GPS augmentation 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 . 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 , 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 System s and Assisted GPS . Background . The Global Positioning System ( GPS ) is a satellite - based system for navigation . 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 . 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 . Precise monitoring . 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 . 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 . 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 . 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 . A second form of precise monitoring is called " Carrier - Phase Enhancement " ( CPGPS ) . The error , which this corrects , arises because the pulse transition of the PRN is not instantaneous , and thus the correlation ( satellite - receiver sequence matching ) operation is imperfect . The CPGPS approach utilizes the L1 carrier wave , 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 between 2 and 3 meters ( 6 to 10 ft ) of ambiguity . CPGPS working to within 1 % of perfect transition reduces this error to 3 centimeters ( 1 inch ) of ambiguity . 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 ) . 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 ) . Timekeeping . While most clocks are synchronized to Coordinated Universal Time ( UTC ) , the atomic clock s 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 second s or other corrections which are periodically added to UTC . 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 correct relativistic effects and keep them synchronized with ground clocks . 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 . 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 . 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 ) . 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 ) . Carrier phase tracking ( surveying ) . 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 . 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 , 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 , 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 . 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 formula _ 3 denote the phase of the carrier of satellite j measured by receiver i at time formula _ 4 . 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 formula _ 5 and to strike a balance between readability and conciseness , let formula _ 6 so as to have a concise abbreviation . Also we define three functions , : formula _ 7 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 . If formula _ 8 is a function of the three integer arguments , i , j , and k then it is a valid argument for the functions , : formula _ 7 , with the values defined as Also if formula _ 13 are valid arguments for the three functions and a and b are constants then formula _ 14 is a valid argument with values defined as Receiver clock errors can be approximately eliminated by differencing the phases measured from satellite 1 with that from satellite 2 at the same epoch . This difference is designated as formula _ 18 Double differencing can be performed by taking the differences of the between satellite difference observed by receiver 1 with that observed by receiver 2 . The satellite clock errors will be approximately eliminated by this between receiver differencing . This double difference is : Triple differencing can be performed by taking the difference of double differencing performed at time formula _ 20 with that performed at time formula _ 21 . This will eliminate the ambiguity associated with the integral number of wavelengths in carrier phase provided this ambiguity does not change with time . Thus the triple difference result has eliminated all or practically all clock bias errors and the integer ambiguity . Also errors associated with atmospheric delay and satellite ephemeris have been significantly reduced . This triple difference is : Triple difference results can be used to estimate unknown variables . 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 the three components of position of receiver 2 . This may require the use of a numerical procedure such as one of those found in the chapter on root finding and nonlinear sets of equations in Numerical Recipes . To use such a numerical method , an initial approximation of the position of receiver 2 is required . This initial value could probably be provided by a position approximation based on the navigation message and the intersection of sphere surfaces . Although multidimensional numerical root finding can have problems , this disadvantage may be overcome with this good initial estimate . This procedure using three time pairs and a fairly good initial value followed by iteration will result in one observed triple difference result for receiver 2 position . Greater accuracy may be obtained by processing triple difference results for additional sets of three independent time pairs . This will result in an over determined system with multiple solutions . To get estimates for an over determined system , least squares can be used . The least squares procedure 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 .