US20090002231A1 - Method for a space-efficient gps receiver - Google Patents
Method for a space-efficient gps receiver Download PDFInfo
- Publication number
- US20090002231A1 US20090002231A1 US12/098,913 US9891308A US2009002231A1 US 20090002231 A1 US20090002231 A1 US 20090002231A1 US 9891308 A US9891308 A US 9891308A US 2009002231 A1 US2009002231 A1 US 2009002231A1
- Authority
- US
- United States
- Prior art keywords
- ephemeris
- satellite
- gps receiver
- almanac
- almanac data
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Images
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
- G01S19/27—Acquisition or tracking or demodulation of signals transmitted by the system creating, predicting or correcting ephemeris or almanac data within the receiver
Definitions
- the present invention relates generally to global positioning system (GPS) technology.
- GPS global positioning system
- the present invention relates to a design of a GPS receiver.
- GPS is a satellite-based radio navigation system.
- Each GPS satellite also called a space vehicle (SV)
- SV space vehicle
- ephemeris i.e., the SV's orbital parameters
- almanac i.e., reduced-precision orbital parameters of all SV's in the GPS constellation
- PRN pseudo-random noise
- a GPS receiver extracts the signal transmit time from each of the satellites it tracks. For each satellite, the difference between the time the satellite's signal is received, as determined by the receiver's clock, and the time the signal is transmitted from the satellite is the apparent transit time.
- the apparent distance between the satellite and the GPS receiver (“pseudorange”) is the product of this apparent transit time and the speed of light in a vacuum.
- An accurate position of the satellite can be calculated based on a valid ephemeris broadcast by the satellite.
- the GPS receiver can determine a 3-dimensional receiver position and the receiver clock bias relative to the GPS Time (GPST).
- the GPS receiver uses ephemeredes and almanac data to search for visible GPS satellites.
- ephemeredes and almanac data are stored in a non-volatile memory circuit in one of the GPS receiver integrated circuits.
- an ephemeris contains a set of ephemeris parameters—valid over a short period of time (e.g., 4 hours)—that enable calculating the satellite's positions and velocities as a function of GPS time.
- Each GPS satellite broadcasts its own ephemeris as 16 Keplerian parameters as follows: (a) “toe”, which is the reference time of the ephemeris; (b) “M o ”, which is the mean anomaly at the reference time of the orbit; (c) “ ⁇ n”, which is the mean motion correction from a computed orbit, (d) “e”, which is the eccentricity of the orbit, (f) “A 1/2 ”, which is the square root of the semi-major axis of the orbit, (g) “ ⁇ O ”, which is a longitude of an ascending node of the orbit plane at a weekly epoch, (h) “i o ”, which is an inclination angle at reference time, (i) “ ⁇ ”, which is an argument of the perigee of the orbit, (j) “ ⁇ dot over ( ⁇ ) ⁇ ” (“Omega-dot”), which is a rate of change of a right ascension, (k) “ ⁇ ” (“I-dot”), which
- Each satellite broadcasts a new ephemeris parameter set every two hours or so.
- An ephemeris parameter set is regarded as current, valid, or unexpired typically within the time window of ⁇ 2 hours of its reference time. Beyond this time window, the accuracies of the satellite's position and velocity, as calculated from this ephemeris, decrease. Thus, the ephemeris is typically regarded as expired outside of the time window.
- Each satellite also broadcasts an almanac that gives ephemeris data at reduced precision for all satellites in the constellation (32) of GPS satellites.
- the time required for a satellite to transmit and for a GPS receiver to receive a complete set of almanac is 12.5 minutes.
- the almanac data for each satellite includes the following 8 parameters: (a) “toa”, which is reference time of the almanac, (b) “M O ”, which is the mean anomaly at reference time of the satellite orbit, (c) “e”, which is the eccentricity of the orbit, (d) “A 1/2 ”, which is the square root of the semi-major axis of the orbit, (e) “d i ”, which is a value that equals I O -0.3 ⁇ , (f) “ ⁇ ”, which is an argument of perigee, and (g) “ ⁇ dot over ( ⁇ ) ⁇ ”(“Omega-dot”), which is a rate of change of a right ascension.
- almanac data is typically regarded as valid for a much longer time than ephemeris data. In fact, almanac data that is up to 2 years old can still be used to determine a satellite's visibility.
- a week number is often attached to an almanac or ephemeris parameter set. This week number is often used to help determine whether or not the almanac or ephemeris is current (i.e., unexpired).
- a GPS receiver can use ephemeris or almanac data to determine approximately when a satellite would rise above the horizon, given an approximate user position and time.
- almanac data is often used to initiate a search list of possible visible satellites.
- the rough satellite positions and velocities calculated from almanac or ephemeris data reduce the time-to-first-fix (TTFF), as the GPS receiver starts up.
- TTFF time-to-first-fix
- the GPS receiver typically saves both ephemeredes and almanac data from the satellites in its on-chip non-volatile memory (e.g., an battery backed-up SRAM). So that, whenever the GPS receiver powers up from a previous shut-down, either an unexpired ephemeris or a valid almanac is available to estimate each satellite's position, so as to generate a list of visible satellites to search for.
- on-chip non-volatile memory e.g., an battery backed-up SRAM
- a method in a GPS receiver saves non-volatile memory space by not storing almanac data for those satellites for which an up-to-date ephemeris is already available in the GPS receiver. That is, a method of the present invention saves on-chip non-volatile memory space by storing in the non-volatile memory, for each satellite, either an up-to-date ephemeris of each satellite, or up-to-date almanac data, but not both.
- the data when more up-to-date ephemeris or almanac data for a satellite becomes available, the data is stored onto the GPS receiver's non-volatile memory.
- the GPS receiver computes a satellite's position to estimate the satellite's visibility, the more up-to-date ephemeris or almanac of the satellite is used.
- Ephemeredes and almanacs are typically provided to the GPS receiver from satellite live signals, a source on an assisted GPS network, or some other data source.
- a method includes: (a) when the GPS receiver has only an ephemeris of a satellite and does not have almanac data for that satellite, the ephemeris—whether current or not—is selected; (b) when the GPS receiver has only almanac data for a satellite and does not have an ephemeris for that satellite, the almanac data is selected; (c) when the GPS receiver has a current or unexpired ephemeris, the ephemeris is selected; (d) when the GPS receiver has both an expired ephemeris and almanac data for a satellite, the GPS receiver compare values for their week number values and selects (1) the almanac data, when the ephemeris and almanac data have the same week number value; and (2) the data with the larger week number value, when the week number values are different. In comparing the week number values, the “rollover” event for the week number is considered. Typically, a larger week number
- data in an expired ephemeris of a satellite is used to create almanac data for the satellite, which can then be used to estimate the position and velocity of the satellite, so as to determine the satellite's visibility.
- the present invention provides the method to transform ephemeris parameters into almanac parameters.
- FIG. 1 shows the errors of a satellite's positions calculated based on its old ephemeredes, relative to a reference ephemeris.
- FIG. 2 shows the errors of a satellite's velocities calculated based on its old ephemeredes relative to a reference ephemeris.
- FIG. 3 shows the errors of a satellite's positions calculated based on its old almanacs, relative to a reference ephemeris.
- FIG. 4 shows the errors of a satellite's velocities calculated based on its old almanacs, relative to a reference ephemeris.
- FIG. 5 shows the errors of multiple satellites' positions calculated based on their old ephemeredes and almanacs, relative to respective reference ephemeredes.
- FIG. 6 shows the errors of multiple satellites' velocities calculated based on their old ephemeredes and almanacs, relative to respective reference ephemeredes.
- FIG. 7 illustrates the method of transforming parameters in an ephemeris to almanac parameters.
- FIG. 8 illustrates a method for determining whether an ephemeris or a set of almanac data is more up-to-date.
- a study conducted by the inventors reveals that satellite positions and velocities calculated using expired ephemeredes are about as accurate as satellite positions and velocities calculated from valid almanac data.
- the positions and velocities of a satellite can be calculated within several meters and several meters per second, respectively. These small position and velocity errors are insignificant for the purpose of correctly predicting whether or not a satellite is visible to the GPS receiver.
- positions and velocities calculated based on unexpired ephemeredes are used as reference values.
- the satellite positions and velocities calculated based on almanac data or old ephemeredes are compared to these reference values, and their differences relative to the reference values are referred to as position and velocity errors.
- FIG. 1 shows three-dimensional position errors 101 , 102 , 103 , 104 and 105 of a satellite (PRN 1 ), calculated using ephemeredes that are 2, 26, 52, 78 and 104 weeks old, respectively.
- PRN 1 satellite
- the position errors at different time points within the four-hour window centered at the reference time (toe) of the reference ephemeris are relatively constant (i.e., there is not a significant degradation in accuracy over the duration of the four-hour window).
- ephemeredes with different reference times (toe's) are selected to avoid any potential bias on calculated position errors.
- the toe of the reference ephemeris is 244800 seconds, while toe's of the expired ephemeredes are 230384, 215984, 201600, 187200, and 172800 seconds (i.e., staggered at a 4-hour offset between ephemeredes).
- FIG. 2 shows the three-dimensional velocity errors 201 , 202 , 203 , 204 and 205 of the same satellite (PRN 1 ), calculated using its ephemeredes with 2, 26, 52, 78 and 104 weeks old, respectively. Again for each expired ephemeris, the velocity errors at different time points within the four-hour window centered at the reference time (toe) of the reference ephemeris are also relatively constant.
- FIGS. 1 and 2 are compared to position and velocity errors obtained using old but still unexpired almanac data.
- FIG. 3 shows three-dimensional position errors 301 , 302 , 303 , 304 and 305 of the same satellite (PRN 1 ), calculated based on almanac data that are 2, 26, 52, 78 and 104 weeks old, respectively. Typically, such old almanac data are still regarded by practitioners as unexpired.
- a comparison of the position errors in FIG. 1 and FIG. 3 shows that the satellite positions calculated using old ephemeredes are roughly as accurate as those calculated using the correspondingly old almanac data.
- FIG. 3 shows three-dimensional position errors 301 , 302 , 303 , 304 and 305 of the same satellite (PRN 1 ), calculated based on almanac data that are 2, 26, 52, 78 and 104 weeks old, respectively. Typically, such old almanac data are still regarded by practitioners as unexpired.
- FIG. 4 shows three-dimensional velocity errors 401 , 402 , 403 , 404 and 405 of the same satellite (PRN 1 ), calculated using almanac data that are 2, 26, 52, 78 and 104 weeks old, respectively.
- PRN 1 satellite
- a comparison of the errors of FIG. 2 and FIG. 4 shows that the satellite velocities calculated using old ephemeredes are roughly as accurate as those calculated using the correspondingly old almanac data.
- FIG. 5 shows the errors in calculating the positions of seven randomly selected satellites (PRNs 1 , 6 , 11 , 16 , 21 , 26 , and 30 ), relative to reference values calculated from current, unexpired ephemeredes, using old ephemeredes (graphs 502 ) and old but still valid almanac data (graphs 501 ).
- PRNs 1 , 6 , 11 , 16 , 21 , 26 , and 30 old but still valid almanac data
- Position errors in degrees are more informative of the severalty of the position errors. Since this conversion is applied to position errors calculated from both ephemeredes and almanacs, the comparison keeps fair. As shown, the position errors due to old ephemeredes are comparable to the position errors due to old almanac data. Thus, expired ephemeredes can replace almanac data of the same age for estimating a satellite's visibility with acceptable accuracy. For example, 26-week old (i.e., 182 days or a half-year) ephemeredes and almanac data both result in position errors of less than 10 degrees.
- FIG. 5 also shows that position errors for 2-week old ephemeredes are consistently about ten times greater than those calculated using 2-week old almanac data. Such a difference, however, is insignificant for our purpose because (1) the errors for 2-week old ephemeredes are as small as 1 degree and (2) the 26-week old almanac data that provides a 10-degree error is still regarded as valid and are routinely used in many GPS receivers.
- FIG. 6 shows the velocity errors for the same set of satellites calculated using old ephemeredes (graphs 602 ) and old almanac data (graphs 601 ). No significant difference is found in velocity errors between those calculated using old ephemeredes and those calculated using the correspondingly old almanac data.
- the estimated velocity of a satellite is used to determine a Doppler search range for the satellite. From this study, expired ephemeredes can also replace old almanac data in estimating satellite velocity, with an acceptable accuracy.
- a GPS receiver may save non-volatile memory space by not storing almanac data for those satellites for which an up-to-date ephemeris is already available in the GPS receiver.
- the GPS receiver stores in the non-volatile memory either an up-to-date ephemeris of each satellite, or unexpired almanac data, but not both.
- the data is stored onto the GPS receiver's non-volatile memory.
- the GPS receiver computes a satellite's position to estimate the satellite's visibility
- the more up-to-date ephemeris or almanac of the satellite is used.
- Ephemeredes and almanacs are typically provided to the GPS receiver from satellite live signals, a source on an assisted GPS network, or some other data source.
- data from an expired ephemeris of a satellite may be used to create almanac data for the satellite, which can then be used to estimate the position and the velocity of the satellite, so as to determine the satellite's visibility.
- the present invention provides the method to transform ephemeris parameters into almanac parameters.
- FIG. 7 shows a method for transforming an expired ephemeris into almanac data.
- ephemeris parameters M O , e, A 1/2 , ⁇ O , ⁇ , and ⁇ dot over ( ⁇ ) ⁇ become almanac parameters of the same names.
- the ephemeris reference time (toe) becomes the almanac reference time (toa).
- Almanac parameter d i is obtained by subtracting 0.3 ⁇ from ephemeris parameter i O .
- the almanac data thus transformed from an expired ephemeris can be used for calculating satellite positions and velocities. In fact, the position and velocity values used in obtaining the data in FIGS. 1 , 2 , 5 , and 6 are all obtained via this transform method.
- the GPS receiver saves at least 1 kilobyte of memory space.
- FIG. 8 illustrates a method for determining whether an ephemeris or an almanac is more up-to-date.
- the ephemeredes and almanac data are obtained by: (a) restoring from the receiver's non-volatile memory, (b) decoding satellite live signals, (c) receiving from an assisted-GPS computer network, or any other suitable source. Only the most up-to-date ephemeris or almanac data is stored into the non-volatile memory of the GPS receiver prior to the GPS receiver shutting down, and only the more up-to-date ephemeris or almanac data is used to estimate the satellite's visibility. Note the same selection rules are applied to each GPS satellite, and different satellites may have a different selection result. As shown in FIG. 8 , the selection rules under this method are provided as follows:
- a GPS receiver determines (at step 801 ) that it has only an ephemeris of a satellite and does not have any almanac data (at step 802 ) for that satellite, the ephemeris data is selected for estimating the satellite's position and velocity or for being stored, regardless of whether or not the ephemeris is current (i.e., unexpired).
- the receiver only has almanac data of the satellite and does not have any ephemeris, the almanac is selected.
- the GPS receiver determines that it has both ephemeris data and almanac data (at step 801 ), it examines whether or not the ephemeris has expired (step 803 ). Unexpired ephemeris is selected over almanac data (step 804 ).
- the receiver needs to compare their week number value (step 805 ), taking into consideration whether or not a rollover event has occurred. Normally, a greater week number indicates a more recent reference time, except immediately after a rollover event. At the rollover event, because the week number has reached the maximum week number, it is reset to the lowest week number value.
- the ephemeris data is selected (step 807 ). Otherwise, the almanac data is selected.
- the selection procedure shown in FIG. 8 compares the age of an ephemeris and almanac data using only their respective week numbers. This is provided only for illustrative purpose.
- the actual age of an ephemeris or an almanac is provided by the combination of its week number and its reference time (toe or toa). When an ephemeris and an almanac both have the same week number, the one with the larger reference time is more recent.
- the procedure of FIG. 8 may be further extended.
- one may associate the respective ages of ephemeredes and almanac data, (e.g., down to the level of seconds) that would provide the same accuracies when used to calculate satellite positions and velocities.
- a table may be provided relating position or velocity estimations with the ages of almanac and ephemeris data (e.g., in seconds or in weeks). In that case, based on the ages in seconds of an ephemeris and an almanac, one may be able to look up their respective accuracies in estimating satellite positions or velocities and determine therefrom which set of parameters should be used for the satellite position or velocity estimation.
- the comparison at step 806 selects the ephemeris or almanac data according to a function that predicts the accuracies in positions and velocities calculated based on their respective ages.
- the ephemeris data is selected when the age of the almanac exceeds the age of the ephemeris by 3 weeks.
Abstract
In a Global Positioning System (GPS) receiver, rather than storing both GPS satellite ephemeredes and almanacs to the GPS receiver's on-chip non-volatile memory, storing into the non-volatile memory only the more recent ephemeris or almanac data for each satellite. When an ephemeris expires, the parameters in the ephemeris are converted into corresponding parameters of an almanac. The ephemeris or almanac is used to determine whether a given satellite is visible to the GPS receiver. A method of the present invention allows efficient use of the memory space in the non-volatile memory. A study by the inventors shows that differences in positions and velocities of satellites estimated using aged ephemeris and almanac data associated with the same week number are insignificant.
Description
- This application is a Continuation application of U.S. patent application Ser. No. 11/368,917, filed Apr. 8, 2008, U.S. Pat. No. 7,355,551, issued Apr. 8, 2008, the contents of which is hereby incorporated by reference in its entirety.
- 1. Field of the Invention
- The present invention relates generally to global positioning system (GPS) technology. In particular, the present invention relates to a design of a GPS receiver.
- 2. Discussion of the Related Art
- GPS is a satellite-based radio navigation system. Each GPS satellite, also called a space vehicle (SV), broadcasts time-tagged ranging signals and navigation data, providing signal transmit time, ephemeris (i.e., the SV's orbital parameters) and almanac (i.e., reduced-precision orbital parameters of all SV's in the GPS constellation) to GPS receivers. Each SV is identified by a unique pseudo-random noise (PRN) code that is embedded in its transmitted signal.
- A GPS receiver extracts the signal transmit time from each of the satellites it tracks. For each satellite, the difference between the time the satellite's signal is received, as determined by the receiver's clock, and the time the signal is transmitted from the satellite is the apparent transit time. The apparent distance between the satellite and the GPS receiver (“pseudorange”) is the product of this apparent transit time and the speed of light in a vacuum. An accurate position of the satellite can be calculated based on a valid ephemeris broadcast by the satellite. When there are four or more satellites in view of the GPS receiver, the GPS receiver can determine a 3-dimensional receiver position and the receiver clock bias relative to the GPS Time (GPST).
- The GPS receiver uses ephemeredes and almanac data to search for visible GPS satellites. Typically, in a GPS receiver implemented on one or more integrated circuits, ephemeredes and almanac data are stored in a non-volatile memory circuit in one of the GPS receiver integrated circuits. Typically, an ephemeris contains a set of ephemeris parameters—valid over a short period of time (e.g., 4 hours)—that enable calculating the satellite's positions and velocities as a function of GPS time. Each GPS satellite broadcasts its own ephemeris as 16 Keplerian parameters as follows: (a) “toe”, which is the reference time of the ephemeris; (b) “Mo”, which is the mean anomaly at the reference time of the orbit; (c) “Δn”, which is the mean motion correction from a computed orbit, (d) “e”, which is the eccentricity of the orbit, (f) “A1/2”, which is the square root of the semi-major axis of the orbit, (g) “ΩO”, which is a longitude of an ascending node of the orbit plane at a weekly epoch, (h) “io”, which is an inclination angle at reference time, (i) “ω”, which is an argument of the perigee of the orbit, (j) “{dot over (Ω)}” (“Omega-dot”), which is a rate of change of a right ascension, (k) “İ” (“I-dot”), which is a rate of change of an inclination angle, and (1) “Cuc”, “Cus”, “Crc”, “Crs”, “Cic”, and “Cis”, which are amplitudes of various harmonic corrections. In addition, an identifying number, called “IODE” (“Issue of Data (ephemeris)”), is assigned to each ephemeris.
- Each satellite broadcasts a new ephemeris parameter set every two hours or so. An ephemeris parameter set is regarded as current, valid, or unexpired typically within the time window of ±2 hours of its reference time. Beyond this time window, the accuracies of the satellite's position and velocity, as calculated from this ephemeris, decrease. Thus, the ephemeris is typically regarded as expired outside of the time window.
- Each satellite also broadcasts an almanac that gives ephemeris data at reduced precision for all satellites in the constellation (32) of GPS satellites. The time required for a satellite to transmit and for a GPS receiver to receive a complete set of almanac is 12.5 minutes. The almanac data for each satellite includes the following 8 parameters: (a) “toa”, which is reference time of the almanac, (b) “MO”, which is the mean anomaly at reference time of the satellite orbit, (c) “e”, which is the eccentricity of the orbit, (d) “A1/2”, which is the square root of the semi-major axis of the orbit, (e) “di”, which is a value that equals IO-0.3π, (f) “ω”, which is an argument of perigee, and (g) “{dot over (Ω)}”(“Omega-dot”), which is a rate of change of a right ascension. A satellite's position and velocity that are calculated from almanac data are less accurate than those calculated from an ephemeris received from the satellite. However, almanac data is typically regarded as valid for a much longer time than ephemeris data. In fact, almanac data that is up to 2 years old can still be used to determine a satellite's visibility. Typically, a week number is often attached to an almanac or ephemeris parameter set. This week number is often used to help determine whether or not the almanac or ephemeris is current (i.e., unexpired).
- A GPS receiver can use ephemeris or almanac data to determine approximately when a satellite would rise above the horizon, given an approximate user position and time. When ephemeredes are unavailable or have expired, almanac data is often used to initiate a search list of possible visible satellites. The rough satellite positions and velocities calculated from almanac or ephemeris data reduce the time-to-first-fix (TTFF), as the GPS receiver starts up.
- To meet demands of a GPS receiver starting under different conditions, the GPS receiver typically saves both ephemeredes and almanac data from the satellites in its on-chip non-volatile memory (e.g., an battery backed-up SRAM). So that, whenever the GPS receiver powers up from a previous shut-down, either an unexpired ephemeris or a valid almanac is available to estimate each satellite's position, so as to generate a list of visible satellites to search for.
- In an advanced GPS receiver, information provided to a GPS receiver to assist it to acquire satellites is often also saved on the same non-volatile memory, which has become a scarce resource. In some designs, it may become necessary to restructure the GPS receiver hardware to obtain more space in the non-volatile memory. A space-efficient design is highly desirable.
- According to one embodiment of the present invention, a method in a GPS receiver saves non-volatile memory space by not storing almanac data for those satellites for which an up-to-date ephemeris is already available in the GPS receiver. That is, a method of the present invention saves on-chip non-volatile memory space by storing in the non-volatile memory, for each satellite, either an up-to-date ephemeris of each satellite, or up-to-date almanac data, but not both.
- In one embodiment of the present invention, when more up-to-date ephemeris or almanac data for a satellite becomes available, the data is stored onto the GPS receiver's non-volatile memory. When the GPS receiver computes a satellite's position to estimate the satellite's visibility, the more up-to-date ephemeris or almanac of the satellite is used. Ephemeredes and almanacs are typically provided to the GPS receiver from satellite live signals, a source on an assisted GPS network, or some other data source.
- A method according to one embodiment of the present invention includes: (a) when the GPS receiver has only an ephemeris of a satellite and does not have almanac data for that satellite, the ephemeris—whether current or not—is selected; (b) when the GPS receiver has only almanac data for a satellite and does not have an ephemeris for that satellite, the almanac data is selected; (c) when the GPS receiver has a current or unexpired ephemeris, the ephemeris is selected; (d) when the GPS receiver has both an expired ephemeris and almanac data for a satellite, the GPS receiver compare values for their week number values and selects (1) the almanac data, when the ephemeris and almanac data have the same week number value; and (2) the data with the larger week number value, when the week number values are different. In comparing the week number values, the “rollover” event for the week number is considered. Typically, a larger week number value denotes a more recent date.
- In one embodiment of the present invention, data in an expired ephemeris of a satellite is used to create almanac data for the satellite, which can then be used to estimate the position and velocity of the satellite, so as to determine the satellite's visibility. The present invention provides the method to transform ephemeris parameters into almanac parameters.
- The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.
-
FIG. 1 shows the errors of a satellite's positions calculated based on its old ephemeredes, relative to a reference ephemeris. -
FIG. 2 shows the errors of a satellite's velocities calculated based on its old ephemeredes relative to a reference ephemeris. -
FIG. 3 shows the errors of a satellite's positions calculated based on its old almanacs, relative to a reference ephemeris. -
FIG. 4 shows the errors of a satellite's velocities calculated based on its old almanacs, relative to a reference ephemeris. -
FIG. 5 shows the errors of multiple satellites' positions calculated based on their old ephemeredes and almanacs, relative to respective reference ephemeredes. -
FIG. 6 shows the errors of multiple satellites' velocities calculated based on their old ephemeredes and almanacs, relative to respective reference ephemeredes. -
FIG. 7 illustrates the method of transforming parameters in an ephemeris to almanac parameters. -
FIG. 8 illustrates a method for determining whether an ephemeris or a set of almanac data is more up-to-date. - A study conducted by the inventors reveals that satellite positions and velocities calculated using expired ephemeredes are about as accurate as satellite positions and velocities calculated from valid almanac data. Using current (i.e., valid, or unexpired) ephemeris, the positions and velocities of a satellite can be calculated within several meters and several meters per second, respectively. These small position and velocity errors are insignificant for the purpose of correctly predicting whether or not a satellite is visible to the GPS receiver. In the inventors' study, described in further detail below, positions and velocities calculated based on unexpired ephemeredes are used as reference values. The satellite positions and velocities calculated based on almanac data or old ephemeredes are compared to these reference values, and their differences relative to the reference values are referred to as position and velocity errors.
-
FIG. 1 shows three-dimensional position errors -
FIG. 2 shows the three-dimensional velocity errors - The results of
FIGS. 1 and 2 are compared to position and velocity errors obtained using old but still unexpired almanac data.FIG. 3 shows three-dimensional position errors FIG. 1 andFIG. 3 shows that the satellite positions calculated using old ephemeredes are roughly as accurate as those calculated using the correspondingly old almanac data. Similarly,FIG. 4 shows three-dimensional velocity errors FIG. 2 andFIG. 4 shows that the satellite velocities calculated using old ephemeredes are roughly as accurate as those calculated using the correspondingly old almanac data. -
FIG. 5 shows the errors in calculating the positions of seven randomly selected satellites (PRNs FIG. 5 , the satellite position errors, which are originally obtained as distance errors (expressed in meters), are converted to angle errors (expressed in degrees relative to the center of the orbit). The following formula is used to transform distance errors to angle errors: -
Angle error=distance error/(5153*5153)*180/π[degrees] - where (5153*5153) meters is the typical length of the semi-major axis of all GPS satellite orbits. Position errors in degrees are more informative of the severalty of the position errors. Since this conversion is applied to position errors calculated from both ephemeredes and almanacs, the comparison keeps fair. As shown, the position errors due to old ephemeredes are comparable to the position errors due to old almanac data. Thus, expired ephemeredes can replace almanac data of the same age for estimating a satellite's visibility with acceptable accuracy. For example, 26-week old (i.e., 182 days or a half-year) ephemeredes and almanac data both result in position errors of less than 10 degrees. As expected, the older data generally results in larger errors.
FIG. 5 also shows that position errors for 2-week old ephemeredes are consistently about ten times greater than those calculated using 2-week old almanac data. Such a difference, however, is insignificant for our purpose because (1) the errors for 2-week old ephemeredes are as small as 1 degree and (2) the 26-week old almanac data that provides a 10-degree error is still regarded as valid and are routinely used in many GPS receivers. -
FIG. 6 shows the velocity errors for the same set of satellites calculated using old ephemeredes (graphs 602) and old almanac data (graphs 601). No significant difference is found in velocity errors between those calculated using old ephemeredes and those calculated using the correspondingly old almanac data. The estimated velocity of a satellite is used to determine a Doppler search range for the satellite. From this study, expired ephemeredes can also replace old almanac data in estimating satellite velocity, with an acceptable accuracy. - Thus, according to one embodiment of the present invention, a GPS receiver may save non-volatile memory space by not storing almanac data for those satellites for which an up-to-date ephemeris is already available in the GPS receiver. In fact, for each satellite, the GPS receiver stores in the non-volatile memory either an up-to-date ephemeris of each satellite, or unexpired almanac data, but not both. When more up-to-date ephemeris or almanac data for a satellite becomes available, the data is stored onto the GPS receiver's non-volatile memory. When the GPS receiver computes a satellite's position to estimate the satellite's visibility, the more up-to-date ephemeris or almanac of the satellite is used. Ephemeredes and almanacs are typically provided to the GPS receiver from satellite live signals, a source on an assisted GPS network, or some other data source. In the GPS receiver, data from an expired ephemeris of a satellite may be used to create almanac data for the satellite, which can then be used to estimate the position and the velocity of the satellite, so as to determine the satellite's visibility. Thus, the present invention provides the method to transform ephemeris parameters into almanac parameters.
-
FIG. 7 shows a method for transforming an expired ephemeris into almanac data. As shown inFIG. 7 , ephemeris parameters MO, e, A1/2, ΩO, ω, and {dot over (Ω)} become almanac parameters of the same names. The ephemeris reference time (toe) becomes the almanac reference time (toa). Almanac parameter di is obtained by subtracting 0.3π from ephemeris parameter iO. The almanac data thus transformed from an expired ephemeris can be used for calculating satellite positions and velocities. In fact, the position and velocity values used in obtaining the data inFIGS. 1 , 2, 5, and 6 are all obtained via this transform method. By not saving satellite almanacs onto receiver non-volatile memory, in one instance, the GPS receiver saves at least 1 kilobyte of memory space. -
FIG. 8 illustrates a method for determining whether an ephemeris or an almanac is more up-to-date. In a GPS receiver, the ephemeredes and almanac data are obtained by: (a) restoring from the receiver's non-volatile memory, (b) decoding satellite live signals, (c) receiving from an assisted-GPS computer network, or any other suitable source. Only the most up-to-date ephemeris or almanac data is stored into the non-volatile memory of the GPS receiver prior to the GPS receiver shutting down, and only the more up-to-date ephemeris or almanac data is used to estimate the satellite's visibility. Note the same selection rules are applied to each GPS satellite, and different satellites may have a different selection result. As shown inFIG. 8 , the selection rules under this method are provided as follows: - At
step 801, when a GPS receiver determines (at step 801) that it has only an ephemeris of a satellite and does not have any almanac data (at step 802) for that satellite, the ephemeris data is selected for estimating the satellite's position and velocity or for being stored, regardless of whether or not the ephemeris is current (i.e., unexpired). Similarly, when the receiver only has almanac data of the satellite and does not have any ephemeris, the almanac is selected. When the GPS receiver determines that it has both ephemeris data and almanac data (at step 801), it examines whether or not the ephemeris has expired (step 803). Unexpired ephemeris is selected over almanac data (step 804). Atstep 803, when the GPS receiver has both an expired ephemeris and almanac data of a satellite, the receiver needs to compare their week number value (step 805), taking into consideration whether or not a rollover event has occurred. Normally, a greater week number indicates a more recent reference time, except immediately after a rollover event. At the rollover event, because the week number has reached the maximum week number, it is reset to the lowest week number value. Atstep 806, if the week number of the ephemeris data is larger than the week number of the almanac data, the ephemeris data is selected (step 807). Otherwise, the almanac data is selected. - The selection procedure shown in
FIG. 8 compares the age of an ephemeris and almanac data using only their respective week numbers. This is provided only for illustrative purpose. The actual age of an ephemeris or an almanac is provided by the combination of its week number and its reference time (toe or toa). When an ephemeris and an almanac both have the same week number, the one with the larger reference time is more recent. - The procedure of
FIG. 8 may be further extended. For example, one may associate the respective ages of ephemeredes and almanac data, (e.g., down to the level of seconds) that would provide the same accuracies when used to calculate satellite positions and velocities. For example, a table may be provided relating position or velocity estimations with the ages of almanac and ephemeris data (e.g., in seconds or in weeks). In that case, based on the ages in seconds of an ephemeris and an almanac, one may be able to look up their respective accuracies in estimating satellite positions or velocities and determine therefrom which set of parameters should be used for the satellite position or velocity estimation. With such data, the comparison atstep 806, for example, selects the ephemeris or almanac data according to a function that predicts the accuracies in positions and velocities calculated based on their respective ages. For example, in one application, the ephemeris data is selected when the age of the almanac exceeds the age of the ephemeris by 3 weeks. - The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous modification and variations within the scope of the present invention are possible. The present invention is set forth in the following claims.
Claims (7)
1. In a GPS receiver, a method for storing satellite orbital parameters, comprising:
receiving an ephemeris and almanac data of a satellite;
comparing the week number and reference time associated with the ephemeris with the week number and reference time associated with the almanac data; and
based on the comparison, selecting the more recent one of the ephemeris and the almanac data to be stored in a non-volatile memory of the GPS receiver.
2. A method as in claim 1 , further comprising, when the GPS receiver is power up, retrieving from the non-volatile memory of the GPS receiver the stored almanac data or ephemeris to compile a list of visible satellites.
3. A method as in claim 1 wherein, for each satellite, only the more recent one of the ephemeris and the almanac data is stored in the non-volatile memory.
4. A method as in claim 1 , wherein the non-volatile memory comprises a random access memory element.
5. A method as in claim 1 further comprising computing a position or a velocity of the satellite using the more recent one of the almanac data and the ephemeris.
6. A method as in claim 5 , further comprising determining whether or not the satellite is visible to the GPS receiver based on the computing.
7. A method as in claim 1 wherein when only an ephemeris but not almanac data is available for the GPS receiver to compute a position or a velocity of a satellite, the GPS receiver computes the position or velocity of the satellite using the ephemeris.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US12/098,913 US20090002231A1 (en) | 2006-03-06 | 2008-04-07 | Method for a space-efficient gps receiver |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/368,917 US7355551B2 (en) | 2006-03-06 | 2006-03-06 | Method for a space-efficient GPS receiver |
US12/098,913 US20090002231A1 (en) | 2006-03-06 | 2008-04-07 | Method for a space-efficient gps receiver |
Related Parent Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US11/368,917 Continuation US7355551B2 (en) | 2006-03-06 | 2006-03-06 | Method for a space-efficient GPS receiver |
Publications (1)
Publication Number | Publication Date |
---|---|
US20090002231A1 true US20090002231A1 (en) | 2009-01-01 |
Family
ID=38471009
Family Applications (2)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US11/368,917 Expired - Fee Related US7355551B2 (en) | 2006-03-06 | 2006-03-06 | Method for a space-efficient GPS receiver |
US12/098,913 Abandoned US20090002231A1 (en) | 2006-03-06 | 2008-04-07 | Method for a space-efficient gps receiver |
Family Applications Before (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US11/368,917 Expired - Fee Related US7355551B2 (en) | 2006-03-06 | 2006-03-06 | Method for a space-efficient GPS receiver |
Country Status (1)
Country | Link |
---|---|
US (2) | US7355551B2 (en) |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US7711481B2 (en) * | 2006-05-26 | 2010-05-04 | Denso Corporation | Navigation system |
GB2483713A (en) * | 2010-09-20 | 2012-03-21 | St Microelectronics Res & Dev | Monitoring positioning satellite positions |
US20120154205A1 (en) * | 2010-12-17 | 2012-06-21 | Seiko Epson Corporation | Method of determining adequacy and adequacy determining device |
Families Citing this family (12)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20080186229A1 (en) * | 2001-06-06 | 2008-08-07 | Van Diggelen Frank | Method and Apparatus for Monitoring Satellite-Constellation Configuration To Maintain Integrity of Long-Term-Orbit Information In A Remote Receiver |
US7355551B2 (en) * | 2006-03-06 | 2008-04-08 | Sirf Technology, Inc. | Method for a space-efficient GPS receiver |
US20090191897A1 (en) * | 2008-01-24 | 2009-07-30 | Cortxt, Inc. | Environment Characterization for Mobile Devices |
JP5753647B2 (en) * | 2008-10-09 | 2015-07-22 | セイコーエプソン株式会社 | Electronic clock and satellite signal receiving method |
JP2010127672A (en) * | 2008-11-26 | 2010-06-10 | Seiko Epson Corp | Position calculating method and position calculating device |
EP2256512B8 (en) * | 2009-05-27 | 2012-09-19 | u-blox AG | A method of determining the position of a mobile device |
US20110133984A1 (en) * | 2009-12-07 | 2011-06-09 | At&T Mobility Ii Llc | Devices, Systems and Methods for Locating a Positioning Satellite |
US9651674B2 (en) | 2009-12-11 | 2017-05-16 | At&T Mobility Ii Llc | Devices, systems and methods for providing location information over a cellular network |
JP5691517B2 (en) * | 2010-12-29 | 2015-04-01 | 富士通株式会社 | POSITION ESTIMATION PROGRAM, POSITION ESTIMATION DEVICE, AND POSITION ESTIMATION METHOD |
JP6015694B2 (en) * | 2014-03-13 | 2016-10-26 | カシオ計算機株式会社 | Radio clock |
US11709277B2 (en) | 2020-03-23 | 2023-07-25 | Caterpillar Inc. | Software-based time rollover detection |
US11841444B2 (en) * | 2020-08-27 | 2023-12-12 | Sony Group Corporation | Resilient ephemeris decoding of GNSS satellite information |
Citations (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6275185B1 (en) * | 1999-09-22 | 2001-08-14 | Trimble Navigation Limited | GPS receiver using coarse orbital parameters for achieving a fast time to first fix |
US20020021245A1 (en) * | 2000-07-20 | 2002-02-21 | Ching-Fang Lin | Integrated GPS/IMU method and microsystem thereof |
US20040263386A1 (en) * | 2003-06-26 | 2004-12-30 | King Thomas M. | Satellite positioning system receivers and methods |
US20060145917A1 (en) * | 2002-06-06 | 2006-07-06 | Matsushita Electric Industrial Co., Ltd. | Gps receiver |
US20060208943A1 (en) * | 2005-03-21 | 2006-09-21 | Sirf Technology, Inc. | Location tagging using post-processing |
US7355551B2 (en) * | 2006-03-06 | 2008-04-08 | Sirf Technology, Inc. | Method for a space-efficient GPS receiver |
-
2006
- 2006-03-06 US US11/368,917 patent/US7355551B2/en not_active Expired - Fee Related
-
2008
- 2008-04-07 US US12/098,913 patent/US20090002231A1/en not_active Abandoned
Patent Citations (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6275185B1 (en) * | 1999-09-22 | 2001-08-14 | Trimble Navigation Limited | GPS receiver using coarse orbital parameters for achieving a fast time to first fix |
US20020021245A1 (en) * | 2000-07-20 | 2002-02-21 | Ching-Fang Lin | Integrated GPS/IMU method and microsystem thereof |
US20060145917A1 (en) * | 2002-06-06 | 2006-07-06 | Matsushita Electric Industrial Co., Ltd. | Gps receiver |
US20040263386A1 (en) * | 2003-06-26 | 2004-12-30 | King Thomas M. | Satellite positioning system receivers and methods |
US20060208943A1 (en) * | 2005-03-21 | 2006-09-21 | Sirf Technology, Inc. | Location tagging using post-processing |
US7355551B2 (en) * | 2006-03-06 | 2008-04-08 | Sirf Technology, Inc. | Method for a space-efficient GPS receiver |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US7711481B2 (en) * | 2006-05-26 | 2010-05-04 | Denso Corporation | Navigation system |
GB2483713A (en) * | 2010-09-20 | 2012-03-21 | St Microelectronics Res & Dev | Monitoring positioning satellite positions |
US8633852B2 (en) | 2010-09-20 | 2014-01-21 | Stmicroelectronics (Research & Development) Limited | Signal verification |
US9140561B2 (en) | 2010-09-20 | 2015-09-22 | Stmicroelectronics (Research & Development) Limited | Signal verification |
GB2483713B (en) * | 2010-09-20 | 2016-04-06 | St Microelectronics Res & Dev | Signal verification |
US20120154205A1 (en) * | 2010-12-17 | 2012-06-21 | Seiko Epson Corporation | Method of determining adequacy and adequacy determining device |
US9354320B2 (en) * | 2010-12-17 | 2016-05-31 | Seiko Epson Corporation | Method of determining adequacy and adequacy determining device |
Also Published As
Publication number | Publication date |
---|---|
US20070205942A1 (en) | 2007-09-06 |
US7355551B2 (en) | 2008-04-08 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US7355551B2 (en) | Method for a space-efficient GPS receiver | |
US7564406B2 (en) | Method and apparatus in standalone positioning without broadcast ephemeris | |
US6975266B2 (en) | Method and apparatus for locating position of a satellite signal receiver | |
US7869948B2 (en) | Method and apparatus in positioning without broadcast ephemeris | |
US6127968A (en) | On-the-fly RTK positioning system with single frequency receiver | |
US8035559B2 (en) | Global positioning system receiver | |
US6798377B1 (en) | Adaptive threshold logic implementation for RAIM fault detection and exclusion function | |
US7117417B2 (en) | Method for generating clock corrections for a wide-area or global differential GPS system | |
US8134500B2 (en) | Method and apparatus for enhanced autonomous GPS | |
US7545317B2 (en) | Method and apparatus for navigation data downloads from weak signals | |
US8538682B1 (en) | Systems and methods for satellite navigation using locally generated ephemeris data | |
US6281836B1 (en) | Horizontal/vertical protection level adjustment scheme for RAIM with baro measurements | |
US6847893B1 (en) | Horizontal/vertical exclusion level determination scheme for RAIM fault detection and exclusion implementation | |
US20070299609A1 (en) | Method and system for ephemeris extension for GNSS applications | |
US8120530B2 (en) | Method and device for using GNSS satellite trajectory extension data in mobile apparatus | |
TWI427315B (en) | Method and system for assisted satellite signal based positioning, and associated apparatus and computer program product | |
US20100198512A1 (en) | Method and apparatus for providing reliable extended ephemeris quality indicators | |
US20060229805A1 (en) | Method and apparatus for navigation using instantaneous doppler measurements from satellites | |
US20020188403A1 (en) | Method and apparatus for generating and distributing satellite tracking information | |
US7535414B2 (en) | Navigational positioning without timing information | |
US20080129593A1 (en) | Determining position without current broadcast ephemeris | |
US20040135721A1 (en) | Method and system for estimating ionospheric delay using a single frequencey or dual frequency gps signal | |
US8248301B2 (en) | Method and apparatus for using GPS satellite state computations in GLONASS measurement processing | |
US20090135061A1 (en) | Method and device for predicting gnss satellite trajectory extension data used in mobile apparatus | |
US20220107427A1 (en) | System and method for gaussian process enhanced gnss corrections generation |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
STCB | Information on status: application discontinuation |
Free format text: ABANDONED -- FAILURE TO RESPOND TO AN OFFICE ACTION |