GPStation6 GISTM Receiver TEC Estimation and Calibration
Contents
1. Component TEC Accuracy TECU Without Calibration After Calibration Satellite known DCB 1 5 lt 0 5 TECU Antenna 14 30 lt 2 TECU GNSS Receiver 6 10 lt 1 TECU The ionospheric fluctuations during calibration will impact the final accuracy of the TEC calibration Moreover this depends on the following factors Region Equatorial vs high latitude Calibration Duration Start and End Time Local time of the day night Season summer winter and Solar activity Sun spot cycle Hence it is strongly recommended to perform the TEC calibration when ionospheric activity is at its minimum i e during early hours 4 6 Calibration Frequency As with any TEC calibration procedure the accuracy of calibration generally degrades due to longer ionospheric changes caused by seasonal changes and from solar activity Hence the GPStation6 TEC calibration should be performed periodically to account for these longer changes A simpler approach is to perform an auto calibration twice a year and apply the new calibration offsets Alternatively the auto calibration can be performed within GPStation6 without interrupting the receiver operation TEC estimation With this approach the auto calibration can be done every few months 2 4 times within a year and can be analyzed externally stored in a file for analysis Note that the changes in TEC calibration offsets will likely include the seasonal solar activity re2. bias is associated with the GPS IIR M satellites broadcasting modernized GPS L2C signal Currently the biases are not reported by CODE but are expected in the future The SETDIFFCODEBIASES command is only applicable to C1P1 and C2P2 biases which are used in L1 C A L2P Y and L1 C A L2C combinations For all other signal combinations there satellite specific differential code biases will be present in the computed TEC value 3 1 3 Receiver Differential Code Bias As with any GNSS receiver the GNSS RF signals propagate through a common path before satellite specific processing i e correlation PSR ADR measurement These delays are common to all SV signals for a specific frequency and manifest in the clock offset computation However the group delays differ between frequencies and manifest as a bias on the geometry free linear combination The following components contribute to the difference in group delays between frequencies on the receiver side GNSS Antenna LNA Band pass filtering GNSS Receiver LNA RF IF Filtering Page 8 June 2015 Note APN 070 Rev A Note that the RF cable is not included as they have a broadband frequency response resulting in constant group delays across the GNSS operating frequency range The differential group delay between L1 and L2 for some geodetic range antennas is summarized in Table 3 2 It is obvious that the group delay difference can introduce significant bias in TEC measurement
3. Re sp where R is the mean radius of Earth is the elevation angle and hsp is the mean height of the ionosphere layer which is usually taken between 350 and 450 km Typical values for Re and hsp are set to 6371 and 450 km respectively Further improvements can be achieved by using a multi shell model instead of single shell model and using appropriate basis functions For example JPL ionospheric TEC estimation uses a three shell model with different mean ionospheric heights TEC5 2 4 Absolute TEC Measurement As discussed in Section 2 2 the TEC values derived from carrier phase measurements are limited by the inherent ambiguity in the number of cycles unknown initialization of carrier phase and cycle slips While code derived TEC is not limited by the above it is still impacted by the satellite and receiver instrument delays and errors Thus to get absolute TEC value these biases must be removed and errors must be reduced A generic expression of code TEC measurement can be expressed as Rec di di TE Ciirx TE Caps gT 6 oad Emp Enoise 5 where TEC is the absolute TEC value ee and oe are the differential inter frequency biases within the satellite and the receiver yp and Enoise are errors caused by the presence of multipath and background noise i e thermal interference Page 4 June 2015 Note APN 070 Rev A 2 5 GPStation6 TEC Measurement GPStation6 provides both the raw an
4. Y The code phase between different RF signals at different frequencies has biases from satellite hardware i e up conversion filtering These differential code biases DCB are specific to a satellite and vary over the life span of the satellite Unlike traditional receivers that use cross correlation between L1 P Y and L2 P Y to track L2 P Y the GPStation6 uses the advanced 2 P Y tracking i e Z tracking The receiver uses the L1 C A tracking for aiding L2 P Y tracking Thus the P2 L2 P Y code measurement is associated with C1 C A code and not P1 L1 P Y Hence the differential code bias associated with P1C1 must be used to get consistent measurements GPStation6 allows the user to enter the GPS satellite specific differential code biases for 32 satellites that will be used internally within TEC calculation C1P1 between L1 C A and L1 P Y which is required for L1 C A and L2 P Y differential TEC estimation C2P2 between L1C A and L2C which is required for L1C A and L2C differential TEC estimation See Section 4 2 8 SETDIFFCODEBIASES OM 20000132 command for further details The bias correction can be applied to the C1P1 combination or C2P2 combination by setting them appropriately in the Code Pair field Monthly mean values of GPS satellite specific corrections for GPS C1P1 and C2P2 are reported by the Centre for Orbit Determination in Europe CODE ref ftp ftp unibe ch aiub CODE Note that C2P2
5. carrier phase However it does provide delta TEC TEC variation that is derived from carrier phase measurement differences between frequencies The code derived pseudorange TEC can be determined from the corresponding L1 and L2 pseudorange observables as 1 frifix TE Chae yoy PR PR 3 where f is the primary L1 frequency and fx is the frequency of the secondary signal PR and PR are the primary and secondary signal pseudorange in metres TEC derived from code pseudorange measurements are noisier compared to carrier TEC but are absolute measurements as they are free from any integer ambiguity Traditional noise reduction approaches includes limiting the code tracking loop bandwidth i e narrow DLL loop bandwidth with the aid of carrier tracking or smoothing i e filtering of code measurements using carrier phase measurements i e Hatch filtering Page 3 June 2015 NovAtel APN 070 RevA 2 3 Slant and Vertical TEC The TEC indicates the total number of free electrons within the ionosphere along the LOS path between the satellite and the receiver This measurement is often represented as slant TEC TEC values To determine the TEC value for different elevation angles the slant TEC must be translated to vertical TEC TEC Using a modified single layer model MSLM for the ionosphere the vertical TEC can be obtained from the slant TEC and vice versa as NIB M e f 4 TEC
6. APN 070 RevA GPStation6 GISTM Receiver TEC Estimation and Calibration Page 1 June 2015 Noite APN 070 RevA 1 Purpose and Scope The purpose of this document is to describe the TEC estimation and calibration feature supported by GPStation6 receiver The scope is limited to generic description and does not provide implementation specific details The document content is relevant to and supplements Section 2 7 TEC Calibration in the GPStation 6 GNSS lonospheric Scintillation and TEC Monitor GISTM Receiver User Manual OM 20000132 2 Total Electron Content TEC 2 1 TEC Measurement The Global Navigation Satellite System GNSS ranging code and carrier are affected differently as the signal interacts with free electrons along its transmission path through the ionosphere The free electrons in the ionosphere advance the GNSS carrier wave by increasing phase velocity while retarding the code modulation by reducing the group velocity As a result the range obtained from the integrated carrier phase is shortened while the measurement obtained from the code ranging is lengthened The magnitude of the ionospheric delay is a function of the refractive index of the ionosphere in the path of the GNSS signal The refractive index is a function of the transmission center frequency and the total electron content TEC The ionospheric delay depends on the number of free electrons present between GNSS receiver and satellite along th
7. d smoothed TEC measurements that are derived from pseudorange The ISMRAWTEC log contains the raw TEC measurements without any kind of carrier smoothing See Section 5 1 4 in OM 20000132 The use of ultra stable oven controlled crystal oscillator OCXO and the narrow delay locked loop DLL bandwidths BW greatly reduces the noise contribution in raw TEC measurements Moreover GPStation6 provides carrier smoothed code TEC measurements as part of the ISMREDTEC log See Section 5 1 6 in OM 20000132 The ISMREDTEC log contains both smoothed code TEC measurements and delta TEC measurements that are derived from carrier phase The use of carrier smoothing further reduces the noise in the code TEC measurements Table 2 1 lists the different signal combinations for which TEC measurement are reported within GPStation6 Table 2 1 GPStation6 supported signal combinations for TEC measurement Signal Combination Primary Signal Secondary Signal Satellite DCB Bias GPSL1CAL2Y GPS L1 C A GPS L2P Y Known All PRNs GLOL1CAL2P GLONASS L1C A GLONASS L2P Unknown GPSL1CAL2C GPS L1 C A GPS L2C Known Selected PRNs GPSL1CAL5 GPS L1 C A GPS L5 Unknown SBASL1CAL5 SBAS L1 C A SBAS L5 Unknown GLOL1CAL2CA GLONASS L1 C A GLONASS L2 C A Unknown TEC measurements derived from the combination of GPS L1 C A L2C and L5 are less noisy and smaller bias from satellite hardware TEC measurements derived from L2P Y in combination with other s
8. e line of sight LOS 40 3 p g 7 TEC metres 1 Where 6 and g are the phase carrier and group code delays TEC is the integrated electron density along the LOS path and is the total number of electrons per square metre f is the center frequency of GNSS RF signal The ionospheric delay is a function of TEC unit TECU and operating frequency For example 1 TECU will introduce a pseudorange delay of 0 163 metres and 0 267 metres in L1 and L2 frequencies respectively Conversely 1 ns of differential delay between L1 and L2 corresponds to 2 852 TECU Page 2 June 2015 NovAtel APN 070 RevA 2 2 Code and Carrier TEC The carrier derived integrated Doppler TEC can be determined from the corresponding L1 and L2 carrier phase observables as _ _ b1it 2 TECy 0 104m TECU 1 2 Where and 2are the carrier phase observables of the respective signals TEC is the column density of electrons measured in TEC units TECU 1 TECU 10 electrons m TEC derived from carrier phase measurements carrier tracking are less noisy compared to code derived TEC Nevertheless the carrier derived TEC are limited due to the inherent ambiguity in carrier cycles Carrier phase derived TEC measurements are also sensitive to cycle slips within carrier phase tracking Note that the carrier phase derived TEC can still be used for relative change in TEC GPStation6 does not provide TEC derived from
9. er it is expected that some residual bias due to unit to unit variation may exist Page 9 June 2015 Noite APN 070 RevA 3 1 4 lonospheric Fluctuations While independent calibration of the receiver antenna cable and receiver is possible it introduces significant difficulty in site deployment Besides absolute calibration of antenna cable and receiver requires a special chamber anechoic and calibrated signal transmission to accurately measure the delay Therefore traditional approaches use the site specific setup and measure the differential code biases along with ionospheric delays by collecting TEC data Subsequently the contribution from ionosphere is removed to obtain the combined satellite receiver specific DCB The TEC data are first obtained over the calibration period which includes the contribution from satellite receiver specific DCBs and that of ionosphere Simpler methods to obtain the receiver satellite differential code biases are to assume TEC of about 3 5 TECU at vertical nighttime data quiet ionosphere Alternatively the ionospheric delay predicted from an independent model i e thin shell single layer can be used to remove the contribution from ionosphere to determine the satellite receiver code bias This approach eliminates the need for complicated receiver calibration but its accuracy depends on the underlying model used for TEC prediction Page 10 June 2015 Noite APN 070 RevA 4 GPStati
10. f calibration data See OM 20000132 ISMREDTECB ISMREDOBSB RXSTATUS RANGE and CLOCKMODEL CLOCKSTEERING O O O O 0 Page 13 June 2015 Noite APN 070 RevA 4 3 Post calibration analysis Once calibration is done it is recommended to review the quality of the calibration by analyzing the TEC data of the satellites that were above the elevation cutoff This includes the number of samples used for TEC calibration the standard deviation of TEC estimates the relative TEC and Delta TEC from ISMREDTEC the average CMC CMC standard deviation S4 and sigma phi from ISMREDOBS If the previous calibration result is available compare any differences between the current calibration and the previous one It is strongly recommended to perform the self calibration over a few days 3 5 days using the same calibration setting The average across these estimates can be used as the final calibration offsets Note that the standard deviation computed from different TEC calibration tests will provide a good indication on the accuracy of the self calibration 4 4 Using Auto Calibration Offsets The GPStation6 does not apply the auto calibration offsets or the satellite specific DCBs The user should enter the following as part of receiver start up The satellite specific DCBs from the CODE P1C1 data base should be applied o Example SETDIFFCODEBIASES gps_c1p1 0 207 0 043 1 0123 40 values The TEC calibration offsets f
11. her interference sources degrade the signal i e C No resulting in degraded pseudorange measurements The presence of multipath results in code tracking error as the receiver correlates the sum of both direct LOS and delayed multipath signal More importantly the error contribution from Emp and that of Enoise is amplified i e doubled due to the combination of pseudorange from different frequencies Therefore measurements noises can potentially degrade the accuracy with which the TEC is measured The use of narrow DLL bandwidth and carrier smoothing significantly reduces the noise contribution GPStation6 uses a wider front end bandwidth of 20 MHz and Pulse Aperture Correlator PAC technology that significantly reduces the multipath error The PAC correlator susceptibility peaks at about 0 05 chips about 5 m and reduces negligibly after 0 1 chip Finally the GNSS antenna and RF cable are exposed to environmental conditions and can be impacted by variations in differential group delay i e group delay variations of individual frequencies caused by Temperature Humidity Air Pressure Mechanical Strain Aging Mismatch Termination Supply voltage Signal power levels i e dynamic range Page 7 June 2015 Note APN 070 Rev A 3 1 2 Satellite Differential Code Bias GPStation6 measures the TEC by differencing the pseudorange measurements between signals from different frequencies i e L1 C A and L2P
12. if not compensated Table 3 2 Reported Differential Group Delay for GNSS Antennas Antenna Model Frequency Band Differential Group Delay ns NovAtel GNSS 750 Upper 1525 1612 MHz 9 25 TECU Lower 1164 1301 MHz NovAtel GPS 702 GG L1 1588 5 23 MHz 5 14 TECU NovAtel GPS 703 GGG L2 1236 18 3 MHz It is expected that the antenna phase center variation PCV can also introduce azimuth elevation specific additional bias However for geodetic antennas the variation is expected to be less than 1 cm Table 3 3 lists the measured differential delays between L1 and L2 see P2 P1 column for a few GNSS receivers The authors TEC7 reported that the measured differential delay varied between individual units of the same receiver type Again it can be seen that the receiver RF IF differential delay can introduce significant bias in estimated TEC if not compensated Table 3 3 Measured group delay for different GNSS receivers ref TEC7 P1 Uncertainty P2 Uncertainty Time Delay ns o 1 ns o 1 Ashtech CNES Rx 28449 039 f 290 71 040 f 62 Z 12T _OPRx 28611 302 58 0 40 1647 Septentrio CNES Rel 19212 043 oa oas f 120 _ PolaRx2 054 Dicom BIPM Rx 9666 037 lon o7 nas GTRSO For GPStation6 the differential bias between L1 and L2 delays are internally corrected using a predefined value This ensures that receiver bias contribution to TEC estimate is minimal Howev
13. ignals i e L1 C A slightly higher probability of error due to the semi codeless of L2 P Y signal However for GPSL1CAL2Y signal combination the satellite differential code biases can be nearly eliminated See Section 3 1 2 and thus may produce the most accurate TEC estimates Note that the L2P Y tracking is achieved with aiding from L1 C A Therefore the biases associated with L1C A should be considered when calibrating for the offset Unlike GPS signals the GLONASS legacy signals uses frequency division multiple access FDMA spread spectrum modulation which introduces additional interfrequency biases Also the pseudorange accuracy of GLONASS signals is slightly less compared to that of GPS signals Thus the measured TEC from GLONASS signals will likely have larger bias and higher probability of error TEC measurement is now possible with SBAS with the launch of second frequency at L5 The primary purpose for SBAS is for integrity and augmentation although it can be used for ranging purposes SBAS does offer a benefit over GNSS systems as the signals are broadcast from Geostationary Orbit GEO satellites Unlike GNSS the SBAS systems such as WAAS and GAGAN use the bent pipe architecture In the bent pipe architecture the SBAS signals are generated in ground based uplink station and is simply Page 5 June 2015 Noite APN 070 RevA broadcasted after frequency translation through commercial communication satellites The cohere
14. lated TEC variations that should be accounted for After accounting for the seasonal variations if the calibration TEC offsets differ by few TECUs then the new calibration offsets can be applied Finally the history of calibration offsets can be potentially used for analyzing longer term systematic effects Page 15 June 2015 APN 070 Noite RevA 5 References 5 1 NovAtel Documents Table 5 1 NovAtel Documents Document Document Title OM 20000132 GNSS lonospheric Scintillation and TEC Monitor GISTM Receiver User Manual Ref http www novatel com assets Documents Manuals om 20000132 pdf OM 20000129 OEM6 Family Firmware Reference Manual Ref http www novatel com assets Documents Manuals om 20000129 pdf 5 2 Other Documents Table 5 2 Other Documents Docume Document Title nt Weather the Storm GNSS and the Solar Maximum Next Generation GNSS lonospheric TEC1 Scintillation and TEC Monitoring Ref http www novatel com assets Documents Papers GPStation 6 White Paper pdf TEC2 Evolution to Modernized GNSS lonospheric Scintillation and TEC Monitoring Ref http www novatel com assets Documents Papers PID2381119 pdf Derivation of TEC and estimation of instrumental biases from GEONET in Japan 2003 TEC3 Annales Geophysicase 21 2083 2093 Ref http www ann geophys net 21 2083 2003 angeo 21 2083 2003 pdf TEC4 http aiuws unibe ch ionosphere Daily JPL Proce
15. n is minimal The GPStation6 auto calibration allows user to enter an elevation cutoff angle to include only satellites with clean TEC data Hence it is desirable to select the days where maximum number of satellites will be available during the calibration period 4 2 2 Correcting for Satellite DCBs As discussed in Section 3 1 2 the GPStation6 allows user to enter the GPS satellite specific DCBs This ensures that the GPS SV specific DCB s are accounted for independently of calibration and are thereby improving the TEC calibration accuracy Note that the SV DCBs are entered as nanoseconds for the 32 GPS SV with remaining set to 0 0 More importantly these values should be used subsequently during operation until another calibration is performed or when it is updated see monthly CODE C1P1 data available from ftp ftp unibe ch aiub CODE 4 2 3 Self Calibration At the start of calibration period ensure that satellite specific DCBs are applied Example SETDIFFCODEBIASES gps_c1p1 0 207 0 043 1 0123 40 values Commence self calibration ISMCALIBRATE o Set start enable and duration of calibration o Set elevation cutoff It is strongly recommended to set the elevation angle is high as possible i e greater than 65 degrees o Example ISMCALIBRATE enable 3600 7200 65 Log the calibration status o Example LOG ISMCALIBRATIONSTATUS ONNEWs The following logs provide useful information when investigation issues with sel
16. ncy between code and carrier are monitored and controlled in a closed loop Therefore the TEC derived from SBAS signal combinations will have higher probability of errors Moreover due to the inherent biases in the ground uplink station and satellite hardware the biases will be significantly large compared to GPS and GLONASS systems In addition these biases tend to vary markedly compared to GNSS systems Page 6 June 2015 Note APN 070 Rev A 3 TEC Calibration Errors amp Biases As with any TEC estimation the GPStation6 receiver uses the geometry free linear combination of pseudorange observables because all geometry related errors are cancelled Table 3 1 classifies the geometry related error sources and uncorrelated error sources in regards to geometry free linear combination Table 3 1 Geometry free linear combination and error sources Correlated Error Sources Geometry Related Uncorrelated Error Sources Geometric Range Measurement noise background noise Satellite clock multipath and group delay variations Receiver clock Satellite specific differential code bias Tropospheric delay Receiver differential code bias antenna RF Common path delays antenna RF cable cable receiver RF IF section receiver RF IF section lonospheric fluctuations from model Relativistic Effects Sagnac effect 3 1 1 Measurement Noise The background noise originating from the receiver thermal and ot
17. on angles The receiver DCB s are reported in the ISMCALIBRATIONSTATUS log See Section 5 1 7 in OM 20000132 The log contains the following Calibration period duration in seconds For each signal combination type see Table 20 in OM 20000132 o Number of samples used for TEC calibration o TEC calibration value in TECU o Standard deviation of the TEC calibration in TECU GPStation6 auto calibration does not automatically apply the satellite and receiver DCB s This should be entered as part of receiver start up see ISMTECCALIBRATION The following sections outline procedures and steps to improve the accuracy of the GPStation6 auto calibration 4 2 1 Calibration Period The calibration period depends on the start time end time and the duration over which the calibration is performed The ideal period for auto calibration is when minimum TEC occurs and there is minimal TEC variation over the calibration duration the maximum number of satellites are above the elevation cut off angle While the ionospheric activity is minimal during the night time i e between 4 00 and 6 00 HRS local time it is strongly recommended to collect relative TEC and Scintillation data during the preselected Page 12 June 2015 Noite APN 070 RevA period and confirm it is the TEC minimum albeit relative and there is negligible scintillation S4 and sigma phi This can be repeated over few days to ensure that day to day variatio
18. on6 TEC Calibration Guideline 4 1 Site Setup Check The following are generic recommendations to ensure that site deployment minimizes the contributions from various error bias sources during TEC calibration and receiver operation Site Survey The GNSS receiver antenna must have a clear open sky view of at least 100 metres to the horizon across all azimuth angles Verify that there are no nearby interference sources for at least 300 metres Verify that there are no nearby signal reflectors such as standing water surfaces flat metal surfaces wire fences large areas of glass or concrete paving Verify the antenna mount and RF cable are supported properly Weather proof i e radome the antenna to limit the impact of solar heating and other environmental factors RF Signal Calibration To ensure optimal receiver operation the RF input power level to the receiver should be within the linear operating range of the receiver Perform a link budget of the setup to determine the expected RF input levels and select the antenna and cable length accordingly After antenna and cable setup measure the RF input level again to confirm that is within the receiver operating range preferably in the middle This can be further verified by observing the RF AGC out of Range Bit Nibble 4 and 5 within the RXSTATUS log See RXSTATUS log in OM 20000132 Ensure the RF cable connectors are properly impedance matched i e 50 ohm Antenna Power Ensu
19. or different signal combination should be applied o Example ISMTECCALIBRATION gpslicaL2Y 12 25 ISMTECCALIBRATION gpslical2c 13 45 Save the calibration offsets within the receiver nonvolatile memory NVM using the SAVECONFIG command see OM 20000132 so that the TEC offsets will be used by default at start up o Example SAVECONFIG 4 5 Generic Error Budget Table 4 1 summarizes the expected accuracy of code derived TEC with and without calibration The TEC accuracy is greatly impacted by the unknown biases in the satellite antenna and the GNSS receiver GPstation 6 HW uses predefined calibration offset to compensate for the inherent RF IF hardware delays Page 14 June 2015 NovAtel APN 070 Rev A The GPstation6 auto calibration feature is expected to provide the most accurate calibration for the GPS L1C A and the L2P Y signal combination when satellite specific DCBs are used For other signal combinations the satellite specific differential code biases are unknown and thus lumped with the receiver DCBs during calibration As the calibration is based on a limited number of satellites that are visible and used for TEC calibration the accuracy of TEC calibration may still be impacted by the variation in DCBs on the satellite side Hence it is better to repeat the TEC calibration for multiple days to maximize the number of satellites used for TEC calibration Table 4 1 Example TEC Biases before and after calibration
20. re that antenna is powered if it is an active antenna see ANTENNAPOWER command in OM 20000132 4 2 GPStation6 Auto Calibration The GPStation6 auto calibration feature allows the user to carry out TEC calibration to compensate for the instrumental biases easily using the existing site setup If the satellite specific differential code bias is unknown or not entered the auto calibration lumps the satellite biases with that of receiver during calibration Page 11 June 2015 Note APN 070 Rev A GPStation6 uses a proprietary TEC model to remove the contribution of ionosphere from the estimated TEC and thus provides the receiver antenna and GNSS receiver associated DCBs It further improves the accuracy by averaging the TEC values computed using different satellites by mapping in to Vertical TEC As part of self calibration GPStation6 allows the user to specify the following auto calibration specific parameters see Section 4 2 4 ISMCALIBRATE in OM 20000132 Binary flag enable disable to start and end the calibration Delayed start for calibration 0 to 604800 in seconds Calibration duration 0 to 604800 Elevation Cutoff angle 90 in degrees that will be used to include the satellite TEC data TEC data from satellites whose elevation is higher will be used for TEC calibration o While elevation cutoff limits the number of satellites used for TEC calibration it greatly reduces the error from using TEC data of lower elevati
21. ssing of 1000 Ground based GPS Receivers to Estimate Interfrequency Biases TECS and Other Practical Applications Ref http igscb jpl nasa gov igscb resource pubs 06_darmstadt IGS 20Presentations 20P DF 12_1 Komjathy pdf The GPS Segment of the AFRL SCINDA Global Network and the Challenges of Real Time TEC TEC6 Estimation in the Equatorial Ionosphere ION NTM 2006 18 20 January 2006 Monterey CA 1036 1047 Proia A Cibiel G Yaigre L 2009 Time Stability and Electrical Delay Comparison of Dual TEC7 Frequency GPS Receivers 41 Annual Precise Time and Time Interval PTTI Meeting pp 293 302 Page 16 June 2015
Download Pdf Manuals
Related Search