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FAQ and Tips

Chapter 9

 

 

 

9.7.4What is PPP used for?

PPP can be used for a number of different purposes, whether for static or kinematic data. It can be used as an alternative to differential processing. In other words, if the achieved accuracy is acceptable, you can use the computed trajectory as your final solution.

Alternatively, you may wish to use the PPP solution as an independent quality control tool for your differential solution. Photogrammetry users, for example, may find this approach useful in situations where the trajectory output at the camera marks from differential processing does not agree with the positions generated through the triangulation procedure. In such a case, the PPP solution can be used to verify whether or not the errors lie with the GPS trajectory.

9.7.5Who should use PPP?

PPP is a viable solution for any application where setting up a base station is either unfeasible or simply uneconomical. However, this does not mean it is suitable for all applications. You must first decide whether or not the accuracies produced through PPP are acceptable for your application. For LIDAR applications and certain scales of photogrammetry, PPP is unlikely to meet the accuracy requirements.

The key to success with PPP is convergence, which is reliant on uninterrupted carrier phase measurements from as many satellites as possible, conditions typically found during airborne acquisition. It is not for applications where numerous cycle slips will occur. When these conditions cannot be met, you should only plan to use PPP if you are able to remain static after re-acquisition in order for the solution to re-converge. Otherwise, you should be prepared to deal with the decreased accuracy associated with the convergence period.

Even when open conditions with continuous tracking are available, you should only use PPP if you are planning to acquire enough data to meet your accuracy needs. In other words, 30 minutes of static data, which can be enough in differential to achieve fixed ambiguities given a reasonable baseline length, should not be expected to provide the same level of accuracy in PPP. By the same token, airborne users should plan to acquire static data before take-off and after landing in order to ensure convergence occurs before the camera events begin recording.

9.7.6Are there any limitations to PPP?

Currently, PPP does not support data from any constellation other than GPS. Other measurements, such as those from GLONASS, will be ignored by the processor. Also, PPP is heavily reliant on the presence of precise ephemeris and clock files, meaning that same-day processing of your acquired data is not possible. The rapid ephemeris files are available from numerous sources with a latency of one day and have been found to produce insignificant differences when compared to processing with the final ephemeris, which is available at a latency of 8 days. However, the rapid high-rate (30-second) clock files, also produced with one day’s latency, are currently only known to be available from one source. In the event that their server is down for an extended period of time, you will be left to wait for 8 days until an alternate source becomes available. Note that, if urgent, you can choose to use the rapid clock file produced by IGS, available with one day latency, but that the corrections here will only be at five-minute intervals. Therefore, if possible, it is suggested that you instead wait for the 30-second file to become available.

You should also be wary of processing any data collected before GPS week 1300 (December 2004). The precise orbit and clock files produced prior to then were not on the same level of accuracy as they are currently. As such, the final results are unlikely to be as accurate as expected. Due to the long convergence time on the tropospheric bias determination, shorter baselines can have degraded accuracies. Be sure to use the multi-pass technique on these baselines. In addition, performing a 15-30 minute static initialization at the beginning and end of the mission can also be helpful.

9.8Common Inquiries

This section contains general information and instructions on how to perform some of the tasks that were not covered in previous sections.

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9.8.1How can I determine the quality of a final solution?

The best method to check the quality of the final solution is to analyze the plots. These can found under the Output menu. The following are some plots that might be useful:

Combined Separation Plot

This should be the one of the first plots to look at. It shows the difference between the forward and reverse solution. An ideal solution should have separation of zero as this indicates that the carrier phase ambiguities have been determined to be exactly the same value in both directions. This plot gives you a general idea of what kind of accuracy you are achieving. For a combined baseline in GrafNav Batch, this plot will always be positive as it shows maximum minus minimum value, and you may wish to also view the Combined RMS plot.

Float/Fixed Ambiguity Status Plot

This plot shows if the solution is float or fixed. Fixed integer ambiguities generally have better accuracies that are, usually less than 20 cm. Ideally, the plot should show two fixes (green), as this indicates a fix in both directions. Furthermore, the separation plot is the most meaningful. A fix in just one direction (cyan) is generally okay too, but it cannot be verified via the combined separation plot as well. Sometimes, a trajectory will lose its fixed status because the DD_DOP became too poor. In such a case, the separation may not be as badly affected as a loss of lock.

Quality Factor Plot

This plot shows the quality of the solution. There are five different quality factors. Increasing quality factors indicate a worse solution. This is not a perfect indication, but it can be useful to isolate problem areas. See Table 4 on

Page 107 for a description.

Estimated Position Accuracy Plot

This plot shows the predicted accuracy given satellite geometry, standard measurement accuracies, and prevalence of cycle slips. It does not account for multi-path or variations in receiver noise. For float solutions, it tends to be optimistic. For fixed integer solutions, it is generally realistic if the fix is correct.

9.8.2How do I copy user files?

User files such as Export Wizard profiles, Favourites stations and coordinates, and antenna, datum and grid definitions can be copied or backed-up. There are two ways to do this:

1.Click the Start button in Windows and navigating to Programs | Waypoint GPS | Utilities | Copy User Files. This utility runs automatically during installation of the software. You are prompted for the source directory.

2.Manually copy the files listed below. These files are copied by the Copy User Files utility, and represent those that should be copied if you choose to do so manually.

Not all the files are available.

User.prf Export Wizard profiles

User.fvt – From Favourites Manager

User.dtm – Datum definitions

User.grd – Grid definitions

Local.fav – Local Coordinate Favourites

Missplan.mpf – Cities defined in Mission Planner

User.dn1 Download Service Data user-defined file

User.adf – Antenna definitions

*.DefOpt – User-defined project/option settings

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9.8.3How do I update manufacturer files?

The pre-loaded datums, station coordinates (Favourites), download stations, grid definitions and IMU definitions will change from time-to-time. Waypoint periodically updates these values on its FTP site. Download these via Help | About in all of Waypoint’s software. Be sure to re-start the program so that any changes will take effect.

New manufacturer files may not be updated for older versions of the software that are now outside of the support period. This feature is only supported by newer versions.

9.8.4How do I produce local coordinates?

The three possibilities for producing local coordinates are the following:

1.Local Cartesian: Creates an orthogonal coordinate system, meaning the X, Y and Z axes are at right angles to each other, that can be used for further computations. This creates a coordinate system where the Z axis and the ellipsoidal height axis are parallel at a central point (or origin). As the observer moves from the origin, a point with the same ellipsoidal height as the origin will have a negative local Cartesian Z value. The advantage of this system is that it is easy to reproduce and is well suited for 3-D applications requiring further transformations. In the case of photogrammetry, such a system would not need an earth curvature correction applied to the image data. However, this is not a “mapping system” and would require an ultimate transformation to UTM, State Plane, and so on.

2.Local Coordinate Grid: In many cases, users wish to reproduce a localized system based on a number of points where coordinates are known in grid or geographic and the local system. The procedure solves for a geometric transformation between grid and local. The following transformations are possible:

1-parameter vertical height shift

2-parameter X and Y shift

3-parameter X, Y and Z shift

4-parameter similarity (or Helmert) transformation (affects only horizontal axes). This solves for scale, rotation and X/Y translation.

6-parameter affine transformation, which solves for scale in X, scale in Y, rotation, shear (or skew) and X/Y translation. Like the similarity, this only affects the horizontal axes

7-parameter similarity transformation, which solves for scale, rotation about X, Y and Z axes, and X, Y and Z translation.

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Such a system is highly dependent on the grid system that the transformation coordinates are based upon. This is because scale and convergence angle (difference between true and grid north) vary differently for each system. Therefore, it is important to match this intermediate grid system if possible. The default system used for internal computations is a Transverse Mercator system with a central meridian scale factor of 1.0 and central meridian and parallel passing roughly through the centre of the project (determined by averaging all of the points’ longitudes).

In the process of creating a local coordinate definition, an LDF file is generated, which can be copied to other computers or shared with other GrafNav users.

3.Scaling, Rotation and/or Translation of Existing Grid System: In some cases, you may just wish to slightly alter an existing grid system. One common technique used by surveyors is to divide the coordinates by the combined scale factor. This removes the map scale factor and applies the height scale factor, which creates true horizontal surface coordinates that will match an electronic distance measurement (EDM) device. Other applications include applying rotations in mine sites and translations to create coordinates that have one axis lined up with a certain linear feature.

9.8.5How do I define a local cartesian coordinate system?

Simply defined, a local Cartesian grid is a plane that is tangent to Earth at a user-defined origin. The orientation of the plane is also entirely dependent on your preference. See Figure 8 below.

Figure 8: Local Cartesian Plane

A grid can be created in any of Waypoint’s programs by performing the following steps:

Create a New Grid Definition

Select Tools | Grid/Map Projection | Define. The Define Grids window should appear, displaying a list of already existing grid definitions. Click the New button and give the grid an appropriate name. Select Local Cartesian as the grid type. Click the Next button.

Define the Origin of the Grid

In the Origin in Geographic Coordinates box, enter the known coordinates of the point where the plane is to be tangent. If the height is unknown, an approximate value can be used, although it should be noted that a value of zero is used in many applications.

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