Sect_4.3.2

4.3.2 GPS Surveying Software:

C0MMENTS ON PLANNING SOFTWARE

It is generally this software component that most impresses, as it makes full use of colourful "menus" and dazzling graphics. It is necessary to distinguish between the comparatively simple satellite availability-visibility software, and the software able to support pre-analysis of a GPS observation session, or of an entire campaign.

The following are some comments on mission planning software:

The software can generate some, or all, of the following:
- graphs of satellite availability-visibility at a site,
- graphs of rise-set times of GPS satellites,
- graphs of GDOP (and other DOP factors),
- graphs of azimuth-elevations of GPS satellites,
- satellite groundtrack plots, and
- skyplots.
A flexible user interface to permit rapid recomputation when the scenario is varied (time of day, satellites to be excluded, site location, etc.). All output is directed, in the first instance, to the screen, with hardcopy output always possible. Such packages are also available these days on the Web.
This data is generated from stored satellite ephemeris or almanac data, hence there are a number of options for inputting this information. The simplest is to install the planning software on the same computer as the data processing software. An automatic procedure would ensure that whenever real data files (phase data, Navigation Message, satellite health and almanac information, etc.) are decoded, a check is made and, if necessary, the computer-resident ephemeris-health-availability information is updated. In general, commercial software requires proprietary file format input for the satellite ephemerides. Some software permit alternative input modes, such as the NASA bulletin data, approximate orbital elements, RINEX files, Yuma almanac files, precise ephemerides or other formats.
The satellite ephemerides need not be very accurate. For high altitude satellites such as GPS the orbits are very predictable, and hence almanacs up to several months old can still be used. (However one should always guard against satellites being withdrawn from service or moved into other orbital positions.)
Planning data, such as that listed above, is specific to a site. However, changes to the coordinates of the site of quite large magnitude (>100km) do not noticeably affect a skyplot, or a satellite rise-set graph. Hence the planning software need only be executed for one centrally located site within the network of points to be surveyed.
Because the GPS orbits are synchronous with sidereal time, the information is valid day after day, but for an approximate time shift four minutes earlier each day.
Pre-analysis software is analogous to network optimisation software used for planning conventional control surveys. In the first instance, an observation scenario is tested to estimate the internal precision ("noise-only") of a GPS survey on a single-session basis. Then the quality of the propagated GPS survey can be tested by considering the manner in which the independent and redundant baselines combine into a network. Such software is generally not provided by GPS manufacturers.
Pre-analysis software to test the external accuracy of a GPS survey is a subsequent step, which makes use of the coordinate information (and its quality) of selected GPS stations from other sources. Generally the geodetic coordinates of some stations are known in the local datum, and as they are often held fixed when GPS results are transformed into the local datum, they can be considered external constraint information. The (simulated) distortion of the GPS-only network to the external control can then be studied. Such software is generally not provided by GPS manufacturers.
Pre-analysis software that takes into account systematic errors in the residual biases (for example, orbits, atmospheric refraction) is not generally available.

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