broadcasts the ephemeris information at 166 for use by other location sensors 2. If the parities do not match (NO at 158), the ephemeris was not properly decoded, and the sensor 2 combines the decoded ephemeris information at 160 with other soft information 60 available from the other location sensors 2 and conducts another parity check at 162. If the parities now match (YES at 162), the location sensor 2 saves and broadcasts the decoded ephemeris information 54 at 166 for other location sensors 2 to use. In certain embodiments, the parity check can be conducted again at 162 in a loop including soft information combination at 160 and parity check at 162, and ending after a defined number of cycles in the hardware or software within the location sensor's memory 30. If after the defined number of parity check loop cycles is conducted the parities do not match (NO at 162), the location sensor 2 broadcasts the available soft information at 164 for other location sensors 2 to use in decoding the ephemeris in the future.
Once the soft information has been broadcast at 164 or the successfully decoded ephemeris 54 has been broadcast at 166, the sensor 2 proceeds to selectively attempt almanac decoding at 136. As seen in
The exemplary sensor apparatus 2 thus operates as part of an efficient expeditious system 12 (e.g.,
In certain implementations, two or more of the location sensors 2 use different sensitivity settings to search for satellites 10. In this manner, some sensors 2 will identify only high signal-to-noise ratio satellites 10, while other sensors 2 will also find lower signal-to-noise ratio satellites 10. Other embodiments are possible, for example, with each sensor 2 being configured to a certain sensitivity setting to simplify the implementation. Moreover, the illustrated embodiments advantageously share acquired information with other sensors 2, where the information sharing may be implemented using broadcast messaging, or messages to specific sensors are possible in certain embodiments. For example, once a satellite 10 has been identified by the location sensor 2, certain parameters are known as soon as a peak is found, and accordingly the sensor 2 provides this information via the wireless transceiver 6 to the other sensors 2 without waiting for ephemeris, pseudorange, almanac, etc. to be computed, where the immediately or initially available information 52 in certain embodiments includes the identified satellite index, the carrier-to-noise or signal-to-noise ratio of the signal received from the satellite 10, the Doppler frequency associated with that satellite 10, etc. In addition, the initially available information 52 reported to the other sensors 2 can include the location within the pseudorandom noise sequence where the peak was found at the time of packet transmission, thereby facilitating fine-time injection for the other sensors 2. In this regard, the Doppler frequency can be used by the other sensors 2 in order to predict where in the pseudorandom number sequence the peak should be found, and the recipient sensors 2 can use this information to perform a search close to the indicated location. In practice, this can advantageously reduce the search by orders of magnitude. In addition, the signal-to-noise ratio can be used by the other sensors to set the sensitivity of their satellite search appropriately.
In certain implementations, the individual sensors 2 receive pseudorange information related to one or more of the satellites 10 from another sensor 2 via the wireless transceivers 6, and store this pseudorange information (information 50 in
As previously noted, moreover, a single sensor 2 takes between 12 and 30 seconds to decode a complete ephemeris for a given satellite 10, whereas the illustrated location sensors 2 decode and broadcast the part of the ephemeris that they have decoded, whereby all the sensors 2 in the system 12 obtain the decoded ephemeris portions 54 and store these in their local electronic memories 30 much faster than a single sensor can, thereby significantly reducing the time to first fix for all the sensors 2 in the system 12. Moreover, the almanac decoding tasks and intermediate results are also shared among the sensors 2 of the system 12. In certain embodiments, for example, each sensor 2 can concatenate its measurements to those it receives in the almanac data 56 of the memory 30 so that the sensor 2 does not need to listen for the full 12.5 minutes, instead the sensors can share the burden of demodulating the almanac so that across a 12.5 minute interval there are a minimal number of sensors actively demodulating at once. Moreover, although decoding the almanac information 56 is not strictly required to perform position determination, having the almanac 56 facilitates longer sleep times in the tracking phase for the sensors 2, and thus the distributed sharing of the almanac information decoding functions facilitates reduced power consumption within the system 12 and within the individual sensors 2 thereof.
Moreover, the sensors 2 advantageously share soft demodulation information 60 about the bits being transmitted, thereby effectively improving system sensitivity when soft information 60 from multiple sensors 2 is combined. For example, as seen at 160 and 162 in
In certain embodiments, the sensors 2 receive initial time of day information 62 (coarse time of day) from other location sensors 2 via the wireless transceiver 6, and store these in the electronic memory 30 as shown in
In addition, once a particular sensor 2 gets a time fix and knows the GPS time with accuracy (e.g., less than 1 ms), that sensor 2 can send a message via the wireless transceiver 6 that includes a timestamp of the time of transmission for that packet according to GPS. Consequently, any receiver 2 within range will then know the time of day very accurately by decoding the packet and noting it's time of arrival, thereby greatly reducing its search space. Moreover, sensors 2 can re-broadcast the fine time of day information 64 after adding a delay associated with re-transmission, thereby ensuring that the entire system 12 can benefit as soon as possible after the first sensor 2 gets a time fix. Thus, this time of day sharing aspect of the present disclosure advantageously reduces the amount of processing overhead and consumed power involved in the acquisition and tracking operation of the sensors 2 of the system 12. Also, once a sensor 2 is synchronized to GPS, it can send out messages regularly with timestamps so that other sensors could synchronize their frequencies, and easily resume tracking operation when transitioning from sleep mode, thereby further facilitating energy conservation within the sensors 2 of the system 12.
The system 12 also advantageously shares position and uncertainty information 66, with the individual sensors 2 in certain embodiments receiving position information from one or more other sensors 2 via the wireless transceiver 6, and storing this information in the electronic memory 30, and providing their individual computed sensor position 66 to the other location sensors 2 via the wireless links 8. This position information sharing aspect will further reduce the search space for the other sensors 2 that have not yet computed their own position 66. In certain embodiments, moreover, the location sensors 2 cooperatively compute distances between individual sensor pairs, thereby facilitating estimation by a given sensor 2 of its position prior to the more accurate position determination possible in tracking mode.
In certain embodiments, the messaging between sensors 2 may have different priorities. For example, messaging relaying the time of day and the announcement of the acquisition or identification of a new satellite 10 may have high priority, as these are particularly helpful to the other sensors 2.
In operation of the system 12, moreover, once all the sensors 2 are tracking the satellites 10 on their own, one, some or all of the sensors 2 may go into a low-power mode in which the GPS receiver 4 is put into a sleep mode. In one possible implementation, a single sensor 2 may remain active and track the satellite broadcasts, and periodically transmit the time of day to the other sensors 2, such that sensors 2 waking up from the low-power mode can quickly lock onto the satellite signals, thereby conserving the overall power consumption within the system 12. In this regard, once at least four satellites have been acquired, and the corresponding ephemeris 54 and almanac 56 have been decoded, the system 12 can conserve power while merely searching for and decoding ephemeris for new satellites 10 that become visible as time passes. One possible implementation conserves system resources by ensuring that at least one sensor 2 always remains awake, thereby ensuring that multiple sensors 2 do not decode an ephemeris associated with a newly visible satellite 10. One possible implementation would be for each sensor 2 to be configured to refrain from going into a sleep or low-power mode until receiving an acknowledgment from another sensor 2 that the other sensor will remain awake.
In other embodiments, the GPS receiver 4 in the individual sensors 2 may implement power saving strategies, including without limitation signal blanking. For example, if the signal blanking intervals are less than 20 ms, data demodulation can still be performed although with degraded sensitivity. Having multiple sensors 2 to cooperate facilitates recovery of the loss and sensitivity due to blanking. Moreover, for longer blanking intervals (e.g., more than 20 ms), multiple sensors 2 can cooperate to successfully complete the data demodulation with their signal blanking intervals being interleaved.
In certain embodiments, the sensors 2 may track the carrier-phase of the satellite signals, to facilitate precise measurements.
In certain embodiments, moreover, the sensors 2 may also share other information. For example, sensors 2 equipped with pressure sensing components may share pressure readings to facilitate estimation of elevation or altitude more accurately.
In certain embodiments, the individual sensors 2 can acquire a limited number of satellites 10 in parallel in order to minimize complexity. In such implementations, the sensors 2 may include dedicated hardware for acquisition, as well as separate hardware for tracking, thereby limiting power consumption during tracking mode operation. In other possible implementations, the same hardware may implement both acquisition and tracking mode operation, but with reduced power consumption during tracking operation. These techniques, moreover, are applicable to any GPS or other satellite-based navigation system receiver architectures, including without limitation delay locked loop (DLL) and frequency locked loop (FLL) approaches.
As discussed above, moreover, the sharing of soft demodulation information can facilitate expeditious decoding of orbital information, including ephemeris information 54. For example, once one sensor 2 has acquired a satellite 10 depending on the signal-to-noise or carrier-to-noise ratio of the acquired signal, most or all of the other sensors 2 can stop looking for that satellite 10 and proceed to search for a different satellite 10. Two or more remaining sensors can cooperate to decode the ephemeris of the acquired satellite 10. In the above described implementations, if the signal has a low signal-to-noise ratio, then more sensors 2 may remain to decode the ephemeris cooperatively, and combining soft information from multiple sensors 2 for the ephemeris will facilitate decoding at a lower signal-to-noise ratio. In certain implementations, moreover, a given sensor 2 may listen to the ephemeris multiple times to further improve sensitivity, and the sensors 10 can optionally enter a sleep mode to conserve power or search for other satellites 10 while the ephemeris is not being broadcast, since only two of the five subframes (subframes 202 and 203 in
The inventors have appreciated that two sensors 2 may not be able to accurately demodulate any of the words in the frame 200 (
The following numerical example illustrates certain advantages of the disclosed sensor apparatus 2 and multi-sensor systems 12, assuming a Doppler uncertainty range of +/−15 kHz due to a low quality receiver clock, where the number of Doppler bins is M=30000*Tcoh/k. The parameters Tcoh and k can be tuned for maximized sensitivity with Tcoh=0.02 and k=0.5, with M=1200 and for minimized sensitivity with Tcoh=0.001 and k=1, with M=30. Assuming that Nnon-coh coherent intervals are combined non-coherently so that the total integration time is T=Nnon- cohTcoh, there are M*N different correlations to be done for each satellite 10 every 1 ms (in GPS), and also assuming that it is equally likely that the satellite 10 could be found in any of the M*N correlation bins. If there are S sensor nodes 2 and each node 2 randomly chooses the time and frequency with which it begins to search, the system 12 can cover S/F correlation bins in T seconds, where F is the overlap in the searches due to non-optimal coverage of the sensor searches. In the GPS example, moreover, there are 32 possible satellites 10 to search. Assuming a time to first fix requirement of 12 hours, there are S sensors 2 cooperating that can each compute C correlations in T seconds, F=0.25, there are Nsv=32 SVs to search, and maximal sensitivity is used with T=1 sec by all sensors 2. Also assuming that once one sensor 2 acquires a satellite 10, the information is propagated quickly through the system 12 so that the time to first fix is dominated by the time to acquire the first satellite 10. Additionally assuming that there are always at least Nmin=4 satellites 10 visible at a given time, the number of sensors 2 required to meet the specification is:
S>T·Nsv·M·N·k/(F·C·TTFF·Nmin)
For C=1 (the simplest possible receiver), then S>909. If fewer sensors 10 or a faster time to first fix is desired, then the individual sensors 2 may be more complex. For example, if a given sensor 2 is capable of searching 1023 correlation bins at a time and the time to first fix requirement is 5 min, then S>128 sensors 2 should be used. Techniques to take advantage of the circular nature of the PRN sequence make this option of C=1023 feasible. Other techniques are available to search multiple Doppler bins with a small degradation in sensitivity due to sinc roll off, so that C=3*1023 is not unreasonable, and this would give S>42.7. If the minimal sensitivity settings are used, then the number of sensors 2 to meet the TTFF requirement reduces by a factor of 40.
At 322 in
Once one of the parallel-searched satellite indexes has been identified (YES at 328 in
If a cancel signal is received at 340 in
If a message is received at 344 in
In this process 300, the use of the orbital information at 312 in
Satellite searches may thus be advantageously updated or started using any available orbital information at 312. For example, available almanac information 56 in the memory 30 may be used to determine whether two particular satellites 2 can be visible at the same time. If the satellite 10 being searched (and not yet acquired) is thus determined to be not visible at the same time as a satellite 10 that has been acquired, then the sensors 2 in the system 12 can stop searching for that satellite 10. In addition, if ephemeris information 54 is available for a given satellite 10, that satellite 10 may be assigned to one of the acquisition channels, and the search can be centered on the Doppler frequency found by another sensor 2 using the code-phase used by the other sensor 2 and the acquisition parameters appropriate for the SNR found by the other sensor 2. This will allow the sensor 2 to acquire the satellite 10 quickly, and move it into its set of satellites 10 being tracked. This example is the same as assigning the index to an acquisition channel at 350 in
In certain situations, once position and time are known, it is possible that a satellite 10 that should be visible cannot be acquired. This may happen, for example, if there are objects in the vicinity of the sensor 2 that block the signal from the satellite 10. In this case, the sensor 2 may stop searching that satellite 10 for a period of time and try again later after the satellite 10 has had sufficient time to move (the obstruction or sensor may also move). In addition, if the sensor 2 has the capability to detect when it moves, it can try again after a certain amount of movement.
The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In addition, although a particular feature of the disclosure may have been disclosed with respect to only one of multiple implementations, such feature may be combined with one or more other features of other embodiments as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.
