General Guide to Static and Rapid-Static - Opti-cal …surveyequipment.com/PDFs/GPS500-General-Guide.pdfGeneral Guide to Static and Rapid-Static -2.0.0en 3 Introduction Overall planning

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General Guide to Static and Rapid-Static

GPS System 500

2 General Guide to Static and Rapid-Static -2.0.0en

Congratulations on your purchase of a newSystem GPS500 from Leica Geosystems.

System GPS500

3General Guide to Static and Rapid-Static -2.0.0en

Introduction

Overall planning for a GPS survey

Mission planning

Observation times and baseline lengths

Field observations

Importing the data to SKI-Pro

Deriving initial WGS 84 coordinates for one point

Data-processing parameters

Baseline selection - Strategy for computation

Interpreting the baseline results

Inspecting the logfile and comparing results

Storing the results

Adjustment, Transformation and output of results

Notes on single-frequency Static and Rapid Static measurements

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Contents

Contents

Introduction ................................................... 6

Overall planning for a GPS survey .............. 7Baseline length ............................................................ 7Temporary reference stations for Rapid Static surveys . 8Check the newly surveyed points ................................ 9Night versus day observations. Measuring long lines .. 10Observation schedule - .............................................. 10best times to observe ................................................ 10Consider the transformation to local coordinates ......... 11

Mission planning ......................................... 13GDOP - Geometric Dilution of Precision ..................... 13Selecting good windows for successful GPS surveying 13

Observation times and baseline lengths... 15

Field observations....................................... 17Reference site ........................................................... 17

Need for one known point in WGS 84 ............................. 18Observing new points ................................................ 19

Use the Stop and Go Indicator as a guide ...................... 19Fill out a field sheet .................................................... 20

Importing the data to SKI-Pro .................... 22Checking and editing during data transfer .................. 22Backing up raw data and projects .............................. 22

Deriving initial WGS 84 coordinates forone point ...................................................... 23

Data-processing parameters ...................... 24Cut-off angle ............................................................. 24Ephemeris ................................................................ 25Data used for processing ........................................... 25Fix ambiguities up to: ................................................. 26Rms threshold ........................................................... 26 Solution type ............................................................ 28Ionospheric model ..................................................... 28Use stochastic modelling ........................................... 29Frequency ................................................................. 29Tropospheric model ................................................... 29

Baseline selection - Strategy forcomputation ................................................. 30

5General Guide to Static and Rapid-Static -2.0.0en

Interpreting the baseline results ............... 32Baselines up to the limitation value............................. 33

Ambiguities resolved ...................................................... 33Ambiguities not resolved ................................................. 34

Baselines above the limitation value ........................... 34

Inspecting the logfile and comparingresults .......................................................... 35

Baselines up to the limitation value............................. 35Baselines above the limitation value ........................... 36Compare the logfile against the field sheets ............... 36Compare the results for double fixes .......................... 36

Storing the results....................................... 37

Contents, continued

Contents

Adjustment, Transformation and output ofresults .......................................................... 39

Notes on single-frequency static and rapidstatic measurements................................... 40


IntroductionAlthough this guide has been writtenspecifically for Leica GeosystemsGPS - System 500 and System 300,much of the information is of ageneral nature and applicable to allGPS surveying. Further informationmay be found in the variousguidelines contained in the System500 or System 300 documentationmaterial.

Introduction

Surveying with GPS has becomepopular due to the advantages ofaccuracy, speed, versatility andeconomy. The techniques employedare completely different however,from those of classical surveying.

Provided that certain basic rules arefollowed GPS surveying is relativelystraightforward and will produce goodresults. From a practical point of viewit is probably more important tounderstand the basic rules forplanning, observing and computingGPS surveys rather than to have adetailed theoretical knowledge of theGlobal Positioning System.

This guide outlines how to carry outStatic and Rapid Static GPS surveysand emphasizes those points towhich particular care has to be paid.

6

7 Overall planning for a GPS surveyGeneral Guide to Static and Rapid-Static-2.0.0en

Overall planning for a GPS survey

Rapid Static surveys feature shortobservation times. It is particularlyimportant for Rapid Static thationospheric disturbances are more orless identical for both sites.

Thus, for all GPS surveying, and forRapid Static in particular, it is soundpractice to minimize baseline lengths.

Baseline length

A GPS receiver measures theincoming phase of the satellitesignals to millimeter precision.However, as the satellite signalspropagate through space to earththey pass through and are affectedby the atmosphere. The atmosphereconsists of the ionosphere and thetroposphere. Disturbances in theatmosphere cause a degradation inthe accuracy of observations.

GPS surveying is a differentialmethod. A baseline is observed andcomputed between two receivers.When the two receivers observe thesame set of satellites simultaneously,most of the atmospheric effectscancel out. The shorter the baselinethe truer this will be, as the morelikely it is that the atmospherethrough which the signals pass to thetwo receivers will be identical.

8Overall planning for a GPS survey General Guide to Static and Rapid-Static-2.0.0en

Temporary reference stations for Rapid Static surveys

In terms of productivity and accuracy,it is much more advantageous tomeasure short baselines (e.g. 5km)from several temporary referencestations rather than trying to measurelong baselines (e.g. 15 km) from onecentral point.

As observation time and accuracyare mainly a function of baselinelength, it is highly recommended thatbaseline lengths should be kept to aminimum.

Depending on the area and numberof points to be surveyed by GPS, youshould consider establishing one ormore temporary reference stations.

Baselines radiating from a temporaryreference station can be severalkilometers in length. Remember,however, that it is advantageous tominimize baseline lengths. The tableon page 16 provides a guide tobaseline lengths and observationtimes.


Check the newly surveyed points

Depending on the accuracy required,the user should be prepared to checknewly surveyed points. This isparticularly important if observationtimes have been cut to a minimumand recommendations regardingGDOP ignored.

For a completely independent check:

Occupy a point a second time in adifferent window. This ensures thatthe set-up, the satellite constellat-ion, and the atmosphericconditions are different.

Close a traverse loop with abaseline from the last point to thestarting point.

Measure independent baselinesbetween points in networks

A partial check can be obtained byusing two reference stations insteadof one. You will then have two fixesfor each point but each will be basedon the same roving-receiverobservations and set-up.

In all types of survey work it is soundpractice to cross check using inde-pendent measurements. In classicalsurvey you check for inaccurate orwrong control points, wronginstrument orientation, incorrectinstrument and target heights, etc.You close traverses and level loops,you fix points twice, you measurecheck distances! Depending on thejob and accuracy needed it is wellworthwhile applying the sameprinciples to GPS surveying.

One should be particularly carefulwith Rapid Static with shortobservation times. If the observationtime is too short, or the satellitegeometry (GDOP) is poor, or theionospheric disturbances are verysevere, it can happen that the post-processing software will resolveambiguities but the results mayexceed the quoted specifications.


Observation schedule -best times to observe

For baselines up to about 20 km, onewill usually attempt to resolve theambiguities using the Rapid Staticalgorithm in SKI-Pro post-processingsoftware.

For baselines over 20 km, it isusually not advisable to resolveambiguities. In this case a differentpost-processing algorithm is used inSKI-Pro. This algorithm eliminatesionospheric influences to a largedegree but destroys the integernature of the ambiguities.

Night versus day observations. Measuring long lines

Generally speaking, the longer thebaseline the longer one has toobserve.

The ionosphere is activated by solarradiation. Thus ionosphericdisturbance is much more severe byday than by night. As a result, thebaseline range for night observationswith Rapid Static can be roughlydouble that of day observations. Or,put another way, observation timesfor a baseline can often be halved atnight.

At the present time ionosphericactivity is increasing in an 11-yearcycle.

The table on page 16 provides aguide to baseline lengths andobservation times under the currentionospheric conditions.

When you inspect the satellitesummary and GDOP plots, you willusually see several good windows(see page 14) distributed through a24 hour period. You should try towork with Rapid Static during goodwindows, and plan your schedulecarefully.

It is impossible to plan GPSobservations to the minute. Ratherthan trying to squeeze the maximumnumber of points into a window bycutting observation times to the bareminimum, it is usually better tomeasure one point less and toobserve for a few minutes longer.Particularly for high-accuracy work, itpays to be conservative and not torisk poor results.


Consider the transformation to local coordinates

The common points should bespread evenly throughout the projectarea. For a correct computation of alltransformation parameters (shifts,rotations, scale), at least three - butpreferably four or more - points haveto be used.

Read the Guidelines to Datum/Map inthe SKI-Pro Documentation fordetails on transformation usingDatum/ Map.

System 500 and System 300 provideaccurate relative positions of pointsthat are observed in a GPS networkand linked in post-processing. Thecoordinates are based on the WGS84 datum.

For most projects it will be necessaryto transform the WGS 84 coordinatesobtained from GPS survey into localgrid coordinates, i.e. into gridcoordinates on the local projectionbased on the local ellipsoid.

In order to be able to compute thistransformation, known points withlocal coordinates have to be includedin the GPS network. These commonpoints, with WGS 84 and localcoordinates, are used to determinethe transformation parameters and tocheck the consistency of the localsystem.


Temporary Reference Stations

In terms of productivity and accuracy, it is usuallypreferable to measure short baselines from severaltemporary reference stations rather than trying tomeasure long baselines from just one central point.

R-Temporary Reference Site

Example:

Establish 6 temporary reference stations using Static orRapid Static.

Check network of temporary reference stations usingdouble fixes or independent baselines.

Fix new points from temporary reference stations usingRapid-Static radial baselines.

Consider the need to check critical points.

Consider the transformation to local coordinates, continued

Overall Planning

ü Plan the campaign carefully

ü Consider the job, number of points, accuracy needed

ü Consider connection to existing control

ü Consider the transformation to local coordinates

ü Consider the best ways to observe and compute

ü For high accuracy, keep baselines as short as possible

ü Use temporary reference stations- Consider the need for independent checks:- Occupying points twice in different windows- Closing traverse loops

ü Measuring independent baselines between points

ü Consider using two reference stations

ü Use good windows

ü Consider observing long lines at night

ü For high-accuracy work, try not to squeeze themaximum number of points into a window

13 Mission planningGeneral Guide to Static and Rapid-Static-2.0.0en

Mission planning

Selecting good windows forsuccessful GPS surveying

Poor windows should only be used tobridge between two or more goodwindows when observing for longperiods of time, e.g. at referencestations and for long lines.

If there are obstructions near a point,use the sky plot to find out if thesignals from a satellite could beblocked. This could cause the GDOPto deteriorate. Check the GDOP byclicking the satellite "off" in theSurvey Design component. A carefulreconnaissance of such sites is wellworthwhile.

GDOP - Geometric Dilution ofPrecision

The GDOP value helps you to judgethe geometry of the satelliteconstellation. A low GDOP indicatesgood geometry. A high GDOP tellsyou that the satellite constellation ispoor. The better (lower) the GDOPthe more likely it is that you willachieve good results.

Poor satellite geometry can becompared with the "danger circle" ina classical resection. If the geometryis poor, the solution in post-processing will be weak.

For Rapid Static you should observewhen the GDOP is less than or equalto 8. A GDOP of 5 or lower is ideal.

For successful, high-accuracy GPSsurveying it is advisable to take theobservations in good windows.Provided that you know the latitudeand longitude to about 1°, thesatellite summary, GDOP, elevation,and sky-plot panels in the SurveyDesign component of SKI-Pro willhelp you to select good windows inwhich to observe.

You should take particular care whenselecting windows for Rapid Staticobservations.

A suitable observation window forRapid Static must have four or moresatellites, with GDOP ≤ 8, above acut-off angle of 15° at both thereference and roving receiver.

14Mission planning General Guide to Static and Rapid-Static-2.0.0en

Selecting good windows for successful GPS surveying, continued

Selecting Good Windows

Window for Rapid Static:

ü 4 or more satellites above 15° cut-off angle.

ü GDOP ≤ 8.

Whenever possible:

ü 5 or more satellites.

ü GDOP ≤ 5.

ü Satellites above 20°.

Always:

ü Use sky plot to check for obstructions.

ü Recompute GDOP if a satellite is obstructed.

ü Be wary if 2 out of 4 or 5 satellites are low (<20°).

Example:

Good window - GDOP low and stable Poor window - GDOP high

Avoid observing during this "spike"

15 Observation times and baseline lengthsGeneral Guide to Static and Rapid-Static-2.0.0en

Observation times and baseline lengthsUnless one is extremely restrictive, it is impossible to quote observation timesthat can be fully guaranteed. The following table provides a guide. It is basedon tests in mid-latitudes under the current levels of ionospheric disturbancewith a dual frequency Sensor.

Ionospheric activity is currently increasing to a high level in an 11-year cycle.As the activity increases it can be expected that observation times have to beincreased or baseline lengths reduced. Ionospheric activity is also a functionof position on the earth's surface. The influence is usually less in mid latitudesthan in polar and equatorial regions.

Note that signals from low-elevation satellites are more affected byatmospheric disturbance than those from high satellites. For RapidStatic observations, it can be worth increasing the observation times iftwo out of four or five satellites are low ( say < 20°).

The observation time required for anaccurate result in post-processingdepends on several factors: baselinelength, number of satellites, satellitegeometry (GDOP), ionosphere.

As you will only take Rapid Staticobservations when there are four ormore satellites with GDOP < 8, therequired observation time is mainly afunction of the baseline length andionospheric disturbance.

Ionospheric disturbance varies withtime and position on the earth'ssurface. As ionospheric disturbanceis much lower at night, night-observation times for Rapid Staticcan often be halved, or the baselinerange doubled. Thus it can beadvantageous to measure baselinesfrom about 20km to 30 km at night.

16Observation times and baseline lengths General Guide to Static and Rapid-Static-2.0.0en

Observation times and baseline lengths, continuedTimes and Baseline Lengths

Observation time depends upon:

• Baseline length• Number of satellites• Satellite geometry (GDOP)• Ionosphere

Ionospheric disturbance varies with time, day/night, month, year, positionon earth's surface.

The table provides an approximate guide to baseline lengths and observationtimes for mid latitudes under the current levels of ionospheric activity whenusing a dual frequency Sensor.

Obs.Method

No. sats.GDOP ≤ 8

BaselineLength

Approximate time

observation

By day By night

RapidStatic

4 or more 4 or more 5 or more

Up to 5 km 5 to 10 km

10 to 15 km

5 to 10 mins10 to 20 minsOver 20 mins

5 mins 5 to 10 mins 5 to 20 mins

Static 4 or more 4 or more

15 to 30 km Over 30 km

1 to 2 hours 2 to 3 hours

1 hour2 hours

17 Field observationsGeneral Guide to Static and Rapid-Static-2.0.0en

Note that the referencereceiver does not have to beset up on a known point. It isfar better to establishtemporary reference stationsat sites that fulfill therequirements listed abovethan to set up the referencereceiver on known points thatare not suitable for GPSobservations.

For computing the transformationfrom WGS 84 to the local system,known points with local coordinateshave to be included in the GPSnetwork. These points do not have tobe used as reference stations. Theycan be measured with the rovingreceiver.

Field observations

Reference site

GPS surveying is a differentialtechnique with baselines being"observed" and computed from thereference to the rover. As manybaselines will often be measuredfrom the same reference station, thechoice and reliability of referencestations are of particular importance.

Sites for reference stations should bechosen for their suitability for GPSobservations. A good site shouldhave the following characteristics:

• No obstructions above the 15° cut-off angle.

• No reflecting surfaces that couldcause multipath.

• Safe, away from traffic andpassers-by. Possible to leave thereceiver unattended.

• No powerful transmitters (radio,TV antennas, etc.) in the vicinity.

The results for all roving points willdepend on the performance of thereference receiver! Thus thereference receiver must operatereliably:

• Power supply must be ensured.Use a fully-charged battery.Consider connecting two batteries.When possible, consider atransformer connected to themains.

• Check that there is ample capacityleft in the memory device forstoring all observations.

• Double-check the antenna heightand offset.

• Make sure that the missionparameters (observation type,recording rate etc.) are correctlyset and match those of the rovingreceiver.

18Field observations General Guide to Static and Rapid-Static-2.0.0en

Need for one known point in WGS 84

The computation of a baseline indata processing requires that thecoordinates of one point (reference)are held fixed. The coordinates of theother point (rover) are computedrelative to the "fixed" point.

In order to avoid that the results areinfluenced by systematic errors, thecoordinates for the "fixed" point haveto be known to within about 20meters in the WGS 84 coordinatesystem. Whenever possible, theWGS 84 coordinates for the "fixed"point should be known to within about10 meters otherwise scale errors ofabout 1 to 2 ppm will be introduced.

This means that for any precise GPSsurvey the absolute coordinates ofone site in the network have to beknown in WGS 84 to about 10meters. WGS 84 coordinates for onesite will often be available or can beeasily derived as explained on page23.

If WGS 84 coordinates for one siteare not known or cannot be derived,the Single Point Position computationin SKI-Pro can be used. Remember,however, that Selective Availability(SA) may be switched on. The onlyway to overcome SA is to observe forsufficient time for the effects of SA tobe averaged out in the Single PointPosition computation.

The reference receiver will usuallyobserve for several hours as therover moves from point to point. Insuch a case, the Single Point Positi-on for the reference receivercomputed in SKI-Pro should berelatively free from the effects of SA.If a Single Point Position is computedfrom only a few minutes ofobservations, the effects of SelectiveAvailability will not be averaged out.The result could be wrong by 100mor more due to SA.

When computing the Single PointPosition for the starting point of anetwork, always compute for a sitefor which you have several hours ofobservations. The resulting WGS 84coordinates should then be correct towithin about 10 meters.

The minimum observation for thecomputation of a reliable Single PointPosition is probably about 2 to 3hours with four or more satellites andgood GDOP. The longer theobservation time, the better theSingle Point Position will be.


Observing new points Use the Stop and Go Indicator as a guide

As the Stop and Go Indicator canonly monitor the roving receiver it canonly provide an estimate for therequired measuring time. It should beused only as a guide.

The operator of the roving receivershould also pay attention to certainpoints. This is particularly importantfor Rapid Static surveys with shortmeasuring times.

• Make sure that the configurationparameters (e.g. recording rateetc.) are correctly set and matchthose of the reference receiver.

• Check the antenna height andoffset.

• Watch the GDOP when observingfor only a short time at a point.

• For 5 to 10mm + 1 ppm accuracywith Rapid Static, only takemeasurements with GDOP ≤ 8.

The Stop and Go Indicator on thesensor provides the roving-receiveroperator with an approximate guideto measuring times for Rapid Staticobservations with four or moresatellites and GDOP less than orequal to 8. It estimates whensufficient observations should havebeen taken for successful post-processing (ambiguity resolution) tobe possible.

At the present time estimates arecalculated for two baseline ranges,0 to 5 km and 5 to 10 km. Theestimates are based approximatelyon the current situation for GPSobservations in mid latitudes andassume that the reference and rovingreceiver are tracking the samesatellites.

20Field observations General Guide to Static and Rapid-Static-2.0.0en

Fill out a field sheet

Reference Stations

ü No obstructions above 15° cut-off angle.

ü No reflecting surfaces (multipath).

ü Safe, can leave equipment unattended.

ü No transmitters in vicinity.

ü Reliable power supply.

ü Ample memory capacity.

ü Correct configuration parameters (e.g. recording rate).

ü Check antenna height and offset.

ü Does not have to be a known point.

ü It is better to establish temporary reference stations atgood sites rather than at unsuitable known points.

For precise GPS surveying, WGS 84 coordinates for onepoint have to be known to about 10 meters.

Roving Receiver

ü 15° cut-off angle.

ü Obstructions should not block signals.

ü No reflecting surfaces (multipath).

ü No transmitters in vicinity.

ü Fully-charged battery.

ü Sufficient memory capacity.

ü Correct configuration parameters (e.g. data-recordingrate).

ü Check antenna height and offset.

ü Observe in good windows.

ü Watch the GDOP ≤ 8.

ü Use Stop and Go Indicator as a guide.

ü Fill out a field sheet.

As with all survey work, it is well worthwhile filling out afield sheet for each site when taking GPS observations.Field sheets facilitate checking and editing at the data-processing stage.


Fill out a field sheet, continued

Practical Hints

ü Tribrachs: check the bubble and optical plummet.

ü Level and center the tribrach and tripod correctly.

ü Check the height reading and antenna offset.

ü An error in height affects the entire solution!

ü Use a radio to maintain contact between reference androver.

ü Consider orienting the antennas for the most precisework.

Field Sheet

Point Id.: Date:

Receiver Serial No.: Operator:

Memory card No.:

Type of set up:

Height reading:

Time started tracking:

Time stopped tracking:

Number of epochs:

Number of satellites:

GDOP:

Navigation position: Lat. Long. Height

Notes:

22Importing the data to SKI-Pro General Guide to Static and Rapid-Static-2.0.0en

6X

7X

3X

5X

7X

Importing the data to SKI-Pro

Checking and editing duringdata transferData can be transferred to SKI-Prodirectly via a PC-card slot, or via acard reader, from the controller(System 300) or receiver (System500), or from a disk with backed-upraw data. During data transfer, theoperator has the opportunity to checkand edit certain data. It is particularlyadvisable to check the following:

• Point identification: Check spelling,upper and lower case letters,spaces etc.

• Make sure that points that havebeen observed twice have thesame point identification. Makesure that different points in thesame project have different pointidentifications.

• Height reading: Compare with fieldsheets.

Note that some of the abovesite-related parameters canbe changed in somecomponents of SKI-Pro.However, the affectedbaselines have then to berecomputed.

Backing up raw data andprojectsAfter reading in a data set alwaysmake a back-up on either a disketteor on the hard disk. You can thenerase and reuse the memory cardbut you still have the raw data. Whenbacking up data from severalmemory cards, it is advisable tocreate a directory for each card.

After importing all the data related tothe project it is often worthwhilemaking a backup of the wholedirectory where the project is locatedbefore starting to process the data.

6X

7X

13X

15X

17X

23 Importing the data to SKI-ProGeneral Guide to Static and Rapid-Static-2.0.0en

Deriving initial WGS 84 coordinates for one pointAs explained on page 18, thecomputation of a baseline requiresthat the coordinates of one point areheld fixed. The coordinates of theother point are computed relative tothe "fixed" point.

For any precise GPS survey theabsolute coordinates of ONE site inthe network have to be known inWGS 84 to about 10 meters. WGS84 coordinates for one site will oftenbe available or can be easily derived.

Using SKI-Pro it is easy to convertthe grid coordinates of a known pointto geodetic or Cartesian coordinateson the local ellipsoid. If theapproximate shifts between the localdatum and WGS 84 are known,WGS 84 coordinates to well withinthe required accuracy can bederived. The local Survey Depart-ment or University will usually be ableto provide approximatetransformation parameters.

As explained on page 17, thereference receiver does not have tobe on a known point. If the referencereceiver was on a new (unknown)point and a known point wasobserved with the roving receiver,simply compute the first baselinefrom the known point (rover) to theunknown point (reference) in order toobtain and store the required initialWGS 84 coordinates for thereference receiver.

If good initial WGS 84 coordinates forthe reference site are not known orcannot be derived as explained in thelast two paragraphs, the Single PointPosition computation in SKI-Pro canbe used. When using the SinglePoint Position computation alwayscompute for a site for which there areseveral hours of observations. Theeffects of Selective Availability shouldthen average out and the resultingWGS 84 coordinates should becorrect to within the required 10meters.

See section "Need for one knownpoint in WGS 84" on page 18 forfurther details.

Always keep in mind that poor initialcoordinates for the reference receiverwill affect the baseline computationand can lead to results outside thequoted specifications.

Deriving initial WGS 84 coordinates

24Data-processing pameters General Guide to Static and Rapid-Static-2.0.0en

Data-processing parameters

In the vast majority of cases, thedefault settings for data-processingmay be accepted and may never bealtered by the operator. On somerare occasions the operator mayneed to modify one or more of thedata processing parameters. Themost common ones are describedbelow.

Cut-off angle

It is common practice in GPSsurveying to set a 15° cut-off angle inthe receiver. 15° is also the systemdefault value in data processing.Avoid cut-off angles less than 15° ifprecise results are to be obtained.

Although you can increase the cut-offangle you should be cautious whendoing so. If the cut-off angle for dataprocessing is set higher than in thereceiver some observations will notbe used for the baseline computationand you may "lose" a satellite. Itcould happen that only threesatellites would be used in thecomputation instead of four. Youcannot expect a reliable answer withonly three satellites.

It can sometimes be advantageous,however, to increase the cut-off angleto about 20° in case of a disturbedionosphere and provided thatsufficient satellites above 20° withgood GDOP have been observed(use the Satellite Availabilitycomponent in SKI-Pro to check theGDOP).

You may sometimes find that abaseline result is outsidespecifications even though fivesatellites have been observed. If oneof the satellites never rises aboveabout 20° the observations to thissatellite may be badly affected by theionosphere. Raising the cut-off angleand computing with only four high-elevation satellites can sometimesproduce a better result.

25 Data-processing pametersGeneral Guide to Static and Rapid-Static-2.0.0en

Ephemeris

SKI-Pro uses the broadcastephemeris recorded in the receiver.This is standard practice throughoutthe world for all routine GPSsurveying. For standard GPS surveywork there is little to be gained byusing a precise ephemeris.

Data used for processing

For precise GPS surveying, one willnormally accept the system defaultsetting "Automatic", which will usuallyuse Code and Phase observations.

"Code only" can be used for the rapidcalculation of baselines when highaccuracy is not required, for instancein exploration or offshore work. If onlycode observations are evaluated theaccuracy cannot be better than about0.3m in position.

For the precise measurement ofbaselines it should make littledifference whether one processescode and phase measurementstogether or "Phase only". The resultsshould be more or less identical.

For long lines above about 100 km,code observations can assist a high-accuracy solution provided that theephemerides are sufficiently good.

If code measurements are corruptedfor some reason, one can processbaselines using "Phase only".

For processing kinematic data,"Automatic" has to be used forprecise results. "Code only" can beused if high accuracy is not required.


Fix ambiguities up to:

With this parameter you candetermine how SKI-Pro shouldcompute baselines. The systemdefault value is 20 km.

For baselines up to this limitationvalue, L1 and L2 measurements areintroduced as individual observationsinto the least-squares adjustment.The Lambda search developed byProf. Teunissen and his co-workersat the TU Delft is used as an efficientapproach to find possible candidatesets of integer ambiguities. Thestatistical decision criteria used hasbeen published previously togetherwith a different search algorithm, theFast Ambiguity Resolution Approach(FARA) by Dr. E. Frei and is nowcalled FARA statistics.

For baselines above this limitationvalue, a so-called L3 solution isperformed. The L3 observable is alinear combination of the L1 and L2measurements. The advantage of theL3 solution is that it eliminates theinfluence of the ionosphere.

However, it also destroys the integernature of the ambiguities, thereforeno ambiguity resolution can becarried out. This is not important,however, as successful ambiguityresolution over long distances is inany case hardly feasible.

Rms threshold

The Rms threshold is used tominimize the possibility of unreliablebaseline results.

During the computation of a baseline,the least-squares adjustmentcomputes the root mean square(rms) of a single-difference phaseobservation (i.e. the rms of unitweight). This value is compared withthe Rms threshold.

For most GPS surveying applicationsone will usually accept the systemdefault "Automatic". This willautomatically select an appropriaterms parameter depending on theduration of your occupation.

The rms of a single-difference phaseobservation is largely dependent onthe baseline length, observation time,and ionospheric disturbance.Ionospheric disturbance is less atnight.


Rms threshold, continued

The following table provides a very approximate guide to the rms of asingle difference that a user could expect:

If the rms of a single-difference observation exceeds the rms threshold,the baseline solution with fixed ambiguities will be rejected and only thefloat solution will be presented (ambiguities not resolved).

Note, that the advanced parameter "Use stochastic modelling"(see page 29) will additionally reduce the rms values of a singledifference.

For Rapid Static observations with up to 10 minutes of measurementtime, one should be cautious about increasing the rms threshold becausean unreasonably high rms value could lead to a weak solution beingaccepted.

For longer observation times - let ussay about 30 minutes or more - therms threshold can be set higherwithout undue risk.

Note that the rms thresholdapplies only to baselines up

to the limitation value (see page 26).For baselines above the limitationvalue ambiguity resolution is notattempted.

Distance Day Observation Night Observation

≤ 10 min > 10 min ≤ 10 min > 10 min

Up to 5 km < 10 mm < 10 mm < 10 mm < 10 mm

5 to 10 km < 15 mm < 25 mm < 10 mm < 15 mm

10 to 20 km < 15 mm < 40 mm < 10 mm < 15 mm


Solution type

The solution type parameter appliesto all baseline up to whichambiguities are attempted to be fixed(see page 26). If solution type "Stan-dard" is chosen, SKI-Pro will attemptto fix ambiguities and applyionospheric corrections as defined inthe parameter "Ionospheric model".

If solution type "Iono free fixed" ischosen then the baselinecomputation is done in two steps.First ambiguities are attempted to befixed, then in the second step anionospheric free solution is calculatedusing fixed L1 and L2 ambiguities.

The advantage of this approach isthat any ionospheric disturbance iseliminated while fixed ambiguities areused; it is recommended to choosethis solution type for all baselinesbetween 5 km and 20 km, inparticular if daylight observationshave been taken.

Ionospheric model

This parameter is only used forbaselines up to the limitation value(see page 26, "Fix ambiguity up to"),that is for baselines for which SKI-Pro will try to resolve ambiguities.

The default parameter is"Automatic", which will automaticallyselect the best possible choice. Ifsufficient observation time isavailable on the reference, this willbe the "Computed model". In anyother case the "Klobuchar model"will be taken provided that almanacdata is available. Typically there isno need to change the default.

A "Computed model" may be usedinstead of the standard model. Thisis computed using differences in theL1 and L2 signal as received on theground at the Sensor.

The advantage of using this model isthat it is calculated according toconditions prevalent at the time andposition of measurement. At least 45minutes of data is required for aComputed model to be used.

The Standard model is based on anempirical ionospheric behaviour andis a function of the hour angle of thesun. When the Standard model ischosen corrections are applied to allphase observations. The correctionsdepend on the hour angle of the sunat the time of measurement and theelevation of the satellites.

For long lines above the limitationvalue (see page 26), the ionosphericeffects are eliminated by evaluating alinear combination of L1 and L2measurements, the so-called L3observable. Ambiguity resolution isnot attempted.


Use stochastic modelling

Using this option may supportambiguity resolution on medium andlonger lines when you suspect theionosphere to be quite active.

You should, however, be careful withshorter baselines since bad data -e.g. data influenced by multipath orobstructions- may be misinterpretedas being influenced by ionosphericnoise.

This is why by default this setting isonly used for baselines longer than10 km.

Note that in order to ensurereliable results this option

will not be used for the processing ofkinematic data.

Frequency

SKI-Pro will automatically select toprocess whatever data is available.Thus there is little point in processingwith anything but "Automatic".

Short observation times with RapidStatic are only possible with dual-frequency observations. Long linescan only be processed successfullyusing L1 and L2 data.

Selecting "Iono free float" makesSKI-Pro compute an L3 solution evenif the baseline length remains underthe limit to fix ambiguities (see p.26).Remember, that for an L3 solutionthe observation time has to be longenough.

Tropospheric model

It will not make much difference tothe end result as to whether youselect the Hopfield or Saastamoinenmodel, but you should never workwith "No troposphere". You cannotexpect to achieve good results if notropospheric model is used.

30Data-processing pameters General Guide to Static and Rapid-Static-2.0.0enBaseline selection - Strategy for computation

Baseline selection - Strategy for computationBefore starting data processing oneshould consider carefully how best tocompute the network. Points to beconsidered include:

• Obtaining good initial WGS 84coordinates for one point.

• Connections to existing control.

• Computing the coordinates oftemporary reference stations.

• Rapid static measurements fromtemporary reference stations.

• Long lines.

• Short lines.

If more than one temporary-referencestation has been used, this "network"of temporary-reference stationsshould be computed first. This mayalso involve the connection toexisting control points. Select andcompute line by line, inspect theresults, and store the coordinates oftemporary reference stations if thebaseline computations are in order.

It is highly advisable to check thecoordinates for each temporary-reference station using double fixesor other means, as all radial rovingpoints depend on temporary-reference stations.

Once the "network" of temporary-reference stations has beencomputed, all remaining baselines -i.e. the radial baselines from thetemporary-reference stations toroving-receiver points - can becomputed.

If baselines of greatly differinglengths have to be computed, it canbe worthwhile making two or morebaseline selections and computationruns. In this way you can select andcompute batches of baselines whichfall into the same category ofparameter sets.

Try to avoid mixing baselines oftotally different lengths in the samecomputation run. And avoid mixingshort-observation "Rapid-Static"baselines with long-observation"Static" baselines.

31 Data-processing pametersGeneral Guide to Static and Rapid-Static-2.0.0en Baseline selection - Strategy for computation

Data Import and Computation

Check and edit during data transfer:

ü Point identification

ü Height reading and antenna offset

ü WGS 84 coordinates of initial point

ü Back up raw data and project

Baseline selection - Strategy for computation, continued

Consider the following carefully:

• How best to compute the network

• The need for good WGS 84 coordinates for one point

• Connection to existing control

• The need to transform to local coordinates

• Computation of network of temporary referencestations

• Computation of new points from temporary referencestations

• Long lines

• Short lines

• Data-processing parameters

32Data-processing pameters General Guide to Static and Rapid-Static-2.0.0enInterpreting the baseline results

When interpreting the results, onehas to distinguish between baselinesup to the limitation value ("Fixambiguities up to") and baselinesabove this value (see page 26).

For baselines up to the limitationvalue, ambiguity resolution using theLambda search and the FARAstatistics is always attempted.

Interpreting the baseline resultsFor baselines above the limitationvalue, a so-called L3 solution (linearcombination of L1 and L2measurements) is performed. Thiseliminates the ionospheric effects butdestroys the integer nature of theambiguities. Thus ambiguityresolution is not carried out.

33 Interpreting the baseline resultsGeneral Guide to Static and Rapid-Static-2.0.0en

Baselines up to the limitation value

These will usually be the "truevalues".

However, one should also be awarethat very severe ionosphericdisturbances can cause systematicbiases in the phase observations. Inthis case, although the results of theleast-squares adjustment will bestatistically correct, they could bebiased away from the true values.

The statistical methods implementedin FARA are based on very restrictivecriteria in order to try to ensure thehighest probability of a reliable result. When the ambiguities are resolved,you know that SKI-Pro has found a"most probable" solution with an rmsvalue that is significantly lower thanfor any other possible ambiguity set.

If the guidelines for baseline lengths,observation windows, number ofsatellites, GDOP, and observationtimes are followed (combinedperhaps with your own experience),the results of baselines for which theambiguities are resolved should bewithin the system specifications.

Nevertheless, as explained above, itis simply impossible to eliminatecompletely the possibility of theoccasional biased result.

Ambiguities resolved

For baselines up to 20 km (systemdefault for "Fix ambiguities up to"),ambiguity resolution should alwaysbe successful if good results are tobe achieved.

For baselines up to the limitationvalue, SKI-Pro searches for allpossible combinations of ambiguitiesand evaluates the rms of a single-difference observation for each set ofambiguities. It then compares the twosolutions with the lowest rms values.If there is a significant differencebetween the two rms values, theambiguity set yielding the lowest rmsvalue is considered as the correctone. This decision is based onstatistical methods.

The reader will realize, of course, thata least-squares adjustment can onlyprovide the "most probable" values.

34Interpreting the baseline results General Guide to Static and Rapid-Static-2.0.0en

Note that for baselines upto 20 km it should normallybe possible to resolve theambiguities provided thatsufficient observations havebeen taken (see page 15for a guide to baselinelengths and observationtimes). If the ambiguitiesare not resolved check therms values in the logfile(see next page).

Baselines above thelimitation valueAmbiguities not resolved

For baselines above the limitationvalue (system default = 20 km), SKI-Pro eliminates the ionospheric effectsbut does not attempt to resolveambiguities.

Thus the result will always show"Ambiguities not resolved" (Ambiguitystatus = no).

Note that there is usually nobenefit in trying to resolveambiguities for lines over20km.

As already explained, ambiguityresolution should always besuccessful for baselines up to 20 kmif good results are to be obtained.

If insufficient observations weretaken or the satellite constellationwas poor, SKI-Pro will not be able toresolve the ambiguities. If theambiguities are not resolved it ismost unlikely that the systemspecifications will be achieved.

If the ambiguities are not resolved inRapid Static (short observationtimes) it is difficult to give anindication of accuracy. However, as arough guide, one could multiply thesigma values for each estimatedcoordinate by 10 in order to obtain anapproximate estimate of the accuracyof the baseline computation.

35 Inspecting the logfile and comparing resultsGeneral Guide to Static and Rapid-Static-2.0.0en

Inspecting the logfile and comparing results

As explained in section "Rmsthreshold" (see page 26), if the rmsfloat exceeds the rms threshold, thebaseline solution with fixedambiguities will be rejected and onlythe float solution will be presented(ambiguities not resolved). Thus ifambiguities are resolved the rms floatand rms fix have to be lower than therms threshold.

The table on page 27 provides anapproximate guide to the rms values(float and fix) that can be expected.

If the rms threshold is lower than therms float or rms fix one can considermanually increasing the rmsthreshold value. However, asexplained on page 27, one shouldexercise a certain amount of cautionwhen doing this for Rapid Staticobservations with up to 10 minutes ofmeasurement time.

The reason is that this could allowunreasonably high rms float and fixvalues and could therefore lead to aweak solution being accepted.

Manually widening the rms thresholdvalue for successful baselinecomputation requires a certainamount of experience andjudgement.

If baselines of greatly differinglengths have to be computed, it isadvisable to make two or morecomputation runs. In this way youcan select and compute batches ofbaselines which fall into the samecategory of processing parametersets.

Baselines up to the limitation value

For baselines up to the limitationvalue, ambiguity resolution using theLambda search and the FARAstatistics is always attempted.

When you look at the logfile, you willfind a summary of the FARAstatistics at the end of each baselineoutput. You should check thefollowing:

• Number of satellites: there shouldalways be at least four.

• The rms float: this is the rms valuebefore fixing ambiguities.

• The rms fix: this is the rms valueafter fixing ambiguities. The rmsfix will usually be slightly higherthan the rms float.

36Inspecting the logfile and comparing results General Guide to Static and Rapid-Static-2.0.0en

Compare the results fordouble fixes

Baselines above thelimitation value

Compare the logfile againstthe field sheets

For baselines above the limitationvalue (system default = 20 km), SKI-Pro eliminates the ionospheric effectsbut does not attempt to resolveambiguities.

When inspecting the logfile check thefollowing:

• The number of satellites observed.

• The rms of unit weight

The rms of unit weight should be lessthan about 20 mm for lines of about20 km to 50 km. For lines over 50 kmthe rms of unit weight will usually behigher due to the minor inaccuraciesin the broadcast ephemeris.

If the results are not as good as youwould expect, it can be wellworthwhile comparing the informationin the logfile with that in the fieldsheets. Check if the number ofsatellites used in the baselinecomputation is the same as thatnoted in the field sheets. Rememberto check the reference station as wellas the rover. If the number of thesatellites is not the same, the GDOPvalues could be higher than youexpected. Check the actual GDOPfor the satellites used in thecomputation using the SatelliteAvailability component of SKI-Pro.

If a point was observed twice indifferent windows or two referencereceivers were operatingsimultaneously, you should comparethe resulting coordinates.

37 Storing the resultsGeneral Guide to Static and Rapid-Static-2.0.0en

Storing the results

After inspecting the summary ofresults and the logfile, store theresults that meet your accuracyrequirements.

The coordinates are averaged(weighted mean) if more than onesolution for a point is stored. Forinstance if you store the coordinatesfor point A from one baseline solutionand then you compute and store thecoordinates for point A again fromanother baseline solution, the storedcoordinates will be updated to theweighted mean values from the twosolutions. The weighted mean istaken provided the coordinates agreein both height and position to withinthe "Limits for Automatic CoordinateAveraging" set in SKI-Pro (default =0.075m).

It follows that you should exercise acertain amount of care when storingpoints that have been fixed in morethan one baseline computation.Compare the results before storing.

38Storing the results General Guide to Static and Rapid-Static-2.0.0en

Storing the results, continued

• Baselines above the limitation value(default = 20 km):

L3 solution, ambiguity resolution not attempted.

Results should meet specifications providedsufficient observations are taken.

Long lines need long observation times.

• Inspect double fixes, independent baselines etc.

• Store results that meet accuracy requirements.

• Coordinates averaged if more than one result stored.

Interpreting and Storing the Results

• For lines up to 20 km, ambiguity resolution should besuccessful if high-accuracy results are to be obtained.

• For long lines over 20 km, the L3 solution withoutambiguity resolution will normally be used.

• Baselines up to the limitation value(default = 20 km):

Ambiguity resolution always attempted.

Ambiguities resolved (Ambiguity status = yes):

SKI-Pro has found most probable solution.

Results should normally meet specifications.

Ambiguities not resolved (Ambiguity status = no):

Float solution presented.

Result outside specifications, inspect logfile.

Consider increasing the rms threshold andrecomputing.

39General Guide to Static and Rapid-Static-2.0.0en Adjustment, Transformation and output of results

6

7

13

15

17

22

23

24X

30X

32X

35X

37X

After the observations have beencomputed, you may wish to adjustthe results if multiple observations topoints exist. This provides the bestestimates for the position of thepoints. See SKI-Pro online help"Adjustment" for further details.

The results of the baselinecomputations are coordinates in theWGS 84 system. Using a"Coordinate System" in SKI-Pro,these coordinates can betransformed into coordinates in anylocal datum or grid system.

Adjustment, Transformation and output of results

40 General Guide to Static and Rapid-Static-2.0.0enAdjustment, Transformation and output of results

6

7

3

5

7

2

3

4X

0X

2X

5X

7X

9X

0

When measuring with the SR510(System 500) or SR9400 / SR261(System 300) there are severalpoints that should be noted in orderthat the measurements aresuccessful and good results can beobtained.

Only observation windows with aminimum of 5 satellites above 15°and a good GDOP (< 8) should beused.

The minimum observation time inStatic or Rapid Static should neverbe less than 15 minutes.

As a rule of thumb the baselineobservation time should be 5 minutesper kilometre of the baseline lengthwith a minimum time of 15 minutes.

Notes on single-frequency static and rapid static measurementsRecommended (minimum)observation times:

A Rapid Static observation canusually be considered to besuccessful when SKI-Pro can resolvethe ambiguities. Providing anestimate of the required observationtime is more difficult for singlefrequency receivers than for dualfrequency equipment as considerablyless information is available for thepost processing software. Never theless, the above table should serve asa guide.

By default, SKI-Pro will not attempt toresolve ambiguities if less than 9minutes of (rapid) static, single-frequency data is available. This isdone in order to avoid unreliableresults. Once the ambiguities areresolved correctly the length of thebaseline will normally be accurate toabout 5 - 10 mm plus 2 ppm. Thesedefault settings can be changed inthe Data Processing component ofSKI-Pro, but this is notrecommended.

Notes on single-frequency/rapid static measurements

Baseline-length Observation time

1 km 15 min.

2 km 15 min

3 km 15 min

4 km 20 min

5 km 25 min

6 km 30 min

7 km 35 min

8 km 40 min

9 km 45 min

10 km 50 min

> 10 km > 60 min

41General Guide to Static and Rapid-Static-2.0.0en Adjustment, Transformation and output of results

6

7

13

15

17

22

23

24X

30X

32X

35X

37X

Notes on single-frequency static and rapid static measurements, continued

Notes on single-frequency/rapid static measurements

If the highest possible accuracyshould be achieved it isrecommended to orient the antennasin a common direction.

On long baselines above 10 km theaccuracy which can be achieved withsingle frequency Sensors is inferiorto that which can be achieved withdual frequency Sensors due toionospheric effects which cannot beeliminated with single frequency data.Users who have previously workedwith dual frequency equipmentshould be aware of this fact.

Leica Geosystems AG, Heerbrugg,Switzerland, has been certified as beingequipped with a quality system whichmeets the International Standards ofQuality Management and QualitySystems (ISO standard 9001) and Envi-ronmental Management Systems (ISOstandard 14001).

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