Real-time differential(1)

Real-Time Differential(1)This paper describes the Real –Time Differential Option that can be installed on ASHTECH XⅡreceivers and its two output formats :Ashtech format and RTCM-104version 2.0 format, In connection with the general description of real –time differential, it covers:1.Major sources of error affecting accuracy2.Age of range corrections3.Issue of orbit data of the base and remote receiversA description of the format for range corrections is followed by an example and a few suggestions in case of trouble.This is followed oby a section devoted to RTCM 104 format. Setting receivers for RTCM format is described in the last half of this paper. (Reading and setting receivers for ashtech format was described in connection with Screen 5.)Real-Time Differential in GeneralReal-Time differential GPS involves a base receiver computing the satellite range corrections and transmitting them to the remote receivers. It transmits these corrections in real time to the remote receivers via a telemetry link. Remote receivers apply the corrections to their measured ranges; they use corrected ranges to compute their position.The base receiver determines range, computed by using the accurate position entered in the receiver. (this accurate position must have been previously surveyed using GPS or some other technique.) The remote receivers subtract the received corrections from their measured ranges and use the corrected ranges for position computation.A stand-alone GPS receiver can compute a position of around 25 meters with Selective Availability off and around 100 meters with SA on. Differential GPS can achieve 1-10 meters at the remote receivers even with SA on.The receiver can be designated as the base or remote station .In base mode, the receiver computes the range errors in every cycle. The communication link can be a radio link , telephone line, cellar phone, satellite communication link or any other medium that can transfer digital data. For testing, connect both receivers via a full handshake null modem RS-232 cable via their B ports.Sources of ErrorThe major sources of error affecting the accuracy of GPS range measurements are orbit estimation, satellite clock estimation, ionosphere, and receiver noise in measuring range. The first four are almost totally removed using differential GPS. Their residual error is in the order of 1 millimeter for every kilometer of separation between base and remote receivers.Receiver noise ,the last error in the list , is not correlated between the base and the remote receiver and is not canceled by differential GPS. However in ASHTECH Ⅻreceivers, integrated Doppler is used to smooth the range measurements and reduce the receiver noise.At the instant a satellite is locked, there is also RMS noise affecting the range measurement. This RMS noise is reduced with the square root of n where n is the number of measurements. For example, after 100 seconds of locking to a satellite, the RMS noise in range measurement is reduced by a factor of 10(1 meter of noise is reduced to 0.1meter) . The noise is further reduced over time.If the lock to a satellite is lost , the noise goes back to 1 meter and smoothing starts from the 1-meter level. The loss of lock to a satellite is rare .It typically happens only when the direct path to the satellite is blocked by an object.Total position error (or error-in-position),is a function of the range errors (or error-in –range), multiplied by the PDOP(there –coordinate position dilution of precision). The PDOP is a function of the geometry of the satellite .Ashtech FormatThe remote receiver needs to know the smooth count of the range corrections .Smooth count is the number of measurements used to compute the current smoothing correction on a given data point .Therefore ,the smooth count for each range correction is also transmitted from base to remote receiver. A range correction with a smooth count of 100 has a noise level of 0.1 meter ,while a range correction with a smooth count of 25 has a noise level of 0.2 m.In using a range correction ,the remote receiver considers its smooth count to determine where or not to use it .The decision is based on the PRECISION that was specified (see the Differential Options in the Screen 4 chain).If the PRECISION was specified as 1 meter the remote receiver will not use the received range corrections that a smooth count less than 25. This means that after locking to a satellite, the first 25 measurements for position computation are ignored and used only for smoothing . Then , after this initial 25 seconds ,the range measurements form the given satellite are used in every second of position computation.The required smooth count is determined by the remote receiver ,using the equation :Smooth count = 1+25/(p2 )Where p is the required precision (in meters )entered in the Differential subscreen.Age of Transmitted Range CorrectionsAnother important factor is the age of the range corrections. The position of GPS satellites and the effect of the environment change with time .The range corrections also change . In addition , they vary depending on Selective Availability.If position accuracy to1 meter is needed and the change in range corrections is 1 meter per hour , the range errors should not be more than 0.2 meters (assuming a PDOP of 5). The range corrections ,therefore ,should not be more than 12 minutes old ,assuming SA is off.Real time differential(2)The remote receiver considers the age of each range correction before using it for position computation . The maximum allowable age is converted by the remote receiver to P minutes where P was specified as the required precision (in meters )entered on the Differential subscreen.Which Issue of the Orbit DataIn differential GPS the base and the remote receivers must use identical versions of orbit data. The version, or Issue of Data Ephemeris (IODE), is designated by the GPS control segment.Along with the range correction, the base receiver transmits the IODE of orbit parameters used for each range correction computation. The remote receiver will not use the received range corrections whose IODE of the orbit parameters of the base receiver.Differential data formatA typical transmitted range error is :＄PASHA,B04,03,+000.00,1422,0868,09,-001.08,1344,0868,11,+003.22,1432,0868,21,-012.42,1445,0098≠0B<CR-LF>Where ＄PASHA is the header. B is the beginning of the differential message ,04 is the number of satellites whose range corrections will follow (shown here as 4 lines of data, broken for readability only).On each subsequent line ,the first number ,the satellite PRN, is followed by its range correction ,then the IODE. The last number is the smooth count used by the base receiver in computing the range corrections . The differential message ends with a checksum 0B, a carriage return and a linefeed .Select a baud rate that can transmit the range correction for all visible satellites within the differential update interval .For example, if the interval is 5 seconds 8 satellites are visible ,it should transmit about 180 characters in 5 seconds ,i.e., a baud rate of more than 720.For example ,assume that PRECISION, for the remote receiver was specified as 2 and these differential data were received .SVS 03 06 09 11 12 13DR/5 00 05 22 14 10 09CCNT 54 99 02 99 99 99IODE 32 31 32 31 32 11IODE 32 31 32 30 32 11In this example ,the receiver neglects data from satellite 9 because its smooth count is less than 1+(25/4)=7; its smooth count will reach an acceptable level in about 5 seconds and the receiver will use it (provided it receives new corrections).The smooth count for satellites in the remote receiver (CNT on Screen 1) should also meet the required minimum.TroubleshootingIf the base receiver does not compute the differential data , check the following equipment and settings .1.If a receiver does not lock on satellites , check the antenna connection and the SatelliteSelection Control(Screen 7) . It is always safe to leave the satellite selection in automaticmode , letting the receiver select satellites automatically.2.If satellites appear to be locked satisfactorily, check the set up on Screen 4.If the remote receiver does not compute position:1.Check Screen 5 . Are corrections being received ?2.Verify (also on Screen 5) that the age of the differential data meets the PRECISIONspecification .You may need to wait a few seconds for it to reach the required level if the lock to a satellite has recently been lost.3.Verify (CCNT of Screen 5)that the smooth count of the base receiver meets thespecification .You may need to wait a few seconds for it to reach the required level if the lock to a satellite has recently been lost .4.Verify (CNT of Screen 1) that the smooth count of the remote receiver meets the specification.You may need to wait a few seconds for it to reach the required level if the lock to a satellite has been lost recently .5.Verify that the two IODE numbers on Screen 5 are identical for all satellites . Otherwise youmay need to wait a few seconds for the base or remote receivers to receiver a new version of orbit data.RTCM 104 Format ,Version 2.0This section describes the RTCM 104 format ,a real-time differential option for ASHTECH Ⅻreceivers . In base mode ,the receiver supplies to rover receivers data for differential corrections .In remote mode ,it operates as user equipment and obtains range corrections from the reference station which are used to correct its position. When a port is dedicated to RTCM ,all other options on that port are disabled.ASHTECH′S RTCM option complies with RTCM 104 version 2.0 standard of the Radio Technical Commission for Maritime Services . If you would like more information on the RTCM 104 standard , please contact:Radio Technical Commission for Maritime ServicesPost Office Box 19087Washington ,D.C. 20036-9087As a Reference StationWhen a receiver is used as a reference station and the RTCM option is installed ,it computes differential corrections for up to 12 satellites converts those corrections to RTCM format and transmits the converted messages via its serial ports .It can generate message types 1,2,3,6, and16.MESSAGE TYPE CONTENTS OF MESSAGE1Differential GPS corrections2Delta differential corrections3Reference station parameters6Null frame9High-rate differential GPS corrections16Special MessageIt uses the “6 of 8 “ format (data bits al through a6 of an 8-bit byte )for communication between the reference station and user equipment .As Remote EquipmentWhen the receiver is used as remote equipment and the RTCM option is installed ,the receiver can accept any type of RTCM message. However it decodes only types 1,2,3,6,9 , and 16 and uses only types 1,2,and 9 for differential corrections. For radio communication ,the receiver in remote mode can recover bit slippage.RTCM Differential Set UpTo use the RTCM differential option ,do the following:。