Military receivers can decode the P(Y)-code

Military receivers can decode the P(Y)-code transmitted on both L1 and L2. Without decryption keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. Sony VAIO VPCCW1S1E/B Battery However, this technique is slow, so it is currently available only on specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors. Sony VAIO VPCCW1S1E/L Battery A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). This corrects the error that arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. CPGPS uses the L1 carrier wave, Sony VAIO VPCCW1S1E/P Battery which has a period of, which is about one-thousandth of the C/A Gold code bit period of , to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to 2–3 metres (6.6–9.8 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimetres (1.2 in) of ambiguity. Sony VAIO VPCCW1S1E/R Battery By eliminating this error source, CPGPS coupled with DGPS normally realizes between 20–30 centimetres (7.9–12 in) of absolute accuracy. Relative Kinematic Positioning (RKP) is a third alternative for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 centimetres (3.9 in). Sony VAIO VPCCW1S1E/W Battery This is done by resolving the number of cycles that the signal is transmitted and received by the receiver by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK). Sony VAIO VPCCW21FX/B Battery While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time (GPST; see the page of United States Naval Observatory). The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. Sony VAIO VPCCW21FX/L Battery GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds). Periodic corrections are performed on the on-board clocks to keep them synchronized with ground clocks.^[92] Sony VAIO VPCCW21FX/R Battery The GPS navigation message includes the difference between GPS time and UTC. As of July 2012, GPS time is 16 seconds ahead of UTC because of the leap second added to UTC June 30, 2012.^[93] Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. Sony VAIO VPCCW21FX/W Battery New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) that, given the current period of the Earth's rotation (with one leap second introduced approximately every 18 months), should be sufficient to last until approximately the year 2300. Sony VAIO VPCCW26EC Battery GPS time is theoretically accurate to about 14 nanoseconds.^[94] However, most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 nanoseconds. As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. Sony VAIO VPCCW26FX/B Battery The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). Sony VAIO VPCCW28EC Battery To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero). Sony VAIO VPCCW28FJ/P Battery Another method that is used in surveying applications is carrier phase tracking. The period of the carrier frequency multiplied by the speed of light gives the wavelength, which is about 0.19 meters for the L1 carrier. Accuracy within 1% of wavelength in detecting the leading edge reduces this component of pseudorange error to as little as 2 millimeters. Sony VAIO VPCCW28FJ/R Battery This compares to 3 meters for the C/A code and 0.3 meters for the P code. However, 2 millimeter accuracy requires measuring the total phase—the number of waves multiplied by the wavelength plus the fractional wavelength, which requires specially equipped receivers. This method has many surveying applications. Sony VAIO VPCCW28FJ/W Battery Triple differencing followed by numerical root finding, and a mathematical technique called least squares can estimate the position of one receiver given the position of another. First, compute the difference between satellites, then between receivers, and finally between epochs. Other orders of taking differences are equally valid. Detailed discussion of the errors is omitted. Sony VAIO VPCCW29FJ/W Battery The satellite carrier total phase can be measured with ambiguity as to the number of cycles. Let  denote the phase of the carrier of satellite jmeasured by receiver i at time  . This notation shows the meaning of the subscripts i, j, and k. The receiver (r), satellite (s), and time (t) come in alphabetical order as arguments of   and to balance readability and conciseness, Sony VAIO VPCCW2AFJ Battery let   be a concise abbreviation. Also we define three functions, : , which return differences between receivers, satellites, and time points, respectively. Each function has variables with three subscripts as its arguments. These three functions are defined below. Sony VAIO VPCCW2AHJ Battery Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch.^[97] This difference is designated as Double differencing^[98] computes the difference of receiver 1's satellite difference from that of receiver 2. Sony VAIO VPCCW2S1E Battery This approximately eliminates satellite clock errors. This double difference is: Triple differencing^[99] subtracts the receiver difference from time 1 from that of time 2. This eliminates the ambiguity associated with the integral number of wavelengths in carrier phase provided this ambiguity does not change with time. Sony VAIO VPCCW2S1E/B Battery Thus the triple difference result eliminates practically all clock bias errors and the integer ambiguity. Atmospheric delay and satellite ephemeris errors have been significantly reduced. This triple difference is: Triple difference results can be used to estimate unknown variables. Sony VAIO VPCCW2S1E/L Battery For example if the position of receiver 1 is known but the position of receiver 2 unknown, it may be possible to estimate the position of receiver 2 using numerical root finding and least squares. Triple difference results for three independent time pairs quite possibly will be sufficient to solve for receiver 2's three position components. Sony VAIO VPCCW2S1E/P Battery This may require the use of a numerical procedure.^[100][101] An approximation of receiver 2's position is required to use such a numerical method. This initial value can probably be provided from the navigation message and the intersection of sphere surfaces. Such a reasonable estimate can be key to successful multidimensional root finding. Sony VAIO VPCCW2S1E/R Battery Iterating from three time pairs and a fairly good initial value produces one observed triple difference result for receiver 2's position. Processing additional time pairs can improve accuracy, overdetermining the answer with multiple solutions. Least squares can estimate an overdetermined system. Sony VAIO VPCCW2S1E/W Battery Least squares determines the position of receiver 2 which best fits the observed triple difference results for receiver 2 positions under the criterion of minimizing the sum of the squares. In the United States, GPS receivers are regulated under the Federal Communications Commission's (FCC) Part 15 rules. Sony VAIO VPCCW2S5C CN1 Battery As indicated in the manuals of GPS-enabled devices sold in the United States, as a Part 15 device, it "must accept any interference received, including interference that may cause undesired operation."^[102] With respect to GPS devices in particular, the FCC states that GPS receiver manufacturers, "must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum."^[103] Sony VAIO VPCF112FX/B Battery The spectrum allocated for GPS L1 use by the FCC is 1559 to 1610 MHz.^[104] Since 1996, the FCC has authorized licensed use of the spectrum neighboring the GPS band of 1525 to 1559 MHz to the Virginia company LightSquared. On March 1, 2001, the FCC received an application from LightSquared's predecessor, Sony VAIO VPCF115FG/B Battery Motient Services to use their allocated frequencies for an integrated satellite-terrestrial service.^[105] In 2002, the U.S. GPS Industry Council came to an out-of-band-emissions (OOBE) agreement with LightSquared to prevent transmissions from LightSquared's ground-based stations from emitting transmissions into the neighboring GPS band of 1559 to 1610 MHz.^[106] Sony VAIO VPCF116FGBI Battery In 2004, the FCC adopted the OOBE agreement in its authorization for LightSquared to deploy a ground-based network that used its allocated frequencies of 1525 to 1559 MHz.^[107] This authorization was reviewed and approved by the U.S. Interdepartment Radio Advisory Committee, Sony VAIO VPCF117FJ/W Battery which includes the U.S. Department of Agriculture, U.S. Air Force, U.S. Army, U.S. Coast Guard, Federal Aviation Administration, National Aeronautics and Space Administration, Interior, and U.S. Department of Transportation.^[108] In January 2011, the FCC conditionally authorized LightSquared's wholesale customers, Sony VAIO VPCF117HG/BI Battery such as Best Buy, Sharp, and C Spire, to be able to only purchase an integrated satellite-ground-based service from LightSquared and re-sell that integrated service on devices that are equipped to only use the ground-based signal using LightSquared's allocated frequencies of 1525 to 1559 MHz.^[109] Sony VAIO VPCF118FJ/W Battery In December 2010, GPS receiver manufacturers expressed concerns to the FCC that LightSquared's signal would interfere with GPS receiver devices^[110] although the FCC's policy considerations leading up to the January 2011 order did not pertain to any proposed changes to the maximum number of ground-based Light Squared stations Sony VAIO VPCF119FC Battery or the maximum power at which these stations could operate. The January 2011 order makes final authorization contingent upon studies of GPS interference issues carried out by a LightSquared led working group along with GPS industry and Federal agency participation. Sony VAIO VPCF119FC/BI Battery GPS receiver manufacturers design GPS receivers to use spectrum beyond the GPS-allocated band. In some cases, GPS receivers are designed to use up to 400 MHz of spectrum in either direction of the L1 frequency of 1575.42 MHz.^[111] However, as regulated under the FCC's Part 15 rules, GPS receivers are not warranted protection from signals outside GPS-allocated spectrum.^[103] Sony VAIO VPCF119FJ/BI Battery The FCC adopted rules in February 2003 that allowed Mobile Satellite Service (MSS) licensees such as LightSquared to construct ground-based towers in their licensed spectrum to "promote more efficient use of terrestrial wireless spectrum."^[112] In July 2010, the FCC stated that it expected LightSquared to use its authority to offer an integrated satellite-terrestrial Sony VAIO VPCF11AFJ Battery service to "provide mobile broadband services similar to those provided by terrestrial mobile providers and enhance competition in the mobile broadband sector."^[113] However, GPS receiver manufacturers have argued that LightSquared's licensed spectrum of 1525 to 1559 MHz was never envisioned as being used for high-speed Sony VAIO VPCF11AGJ Battery wireless broadband although there is no regulatory or legal backing of this claim.^[114] To build public support of efforts to reverse the 2004 FCC authorization of LightSquared's network, GPS receiver manufacturer Trimble Navigation Ltd. formed the "Coalition To Save Our GPS."^[115] Sony VAIO VPCF11AHJ Battery The FCC and LightSquared have each made public commitments to solve the GPS interference issue before the network is allowed to operate.^[116][117] However, according to Chris Dancy of the Aircraft Owners and Pilots Association, airline pilots with the type of systems that would be affected "may go off course and not even realize it." Sony VAIO VPCF11JFX/B Battery The problems could also affect the Federal Aviation Administration upgrade to the air traffic control system, United States Defense Department guidance, and local emergency services including 911.^[118] On February 14, 2012, the U.S. Federal Communications Commission (FCC) moved to bar LightSquared's Sony VAIO VPCF11M1E Battery planned national broadband network after being informed by the National Telecommunications and Information Administration (NTIA), "the federal agency that coordinates spectrum uses for the military and other federal government entities," that "there is no practical way to mitigate potential interference at this time."^[119][120] LightSquared is challenging the FCC's action. Sony VAIO VPCF11M1E/H Battery GPS augmentation refers to techniques used to improve the accuracy of positioning information provided by the Global Positioning System, a network of satellites used for navigation. Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. Sony VAIO VPCF11MFX/B Battery There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, Sony VAIO VPCF11S1E Battery while a third group provide additional navigational or vehicle information to be integrated in the calculation process. Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, Inertial Navigation Systems and Assisted GPS. The Global Positioning System (GPS) is a satellite-based system for navigation. Sony VAIO VPCF11S1E/B Battery Receivers on or near the earth's surface can determine their locations based on signals received from any four or more of the satellites in the network. All satellites in the work broadcast on the same two frequencies, known as L1 (1575.42 MHz) and L2 (1227.60 MHz). The network uses code division multiple access(CDMA) to allow separate messages from the individual satellites to be distinguished. Sony VAIO VPCF11Z1E Battery Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P) code, that is encrypted so that only the U.S. military can access it. The messages sent from each satellite contain information ranging from the satellite health, the satellite's orbital path, the clock state of the satellite, and the configuration of the entire satellite network. Sony VAIO VPCF11Z1E/BI Battery The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways. After Selective Availability was turned off by the U.S. government, the largest error in GPS was usually the unpredictable delay through the ionosphere. Sony VAIO VPCF11ZHJ Battery The spacecraft broadcast ionospheric model parameters, but they are necessarily imperfect. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency. Sony VAIO VPCF127HGBI Battery Receivers with decryption keys can decode the P(Y)-code transmitted on both L1 and L2. However, these keys are reserved for the military and authorized agencies and are not available to the public. Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information. Sony VAIO VPCF137HG/BI Battery However, this technique is slow, so it is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization). Then all users will be able to perform dual-frequency measurements and directly compute ionospheric delay errors. Sony VAIO VPCS111FM/S Battery A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRNis not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. Sony VAIO VPCS115EC Battery By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy. Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, determination of range signal can be resolved to a precision of less than 10 centimeters (4 in). Sony VAIO VPCS115FG Battery This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK). Sony VAIO VPCS117GG Battery While most clocks are synchronized to Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. Sony VAIO VPCS117GGB Battery GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset withInternational Atomic Time (TAI) (TAI - GPS = 19 seconds). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks. Sony VAIO VPCS118EC Battery The GPS navigation message includes the difference between GPS time and UTC, which as of 2009 is 15 seconds due to the leap second added to UTC December 31, 2008. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. Sony VAIO VPCS119FJ/B Battery The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, given the current rate of change of the Earth's rotation (with one leap second introduced approximately every 18 months), should be sufficient to last until approximately the year 2300. As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. Sony VAIO VPCS119GC Battery The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). Sony VAIO VPCS11AFJ Battery To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field, which only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero). Sony VAIO VPCS11AGJ Battery Utilizing the navigation message to measure pseudorange has been discussed. Another method that is used in GPS surveying applications is carrier phase tracking. The period of the carrier frequency times the speed of light gives the wavelength, which is about 0.19 meters for the L1 carrier. Sony VAIO VPCS11AHJ Battery With a 1% of wavelength accuracy in detecting the leading edge, this component of pseudorange error might be as low as 2 millimeters. This compares to 3 meters for the C/A code and 0.3 meters for the P code. However, this 2 millimeter accuracy requires measuring the total phase, Sony VAIO VPCS11AVJ Battery that is the total number of wavelengths plus the fractional wavelength. This requires specially equipped receivers. This method has many applications in the field of surveying. We now describe a method which could potentially be used to estimate the position of receiver 2 given the position of receiver 1 using triple differencing followed by numerical root finding, Sony VAIO VPCS11J7E/B Battery and a mathematical technique called least squares. A detailed discussion of the errors is omitted in order to avoid detracting from the description of the methodology. In this description differences are taken in the order of differencing between satellites, differencing between receivers, and differencing between epochs. Sony VAIO VPCS11M1E/W Battery This should not be construed to mean that this is the only order which can be used. Indeed other orders of taking differences are equally valid. The satellite carrier total phase can be measured with ambiguity as to the number of cycles. Let   denote the phase of the carrier of satellite j measured by receiver i at time  . Sony VAIO VPCS11V9E Battery This notation has been chosen so as to make it clear what the subscripts i, j, and k mean. In view of the fact that the receiver, satellite, and time come in alphabetical order as arguments of   and to strike a balance between readability and conciseness, let   so as to have a concise abbreviation. Sony VAIO VPCS11V9E/B Battery Also we define three functions, :  which perform differences between receivers, satellites, and time points respectively. Each of these functions has a linear combination of variables with three subscripts as its argument. These three functions are defined below. Sony VAIO VPCS11X9E/B Battery,Sony VAIO VPCS123FGB Battery,Sony VAIO VPCS125EC Battery