CARTMELL ANDREW PETER JOHN (US)
| US5223843A | ||||
| US4450585A |
Cl aims
1 * A method of dynamic switching of a plurality of carrier loops in a radio receiver; each said channel comprising a plurality of carrier tracking loops? each said carrier tracking loop configured to track a radio source; each said carrier tracking loop leaving a predetermined accumulation period; said method comprising the steps of; (A) receiving a radio signal from said radio source by using one said carrier tracking loop;
(B) substantially continuously estimating a signal- to-noise ratio (SMR) of said received radio signal from S-aid radio source; (C) if said signal-to-noise ratio (SKFR) of said received radio signal from said radio source is above an initial predetermined threshold, performing carrier tracking of said received carrier signal by an initial carrier tracking loop having an. initial accumulation period; and
(D) if said signal -to-noise ratio CSHR) of said received radio signal from said radio source falls below said initial predetermined threshold; switching from said initial carrier tracking loop having said initial accumulation period to a. subsequent carrier tracking loop having a subsequent accumulation period; ^a.nu performing carrier tracking of said received carrier signal by said subsequent carrier tracking- loop having said subsequent accumulation period.
2. The method of claim 1 further comprising the step of: (-E) repeating said steps (B-D) .
3. The method of claim 1, wherein said step (A) further compiuses the step of:
(Al) -cracking said radio source o£ said, received radio signal, wherein said radio source is selected from the group consisting of: {a GPS satellite,' a GLOWASS satellite; a GALILEO satellite; and a pseudolite} .
4. The method of claim 1 , wherein said step (B) further comprises the steps of i (Bl) substantially continuously estimating said signal-to-noise ratio (SHR) of said received, radio signal from said radio source ; and
(B2) selecting said initial carrier tracking loop having said initial accumulation period based on said estimated SNR.
5. The method of claim 4, xvherein said step (B2) further comprises the step of :
{B2, 1} if said estimated SNR is above a first predetermined threshold,, selecting a first carrier tracking loop having a first accumulation period to implement said initial carrier tracking loop having said initial accumulation period.
6. The method of claim 4, wherein said step (B2) further comprises the step of :
{B2, 2} if said estimated SNR falls below said first predetermined threshold but is εibσve a second predetermined threshold, selecting a second carrier tracking loop having a second accumulation period to implement said initial carrier tracking loσp having said initial accumulation period.
7. The method of claim 4, xvherein said step (B2) further comprises the step of?
(B2, 3) if said estimated SNR falls below said second predetermined threshold but is above a third predetermined threshold, selecting a third carrier tracking loop having a third accumulation period to implement said initial carrier tracking loop having said initial accumulation period.
8. The method of claim i, wherein said step (C) further comprises tJae steps of;
(Ci) performing a standard quadrature mixing operation of I and Q samples of said received radio signa.1 with I and Q components of a local carrier replica. generated by a carrier NCO by using an Image Rejection Mixer to generate 1 and Q baseband samples,-
(C2) correlating said I and said Q baseband samples with a local copy of a code signal aligned to said received radio signal by a code -tracking loop to generate I and Q correlated signals;
(C3) accumulating said ϊ and Q correlated signals over a period of time T;
(C4) computing a raw carrier tracking error signal by using said I and Q accumulated signals; (C5J filtering said raw carrier tracking error signal by " using a loop filter to obtain a filtered carrier tracking error signal?
(CS) inserting a loop delay T into said filtered carrier tracking error signal to generate a feedback error signal; wherein said loop delay r represents latency of said carrier tracking loop; and
(C7) closing said carrier tracking loop by applying said feedback error to an input of said carrier NCO to control a frequency of said local carrier signal . $. The method, of S, wherein said step (CS) of inserting said loop delay into said filtered carrier tracking error further comprises the step of :
(C€, 1) controlling said loop delay to be a fixed period,
10, The method of claim Q, wherein said step (Cβ) of inserting said loop delay to said filtered carrier tracking error further comprises the step of: (CSj 2) selecting said loop delay from the group consisting of: (a J oop update? an accumulatXOϊI period of said first tracking loop; an accumulation period of said second tracking loop; and an accumulation period of said third tracking loop).
11. The method of claim 1, wherein said step (D) of switching from said initial carrier tracking loop having said initial accumulation period to said subsequent carrier tracking loop having said subsequent accumulation. period further comprises the steps oft
{01} modeling said loop filter as a third order digital filter; wherein a numerator of said third order digital filter contains a memory of said previous carrier tracking errors; and wherein a denominator of said third order digital filter contains a memory of said previous loop outputs used to update said carrier NCO;
(D2) estimating said SNR and selecting said subsequent carrier tracking loop based on said estimated SMR; wherein said subsequent carrier tracking loop include^ a shortest accumulation period corresponding to said estimated SlSfR;
(D3) checking whether said selected carrier loop is different from a currently used loop; and (D4) if said selected carrier loop is different from said currently used loop, resetting said numerator terms to sero, and setting said denominator terms to be eαual to a last value output by said loop filter.
12» An apparatus for dynamic switching of a plurality of carrier loops in a radio receiver; each said channel comprising a plurality of carrier tracking loops; each said carrier tracking loop configured to track a radio source; each said carrier tracking loop having a predetermined accumulation period; said apparatus comprising:
(A) a means for receiving a radio signal from said radio source; (B) a means for substantially continuously estimating a signal-to-noise racio (SWR) of said received radio signal from said radio source;
(C) a means for performing carrier tracking of said received carrier signal; and (D) a means for switching from said initial carrier Cracking loop having said initial accumulation period to a subsequent carrier tracking loop having a subsequent accumulation period.
13. The apparatus of claim 12, wherein said means (A) further comprises ;
(Al) a receiving means configured to receive a radio signal from said radio source.
14. The apparatus of claim 12, wherein said means ($.1) further comprises :
(Al, l) a receiving means configured to track said radio source of said, received radio signal, wherein said S radio source is selected from the group consisting of: {a GPS satellite ? a GLOHASS satellite; a GALILEO satellite; and a pseudαlite} .
15. The apparatus of claim 12, wherein, said means (B) ID further comprises:
(Bi) a means for selecting said initial carrier tracking loop having said initial accumulation period based on said estimated SNR.
IB 16- The apparatus of claim 15 f wherein said means (Bl) further comprises :
(Bl, 1) a means for selecting a first carrier tracking loop having a first accumulation period to implement said initial carrier tracking loop having said
20 initial accumulation period, if said estimated SNR is above a first predetermined threshold.
17. The apparatus of claim 15, wherein said means (Bl) further comprises '.
2S (Bl, 2} a means for selecting a second carrier tracking loop having a second accumulation period to implement said initial carrier tracking loop having said initial accumulation period, if said estimated SNR falls below said first predetermined threshold but is above a
30 second predetermined threshold.
18. The apparatus of claim 15, wherein said means (Bl) further comprises :
(Bl, 3) a means for selecting a third carrier tracking loop having a third accumulation period to implement said initial carrier tracking loop having aaiςl initial accumulation period, if said estimated SNR falls below said second predetermined threshold but is above a third predetermined threshold.
19. The apparatus of claim 12, wherein said means (C) further comprises :
(Cl) an initial carrier tracking loop having an initial accumulation, period configured to perform carrier tracking of said received carrier signal.
20. The apparatus of claim 12, wherein said means " ' (C) further coracαrises:
(C2) a Phase-Locked Loop configured to align a local carrier signal with said received carrier signal.
21. The apparatus of claim 12, whereiu said means (C) further comprises:
(C3) a means for performing a standard quadrature mixing operation of I and Q samples of said received radio signal with I and Q components of a local carrier replica, generated by a carrier NCO to generate I and Q baseband samples;
(C4) a means for correlating said I and said Q baseband samples with a local copy of a code signa.1 aligned to said received radio signal to generate I and Q correlated signals;
(C5) a means for accumulating said I and Q correlated signals over a period of time T;
(C£) a means for computing a raw carrier tracking error signal by using said 1 and Q accumulated signaj| , |i (C7) a means far filtering said raw carrier tracking error signal to obtain a filtered carrier tracking error signal;
(CS) a means for inserting a loop delay r into said filtered carrier tracking error signal to generate a feedback error signal; wherein said loop delay T represents latency of said carrier tracking loop; and
(CS) a means for closing said carrier tracking loop by applying said feedback error to an input of said carrier NCO to control a frequency of said local carrier signal .
22. The apparatus of claim 21 e wherein said means (C3) further comprises : (C3, 1} an Image Rejection Mixer configured to perform a standard quadrature mixing operation of I and Q s&raples of said received radio signal with. I and Q components of a. local carrier replica generated by a carrier BTCO to generate I and Q baseband samples -
23. The apparatus of claim 21, wherein said means (C4) further comprises :
(C4 , 1) a code tracking loop -configured to correlate said I and said Q baseband saϊftples with a local copy of a code signal aligned to saxd received radio signal by to generate I and Q correlated signals,
24. The apparatus of claim 21, wherein said means (C7) further comprises; (C7, 1) a loop filter configiired to filter said raw carrier tracking error signal to obtain a filtered carrier tracking error signal .
25. The apparatus of claim 21 t wherein said αieans [CB) further comprises ;
(C8, l) a means for controlling said loop delay to be a fixed period.
26. The apparatus of claim 21 , wherein said means (CS) further comprises :
(CS, 2) a means for selecting said loop delay from the -group consisting of: {a loop update; an accumulation period of said, first tracking loop; an accumulation period of said second tracking loop, and an accumulation period of said third tracking loop} ,
21. The apparatus of claim 12, wherein said means (D) further comprises :
(Dl) a third order digital filter; wherein a numerator of said third order digital filter contains a memory of said previous carrier tracking errors; and wherein a denominator of said third order digital filεíβr contains a memory of said previous loop outputs used to update said carrier KCO;
(D2) a means for estimating said SNR / (D3) a means for selecting said subsequent carrier tracking loop based on said estimated SHR; wherein said subsequent carrier tracking loop includes a shortest accumulation period corresponding to said estimated SNR/ and
(D4) a means for resetting said numerator terms to sex'o, and for setting said denominator terms to be equal to a last value output by said loop filter, if said selected carrier loop is different from said currently used loop.
28. The app aratus of claim 27, wherein sais means (D1) further comprises :
(D1, 1) a softwarte program implementing said third order digital filter. |
Description
DYNAMIC SWITCHING OF CARRIER TRACKING LOOPS
WITHOUT LOSS OP TRACKIHG INFORMATION
TECHNICAL FIELD
The present invention is in the field of signal carrier tracking, and more specifically, is directed to dynamic switching of carrier tracking loops without loss of tracking information,
BACKGROUND ART
A mechanical fastener (for example, MS9880) is interacted for use in the construction market - A. unit is mounted on a masc on each end of a *dozer blade- Using RTK / the position of the ^dozer and the attitude of the blade can foe determined. This information is used to update the operator on required levelling needed based on, maps of the construction site, Although the 'dozer itself does not move quickly, the λ doser blade sees significant, short period dynamic activity as it Jbdts buried rocks or ±& used to scrape areas. A mechanical fastener (for example, MS9880) mounted on a mast on the ends of the A dozer blade also experiences significant dynamics, some of which are amplified by the flexing of the roast itself. More specifically, acqelerorneters mounted on a HS980 unit during development estimated Shock pulses in excess of 300 gm/s 2 during operation. The accelerations where of short duration and hence significant jerk terms were also present .
The conventional tracking methods originally designed for static receivers performed unreliably with the dynamics present .
What is needed is new tracking techniques that would perform reliably with the dynamics present,
SUMMARY OF THE INVENTION The present invention discloses a radio receiver including a plurality of carrier tracking loops, and a method of dynamic switching of the plurality of carrier tracking loops to optimize the receiver' s performance in a situation when dynamics is present. One aspect of the present invention is directed to a method o£ dynamic switching of a plurality of carrier loops in a radio receiver * Each channel comprises a plurality of carrier tracking loops » Each carrier tracking loop is configured to track a radio source. Each. carrier tracking loop has a predetermined accumulation period .
In one embodiment f the method of the present invention comprises the following seeps of: (A) receiving a radio signal from a radio source by using a carrier tracking loop ? (B) substantially continuously estimating a signal-to-tιoise ratio (SWEt) of the received radio signal from the radio source; (C) if the signal-tσ-noise ratio (SNR) of the received radio signal from the radio source is above an initial predetermined threshold, performing carrier tracking of the received carrier signal by an initial carrier tracking loop having an initial accumulation period; and (D) if the signal -fco- noise ratio (SNR) of the received radio signal from the radio source falls below the initial predetermined threshold, switching from the initial carrier tracking loop having the initial accumulation period to a. subsequent carrier tracking loop having a subsequent accumulation period, and performing carrier tx~acking of the received carrier signal by the subsequent carrier trackinα loop havincj the subsequent accumulation, p
The steps (B-D) are preferably repeated when the dynamics changes «
In one embodiment of tlie present invention, the øtep (A) further comprises the step (Al) of tracking the radio source of the received radio signal, wherein the radio source is selected from the group consisting of ♦ {a GPS satellite ? a GLQNASS satellite ? a GALILEO satellite; and a pseiidolite} . in on& embodiment of the present invention, the step (B) further comprises the following steps: (Bl) substantially continuo-usly estimating the signal-to-noise ratio (SNR) of the received radio signal from the radio source; and (B2) selecting the initial carrier tracking loop having the initial accumulation period based on the estimated SNR.
In one embodiment of the present invention, the step (B2) further cotnprises the following Step; (B2, 1) if the estimated SNR. is above a first predetermined threshold, selecting & first carrier tracking loop having a first accumulation period to implement the initial carrier tracking loop having the initial accumulation period,
In one embodiment of the pi'βsenc invention, the step (B2) further comprises the following step: (B2 , 2} if the estimated SNH falls below the first predetermined threshold but is above a second predetermined threshold, selecting a second carrier tracking loop having a second accumulation period, to implement the initial carrier tracking loop having the initial accumulation period-
In one embodiment of the present invention, the step (S2) further comprises the following step; {B2, 3) if the estimated SKR falls below the second predetermined tbxeshold but is above a third predetermined threshold, selecting a third carrier tracking loop having a thi|!| ! f;§|jS
accumulation period to Implement the initial carrier tracking loop .having the initial accumulation period.
In one embodiment of the present invention, the step (C) further comprises the following steps: (Cl) performing a standard quadrature mixing operation of I and Q samples of the received radio signal with 1 and Q components of a local carrier replica generated by a carrier HCO by using- an Image Rejection Mixer to generate I and Q baseband samples; (C2) correlating the I and the Q baseband samples with a local copy of a code signal aligned to the received radio signal by a code tracking loop to generate I and Q correlated signals; (C3) accumulating the I and Q correlated signals over a period of time T; ^C4) computing a raw carrier tracking error signal by using the ϊ and Q accumulated signals; (CS) filtering the raw carrier tracking error signal by using a loop filter to obtain a filtered carrier tracking error signal; (C6) inserting a loop delay r into the filtered carrier tracking error signal to generate a feedback error signal; wherein the loop delay r represents latency of the carrier tracking loop; and (C7) closing the carrier tracking loop by applying the feedback error to an input of the carrier NCO to control a frequency of the local carrier signal , In one embodiment of the present invention, the step (CS) of inserting the loop delay into the filtered carrier tracking error further comprises the step: {Cβ, 1) of controlling the loop delay to be & fixed period to minimise transient noise associated with switching between the plurality of carrier tracking loops within a radio channel .
In one embodiment of the present invention, the step (C6) of inserting the loop delay into the filtered carrier tracking error further comprisea the step: {C6 r 2) of. selectincj the loop delay from the gi-oup con8IsJwJiPs^
- S -
of t [a loop update; an accumulation period of the first tracking loop; an accumulation period of the second tracking loop; and an accumulation period of the third tracking loop} . In one embodiment of the present invention, che step (D) of switching from the initial carrier tracking loop having the initial accumulation period to the subsequent carrier tracking loop having the subsequent accumulation period further comprises the following steps: (Dl) modeling the loop filter as a third order digital filter; wherein a numerator o£ the third order digital falter contains a memory of the previous carrier tracking errors; and wherein a denominator of the third order digital filter contains a memory of the previous loop outputs used to update the carrier NCO,- (D2} estimating the SNR and selecting the subsequent carrier tracking loop based on the estimated SNR; wherein the subsequent carrier tracking loop includes a shortest accumulation period corresponding to the estimated SNR; (D3) checking whether the selected carrier loop is different from a currently used loop; and (D4) if the selected carrier loop is different from the currently used loop, resetting the numerator terms to zero, and setting the denominator terms to be equal to a last value output by the loop filter.
Another aspect of the present, invention is directed to an apparatus for dynamic switching of a plurality of carrier loops in a radio receiver.
In one embodiment, the apparatus of the present invention comprises : (A) a means for receiving a radio signal from, a radio source; (B) a means for substantially continuously estimating a signal-to-noise ratio (SNR) of the received radio signal from the radio source ? (C) a means for performing carrier tracking of the received carrier siqnal? and (D) a means for switching from tftp!^§||
initial earner tracking loop having the initial accumulation period to a subsequent carrier tracking loop having a subsequent accumulation period.
Itϊ one embodiment of the present invention, the means (A) further comprises: (Ai) a receiving means configured to receive a radio signal from the radio source .
In one embodiment of the present invention, the means (Al) further comprises; (Al, 1) a receiving means configured to track the radio source of the received radio signal, wherein the radio aoxixce is selected from the group consisting of: {a GPS satellite; a GLOMASS satellite? a GALIItEO satellite; and a pseudolite} .
In one embodiment of the present Invention, the means (B) further comprises t (Bl) a means for selecting the initial carrier tracking loop having the initial accumulation period based on the estimated SNR,
In one embodiment of the present invention, the means (Bi) further comprises J (Bl, D a means for selecting a first: carrier tracking loop having a first accumulation period to implement the xnitial carrier tracking loop having the initial accumulation period, if the estimated, ISKR is above a first predetermined threshold. In one embodiment of the present invention, the means (Bl) further comprises; (Bl, 2) a means for selecting a second carrier tracking loop having a second accumulation period to implement che initial carrier tracking loop having the initial accumulation period, if the estimated SNR falls below the first predetermined threshold but is above a second predetermined threshold.
In one embodiment of the present invention, the means (Bl) further comprises ; (Bl, 3) a, means for selecting a third carrier- tracking loop having a third accumulation period to implement the inicial carrier*'
trackittg loop having the initial accumulation period, if the estimated SNi?. falls below the second predetermined threshold but is above a third predetermined threshold.
In one eπtbodiment of the present Invention,- the means (C) further comprises . (Ci) an initial carrier tracking loop having an initial accumulation period configured to perform carrier tracking of the received carrier sicpnal ,
In on® embodiment of the present invention, the means (C) further comprises; (C2) a Phase-Locked Loop configured to align a local carrier signal with the received carrier signal *
In one embodiment of the present invention, the means (C) further comprises; (C3) a means for performing a standard guadratm~e mixing operation of ϊ and Q samples of the received radio signal with I and Q components of a local carrier replica generated by a carrier NCO to generate I and Q baseband samples; (C4) a means for correlating the I and the Q baseband samples with a local copy of a code signal aligned to the received radio signal to generate I and Q correlated signals; (C5) a means for accumulating the 1 and Q correlated signals over a period of time T ? 1,06} a means for computing a raw carrier tracking error signal by using the I and Q accumulated signals; (C7) a means for filtering the raw carrier tracking error signal to obtain a filtered carrier tracking error signal ? (C8) a means for inserting a loop delay T into the filtered carrier tracking error signal to generate a feedback error signal; wherein the loop delay r represents latency of the carrier tracking loop; arεd (C9) a means for closing the carrier tracking loop by applying the feedback error to an input of the carrier NCO to control a frequency of the local carrier signal .
In one embodiment of the present invention, the means (C3) further comprises t (C3 f 1) an Image Rejection Mixer configured to perform a standard quadrature mixing operation of I and Q samples of the received radio signal with I and Q components of a local carrier replica generated by a carrier NCO to generate I and Q baseband samples ♦
In one embodiment of the present invention, the ■ means (C4) further comprises: (C4, 1) a code tracking loop configured to correlate the I and the Q baseband samples with a local copy of a code signal aligned to the received radio signal by to generate I and Q correlated signals »
Jn one embodiment of the present invention, rhe means (C7) further comprises? (C7, 1) a loop filter configured to filter the raw carrier tracking error signal to obtain a filtered carrier tracking error signal ,
In one embodiment of the present invention, the means (CB) further comprises: (CS 1 - X) a means for controlling the loop delay to be a fixed period to minimise transient noise associated with switching between the plurality of carrier tracking loops within a radio channel . In one embodiment of the present invention, the means {CS} further comprxsesj (C8, 3) a means for selecting the loop delay frotn the group consisting of; {a loop update; an accumulation period ό£ the first tracking lqop; an accumulation period of the second tracking loop ? and an accumulation period of the third tx-acking loop} .
In one embodiment of the present invention, the ■ means (D) furthex" comprisest (DlJ a third order digital filter; wherein a numerator of the third order digital filter contains a memory/ of the previous carrier tracking errors; and wherein a denominator of the third order 1'
digital filter contains a memory of the previous loop outputs used to update the carrier- NCO; CD2) a means for estimating the SNR; (D3) a means for selecting the subsequent carrier tracking loop based on the estimated SNR; wherein the subsequent carrier tracking loop irtcli-.ci.es a shoxtest accumulation period corresponding to the estimated SNRj and (D4) & means for resetting the numerator terms to zero, and for setting the denominator t-erms to toe equal to a last value output by the loo£> filter, if the selected carrier loop is different from the currently used loop.
In one embodiment of the present invention, the means (Dl) further comprises; (Dl, 1} a software program implementing the third order digital filter; wherein a numerator of the third order digital filter contains a memory of the previous carrier tracking errors; and wherein a denominator of the third order digital filter contains a memox-y of the previous loop outputs usod to update the carrier NCO.
BRIEF DESCRIPTION OF THB DRAWINGS
The aforementioned advantages of the present invention as well as additional advantages thereof will be more clearly understood hereinafter as a result of a detailed description of a preferred embodiment of the invention when taken in conjunction with the following drawings .
FI<3. 1 depicts an apparatus o£ the pr^s&nt invention for dynamic switching of a plurality of carrier loops in a radio receiver.
FIG. 2 illustrates theoretical switching thresholds between 1 ms loop, 5 ms loop, ^nd 10 ms, and minimum CNR for 10 ms tracking loop for the purposes of the present invention.
DETAILED DESCRIPTION OF THE IWE&TION
Reference will now be made in detail to the preferred embodiments- of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be comprised within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention,, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced %<?ithout these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in. detail as not to unnecessarily obscure aspects of the present invention.
In one embodiment, VlG. 1 depicts an apparatus 10 of the present invention for dynamic switching of a plurality of carrier loops in a radio receiver. Code tracking is inherently more robust {than carrier tracking) to signal dynamics. Therefore, an apparatus 10 uses the standard prior art code tracking fueαctionality (not shown) .
The input radio signal l€ is sampled in in.- phase (i) 12 and quadrature-phase (Q) 14 components by tiie RP part 18 of the apparatus 10. In one "embodiment of the present invention., the radio receiver is configured to track a radio source of the received radio signal , wherein the radio source is selected from the group consisting of: {a GPS satellite; a GLOKTASS satellite; a GALILEO satellite; and a pseudolits} .
Global Navigational Satellite System (GNSS) is a general term of art comprising GPS, or Q-LONASS, or GALILEO f or any combination of GPS, GLONASS and GALILEO systems . The Global Positioning System (GPS) is a system of satellite signal transmitters that transmits information from which an observer's present location and/or the time of observation can be determined. Another satellite-based navigation system is called the Global Orbiting Navigational System (GLONASS) , which can operate as an alternative or supplemental system.
The GPS was developed joy the United States Department of Defense (DOD) under its BfAVSTAR satellite program, A fully operational GFS includes more than 24 Earth orbitinø satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55° relative to the equator and being separated from each other by multiples of 60° longitude. The orbits have radii of 26,560 kilometers and. are approximately circular. The orbics are non-geosynchronous, with 0.5 sidereal day (11,967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Generally, four or more GPS satellites will be visible from røost points on the Earth's surface, which can be used to determine an observer's position anywhere oix the Barth's surface. Each satellite carries a cesium or rubidium atomic clock to- provide timing information for the signals transmitted by the satellites. An interna.! clock correction is provided for each satellite clock.
Each GPS satellite continuously transmits two spread spectrum, Ii-band carrier signals: an l»l signal having a frequency fl ■= 1575.42 MRz (approximately nineteen centimeter carrier wavelength) and an L2 si-gnal havinq a frequency £2 = 1227.6 MHg (approximately
four centimeter carrier wavelength.) . These two frequencies are integral multiplies fl = X, 540 fO and £2 - 1,200 fO of a base frequency f0 =■ 1.023 MHz. The Ll signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and P- code. The L2 signal from each satellite is BPSK modulated by only the P-code. The nature of these PFB codes and accepted methods for generating the e/A-code and p-code are set forth in the document ICD-OPS-200 : GPS interface Control Document, ARISfG Research, 1957, GPS Joint Program Office, which is incorporated by reference herein.
The GPS satellite bic stream includes navigational information on the ephemeris of the transmitting GPS satellite {which includes orbital information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which, includes a less detailed orbital information about all satellites) . The transmitted satellite information also includes parameters providing corrections for ionospheric signal propagation delays (suitable for frequency receivers) and for an offset time between satellite clock titne and true GPS time, The navigational information is transmitted at a rate of 50
A second satellite-based navigation system is the Global Orbiting Navigation Satellite System (QLOHASS) , placed in orbit by the former Soviet Union and now maintained by the Russian Republic, GLOHASS uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8° relative to the equator, and the three orbital planes are separated from each other by multiples of I2θ ύ longitude. The GLONASS satellite© have circular orbits with a radii αf aboui^i
25,510 kilometers and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours) . A GLOHASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively r around the Earth every 8 days. The GLOMASS system uses two carrier signals Ll and L2 with frequencies of fl - (1,602 + 91c/16) GHz; and £2 =* (1.246 + 7k/16) GHz, where k =(1,2,...24) is the channel or satellite ϊmtnber. These frequencies lie in. t"WQ bands at 1.597-1,617 GHz (Ll) and 1,240-1,260 GKK (L2) . The Ll signal is modulated by a C/A- code {chip rate - Q .511 MHz) and fay a P-code (chip rate = 5.11 MHs) . The L2 signal is presently modulated only by the P-code . The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and tiie C/A-code is the same, for each, satellite. The methods for receiving and demodulating the GItGNASS signals are similar to the methods used for the (SPS signals. As disclosed in, the European Commission "White
Paper- on European transport policy for 2010 !l , the European Union will develop an independent satellite navigation system Galileo as a part of a global navigation, satellite infrastructure (GMSS) . The GALILEO system is based on a constellation of 30 satellites and ground stations providing information concerning the positioning of users in many sectors such as transport (vehicle location, route searching, speed control, guidance systems, etc.), social services (e.g. aid for the disabled or elderly) , the justice system and customs services (location of suspects, border controls) , public works (geographical information systems) , search and rescue systems, or leisure (direction-finding at sea or in the mountains, etc.) .
GALIIJSO will offer several servxce levels, from open access to restricted access of various levels:
(A) An open, free hasic service, mainly involving applications for the general public and services of general interest. This service is comparable to that provided by civil GPS, which is free of cost for these applications, but with improved quality and reliability.
{B) A commercial service facilitating the development of professional applications and offering enhanced performance compared with the basic service, particularly in terms of service guarantee»
(C) A "vital" service (Safety of Life Service) of a very high, quality and integrity for safety-critical applications, such as aviation and shipping, & search and rescue service will greatly improve existing relief and rescue services,
(D) A public regulated service (PRS) , encrypted and resistant to jamming and interference, reserved principally for the public authorities responsible for civαl protection, national security and law enforcement which demand a high level of continuity. It will enable secured applications to be developed in the European Union, and could prove in particular uo be an important tool in improving the instruments used by the European Union to combat illegal exports and illegal immigration.
The real needs of future -GJUJIIJEQ users need to be identified before the characteristics of the package of services can. be decided. Studies have already been carried out in various standardisation institutes and international bodies, such as the International Civil Aviation Organization, the International Maritime Organisation, etc,
The range of GALILEO services is designed to meet practical objectives and expectations, from improving the coverage of open-access services in
environments (to cover 95% of urban districts compared with the 50% currently covered by GPS alone) which will benefit the 160 million private vehicles in Europe, or enabling the use of -satellite navigation applications "indoors'% in buildings and even in tunnels, or indeed mobile telephone services based on identifying the caller's position.
Reference to a HADPS herein refers to a Global Positioning System, to <& Global Orbiting Navigation System, to GALILEO System, and to any other compatible Global Navigational Satellite System (GMSS) satellite- based system that provides information by which an observer '3 position and the time of observation can be determined, all of which meec the requirements of the present invention, and to a ground based radio positioning system such as a system comprising of one ox more pseudolite transmitters.
After the KiϋDPS receiver determines the coordinates of the i-th RADPS satellite by demodulating the crcrøsraitted epJaemeris parameters, the RADPS receiver can obtain the solution of the set of the simultaneous equations for its -unknown coordinates (x $ , yo, s 0 } and for unknown time bias error (cb) , The RADPS receiver can also determine velocity of a raoving platform. A pseudolite comprises a ground based radio positioning system working in any radio frequency including but not limited to the GI?S frequencies and the ISM (industrial scientific medical) unlicensed operation band, including 300 MHZ, 2.4 GHs, or 5.8 GHE bands ISM bands, or in a radio location band such, as the (9,5-10) GHz band. Pseudolites can be used for enhancixtg the GPS by providing increased accuracy, integrity, and availability.
The complete description of the pseudolite transmitters in GPS band can. be found in "Global
Positioning System: Theory and Applications ; Volume IX", edited by Bradford W. Parkinson and James J. Spilker Jr., and published in Volume 164 in "PROGRESS XN ASTRONAUTICS MTD AERONAUTICS", by American Institute ox Aeronautic and Astronautics, inc., in 1966 *
The following discussion is focused on a GPS receiver, though the same approach can be used, for a GIJONASS receiver, for a GPS/GLONASS combined receiver, GALILEO receiver, or any other RM)PS receiver, εeferring still to FIG. 1, I 22 and Q 24 components of a local copy of the GPS carrier (the local copy refers to a signal generated internally within the GPS receiver) are generated by the UCO 20 and used to mix X 12 and Q 14 samples down to baseband (zero Herts) in the Image Rejection Mixer 24.
The apparatus 10 (of FIG. 1) of the present invention employs a Phase-Locked Loop to align I 22 and Q 24 components of a local carrier generated by NCO 20 with I 12 and. Q 14 components of the received carrier. As the local carrier and the received carrier are aligned when successfully phase-locked, ∑ne-asuring the phase of the local carrier will also provide the pha.se of the received carrier. Phase tracking is a requirement for RTK.
More specifically, referring still to FIG, 1, in one embodiment of the present invention, the apparatus 10 comprises Image Rejection Mixer 26 configured to perform a standard quadrature mixing operation of I 12 and Q 14 samples of the received radio signal 16 with I 22 and Q 24 components of the local carrier replica generated by the carrier NCO 20 to generate I 28 and Q 30 baseband samples *
In one embodiment of the present invention, the apparatus 10 further comprises I correlator 32 configured to correlate I bδ&ebaϊid sample 28 with a local copy of a cod® siαnal aliαned to the received radio sicjπal to
generate I correlated signal 33 , and Q correlator 34 configured to correlate Q baseband sample 30 with a local copy of a code signal aligned to the received radio signal to generate Q correlated signal 35. More specifically, the phase error between the received ,aiαd local carrier can be obtained from the correlation of the known PET code for a given satellite with I 28 and Q 30 baseband signals. The code tracking loop, not shown, handles alignment between the received a,∑id local copies of the PN codes. The simple mathematical identity is as follows :
^baseband - A . COβ (ωt + φ ) (Kq - I)
Qb««b * -a * A . sin iφt + φ) (Eg . 2 )
where A is the signal amplitude, <y is frequency of the tracking error signal, and φ is the phase of the tracking error signal . Referring still to PXG ► 1, in one embodiment of the present invention, the apparatus 10 further comprises; I accumulator 36 configured to accumulate I correlated signal 33 over a period o£ time T, and Q accumulator 38 configured, to accumulate Q correlated signal 35 over the same period of time T.
In one embodiment α£ £h& present invention, the apparatus 10 further comprises axσt ^ an (Q/∑) means 44 configured to compute a raw carrier tracking error signal 45 by using 1 40 and Q 42 accumulated signals, Assxiπύϊig that the frequency error is zero, the phase error is given by:
φ - arctan (Gkasebanςl / I^asa-bam.) - (Eq . 3 )
- IS -
According to (Eq. 3), the minimum phase error (zero) is achieved if the carrier tracking- places all of the signal energy in the l-phase baseband samples.
Referring still to FIG- 1, in one embodiment o£ the present invention, the apparatus 10 further comprises a loop filter 4S configured to filter the raw carrier tracking error {phase error) 45 signal to obtain a filtered carrier tracking error signal 47. in one embodiment of the present invention, the apparatus 10 further comprises a loop delay block 48 configured to Insert a loop delay r into the filtered carrier tracking error signal 47 to generate a feedback error signal 50, The loop delay r represents latency of the carrier tracking loop. The feedback error signal SO is used to update the digital HCO 20 that generates the local carrier copy that is phase-locked to the receiver carrier signal *
During the normal tracking operation, the code correlation is accumulated over 20 ms, one GPS data bit, The carrier tracking error and carrier NCO are also updated at this rate. The longer accumulation is not possible without first removing the data bits- The shorter accumulation is possible but with the penalty of a reduced energy in the error signal 45 generated by the arctan (Q/l) block.44 thus making the tracking loop more susceptible to noise and thereby limiting the signals that can be tracked.
Under statiσ conditions, the 20 trts accumulation is perfectly adequate. However, when undex" dynamics the received carrier is changing much faster than it can be detected and accounted for by the phase-locked loop. With respect to the local carrier, the x-eceived signal is seen to rotate energy around I and Q baseband samples . The result is that the computation performed hy the arctan
w w J I-, JO? • * •
(Q/I) block. 44 becomes very noiøy and can cause poor signal tracking εmd even signal unlocking.
One technique to handle dies increased dynamics is to use a shorter accumulation period. The shorter accumulation period allows for a faster loop update rate, for a quicker reaction to the dynamics and for a lesser effective rotation of I and Q baseband samples within the accumulation period. This is the approach used in the current invention. The shortest accumulation period that can be easily achieved for GPS is 1 ms which is a C/A code period. The carrier tracking loop having 1 ms accumulation period provides updates 20 times faster than fcHe sta-ϊidard carrier tracking loop having 20 ms accumulation period. However, if the carrier tracking is performed by using the carrier tracking loop having 1 ms accumulation period, the signal to ttoise ratio (SNR) is 20 times lower compared with the SNR resulted from the carrier tracking performed by usαng the carrier tracking loop having 20 rnø accumulation period. This is a 13 dB loss. Xhus, the carrier tracking is performed by using the carrier tracking loop having 1 ms accumulation period results in noisier measurements axiά. an increase in the lower CNR (SNR normalized to 1 Hz bandwidth) boundary of signals that can be tracked compared with the carrier tracking performed by -using the carrier tracking loop having 20 ras accumulation period.
To overcome this limitation, in one embodiment of the present invention, the apparatus 10 employs three independent Carrier tracking loops, each with a different accumulation period.
In one embodiment of the present invention, the three carrier tracking loops are; (A) 1 ms loop; (B) S ms loop; and {O 10 ms loop. The 1 ins loop has the best dynamic performance but requires high SNR to
5 ms loop provides a good dynamic performance and requires moderate SKR to operate. ThelG ms loop has the lower dynamic performance but can operate even, on low SitfR signals Only one of the loops is operating at any one time for a given signal. The software dynamically switches between the loops based on changes in the received CNJR.
As shown in FIQ, 2, the theoretical switching thresholds are approximately as follows: che β8 CNR threshold 40 dB becween 1 ms loop 82 and 5 ms loop 84; the 90 CNR chreshold 35 dB between S ms loop 84 and 10 ms loop 86; and the 92 CETR lower boundary 30 dB of the signal to be tracked by the 10 ms tracking loop 86. In one embodiment of the present invention, the tracking loop to be used for the next aeeumulacion is selected based on the most recent estimated CHR, Switching between loops is perϋoi-jned as needed.
However, the theoretical thresholds 88, 90, and 92 of FIG, 2 cannot be used in practice due to the fact, that the dynamics causes extra noise that has to be estimated during the tracking process itself. For example, the thresholds for the IVIS98G implement are greater than the theoretical thresholds SS, 90, and ' 92 fay 3 to 4 dB.
The theoretical thresholds 88, 90, and 92 of PXG, 2 also include a 2 dB hysteresis biiilt into the thresholds. Therefore, for a presently used loop to be upgraded to a tracking loop with a shorter period, the CNR must be 2 dB higher than that CNR needed to downgrade the presently used loop to a tracking loop with a longer period. This is done to prevent the loops from rapidly switching when the CNR approaches one of the thresholds. The apparatus i0 {of PIG, I) of the present invention should switch between different loops smc
without significant transients. Indeed, transients can cause cycle slips (resulting in removing of the satellite for which tracking slips from the RTK solution for a while thus requires extra processing to * fix' back into 5 the solution again) , can. cause loss of data, or can cause the tracking to unlock. in one embodiment of the present invention, the apparatus 10 {of FIG. 1) switches between, different loops smoothly without significant transients by using the 1 ms 0 accumulation blocks 36" and 38 implemented in hardware, and by implementing the accumulation over S ros and 10 ms needed for the other loops in software *
In one embodiment of the present invention, the apparatus 10 (of FIG, 1) includes a loop delay block 43 S coupled between the loop filter- output 47, The loop delay block 48 updates the WCO 20. The loop delay r is needed to account for latency in reading the accumulator outputs into software and for the time taken by the arct&n {Q/I) blocJc 44 and by the loop filter 46 to perform, necessary 0 calculations,
The loop delay block 48 forms part of the closed-loop transfer function, of the phase-locked loop of the apparatus XO of FIG. 1. If the delay is allowed to vary uncontrolled with time or between tracking channels S then each tracking loop would exhibit undesired, different, and unmodelled behavior. Thus, to control this delay term is critical for the functioning of the apparatus 10 {of FIG. 1) of the present invention. Indeed, control of the delay term allows switching 0 between different tracking loops with miniπial transients.
The GPS cracking software is normally controlled so that the loop delay r is of a fixed period. In one embodiment of the present invention, the loop delay T is normalised to values of T, equal to the 5
accumulation period {and update period) of the tracking loop.
Software limitations mean that the smallest loop delay for the 1 ms loop is X us. The optimum loop delay of 5 ms should be used fox the 5 ms loop, and the optimum loop delay of 10 ms should be used fox * the 10 ms loop.
The usage of a common normalized loop delay T for a.11 there loops (T * =1 ms for 1 me loop; T =5 TUB for 5 ms loop; and T *=1Q ms for 10 ms loop) would result in the similar closed-loop Z-domain representations for all three tx * ac3cixig loops,. The similarity of closed-loop S-domain representations for all three tracking loops results in the smooth switching between loops with. minimum transients. Another similarity requirement in the loops design is that the closed-loop bandwidth needs to be the same for each loop.
On the other hand, using the same 1 ms loop delay for all three loops would result in a faster loop update for all loops and would achiere slightly better dynamic pexform&nce for 5 tns and 10 ms loops. However, the difference in loop delays (T for 1 ms loop, T/5 for 5 tns and T/10 for 10 ms} in terms of their 2-dαπisύn representations can result in significant transients when a loop switch is performed.
Referring gtill to FIG. 1, n one embodiment of the present invention, the loop filter 46 is implemented by using is a standard bhird-order digital filter. It can be represented as follows;
B(z) = (a 2 .a "2 + a^sf 1 + a 0 ) / (b 2 -zf 2 + Jb 1 -Z "1 + b 0 ) (Bq- 4)
where a* and b x terms are the loop coefficients.
"5 "i
The numerator contains a memory of the previous carrier tracking errors . The denominator- contains a memory of the previous loop outputs (the values used to update the carrier NCO) > Whenever a new CNR value is calculated the tracking software selects the optimum tracking loop which has the shortest period for the given C!HR. The selected loop is used the next time the loop closes ,
If the loop selected is different from the loop currently used, then the tracking software should perform extra tasks to switch between loops with minimum transients. This basically requires two operations; (1} the numerator terms are reset to aero thereby clearing the filter memory of previous phase errors; (2) the denominator terms are all set equal to the last value output by the filter, thereby clearing the filter memory o£ previous outputs while maintaining the best current estimate of the signal frequency needed to maintain phase-lock, These two operations are performed at all loop switches. The switching can be performed from any of the three loops to any of the other loops , Loop switches can occur at any time, as the CNR changes, with the exception of a short period directly after each, switch to allow the new loop to settle.
One aspect of the present invention is directed to a method of dynamic switching of a plurality of carrier loops in a radio receiver,
In one embodiment , the method of the present invention comprises the following steps- (not shown) ; {h) receiving a radio signal from a radio source toy using a carrier tracking loop; (B) substantially continuously estimating a signal -to-noise ratio (SNH) of the received radio signal from the radio source; (C) if the signal-to- noise ratio (SHR) of the received radio signal from
radio source is above an initial predetermined threshold, performing carrier tracking of the received carrier Signal by an initial carrier tracking loop having an initial accumulation period; and (P) if the signal -to- noise ratio (SNR) of the received radio signal from the radio source falls below the initial predetermined threshold, switching from the initial carrier tracking loop having the initial accumulation period to a siibsequent carriex* tracking loop having a subsequent accumulation period, and performing carrier tracking of the received carrier signal by the subsequent carrier tracking loop having the subsequent accumulation period. The steps (B-D) are preferably repeated when the dynamics changes . In one embodiment of the present invention, the step (A) further comprises the step (Al) of tracking the radio source o£ the received radio signal, wherein the radxo source is selected from the group consisting of: {a GPS satellite; a GLONASS satellite; a GALILEO satellite; and a pseudolite} .
In one eitibo&iment of the present invention, the step (B) further comprises the following step: (Bl) substantially continuously estimating the signal-to-noise ratio (SKER) of the received radio signal from the radio source; and (B2) selecting the initial carrier tracking loop having the initial accumulation period based on the estimated SNR.
Xn one embodiment of the present invention,- the step {B2} further comprises the following steps (B2, 1) if the estimated SNR is above a first predetermined threshold, selecting a first carrier tracking loop having a first accumulation period to implement the initial carrier tracking loop having the initial accumulation period,
In one etnbodittient of the present invention, the step (B2) further comprises the following step: {B2, 2) if the estimated SNR falls below the first predetermined threshold but is above a second predetermined threshold, selecting a second carrier tracking loop having a second accumulation period to implement the initial carrier tracking loop having the initial accumulation period,
In one embodiment of the present invention, the step (B2) further comprises the following step: (B3, 3} if the estimated SMR falls below tins second predetermined threshold but is above a third predetermined threshold} selecting a. third carrier tracking loop having a third accumulation period to implement the initial carrier tracking loop having the initial accumulation period. In one embodiment of the present invention, the step (C) further comprises the following steps: (Cl) performing a standard quadrature mixing operation of I and Q samples of the received radio signal with I and Q components of a local carrier replxca generated by a carrier NCO by using an Image Rejection. Mixer to generate 1 and Q baseband samples; (C2) correlating the I and the Q baseband samples with a local copy of a code signal aligned to the received radio signal by & code tracking loop to generate I and Q correlated signals; (C3) accumulating the 1 and Q correlated signals over a period of time T; (C4) computing a raw carrier tracking error signal by using the I and Q accumulated signals; (CS) filtering the raw carrier tracking error' signal by using a loop filter to obtain a filtered carrier tracking error signal; (C6) inserting a loop delay T into the filtered carrier -tracking error signal to generate a feedback error signal? wherein the loop delay T represents latency of the cax-rier tracking loop? and [CT) closing the carrier tracking loop by applying the feedbaσk error to
an input of the carrier SfCO to control -a frequency of the local carrier signal . in one embodiment of the present invention, the step (C6) of inserting the loop delay into the filtered carrier tracking error further comprises the step: (CS 1
1) of controlling the loop delay to be a fixed period to minimise transient noise associated with switching between the plurality of carrier tracking loops within a radio channel. In one embodiment of the present invention, the step (CS) of inserting the loop delay into the filtered carrier tracking error further comprises the step: {C6,
2) of selecting the loop delay fz-om the group consisting of: {a loop update; an accumulation period of the first tracking loop,- an accumulation period of the second tracking loop; and an accumulation period of the third tracking loop} .
In one embodiment of the present invention, the step ( D) of switching from the initial carrier tracking loop having the initial accumulation period to tlie subsequent carrier tracking loop having the subsequent accumulation period further comprises the following steps; (Dl) modeling the loop fxlter as a third order digital filter; wherein a numerator of the third order digital filcer contains a memory of the previous carrier tracking errors; and wherein a denominator of the third order digital filter contains a memory of the previous loop outputs used to update the carrier NCO; (D2) estimating the SNR and selecting the subsequent carrier tracking loop based on the estimated SMR; wherein the subsequent carrier tracking loop includes a shortest accumulation period corresponding to the estimated SNR; (D3) checking whether the selected carrier loop is different from a currently used, loop,- and (D4) if the selected carrier loop is different from the currently
used loop, resetting the numerator terms to aero, and setting the denominator ce-rtns to be equal to a last value output by the loop filter.
The foregoing description of specific embodiments of the present invention have h&&n presented for purposes of illustration and description * They are not intended to be ■exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in. the art to best utilize the invention and various embodiments with various modifications a,s are suited to the particular use contemplated. Xt is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.