CHAPTER SIX GLOBAL POSITIONING SYSTEM (GPS)

Transcription

1 CHAPTER SIX GLOBAL POSITIONING SYSTEM (GPS) 6

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3 GLOBAL POSITIONING SYSTEM (GPS) 6 OVERVIEW Glbal Psitining System (GPS) is a tl that the cnstructin industry has adpted frm the United States Military. It is a cnstellatin f at least 24 satellites that prvide accurate psitin crdinates. GPS uses satellites and cmputers t cmpute psitins anywhere n the Earth. There are three main segments t the GPS. The first being space. This medium encmpasses a minimum f 24 satellites that rbit the earth every 12 hurs at an altitude f abut 12,551 miles. The secnd segment is cntrl. There is a master statin lcated at Schriever Air Frce Base in Clrad Springs, Clrad. Each satellite passes ver ne f fur mnitring statins twice a day. The master statin calculates crrectins and synchrnizes the atmic clcks abard the satellites. The third and final segment is the user. The user is, simply put, anyne wh has a GPS receiver and can access the signal. GPS survey methds include, but are nt limited t: Static GPS surveys Fast-static GPS surveys Real-Time GPS surveys (RTK) Pst-Prcessed kinematic GPS surveys The type f GPS survey used mstly fr cnstructin is Real-Time GPS survey (RTK). RTK is similar t a Ttal Statin radial survey. RTK surveys measure the baselines frm the reference statin t the rving receivers pint. A radi at the reference statin bradcasts the psitin f the reference pint t the rvers, and the system prcesses the baselines in Real Time allwing fr prject crdinate infrmatin t be gathered and analyzed during the actual field survey. RTK surveying prvides centimeter-level precisin withut pst-prcessing. There are three types f survey methds in RTK surveys: tp pints, cntinuus surveys, and stakeuts. Tp pints are shrt (usually 3-15 secnd) ccupatins, e.g., ver a sample site r survey marker. Cntinuus survey mde allws nging data cllectin at a specified lgging interval, e.g., every 5 secnds, r a specified distance interval, e.g., 1 meter. Cntinuus mde is used fr mapping. Stakeut mde allws navigatin t predetermined crdinates. The fllwing are sme f the items that will affect the accuracy f a GPS survey: Satellite Gemetry. A minimum f fur satellites are required t survey with GPS. A minimum f five satellites is recmmended. The cnfiguratin f the visible satellites the receiver is able t track in relatin t each ther will make a significant difference in the data that is being cllected. Satellite gemetry is expressed as a numeric value knwn as Dilutin f Precisin (DOP). Gd satellite gemetry will have small DOP values while pr satellite gemetry will have large DOP values. As a guideline, DOP values f six r lwer are required fr NDOT GPS surveys. The ideal satellite gemetry is ne which has the visible satellites distributed thrughut the sky. Gd satellite gemetry will yield a higher precisin. Satellite gemetry factrs that must be cnsidered when planning a GPS survey are: Number f satellites available. Minimum elevatin angle abve the hrizn (elevatin mask). Obstructins limiting satellite visibility. Psitin Dilutin f Precisin (PDOP). Vertical Dilutin f Precisin (VDOP). Hrizntal Dilutin f Precisin (HDOP). Gemetric Dilutin f Precisin (GDOP). State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember

4 6 GLOBAL POSITIONING SYSTEM (GPS) Weather Cnditins. Generally, weather cnditins d nt affect GPS surveying; hwever, the fllwing cnditins must be cnsidered when planning a GPS survey: GPS bservatins shuld never be cnducted during an electrical strm. Significant changes in weather r unusual weather cnditins shuld be nted either in the field ntes, data cllectr, r receiver. Hrizntal and vertical GPS bservatins can at times be affected by severe snw, hail and rain strms. Therefre, high accurate GPS surveys shuld nt be cnducted during these perids. Sunspts r magnetic strms can affect GPS bservatins; care needs t be taken t avid GPS surveying during these perids. Elevatin Mask Angle. Nearly all GPS receivers, inexpensive r expensive, have a Mask Angle setting. This means that the receiver can be set t ignre any satellite signals that cme frm belw a user-definable angle abve the hrizn, r mask them ut. The mst typical mask angle is usually smewhere between 10 and 15 degrees. The drawback here is that setting the mask angle t high might exclude satellites needed t acquire the necessary minimum f fur. It s a trade-ff. Are yu s desperate fr a psitin at that exact time that yu re willing t accept a degraded signal? It des happen. In that case, the mask angle culd be set t maybe 5 degrees, r even t zer if there s a clear view f the hrizn, such as at sea, and simply accept a degraded signal and pssibly (prbably) a prer accuracy as a result. In mst cases, it s better t keep the mask angle at that upper end f arund 15 t (at mst) 20 degrees and just wait fr a sufficient number f satellites t becme available abve the mask. Nw that the full GPS cnstellatin is cmplete, there will rarely be times with t few satellites sufficiently high in the sky t get a gd psitin. Anther ptential surce f errr is receiver nise, r electrnic nise prduced by the receiver itself that interferes with the very weak incming signal. While this errr is highly variable amng receiver brands, mst have sme kind f internal filtering designed t minimize the prblem sme better than thers. Elevatin mask als helps t minimize the atmspheric nise in the data. Satellites that are high in the sky will have less atmspheric nise than satellites lw in the sky and very clse t the bserver s hrizn. By having an elevatin mask set, the nise in the GPS satellite signal is kept t a minimum. Multi-Path Errrs. Anther ptential, thugh relatively minr, surce f signal errr is Multi-Path. Multi-Path is simply the receptin f a reflected satellite signal. With multi-path receptin, the receiver cllects bth the direct signal frm the satellite and a fractinally delayed signal that has bunced ff f sme nearby reflective surface then reached the receiver. This is the same kind f thing seen in televisin ghsts. The prblem is that the path f the signal that has reflected ff sme surface is lnger than the direct line t the satellite. This can cnfuse sme lwer-end receivers resulting in an incrrect range measurement and, cnsequently, an incrrect psitin. Mst receivers have sme way f seeing and cmparing the crrect and incrrect incming signal. Since the reflected multi-path signal has traveled a lnger path, it will arrive at a fractin f a secnd later, and a fractin weaker than the direct signal. By recgnizing that there are tw signals ne right after anther, and that ne is slightly weaker than the ther, the receiver can reject the later, weaker signal, minimizing the prblem. This ability is referred t as the receiver s multi-path rejectin capability. Dilutin f Precisin (DOP). The DOP is a measure f the gemetry f the visible satellite. The ideal rientatin f fur r mre satellites wuld be t have them equally spaced all arund the receiver, including ne abve and ne belw. Because we re taking ur psitin frm nly ne side f the Earth, that s really nt pssible since that part f space is blcked by the planet itself. The next best rientatin is t have ne satellite directly abve and the ther three evenly spaced arund the receiver and elevated t abut 25 t 30 degrees (t help minimize atmspheric refractin). This wuld result in a very gd DOP value. If all the satellites are clustered tgether, it wuld result in a pr DOP value and yur readings culd be suspect. A lw numeric DOP value represents a gd satellite cnfiguratin, whereas a higher value represents a pr satellite cnfiguratin. The DOP at any given mment will change with time as the satellites mve alng their rbits. When the satellites are widely spaced, the verlap area f the tw znes f pssible satellite range errr is relatively small. The diagram belw n the left illustrates a pair f widely spaced satellites which wuld result in a gd r lw DOP value. The diagram n the right in Figure 6-1 illustrates pr satellite gemetry resulting in pr r high DOP. 6-2 State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember 2017

5 GLOBAL POSITIONING SYSTEM (GPS) 6 Figure 6-1: Dilutin f Precisin (DOP) A DOP value f less than 2 is cnsidered excellent-abut as gd as it gets, but it desn t happen ften, usually requiring a clear view f the sky all the way t the hrizn. DOP values f 2 t 3 are cnsidered very gd. DOP values f 4 r belw are frequently specified when equipment accuracy capabilities are given. DOP values f 4 t 5 are cnsidered fairly gd and wuld nrmally be acceptable fr all but the highest levels f survey precisin requirements. A DOP value f 6 wuld be acceptable nly in lw precisin cnditins, such as in carse psitining and navigatin. Psitin data shuld nt be recrded when the DOP value exceeds 6. It s imprtant t carefully cnsider where the data is t be cllected. Is the area f interest n Main Street f a large city? If s, the receiver is likely t be surrunded by tall buildings that restrict satellite visibility resulting in pr DOPs, since the nly satellites that the receiver can see will be nearly straight up. That is, prvided it s even pssible t see enugh satellites t get a psitin at all. In additin, the glass-sided structures all arund the receiver act as nearly perfect multi-path reflectrs. It s pssible that, because f the efficiency f the buildings t reflect the incming satellite signal, the receiver s multi-path rejectin capability may actually be verladed. These are very difficult prblems t vercme, particularly in dense urban areas with many tall buildings. And the prblems aren t just in the cities. Even ut in the cuntry with wide pen spaces there are cnditins t be cnsidered. Clse prximity t high-pwer lines is a prblem. The electrmagnetic radiatin surrunding the lines can interfere with the satellite signal, cntributing an errr that is nearly impssible t cmpensate fr r mdel. Frests with dense canpy cver can bscure the sky and interfere with the incming satellite signal. The prblem is even wrse if the vegetatin is wet, since the liquid water itself can als interfere with the signal. Human Errr. The greatest cntributr t errr in GPS measurement is human errr. Care must be taken while perfrming any GPS survey t keep human errr t a minimum by prper prcedures, redundant checks, repeat measurements and GPS bservatin lg reprts. The fllwing are sme cmmn examples f human errr: Misreading antenna height measurements Transpsing numbers entered electrnically Rushing bservatins Pr centering and leveling ver pints Observing the wrng survey pint (fr example, bserving a reference mark instead f the actual mark itself) Incrrect equipment cnfiguratin settings State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember

6 6 GLOBAL POSITIONING SYSTEM (GPS) EQUIPMENT NDOT is currently using GPS equipment frm varius manufacturers such as Leica and Trimble and als varius generatins f ld and new. Therefre, yu will need t refer t the perating manuals n the actual setup and peratin f yur equipment. RECEIVERS RTK surveys utilize tw r mre receivers. One is used as a reference r base statin ver a cntrl mnument. The ther receiver r (rver) is mved frm pint t pint cllecting data. Additinal receivers can be emplyed t achieve better prductivity. BASE RADIO A radi r cellular link between the receiver and the rver is required. Sme receivers utilize an internal radi while thers need an external base radi t transmit t the rvers. The rver has a built-in radi t receive data frm the base. Mst external base radis include an antenna that munts n a standard tripd and are pwered by its wn battery. The rver units have a small whip antenna and d nt require an external pwer surce. Figure 6-2: Trimble R8 GPS DATA COLLECTOR The Data Cllectr is needed fr running the rver receiver. The Data Cllectr gives cntrl ver the survey and recrds data. It cmmunicates with the rver by either cable r by Bluetth. Mst Data Cllectrs wrk exclusively n their manufacturer s equipment and will nt cmmunicate with ther brands. Mst Data Cllectrs have an internal battery. Each manufacturer has its wn menu s and prcedures. Therefre, the peratr s manual must be referenced. 6-4 State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember 2017

7 GLOBAL POSITIONING SYSTEM (GPS) 6 SOFTWARE Each manufacturer has its wn versin f sftware fr dwnlading data frm the Data Cllectr and receiver. Data can then be manipulated frm yur persnal cmputer and transferred back t the Data Cllectr fr stakeut. MISCELLANEOUS Observe the fllwing recmmendatins and precautins: Use a fixed height rd fr bth the base and the rver t eliminate height f instrument mistakes. Use bi-pd leg attachments when perfrming calibratin shts. Check fish eye level bubbles frequently fr plumb. Always take cautin when winding up crds as they have glass encasements inside that if kinked will break and malfunctin. BATTERIES AND BATTERY CHARGERS Supply the electricity required t run GPS equipment. Plan ahead t keep yur batteries in the best cnditin and fully charged. Each supplier that delivers yur GPS units include chargers that crrespnd with that equipment. D nt mix and match chargers and/r cables nt meant fr each particular unit. The life f the batteries can be affected by temperature. During very cld cnditins, place hand warmers r ther suitable devices inside the base/radi case. Keep rver batteries in a heated truck r in an inside pcket t keep them warm until needed. Cnversely, when surveying in very ht cnditins, keep the equipment ff the direct surface f the grund by using a blanket and make sure t keep the equipment shaded as much as pssible. It may be necessary t adjust yur surveying times t reflect the clest pssible times. There is ne ther issue with batteries that might need t be mentined. This is the slw deteriratin that ccurs ver time. When battery life declines by half, get rid f the batteries and replace them with new nes. The lst time and maneuvering in the field t keep changing batteries is nt cst effective. EQUIPMENT MAINTENANCE At the beginning f any survey and at least every 6 mnths, all survey equipment shuld be checked and adjusted if needed. Checks and adjustments shall include but are nt limited the fllwing: Tripds - nuts and blts are tight, n lse r brken legs, tripd head is tight, flat, and nt damaged. Fixed Height Tripds - level bubbles are in adjustment, rd is nt bent r damaged, height f rd is crrect as reprtedly measured, and legs are secure. Rds - level bubbles are in adjustment, rd is nt bent r damaged, height f rd is crrect as reprtedly measured, and adjustable rd height clamps are secure. Tribrachs - ptical plummets are in adjustment, level bubble is in adjustment, n lse legs, n lse r missing screws, bttm head is flat and nt damaged. Cables - n cuts, breaks, pinch marks r damage. Receivers - n cracks r visible signs damage. Receiver Antennas - if equipped with a grund plane, it is nt bent r warped, n cracks r visible signs f damage. Fllw the manufacturer s recmmendatins n the care and strage f yur equipment. Stre the equipment in a secure area and d nt stre the equipment in a wet case. State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember

8 6 GLOBAL POSITIONING SYSTEM (GPS) PROCEDURES Prper planning is an imprtant step in an RTK survey. The first step in beginning a survey is t lcate cntrl mnuments in yur prject area. These can be fund in the cntract plans prject cntrl sheets (refer t Mnument Identificatin, in Chapter 2, fr mre infrmatin). Use at least six t eight cntrl pints fr a site calibratin (the minimum is three fr hrizntal and fur fr vertical). Place the calibratin pints arund the perimeter f the jb site. D nt survey utside f the area enclsed by the calibratin pints as the calibratin is nt valid beynd this perimeter. There are times f the day when the number f satellites available will vary. The psitins f the satellites at varius times f the day are als a factr. Planning yur wrk arund these times greatly increases prductivity and the quality f yur results. The selectin f the base statin sites will als affect the success f the RTK bservatins. The base shuld be situated in a lcatin that minimizes bstructins. A prblem at the base will affect all rvers. In general, a clear view f the sky abve 15 degrees is desired. At least five healthy satellites must be bserved and the PDOP shall nt exceed six during any GPS survey bservatins. Cnsider the fllwing when chsing a site: Sites shuld be free f vertical bstructins blcking the hrizn such as buildings, verhangs, terrain, trees, fences, utility ples, verhead lines, r any ther visible bstructins. Nn-bstructed skies 15 degrees abve the hrizn are best. Sites shuld nt be lcated clse t radi transmitters including cellular phne equipment, because they may disrupt satellite signal receptin. Sites clse t large flat surfaces such as signs, fences, glass, r utility bxes shuld be avided. If feasible, sites shuld nt be disturbed by future cnstructin activities and shuld be utside the design cnstructin limits and tp f cuts fr the cntract. PREPARING THE DATA COLLECTOR Set up the data cllectr fr an RTK survey. The methds will vary depending n the manufacturer. Therefre, the peratr s manual shuld be referenced. Trimble uses a feature called a survey style which is a template f settings fr different types f surveys. Each style cntains dzens f settings fr receivers, base and rver radis, etc. A feature cde library shuld be laded int the data cllectr. (Refer t file:\\datsrv1\017public\featurecdelibraries\sue fr mre infrmatin.) Set up a jb n the data cllectr. Enter the names, crdinates and rth elevatins f the cntrl pints that were selected. When naming pints, use different names fr Grid and WGS-84 crdinates with grid being keyed in frm the prject cntrl sheets and WGS-84 frm 3-minute field bservatins. Fr example, a pint named S use S fr grid crdinates and G S fr WGS-84 crdinates. BASE STATION SET- UP T set up the base statin fr RTK survey: 1. Select a lcatin ver a cntrl pint where there is a clear and unbstructed view f the sky and preferably this lcatin is higher than the area t be surveyed. 6-6 State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember 2017

9 GLOBAL POSITIONING SYSTEM (GPS) 6 2. Set the GPS receiver antenna ver the cntrl pint and face it t the nrth. GPS antennas shuld be set up ver the pints using fixed-height antenna tripds. When using standard tripds with a tribrach, the antenna slpe-height will be measured multiple times (per manufacturer's directins) and the average recrded. Figure 6-3: Base Statin Setup 3. Attach the GPS antenna cable t the GPS antenna and then int the GPS receiver. (In sme units, the receiver is built int the antenna. Therefre, this step is unnecessary). 4. Attach the data cable t the receiver. 5. Attach the battery t the base receiver. 6. Set up a tripd and place the radi antenna n it apprximately 20 feet away frm yur GPS antenna. 7. Plug the radi antenna cable int the antenna prt n the Trimmark 3 r ther radi. 8. Attach the pwer cable t the external radi battery and plug the ther end int the radi. 9. Plug ne end f the radi cable int the base receiver and then the ther int the radi. 10. Turn n the receiver. T start a Trimble base receiver: 1. Start a file in yur data cllectr by selecting Files frm the main menu. Select New Jb. Name yur jb and then tap crdinate system. Fur chices will cme up. Fr this time, select n prjectin/n datum. The screen will change and make sure the crdinates are selected Grund. Enter in the prject height (can be fund n the LPN sheet) and select Use Geid Mdel. Select the prper geid mdel and hit Stre. Make sure yur feature cde library is selected (refer t file:\\datsrv1\017public\featurecdelibraries\sue fr mre infrmatin) and that US survey feet is entered in the units categry. Enter in the LPN number and yur name and hit Accept. 2. Cnnect the data cllectr t the data cable already attached t the Trimble base receiver and select Survey. Then select the apprpriate RTK survey style. Then select START BASE RECEIVER. State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember

10 6 GLOBAL POSITIONING SYSTEM (GPS) 3. It will ask fr a pint number. Key in the pint name and cde. Select the Here buttn dwn n the bttm. This will give yu a general lcatin f yur base statin and allw yu t calibrate n the cntrl pints. 4. Discnnect data cable and key in Grid Crdinates and rth elevatins f the cntrl pints that were selected. Use the pint names frm the prject cntrl sheets. 5. Turn n the rver and wait fr it t initialize befre yu leave the base. CALIBRATION T btain a site calibratin: 1. Attach the bipd legs t yur rver rd (this is mandatry). 2. Face the antenna t the Nrth fr each measurement. 3. G t cntrl pints that are utside the limits f yur jb usually befre, after, in the middle and utside f the Right-f- Way if pssible. Yu want t try and surrund yur jbsite if pssible. Sht at least six t eight cntrl pints fr the calibratin and remember that yu shuld nly sht within 6 miles f yur base. Always remember t name the shts yu d in the calibratin the number that is in the LPN sheet with a G placed in frnt f it t keep the pints straight. 4. At the ccupied pint, face yur predetermined directin and place the rd tip in the divet, firmly step n the bipd feet t sink them int the grund, level up, select Start Survey, then wait fr the rver t initialize. Select Measure Pints and select Observed Cntrl Pint fr yur chice f sht type. 5. After the 3-minute sht, cllapse the legs and pick up and g t the ther mnuments fr their shts. 6. Once yu have cmpleted all f the shts yu want t d, yu may start the field calibratin in yur data cllectr. 7. Select Key in / Pints frm the main menu. Set the type field t Crdinates. Check that the crdinate fields are Nrth, East, and Elevatin. Enter in any cntrl pints that have nt been entered already. 8. Select the pint pairs that yu want t use fr the calibratin by selecting the LPN crdinates number and then yur survey shts with the same name and the G prefix. 9. Once yu select the pint pairs, evaluate the vertical and hrizntal residuals. Yu want t d the hrizntal first. Try t hld the furthest pints ut frm the jb t get the widest calibratin pssible. Tggle n/ff the hrizntal prtin f the selectins n each f the pairs t determine if yu get better residuals. Once yu get the hrizntal residual t be.03 r less, wrk n getting the vertical as lw as pssible. Once yu get bth as gd as yu can get them, fix the scale factr t 1 and then hit Apply. 10. Once the calibratin is cmplete and yu accept the results, never re-calibrate n this area as yu d nt want t change the relatinship between all f the cntrl pints and the subsequent pints yu sht. Multiple calibratins alng a radway shuld be cnnected at the ends by using ne r tw f the same cntrl pints in each f the assciated calibratins. This enables the tw calibrated surfaces t be held tgether at this pint and remves the pssibility f elevatin breaks that can plague sme cntracts. 11. Once yu have a calibrated site, yu shuld be able t set up yur base receiver n any f yur cntrl pints yu have in yur data cllectr. FIELD OBSERVATION RTK GPS surveying is similar t a Ttal Statin radial survey. The prtcls used in pint cllectin are the same in bth methds f surveying. Once the gear is set up and yu have started the base receiver, yu can initialize and start the survey. It is generally best t make sure yu have a radi signal frm the base, and the rver is initialized befre leaving the area. 6-8 State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember 2017

11 GLOBAL POSITIONING SYSTEM (GPS) 6 Measuring pints can take a cuple f different frms: Tp Pint methd is a sht that takes apprximately three secnds t take nce yu hit measure. This is a very accurate sht and shuld be taken when shting cncrete and/r plantmix bituminus surfaces, flw lines f pipe r drp inlets, and basically any time that gd elevatin is required. Cntinuus Tp is anther ptin where the vehicle r rd munted rver can take a sht in a selected time r distance interval. Rapid Pint has the least quality f the methds. It nly takes abut a secnd t take ne f these shts, and this methd shuld nly be used n crss sectins fr brrw pits and riginal surfaces n dirt. Stake ut. Once calibrated, the prject s alignment can be entered int the data cllectr r dwnladed frm a persnal cmputer in the ffice. Any pint n the alignment can be staked ut and ffset as lng as it is inside the calibrated area. FEATURE CODES Always remember t use the prper feature cde n yur pints. (Refer t file:\\datsrv1\017public\featurecdelibraries\sue fr mre infrmatin.) Imprper cding is the number ne prblem when trying t create breaklines and surfaces and can lead t cstly mistakes. Features shuld be sht sequentially whenever pssible t reduce the amunt f editing n breaklines in the ffice. TROUBLESHOOTING If yu are having truble getting yur base and rver t cmmunicate via radi link, cnsider the fllwing: Are bth the base and the rver set t CMR+ (in "Bradcast frmat" in the survey style n the cntrller)? Is the crrect antenna type in the survey style fr bth the base and the rver? Is the crrect radi type in the survey style fr bth the base and the rver? Is the pwer setting fr the base radi apprpriate fr the distance between the base and rver? Des the base radi frequency and wireless mde settings match n the base and the rver? Is the crdinate system apprpriate fr the regin? Is the base radi in the right bradcast mde? Check that all cables are crrect. Check batteries. Pwer the receivers and Data Cllectr ff and then back n and restart the base with the Data Cllectr. Smetimes a pwer cycle is all that is needed. NOTE: Different manufacturers may refer t these methds by different names s read thrugh the peratr s manual fr the crrect prcedures fr yur equipment. This guide was created with Trimble GPS equipment in mind. State f Nevada Department f Transprtatin Cnstructin Divisin Cnstructin Survey Manual Nvember

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