GrafNet

3.1GrafNet Overview

GrafNet is a static network processing package that creates a single network by tying all the points of static GPS baselines together. Within minutes, GrafNet processes the entire project in a single operation. When the processing is completed, GrafNet color codes the baselines so irregular ones are isolated from the project and can be easily analyzed. GrafNet allows 3 types of static baseline processing solutions, including fixed static, float and ionospheric free.

This chapter tells you about the types of common networks, the different solutions and the methods to produce coordinates for each station in GrafNet. This chapter also describes how to get started with GrafNet, goes through each menu of its interface and provides step-by-step instructions for first-time users.

3.1.1Types of Networks

Closed Loop Network

Surveyors often use this style of network because of increased reliability. Due to the closing of the loops, any baseline determination errors will show up as tie point error. Such closure values can be seen via

Process | View Traverse Solution. If just two GPS receivers are employed, then a method called “leapfrogging” can be used to collect the data. In this procedure, starting from a known point, the lead receiver is placed on the first point to be surveyed. After the first session is complete, the trailing receiver is moved ahead of the lead receiver so that it now becomes the lead. The next baseline is observed and this procedure is repeated until small (4-6) loops are closed.

Radial Network

Also referred to as Single Base Station. Applications where productivity is more important, like GIS, do not need the same degree of reliability as the closed loop network. For these situations, use open loop networks. An example of this a network is in the shaded box. For this method, one receiver is left stationary over a reference or control point. One of more remote GPS receivers are moved from point to point being surveyed.

GPS Baseline

station

Figure 1: Closed Loop Network

Antenna height measurement errors will often cancel with this method and should therefore be double-checked. Methods involving more than two receivers become quite complex, and are past the scope of this chapter.

Figure 2: Radial Network

Table 11: Solution Types

Ionospheric correction is applied if the Iono Noise model is used, which depends on the baseline distance, or if you manually select the L2 Noise model. In such cases, the solution type would appear as L1L2-IonoFixed or L1L2-IonoNewFixed.

3.1.2Solution Types

GrafNet automatically determines concurrent sessions by examining the start and end times of the observation files. GrafNet tries to form sessions using an extended time scale to resolve any conflicts stemming from differing week numbers. This requires that a valid ephemeris file (EPP) be present for each corresponding observation file (GPB). For more than two receivers simultaneously collecting data, unwanted session connections can be ignored using the Session Menu, under Process | View All Sessions. The vector for each session is resolved independently. There are three modes of static processing includes the following:

Fixed Solution

This process uses single or dual frequency data, but assumes integer ambiguities. This mode delivers the best accuracies and is also the default static processing mode. Single frequency will fix reliably on baselines less than 10 km, while dual frequency will work well up to 25 km. Longer baselines will require longer observation times.

Float Solution

This method does not solve for integer ambiguities, and therefore, has no baseline length restrictions. Regardless, long single frequency baselines will have much poorer accuracies than their dual frequency iono-free counterparts. Normally, the float solution is only used when either fixed static solutions cannot be made to pass or for long single frequency baselines of 10 km or more.

Iono-free Solution

This is a float solution with the ionospheric effect largely removed from the carrier phase by combining L1 and L2 carrier phases in the iono-free combination. This option requires dual frequency data at both stations. Iono-free should normally be used if the fixed solution fails (on longer baselines), or on very long baselines (greater than 50km) where fixed static is unreliable.

GrafNet also offers an Automatic mode, which chooses between the three processing modes by examining baseline lengths and the type of measurements available. Table 11, in the shaded box, contains a list of solution types attainable in GrafNet.