Archtecture and Performance Testng of a Software GPS Recever for Space-based Applcatons,2 Kenn Gold and Alson Brown NAVSYS Corporaton 496 Woodcarver Road Colorado Sprngs, CO 892 79-48-4877 kgold@navsys.com and abrown@navsys.com Abstract Space-based GPS technology presents sgnfcant challenges over Earth-based systems. These nclude vsblty ssues for rotatng platforms and trackng of GPS satelltes from spacecraft that are n hgher orbts than the GPS, realtme resoluton of carrer phase ambgutes, and dfferent dynamcs durng varous msson phases. NAVSYS has developed a software GPS recever that makes use of 3-dmensonal Dgtal Beam Steerng technology and nertal adng to address these ssues. Ths approach offers several advantages ncludng all around vsblty for spnnng satelltes, trackng of weak GPS sgnals, reducton of multpath, and reprogrammablty to accommodate dfferent msson phases. Addtonally, a sute of smulaton tools based around the NAVSYS Matlab Toolbox and Advanced GPS Hybrd Smulaton products have been bult to allow testng for smulated space envronments. The recever archtecture and test tools are descrbed n ths paper. TABLE OF CONTENTS.... INTRODUCTION... 2. MISSION PHASES OF INTEREST... 3. HIGH-GAIN ADVANCED GPS RECEIVER...2 4. SOFTWARE REPROGRAMMABLE GPS RECEIVER...3 5. SIMULATION FOR THE SPACE ENVIRONMENT...4 6. MATLAB GPS TOOLBOX...5 7. ADVANCED GPS HYBRID SIMULATOR...5 8. INTERNAV SOFTWARE...7 9. IMU SIMULATION...7. ALL-AROUND SATELLITE VISIBILITY...8. HIGH GAIN SATELLITE TRACKING... 2. CONCLUSION...2 REFERENCES...2. INTRODUCTION NAVSYS Corporaton has developed the desgn for a flexble, hgh performance Space-based Software GPS Recever (SSGR), and s currently buldng an Engneerng Development Unt to demonstrate ts next generaton capabltes for space applcatons. The SSGR wll provde a flexble, ntegrated precson navgaton and atttude determnaton soluton for space applcatons ncludng Low Earth Orbt (LEO), hghly eccentrc orbt (HEO) and Geostatonary Earth Orbt (GEO) mssons. The ablty to track low power GPS satelltes wll extend the use of GPS for precson navgaton and tmng, partcularly for hgh alttude space mssons (above the GPS satellte constellaton). The SSGR wll be sutable for supportng multple space mssons, ncludng GPS metrc trackng durng launch, orbt determnaton durng transfer to geostatonary orbts, and hgh accuracy navgaton, atttude control and tmng. The flexblty of the SSGR desgn wll allow t to be re-programmed for use n launch and orbt entry, staton-keepng and autonomous orbt estmaton applcatons. In desgn of a space-based GPS recever, the dffculty comes n testng, snce the dynamcs nvolved are radcally dfferent from anythng achevable on the ground. The mult-element Advanced GPS Hybrd Smulaton (AGHS) capablty avalable at NAVSYS addresses many of these concerns. The AGHS generates smulated dgtal sgnal sets usng profles generated by NAVSYS MATLAB Sgnal Smulaton capablty, whch s n turn drven by trajectory and atttude nformaton generated wth Satellte Tool Kt (STK). The AGHS can be used to generate dgtal representatons for the GPS sgnals under the varous scenaros for playback ether nto an RF Re-modulator or drectly nto the GPS recever. The NAVSYS MATLAB Toolbox has been augmented wth varous new tools to allow easy smulaton of varous space-based msson profles. The new features nclude tools to determne the vsblty and expected sgnal strength of GPS sgnals that wll be receved n each scenaro and the ablty to drve the AGHS under each of these scenaros. Orbt and atttude nformaton s easly entered nto the tool through a text-based fle. 2. MISSION PHASES OF INTEREST The SSGR s based on the NAVSYS Hgh-gan Advanced GPS Recever (HAGR) software reprogrammable recever, and ncludes addtonal functonalty to address varous -783-855-6/4/$7. 24 IEEE 2 IEEEAC paper #387, Verson 4 Updated December 7, 23 Proceedngs of IEEEAC, Bg Sky, Montana, March 24
complcatons of space-based GPS usage. The recevermust have a capablty to mantan lock through dynamc maneuvers both durng launch and through orbt transtons. GPS vsblty must be mantaned even for spnnng satelltes and when the satellte s n hgher than GPS orbt. These ssues are addressed by addng nertal data to ad the GPS trackng and recovery durng outages due to dynamcs and wth the use of beam steerng capablty. The Dgtal Beam Steerng capablty utlzed n the SSGR allows for Table Testng for Varous Msson Phases the constructon of a composte GPS sgnal from multple non-coplanar antenna elements placed around the spacecraft. The beam steerng/null formng functonalty also allows for trackng of weak GPS sgnals (such as GPS sdelobes) from hgher than GPS orbt. Table summarzes the enhancements that wll be requred for the SSGR and the testng that wll be done for each msson phase. Msson/ Capablty Launch & Orbt Entry Staton-keepng Formaton-Flyng Recovery & Landng 3-D Beam-steerng Mantans SV vsblty at all atttudes Provdes gan towards GPS SVs Provdes gan towards GPS SVs Mantans SV vsblty at all atttudes Inertal-adng Hgh dynamc aded trackng for data contnuty and navgaton through SV outages N/A N/A Hgh dynamc aded trackng for data contnuty and navgaton through SV outages Precson GPS Navgaton Provdes hgh accuracy code/carrer observatons for Wde Area Dfferental GPS (WADGPS) soluton Provdes hgh accuracy code/carrer observatons for WADGPS soluton Provdes hgh accuracy code/carrer observatons for Knematc GPS (KGPS) soluton Provdes hgh accuracy code/ carrer observatons for WADGPS and KGPS soluton Atttude Determnaton N/A Provdes nterferometrc atttude data from array Provdes nterferometrc atttude data from array N/A 3. HIGH-GAIN ADVANCED GPS RECEIVER NAVSYS Hgh-gan Advanced GPS Recever (HAGR) [] s a software reprogrammable, dgtal beam steerng GPS recever. The HAGR components are llustrated n Fgure. Wth the HAGR dgtal beam steerng mplementaton, each antenna RF nput s converted to a dgtal sgnal usng a Dgtal Front-End (DFE). The HAGR can be confgured to operate wth up to 6 antenna elements (L and L2) wth the 2 antenna elements nstalled n any user specfed antenna array pattern. Each DFE board n the HAGR can convert sgnals from 8 antenna elements. The dgtal sgnals from the set of the antenna nputs are then provded to the HAGR dgtal sgnal processng cards. The HAGR can be confgured to track up to 2 satelltes provdng L C/A and L and L2 P(Y) observatons when operatng n the keyed mode. The dgtal sgnal processng s performed n frmware, downloaded
from the host computer. Snce the dgtal spatal processng s unque for each satellte channel, the weghts can be optmzed for the partcular satelltes beng tracked. The dgtal archtecture allows the weghts to be computed n the HAGR software, then downloaded to be appled precorrelaton to create a dgtal adaptve antenna pattern to optmze the sgnal trackng performance. Antenna DFE Sampled RF Spectru m Dgtal Recever CAC DBS (Xlnx) I & Q Data Control Host Computer I/O (Seral) Up to 6 Antenna Elements DFE Module DFE Module DFE Module DFE Module To All Modules Local Oscllator 6 to 2 Processng Channels Array Weghts Logc Processng Channel Antenna Element Output Bus Weghts & CorrelatorControl Sample Clock and Reference Clock to All Crcuts Correlator Logc Processng Channel Processng Channel Calbraton Logc Control Computer I/Q Data N C B Atttude Sensor Fgure P(Y) HAGR System Block Dagram 4. SOFTWARE REPROGRAMMABLE GPS RECEIVER The flexble Software GPS Recever (SGR) archtecture leveraged by the HAGR allows the GPS sgnal processng software and frmware to be easly ported to run on space qualfed sgnal processng and host computer cards [2]. The GPS software rado archtecture adopted by the HAGR shown n Fgure 2 allows the recever confguraton to be optmzed dependng on the phase of flght [2]. For example, dfferent antenna nputs and navgaton modes could be used durng launch and orbt entry than durng the remanng msson lfe where the recever could be optmzed for autonomous orbt estmaton and staton keepng. The SSGR Archtecture utlzes 4-p sterdan feld of vew usng 3-D dgtal beam steerng provdng contnuous trackng durng maneuvers whle smplfyng antenna nstallaton through the use of a dgtal nterface. Dgtal beam steerng provdes addtve gan n the drecton of the GPS satellte tracked mprovng sgnal recepton at hgh alttude orbts (e.g. transfer orbts or GEO). Dgtal beam steerng can provde antenna drectvty towards both the GPS satellte man-lobes and sde-lobes ncreasng the numbers of satelltes that can be tracked. Modular dgtal beam-steerng archtecture can be confgured for crossstrappng to provde added redundancy. Software reprogrammable GPS approach can be mplemented on radaton hardened sgnal processors and re-used durng the msson lfe to support multple msson requrements. HAGR Functons Receve Sgnals 2 to 24 MHz bandwdth Reference and LO Phase Coherent Down Converson Internal Tme Synch. Beam Steerng Correlaton Securty Track Satelltes Baseband Processng Loop Control Calbraton Network Connectvty Fgure 2 NAVSYS Software GPS Recever Archtecture Dgtal Beam Steerng The dgtal sgnal from each of the HAGR antenna elements can be descrbed by the followng equaton. y ( t) = k Ns s ( x, t) + n ( t) + Nj k k = k = j ( x, t) where s (x k,t) s the th GPS satellte sgnal receved at the kth antenna element n k (t) s the nose ntroduced by the kth DFE j j (x l,t) s the fltered jth jammer sgnal receved at the kth antenna element The GPS satellte sgnal at each antenna element (x k ) can be calculated from the followng equaton. 2π T s ( x k, t) = s (, t)exp{ x k ) = s (, t) e λ where s (,t) s the satellte sgnal at the array center and s the lne-of-sght to that satellte e sk are the elements of a vector of phase angle offsets for satellte to each element k The combned dgtal array sgnal, z(t), s generated from summng the weghted ndvdual fltered DFE sgnals. Ths can be expressed as the followng equaton. Ns Nj z( t) = w y( t) = w s ( t) e s + n( t) + j = l= j k j sk ( t) e jl 3
Wth beam steerng, the optmal weghts are selected to maxmze the sgnal/nose rato to the partcular satellte beng tracked. These are computed from the satellte phase angle offsets as shown n the followng equaton. 2π T exp{ x) λ w BS =. = e 2π T exp{ x M ) λ In Fgure 3 and Fgure 4 the antenna patterns created by the dgtal antenna array are shown for four of the satelltes tracked. The HAGR can track up to 2 satelltes smultaneously. The antenna pattern provdes the peak n the drecton of the satellte tracked (marked x n each fgure). The beams follow the satelltes as they move across the sky. Snce the L2 wavelength s larger than the L wavelength, the antenna beam wdth s wder for the L2 antenna pattern than for the L. s 5. SIMULATION FOR THE SPACE ENVIRONMENT The complex smulaton envronment that must be modeled for hgher than GPS trackng s shown n Fgure 5. The receved sgnal power from the GPS satelltes s a functon of the GPS angle of drectvty α. Ths must be computed based on the sgnal avalable from both the man lobes of the GPS satellte antenna pattern and the sdelobes, as shown n Fgure 6. The model must also take nto account earth blockage as well. As an example, a plot of the user (Geostatonary Operatonal Envronment Satellte (GOES) and lne-of-sght vectors to every avalable GPS satellte at the begnnng of the smulaton s shown n Fgure 7 (Earth not to scale). Based on a mnmum receved C/No of 2 db-hz, seven GPS satelltes were vsble, whch are shown n red (PRN 7, 8,, 3, 2, 22, and 27). x r user β = 9 + el d R user θ α x r GPS O R GPS Earth (not to scale) Fgure 3 L Antenna Pattern Fgure 5 HEO orbt scenaro Fgure 4 L2 Antenna Pattern Fgure 6 Modeled relatve antenna attenuaton of the GPS transmttng antennas 4
User Poston (nc tme) sol2rng.m Generate RNG data from trajectory nputs RNG SV Data gpsmsg.m Generate Navgaton Message data RNG MSG Fgure 7 Lne-of-sght vectors from GOES satellte to each avalable GPS satellte. Vsble GPS satelltes based on a C/No threshold of 2 db-hz are shown n red 6. MATLAB GPS TOOLBOX The AGHS GPS sgnals are generated wth nputs from the NAVSYS Matlab GPS Tools. The sgnal flow employed n ths generaton process s shown n Fgure 8. As the fgure llustrates, the Matlab tools allow the user to have total control over the GPS sgnals that are smulated. The ntal smulated trajectory s nput as a user defned soluton/trajectory profle. The Matlab tools then convert ths trajectory nto an RNG format usng the sol2rng functon, whch ncludes the pseudo-range, sgnal power, Doppler frequency shft and carrer phase of the smulated sgnals. The GPS navgaton message data to be modulated on each smulated sgnal s also generated. The aghssm functons provde the low level control of the sgnal generaton. Ths provdes the raw sgnal ampltude and code phase and carrer phase nformaton needed to generate the C/A and P(Y) code sgnals. There are two modes of operaton for the aghssm functonalty. The frst s the software sgnal generaton mode where the Matlab functons are used to drectly generate a smulated Dgtal Sgnal Fle (DSF), whch s a dgtal representaton of the GPS smulated sgnals. The second mode s where the Correlator Accelerator Card (CAC) s controlled through the Matlab drvers to generate the dgtal smulated sgnal n real-tme. In ether of these modes, the AGHS generated sgnals can be recorded n the DSR and/or remodulated onto an RF carrer for output to a GPS recever under test. Dgtal Output to RFM aghssm Generate khz CAC Downloads (PR,CPH,D(t)) cacsssg.m Emulate CAC Dgtal Sgnal Generaton Save to Dsk as DSF Fle DSR Data Playback Fgure 8 Sgnal Smulaton Flowchart 7. ADVANCED GPS HYBRID SIMULATOR The AGHS archtecture s llustrated n Fgure 9. Ths ncludes the followng advantages over prevous analog smulators that wll be leveraged n the SSGR test actvtes. o Access to all levels of satellte sgnal generator control through NAVSYS Matlab satellte and sgnal generaton scrpts o Software nterface for nserton of future GPS sgnals or smulated jammer waveforms onto composte dgtal satellte sgnal profle. o Dgtal data storage for exact reconstructon and playback of sgnal smulaton profles o Dgtal output from the smulator of pre-recorded or real-tme smulated sgnals o Dgtal trackng of the recorded sgnals for hgh fdelty sgnal reconstructon and analyss o Hgh fdelty, phase coherent RF remodulaton of dgtal sgnals for output to a GPS recever or mult-element Controlled Recepton Pattern Antenna (CRPA). 5
elements. Many satelltes are avalable n a database ncluded wth the program and others can be downloaded from the Analytcal Graphcs webste. Several orbt ntegrators are avalable to generate the orbt trajectory from the ntal state. Fgure 9 AGHS System Archtecture In the profle generaton mode, the AGHS system generates the satellte sgnal profle to be smulated. The user trajectory s nput ether from a pre-defned soluton fle (SOL) or n real-tme through the web nterface. Once the profle s defned, the AGHS system generates the dgtal smulated sgnals that emulate the dgtal outputs from each of the SSGR Dgtal Antenna Elements (DAEs) that would be appled to the SSGR sgnal processor. Ths mode wll run n real-tme and can also to be used to generate recorded data fles n the Data Logger (DL) subsystem for playback n the SSGR. We wll record data lbrares for each of the msson profles generated to allow ths testng to be repeated wth dfferent software confguratons of the SSGR. STK provdes many vsualzaton tools whch are useful n determnng that an orbt has been properly generated. Fgure shows a ground track for the MMS satellte, whch s n a hghly ellptc orbt. The apogee of ths orbt s hgher than the orbts of the GPS satelltes. Parameters for Hghly-Eccentrc-Orbt Satellte Propagator: TwoBody Start Tme: 24 Oct 23 ::. UTC Stop Tme: 25 Oct 23 ::. UTC Step sze: 6 sec Orbt Epoch: 2 June 23 ::. UTC Semmajor Axs: 4457. km Eccentrcty:.53846 Coord. Type: Classcal Coord. System: J2 Inclnaton: 28.5 deg Argument of Pergee:. deg RANN: 9. deg MEAN ANOMALY:. DEG In the dgtal sgnal playback mode, the data recorded n the DSF s regenerated dgtally for playback nto the SSGR sgnal processor. As an opton, t can also be modulated onto an RF sgnal by the RF Modulator (RFM) subsystem for playback nto the ndvdual antenna elements. An Inertal Navgaton System (INS) model component has also been added to the AGHS so we can generate smulated nertal-measurement unt (IMU) delta-theta and delta-v nputs nto the SSGR durng testng. Satellte Tool Kt Interface Satellte Tool Kt (STK) standard s the core of the STK software sute and t s free to all government, aerospace, and defense professonals. STK provdes the analytcal engne to calculate data and dsplay multple 2-D maps to vsualze varous tme-dependent nformaton for satelltes and other space-related objects, such as launch vehcles, mssles, and arcraft. STK's core capabltes nclude orbt/trajectory ephemers generaton, acquston tmes, and sensor coverage analyss for any of the objects modeled n the STK envronment. To extend the analytcal capabltes of STK, Analytcal Graphcs Inc. (AGI) also offers STK Professonal (STK/PRO), a collecton of addtonal orbt propagators, atttude profles, coordnate types and systems, sensor types, nvew constrants, and cty, faclty, and star databases. Orbt trajectory generaton s a feature ncluded n the core STK program. Intal orbt state can be entered n Cartesan (Earth Fxed or Inertal) coordnates or wth orbt Kepleran Fgure Groundtrack of HEO orbt Fgure Hghly Eccentrc Orbt generated wth STK 6
STK s capable of generatng a report whch contans the poston and velocty of a satellte at user defned tme steps. Fgure shows a porton of the trajectory generated n the Earth Centered Fxed coordnate frame at sec ntervals. Ths trajectory s used wth a modfed verson of the sol2rng.m Matlab functon to generate the range vectors and to drve the smulaton for GPS data n orbtal scenaros. Varous atttude profles are also avalable n the STK trajectory smulatons, and atttude reports can be generated usng Euler angles or quaternons. The modfed AGHS smulaton profles make use of quaternons to descrbe the rotaton of a satellte. The quaternons defne the orentaton of a vector n the spacecraft body-fxed frame wth respect to the Earth Centered Inertal frame. Thus, a normal vector and n plane vector for a GPS antenna, defned n the Body fxed frame, relatve to the spacecraft center of mass can be rotated to nertal coordnates. The GPS ToolBox contans functons to create a drecton cosne matrx for ths rotaton from the quaternon set. The resultng vector can then be rotated to the ECF frame that descrbes the GPS orbts, allowng vsblty and lne of ste calculatons to be performed. 8. INTERNAV SOFTWARE The NAVSYS InterNav software wll be used to calculate combned GPS/Inertal navgaton solutons. Ths ncludes the software functons llustrated n Fgure 2. The recever nterface module handles the nterface to the SSGR trackng software, whch provdes the GPS pseudo-range and carrer observatons for processng n the Kalman Flter. The IMU nterface module formats the IMU delta-theta and delta-v observaton for the nertal navgaton soluton. The nertal navgaton soluton s based on a quaternon ntegraton algorthm to compute the body-to-navgaton transformaton drecton cosne matrx and ntegrate the acceleraton to propagate poston and velocty n a wander-azmuth navgaton frame. he ntalzaton and algnment procedure followed at start up s llustrated n Fgure 3. SATELLITE POSITION COMPUTATION In the rough levelng mode (system state=), the GPS updates are used to estmate where the local level frame s. Once local level has been determned, the system transtons to the rough algnment mode (system state=2) to generate an ntal estmate of the wander azmuth angle (and headng). Once the headng of the INS has been observed, the system transtons to the navgaton mode (system state=3) where the accelerometer and gyroscope errors are further refned usng a small-angle model for the Kalman Flter. System Intalzaton Rough Levelng Rough Algnment GPS/Inertal Navgaton Fgure 3 InterNav System Modes The InterNav software wll be ntegrated wth the SSGR navgaton software generatng a tghtly ntegrated GPS/nertal navgaton soluton and atttude data, whch s used by the dgtal beam-steerng software module. The nertal poston, velocty, acceleraton and atttude data wll also be provded to the GPS trackng loops to be used for optmzng satellte selecton and also adng the trackng loops durng hgh dynamcs to mnmze sgnal drop-out tmes. 9. IMU SIMULATION RECEIVER INTERFACE IMU INTERFACE RX MEAS * PR / DR RPR,RDR, V QUAT V N INTEG C N B MEASUREMENT RESIDUAL INS NAV G^ X INS G^ X INS GPS / INS KALMAN FILTER INS ERROR CORRECTION To support future applcatons nvolvng tghtly ntegrated GPS and nertal systems and ultra-tghtly-coupled GPS/nertal recevers, the AGHS has been upgraded to add nertal smulator capablty. Ths wll operate n conjuncton wth the AGHS satellte sgnal smulaton capablty as shown n Fgure 4. RAW IMU INS NAV * DIFFERENTIAL CORRECTIONS APPLIED # INS / SMOOTHER DATA COMBINED POST-TEST # CORRECT INS NAV INS CORR NAV Fgure 2 InterNav Software Archtecture 7
Atttudes 3 SV Errors & Constellaton GPS Satellte Sgnal Smulator 25 2 Smulated Trajectory UTC GPS/Inertal System under test Angle (degree) 5 5 Trajectory: Ptch Trajectory: Roll Trajectory: Headng IMU: Ptch IMU: Roll IMU: Headng Inertal Errors Inertal Smulator -5 2 3 4 5 6 Tme (sec) Fgure 4 IMU Smulaton Archtecture For ths launch, the trajectory duraton s approxmately.2 mnutes. In order to make the launch more applcable and realstc, a 2-second statonary data s added to the begnnng of trajectory to smulate the ntal poston of the launch. Ths s necessary for two reasons: () makng sure that IMU s n fnal algnment before ntatng the launch and (2) verfyng sngularty does not occurred n IMU smulaton by nspectng the fnal results. By the end of the launch wndow, the fnal alttude of the orbter s,5 km, whch places the vehcle n Low-Earth-Orbt. Fgure 5 shows the flght path. Fgure 6 Atttude Plot of Trajectory and Corrected Navgaton Data Dfference (degree).7.6.5.4.3 Dfference n Ptch, Roll, and Headng Ptch Roll Headng.2. 2 3 4 5 6 Tme (sec) Fgure 7 Dfference n Trajectory and Corrected Navgaton Data In transent state, the atttude dfference s n the order of tenths of a degree. The atttude error shows a convergence toward zero as the soluton dfference approaches steadystate response. Wth 2-second ntal statonary trajectory, no sngularty s observed and, n addton, the output shows an mprovement over results wthout statonary ntal poston (not plotted here). Wth perfect ntalzaton, the smulaton provdes expected results for corrected navgaton poston, alttude, and atttude. Fgure 5 Vehcle Launch Path Note n Fgure 6 the sharp drop-off n roll s due to realgnment of the space vehcle and not obvous. Fgure 7 shows the dfference between the nput atttude and the corrected navgaton atttude.. ALL-AROUND SATELLITE VISIBILITY Testng was completed to demonstrate the capablty of the HAGR to provde all-around satellte vsblty usng multple antenna elements. Ths testng was performed to show that a composte sgnal could be formed from the multple elements. The test confguraton s shown n Fgure 8 and a pcture of the test fxture s shown n Fgure 9. 8
AE #4 AE # AE #3 AE #2 Fgure 8 Four-Element All-around Vsblty Antenna Testng Table 2 All-around Satellte Vsblty Test Data Summary PRN AZ EL C/N 55 23 42 2 245 9 44 3 55 3 48 7 25 3 39 8 35 3-3 - 25 37 3 294 5 45 5 98 2 43 8 89 6 47 9 34 72 44 27 35 45 46 3 53 47 3 33 8 27 3 3 3 45 6 3 6 9 27 9 3 24 2 8 2 2 7 5 8 Fgure 9 Satellte Test Fxture Fgure 2 Skyplot of 3-D Beam steerng Satellte Vsblty In Fgure 2, a sky plot s shown wth the locatons of the GPS satelltes tracked durng the test. In Fgure 2, the satelltes (dentfed by PRN number) that were tracked durng the test are plotted aganst tme, and n Table 2, the sgnal-to-nose ratos of the satelltes tracked durng the test are lsted. From ths test data, t s evdent that the 3-D beam formng s functonng correctly. All of the satelltes above the horzon were tracked wth the excepton of satelltes 8 and, whch were not selected by the 8-channel GPS recever. The sgnal-to-nose rato s also comparable wth normal GPS operaton ndcatng no notceable degradaton from the 4π sterdan sgnal combnng. 9
C/N (db-hz) 58 56 54 52 5 48 46 44 P(Y) HAGR 2 3 4 7 8 9 3 4 8 2 2 22 23 25 26 27 28 29 3 3 42 4 38 2 4 6 8 2 4 Tme (hrs) Fgure 2 All-around Vsblty Tests - SVs tracked. HIGH GAIN SATELLITE TRACKING The drectvty of the dgtal beam formng provdes gan n the drecton of the GPS satelltes. Ths mproves the ablty of the dgtal beam steerng recever to be able to track GPS satelltes wth low sgnal power, for example, from a space platform located above the GPS satellte constellaton. Wth a 6-element array, the beam steerng provdes up to 2 db of addtonal gan. Wth a 7-element array, up to 8.45 db of addtonal gan s provded. A data set was collected to observe the sgnal-to-nose rato on the C/A and P(Y) code HAGR data over a perod of 2 hours. From ths data (Fgure 22 and Fgure 23), t can be seen that the beam steerng ncreases the GPS sgnal strength to a value of 56 db-hz on the C/A code. As expected the P(Y) code observed sgnal strength s 3 db lower. Fgure 23 P(Y) HAGR Sgnal-to-Nose (db-hz) Measurement Nose And Multpath Error Reducton The dgtal beam steerng also mproves the measurement accuracy and decreases the effect of multpath errors from sgnal reflectons receved from the spacecraft structure (e.g. solar panels or antenna arrays). The GPS L pseudo-range and carrer-phase observatons are descrbed by the followng equatons. PR ( m) = R + b + I + + τ + n CPH ( m) = N λ + n ( R u + b u CPH I + T T M + λ θ ) M PR The followng errors affect the pseudo-range and carrer phase observatons. C/N (db-hz) 62 6 58 56 54 52 5 48 C/A HAGR 2 3 4 7 8 9 3 4 8 2 2 22 23 25 26 27 28 29 3 3 o Ionosphere errors (I) o Troposphere errors these are the same on all of the observatons ( T ) o Recever Measurement Nose these are dfferent on each of the observatons ( n PR, n CPH ) o Multpath Nose these are dfferent on each of the observatons ( τ M, λ θ M ) o Satellte and Staton Poston error - these affect the ablty to correct for the Range to the satellte (R ) o Recever clock offset (bu) 46 44 42 2 4 6 8 2 4 Tme (hrs) Fgure 22 C/A HAGR Sgnal-to-Nose (db-hz) From ths equaton, the L pseudo-range + carrer phase sum cancels out the common errors and the range to the satellte and observes the pseudo-range and multpath errors as well as the change n the onospherc offset. PR + CPH ( m) = 2I + τ M = C + 2I + τ C + 2I + τ + n M M PR + n + n + N λ + n PR PR + ( n CPH CPH λ θ λ θ M M ) The PR+CPH s plotted n Fgure 25 for SV 25 and each of the recever data sets. The short term (< sec) whte recever nose was removed by passng the PR+CPH
observaton through a lnear flter. The drft caused by the onosphere on each observaton was removed usng a polynomal estmator. The remanng cyclc error s an estmate of the multpath pseudo-range errors. The RMS whte nose on the pseudo-range observatons was computed by dfferencng the PR+CPH measurement. Ths s shown n Fgure 26 and Fgure 27 for all of the satelltes tracked for the C/A and P(Y) code observatons. The observed PR nose shows good correspondence wth the predcted values, based on analyss of the trackng loops, shown n Fgure 28. For C/N values above 52 db-hz, the P(Y) code HAGR provded pseudo-range accuraces of 5 cm (-sgma) whle for C/N values above 55 db-hz the C/A code observatons were accurate to 5 cm. These values are for -Hz observatons wthout any carrer smoothng appled. The mean observed RMS accuraces are summarzed below n Table 3 wth the average peak multpath PR errors observed. The short term cyclc varatons shown n Fgure 25 are caused by multpath errors. The peak-to-peak cyclc PR varaton for each of the recever data sets was calculated to estmate the errors observed for each satellte from the pseudo-range multpath []. These errors are lsted n Table 3 for each of the satelltes. The HAGR spatal sgnal processng can also be used to detect the presence of multpath and adapt the antenna pattern to further mnmze these errors[3,4]. In Fgure 24 spatal nformaton from a 7- element phased array s shown that dentfes the source of a strong multpath sgnal through drecton of arrval (DOA) estmaton usng the MUSIC algorthm[5].. Testng has shown that the dgtal beam steerng and spatal processng sgnfcantly reduces the multpath errors on the carrer phase observatons. Ths s mportant for space applcatons whch rely on the GPS carrer phase nformaton, such as nterferometrc atttude determnaton. Table 3 Mean PR Nose and M-path Peak Errors (m) (6-element array) SVID C/A HAGR RMS PR C/A Mean Mpath PR P(Y) HAGR RMS PR P(Y) Mean Mpath PR.239.259.54.22 3.284.494.56.337 8.2.278.45.22.278.535.59.287 3.252.32.59.26 4.24.359.49.35 2.222.267.5.64 2.252.26.58.33 22.248.38.47.27 25.22.362.44.265 27.83.27.44.78 28.236.366.55.272 29.225.32.5.27 3.477.79.89.624 3.325.266.55.35 PR+CPH (m) -5 - -5-2 -25 SV 25 -Antenna -Choke Rng HAGR C/A HAGR P(Y) -3 5 5. 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6 Tme snce : (hrs) Fgure 25 PR+CPH (m) - SV 25 RMS PR Nose (m).9.8.7.6.5.4.3 C/A RMS PR Nose (m) 2 3 4 7 8 9 3 4 8 2 2 22 23 25 26 27 28 29 3 3.2 Fgure 24 MUSIC drecton of arrval estmaton. 2 4 6 8 2 4 Tme (hrs) Fgure 26 HAGR C/A Code Pseudo-Range Nose (m) (6-element array no carrer smothng )
RMS PR Nose (m)..9.8.7.6.5.4.3.2 P(Y) RMS PR Nose (m) 2 3 4 7 8 9 3 4 8 2 2 22 23 25 26 27 28 29 3 3 NAVSYS s currently developng a desgn for a space-borne verson of our reprogrammable, dgtal beam steerng GPS recever product under contract to AFRL/VS and NASA Goddard Space Flght Center (GSFC). Ths modular, flexble archtecture s desgned to be ported from our nhouse test-bed to a varety of space-qualfed sgnal processng boards and host computers to provde an embedded GPS capablty. The desgn also allows the recever to be reconfgured n-flght to optmze the GPS trackng performance dependng on the needs of each phase of the msson.. 2 4 6 8 2 4 Tme (hrs) Fgure 27 HAGR P(Y) Code Pseudo-Range Nose (m) (6-element array no carrer smoothng) To ad n testng of ths space qualfed recever, the NAVSYS GPS Toolbox, and AGHS products have been augmented to nclude tools to allow smulaton of GPS data n a space envronment. In addton, the IMU smulaton of the InterNav product has also been ncorporated nto the AGHS desgn. HAGR C/A HAGR P(Y) REFERENCES RMS PR Error (m) - [] A. Brown, N. Geren, "Test Results from a Dgtal P(Y) Code Beamsteerng Recever for Multpath Mnmzaton," ION 57 th Annual Meetng, Albuquerque, NM, June 2. [2] N. Geren, A. Brown, Modular GPS Software Rado Archtecture, Proceedngs of ION GPS 2, Salt Lake Cty, Utah, September 2. -2 4 42 44 46 48 5 52 54 56 58 6 C/N (db) [3] A. Brown, Performance and Jammng Test Results of a Dgtal Beamformng GPS Recever, Jont Navgaton Conference, Orlando, FL, May, 22. Fgure 28 C/A and P(Y) HAGR RMS PR error versus C/N 2. CONCLUSION The test data presented n ths paper has shown that the dgtal beam steerng archtecture has advantages n: ncreasng the receved GPS sgnal/nose rato, whch mproves the trackng performance for low power satellte sgnals; mprovng the measurement accuracy for precson applcatons such as rendezvous, dockng or formaton flyng; mnmzng carrer phase multpath errors whch can result n mproved nterferometrc atttude determnaton. [4] D. Sullvan, R. Slva, and A. Brown, Hgh Accuracy Dfferental and Knematc GPS Postonng usng a Dgtal Beam Steerng Recever, Proceedngs of 22 Core Technologes for Space Systems Conference, Colorado Sprngs, CO, November 22. [5] A. Brown and K. Stolk; Rapd Ambguty Resoluton usng Multpath Spatal Processng for Hgh Accuracy Carrer Phase, Proceedngs of ION GPS 22, Portland, OR, September 22. 2
BIOGRAPHY Kenn Gold s a Product Area Manager at NAVSYS Corporaton for the Advanced Systems and Smulaton Tools group. Hs work ncludes development of spaceborne GPS recevers, ntegrty montorng algorthm development, and GPS smulator desgn. He holds a PhD from Unversty of Colorado n Aerospace Engneerng. Alson Brown s the Presdent and Chef Executve Offcer of NAVSYS Corporaton. She has a PhD n Mechancs, Aerospace, and Nuclear Engneerng from UCLA, an MS n Aeronautcs and Astronautcs from MIT, and an MA n Engneerng from Cambrdge Unversty. In 986, she founded NAVSYS Corporaton. Currently she s a member of the GPS-III Independent Revew Team and Scentfc Advsory Board for the USAF and serves on the GPS World edtoral advsory board. ACKNOWLEDGEMENTS Ths work s beng sponsored under an SBIR contract to AFRL/VS and to NASA GSFC. The authors would lke to express ther apprecaton for the support of these organzatons and the techncal ponts of contact, Dr. Alan Lovel and Dr. Mchael Moreau, n the development of ths new technology. 3