GPS Navigation in Land Search

 GPS Navigation in Land Search

Donald C. Bower
Kenneth A. Hill

Waverley Ground Search and Rescue
Nova Scotia, Canada

Many of the events critical to the success of the land search mission involve determining the location of search resources.

Keeping accurate records of searchers’ positions has more than a logistical significance; it bears directly on the accuracy of probability of detection (POD) estimates, and is therefore fundamental to search planning.

For this reason, considerable effort is devoted to navigational chores, and a significant proportion of radio time is spent determining the locations of searchers in the field. Unfortunately, even the most skilled compassman is not always able to specify his or her exact location at any given time. The continuous monitoring of position (dead reckoning), especially at night, is a full-time job involving constant reference to map, compass, and terrain as well as counting paces.

Consequently, most land navigators resort to periodic updating through triangulation or other methods. Normally, this loss of precision may not be a problem, but there are occasions on which it may be hazardous to the lost person’s health.

Consider, for example, the following:

  • A driver transporting searchers to a segment turns on the wrong road and unknowingly delivers them to the wrong woods. The searchers complete their “task,” return to the search base, and report POD estimates for a segment that hasn’t been searched.
  • A compassman fails to compute declination into his bearing and leads his grid team in a direction that is not parallel to the sweep of a neighboring team, resulting in a wide gap in coverage.
  • An inexperienced team leader gets “turned around” and requests assistance from the command post in getting reoriented.
  • A footprint found earlier in the search and discounted at that time suddenly becomes significant. However, records yield only an approximate location of the clue.
  • Late at night the injured victim is located by a two-person team following a contour around a lake. Unfortunately, the team leader cannot specify her precise location and it takes rescuers over an hour to reach the scene.

These are not uncommon occurrences and most experienced search responders will likely find them familiar. In this paper we shall describe a means for reducing navigational errors and simplifying procedures for communicating position.

The Global Positioning System

The Global Positioning System (GPS) allows for quick and accurate identification of one’s position anywhere in the world.

The GPS unit, which may vary in size from that of a CD disc player to a handheld calculator, is a radio receiver that “listens” to microwave transmissions emitted by Navstar satellites circling the earth at 11,000 miles altitude (see Figure 1).

Encoded within the message it receives is the precise location of each satellite. Since the receiver “knows” the travel time of a radio signal, it can calculate the distance to a satellite, and with the reception of signals from at least three satellites it can instantly determine its own position on the Earth’s surface (with a fourth satellite it can also determine its elevation with respect to sea level).

As there are presently 21 such satellites in orbit, with more scheduled for launching, the GPS receiver does not normally have difficulty receiving enough signals to calculate a fix on its location. Because of its reliability and accuracy, GPS has become very popular with pilots and ocean navigators, as well as surveyors and geologists.

The future of GPS was ensured by its success in the Gulf War, in which the U.S. and French military applied it extensively during air and ground missions.

[Figure 1 is missing in this text version]

GPS is expected to replace Loran-C for most navigational uses by the year 2001, if not sooner (Kunzig, 1988).

Unfortunately, GPS receivers are considerably more expensive than Loran-C units, with GPS prices on the low end at about $2,000.

However, prices are expected to drop significantly over the next several years, and possibilities of leasing may ease the financial burden of adapting GPS technology to SAR. Even the less costly units display latitude and longitude to fractions of a second, and are accurate to approximately 60-100 meters when the receiver is in motion (more precision is possible when the receiver is stationery for a period of time).

While GPS is capable of 15-meter position accuracy, the U.S. Department of Defense has implemented a policy of “selective availability”, which introduces error into civilian units so that they cannot be used for military purposes (see Nordwall, 1988). However, studies have shown that even a 100-meter error is tolerable for most navigational functions.

In addition to determining geographical position, most GPS receivers can also calculate their direction of travel and display it in degrees, and can be programmed to emit a warning sound when they deviate from a particular course, or when the user is approaching hazardous areas, such as cliffs or mine shafts.

It accomplishes these feats by retaining in memory specific coordinates or “waypoints” which it can continuously reference as new location information is processed. The receiver can automatically compute what bearing will be necessary to intersect a particular waypoint (in the case of using the receiver to stay on course), or it can be programmed to “beep” when the user approaches within (say) 100 yards of a hazard.

Indeed, one important waypoint for SAR personnel could be the position of an injured subject, as communicated to rescuers from a search team. The rescue unit could enter the coordinates in a receiver, which then would display the correct compass bearing to reach the subject, as well as signaling when the unit was within voice contact of the search team.

GPS in the Woods

Members of the Waverley Ground Search and Rescue Team have been working on the possibility of applying GPS technology to land search operations. In so doing, we have identified a number of issues which require addressing before GPS can be usefully employed. One concern is the possibility that heavy forest canopy might adversely affect radio signals, due to the occasional low-angle position of satellites. While brief testing with a borrowed unit under a moderate canopy has somewhat allayed this concern, research is required to determine whether GPS accuracy is affected by wet, heavy foliage or other possible sources of interference, such as narrow ravines.

A more general issue is the manner in which GPS would be applied during the land search. At the simplest level, GPS receivers could be just another navigational aid for searchers in the field, much like the compass, with the added benefit of warning its user when he or she is deviating from a bearing or a predetermined sequence of waypoints.

However, we envision a much more comprehensive application of GPS, with full integration into search management. In this view, each team in the field would be equipped with a GPS unit interfaced with a portable radio.

Upon request from the search command post, either verbally or electronically, the field unit would transmit its location to the base station, which would then relay the information via modem to a computer. The computer would graphically display the current locations of all search resources as well as record all past positions for each resource. This latter function should be extremely useful for POD estimates and other search planning functions, as well as post-mission analysis.

Figure 2 is missing in this text version


The potential benefits of adapting GPS technology to land search will accrue to searchers concerned with navigational accuracy, rescuers wanting to reach and evacuate a victim in the shortest possible time, search coordinators needing to monitor the “big picture” with respect to resources, and, most importantly, the lost or injured subject. As both the size and price of GPS receivers diminish, SAR responders can reasonably expect to implement this important technological development in the land search mission of the near future.

Suggested Readings

Bower, D. C. “GPS and the Land Search.” SAR Scene, October, 1991, p. 6.

Henderson, B. W. “Ground Forces Rely on GPS To Navigate Desert Terrain.” Aviation Week & Space Technology, February 11, 1991, pp. 77-78.

Kiernan, V. “Guidance From Above in the Gulf War.” Science, March 1, 1991, pp. 1012-1014.

Kunzig, R. “Knowing Your Place.” Discover, November, 1988, p. 86.

Nordwall, B. D. “Civilian GPS Users Fear Pentagon’s Ability to Degrade System Accuracy.” Aviation Week & Space Technology, October 24, 1988, pp. 69-71.

West, G. “The Global Positioning System On the Air!”, Radioscan, August, 1991, pp. 22-26.

The authors:

Donald C. Bower is a technology specialist with Maritime Tel & Tel, a ham operator (VE1 AMC), and Past Chief Director for Waverley Ground Search and Rescue.

Dr. Kenneth A. Hill is Professor of Psychology at Saint Mary’s University in Halifax, Nova Scotia, Search Director for Waverley Ground Search and Rescue, a ham operator (VE1 ICS), and a member of NASAR’s Board of Directors.

[Figure 1 caption]
Navstar satellites at 11,000 miles altitude broadcast their position to a GPS receiver on land. The receiver triangulates its location and displays it in longitude and latitude and/or UTM coordinates. With a fourth satellite in view, the receiver can also determine its altitude.

[Photo Caption]
SAR navigation can be a time-consuming chore, requiring consultation, constant reference to maps, and considerable time on the radio – all of which may detract from the process of searching.

[NOTE: This is a plain text version of the original article, which appeared in RESPONSE Magazine, 1992, vol. 11]