GPFMS System Overview

The ATM sensor was originally developed to measure changes over vast expanses of featureless Arctic ice cover. While many other applications for the ATM have arisen over the years, most of these applications share a requirement with the original one - the need to steer the aircraft, and the sensor swath swept along beneath it, on a precisely-defined path. ATM has mainly been used for change-detection, so typically it is used first to make a baseline measurement, then make a repeat measurement in the same location years later. Thus it is critical that new measurements be geographically consistent with past measurements.

This requirement originally centered around the need to re-occupy long straight survey swaths stretching for as much as hundreds of miles across the Greenland ice sheet. The ATM's swath at that time was usually about 200 m in width, with the accuracy of the individual measurements better at the center of the swath than near the edges. Thus in order to measure ice surface change with high confidence, it was necessary that the swath of a new measurement set overlap the center of an old swath as much as possible. This translated into a cross-track navigation accuracy requirement of a few 10s of meters or better.

Fortunately in the early 1990s the GPS constellation was becoming operational, and with some advance planning to avoid times of poor coverage it could be relied upon to provide the needed real-time positioning accuracy. But how could the GPS signal be utilized to steer a large aircraft (NASA's P-3B Orion) so precisely? The NASA-developed CDI system provided the answer. CDI proved to be a reliable tool which enabled the ATM project to conduct the first island-wide multi-year surface change survey of the Greenland ice sheet.

GPFMS rack aboard the NASA P-3.

As the results of the Greenland change surveys came in, it gradually became clear that most of the change occurring on the ice sheet was happening around the edges, and especially along dozens of sinuous and relatively narrow outlet glaciers. Simultaneously, new applications for the ATM emerged. These included mapping of coastlines and of irregularly-shaped land features. The CDI system, geared toward steering an aircraft along straight lines dozens or hundreds of miles long, was not easily applicable to these kinds of surveys, nor was it ideal for steering an aircraft down a steep, winding glacier. Note that the "straight-line" paths we refer to here are actually segments of great-circle routes, and are treated as such by all of our navigation techniques.

In response to these emerging problems, NASA developed a second, and complementary, navigation technique for the ATM project. This system eventually became known as Soxmap. While both CDI and Soxmap provide visual cues to help a pilot steer an airplane in order to meet its science goals, Soxmap is more suitable for non-straight survey lines and for area mapping, while CDI provides better repeatability and more automation for straight-line surveys. The ATM team often employs the two systems in tandem, with the navigation operator simply switching the flight crew's display between the two systems, selecting whichever system is most appropriate for a given situation.

Combined GPS/GPFMS rack being installed aboard a commercial Twin Otter.

The ATM project, depending on the application, either utilizes Soxmap as a standalone navigation system (usually on smaller aircraft such as Twin Otters), or packages CDI and Soxmap together in a single equipment rack and uses the two systems as complements to each other (most often on larger aircraft such as P-3s). In whichever form, the ATM's navigation hardware has become known as the "GPFMS" system. The origins of this name or acronym are unclear. FMS refers to a flight management system, and GP perhaps devolved from the GPS basis of the navigation techniques. At any rate, the term "GPFMS" somehow circulated among NASA's airborne science community and this generic navigation capability developed under the ATM project now carries the label GPFMS.

The components of the GPFMS system include an equipment rack populated with one or more computers and ancillary equipment which might include an uninterruptible power supply (UPS), signal generator and GPS receivers. It also includes a remote display for the flight crew, and in some cases an electronic interface between the GPFMS rack and the aircraft's instrument landing system, or ILS.