External vs. Integral GPS

The location protocol beacons tested obtain their GPS location via two dissimilar means that offer various advantages and disadvantages. The operation of beacons using an external GPS source and those with an internal, or integral, GPS receiver are markedly different and not always directly comparable.

Beacons using an external GPS source have generally been less expensive than those with an internal GPS receiver. Especially for a consumer who already owns a GPS receiver and for one who uses a GPS for navigation purposes, this can result in a more economical total purchase cost with similar distress signaling benefits.

Having an external GPS source requires that the owner/operator connect the GPS to the beacon. All current beacons require a physical connection. The ACR external GPS beacons evaluated come with a proprietary infrared adapter that terminates in two bare wires. This adapter provides a waterproof connection to the beacon as there is no physical connection between the interior and exterior of the beacon case, but the wires must be connected to an adapter to fit the external GPS.

On some units by other manufacturers that were not evaluated, the adapter is not included and must be purchased. On some units the connection is a plug and receptacle that may not be waterproof, not via an infrared connector.

There is no universal standard for the GPS data output connector used for this purpose; sometimes not even within a GPS manufacturer's product line. As a result, the owner must acquire a GPS adapter from the manufacturer or elsewhere and either assemble their own interface cord or have one manufactured for them using the beacon's adapter. In most cases, the GPS adapter is not waterproof or submersible, although most appear to be very water-resistant. The interface cord for our evaluation was provided by the ACR Electronics representative attending the evaluation who had personally assembled it.

External GPS receivers have the potential, at least when compared to the current generation of GPS enhanced beacons, of being more capable of acquiring a location under difficult conditions. This can be attributed to a combination of any or all of better receiver, software, and antenna.

EPIRBs using an external GPS source in most marine installations use the boat's own generally very capable high-performance GPS receiver that is permanently connected to the EPIRB held in its storage bracket. In such installations the GPS is generally equipped with a high-performance external GPS antenna that provides much-improved reception compared to the internal antennas on a handheld GPS receiver or within an integral GPS beacon. In such an installation, the EPIRB is constantly being updated with the latest GPS location and would be expected to have received a GPS location from the GPS prior to being activated and deployed. This would not be the case for an EPIRB in an abandon-ship bag or in a life raft survival equipment pack.

In situations where a user of a beacon, typically a PLB in this case, is going to be entering an area or circumstances where they know that it is unlikely that a GPS location can be acquired, an area with heavy overhead canopy, a narrow canyon, or where they might fall into a crevasse, for example, they can load a location into the beacon beforehand, possibly providing a nearby location for SAR to work with and the attendant advantages when they might otherwise not be able to do so. Note, that while the beacons tested retain this location in memory forever or until the beacon is activated and then deactivated again, the COSPAS-SARSAT standard has been changed since they were originally approved and any recently approved model beacons discard the location after four hours.

A beacon and an external GPS generally represent a bulkier and weightier package than a single integrated device. With an external GPS beacon, the user must contend with two devices and a connecting cord, at least with existing beacon and GPS interfaces. Deployment and activation can be more difficult when having to handle multiple devices. The survivor must know how to switch on and properly orient the GPS for satellite reception. In a situation where the owner or normal operator of the beacon is incapacitated or unavailable, and the person operating the distress signaling gear is not familiar with the gear, these issues can become a bigger liability.

Beacons using external GPS do not update their position unless manually switched off and on again while a GPS is still connected. Typically, the user isn't aware of this as an option as it is usually only explained in the operator's manual. Experience suggests that many owners do not read the operator's manual or review it only cursorily. This is not a disadvantage for inland use where survivors will typically remain in the same location until rescue, or at least until contact is made with SAR. It is a potential deficiency in marine use, but as noted in the following discussion of integrated GPS beacon advantages, this is of practical use in only a very limited number of SAR scenarios.

The primary advantage of an integral GPS beacon is in the packaging—everything is self-contained. This is an advantage especially for PLBs where size and weight typically are major considerations.

Another advantage is that there is no need for a user to be familiar with the operation of the GPS receiver, or how to connect the two devices together. Simply activating the beacon also activates the GPS. In general, operation of the beacon is easy and self-evident to a degree, although most beacons examined by the authors do not do a very good job of instructing the user in optimum operation with regards gaining a GPS location.

Beacons with integral GPS are allowed to update their position every 20 minutes, though this isn't a requirement. This is of little advantage for inland use, but it is a potential advantage in maritime use in a minority of survival circumstances.

The value of the ability to update location under current search and rescue protocols used in the U.S. SAR community is limited, even in the maritime environment where movement due to wind, current, and waves is the norm. Current protocol is to provide the initial location to SAR forces who launch on that information. In the vast majority of circumstances in response to a GPS-enabled 406 MHz alert, SAR resources will arrive on scene within an hour or two. Unless a new location is significantly distant from the original, they will not be provided with it enroute. Typically, they find the survivor(s) within viewing distance of the original location as drift is generally slow enough that they will not have moved a significant distance in the interval, or SAR on scene can quickly determine the direction and speed of drift and can thereby locate the survivor(s).

If the survivor(s) is a person in the water in a PFD, rather than an easier-to-detect vessel or life raft, then the probability of detection is much lower and the value of updated location information becomes greater. If the SAR resource that first arrives on the scene does not promptly locate the survivors, they may contact their operations control and should receive an updated position at that time. Movement due to drift is more significant an issue in blue water conditions far from land, where the time to arrive on scene may be measured in hours or even days. Extreme conditions can also increase the rate of drift to a sometimes surprising degree. In such instances, updated location has the potential to be much more valuable.

It is also expected that the on-scene SAR resource will use their 121.5 MHz homing capability to locate the survivors upon arrival, if necessary. In some instances crews do not even turn on direction-finding equipment unless they fail to locate survivor(s) initially. The instances when this tool is useful with 406 MHz location protocol alerts are relatively few because the overall location accuracy is so good. However, in those cases where it is needed, there are a number of potential problems with this strategy, not the least of which being both the poor overall performance of 121.5 homing in some conditions, and the poor performance of some aircrews in use of existing direction-finding equipment to quickly locate 121.5 MHz transmission sources.

It is expected that the increased utilization of self-locating beacons will engender a change in strategy to provide enroute updates more readily and resultant potential for improved rescue response times in such scenarios. In addition, new direction-finding equipment has been introduced that directly receives and translates the location data, and provides improved directional guidance using the more robust 406 MHz data burst, which will provide on-scene communication directly from the beacon to SAR resources. This improved capability is just beginning to be put into service, but will eventually make its way into the majority of the SAR fleet. Once available, it will likely become a primary location tool as SAR resources approach the scene.

Integral GPS beacons are generally more expensive than those relying upon an external GPS. In many situations the ability of the internal GPS to acquire a location under poor reception conditions, at least that we have seen in current generation beacons, may not be as good as that available from a high quality external GPS. With packaging limitations, that may be the situation for some time to come, but it should not be considered inherently so.

Operation of the GPS receiver is a significant drain on the battery, as users of handheld GPS units have often discovered to their dismay when they have no spare batteries available. Manufacturers have developed proprietary operating schemes that minimize the operation of the GPS receiver, while at the same time theoretically providing adequate time to acquire a location. This is an effort to limit battery consumption, and thereby the size of the battery, which is a prime component that determines the overall size of the beacon, and to a lesser degree, the cost.

As COSPAS-SARSAT specifications allow the transmitted location to be updated no less than every 20 minutes, there is no need to operate the GPS continuously. Between operating periods, the GPS receiver is put into "sleep" mode to conserve power, waking up to check for location and, if necessary, update the location, every 20 minutes in the models tested. The initial operating period of the GPS receiver may be longer than subsequent periods, although not necessarily so, to allow additional time to acquire and download the ephemeris data and almanac.

Some industry observers have suggested that the difficulty some integral GPS beacons may have in acquiring a location could be related to an initial operating period that is too short. In the Key West Test report's summary (see Appendix 1), one suggestion is that "beacons be designed to try to acquire GPS locations for time periods of at least [15] minutes." The ACR beacon already complies with this suggestion. The Techtest beacon initially attempts to acquire for a total of four 5-minute periods alternating with 5-minute sleep periods. McMurdo has an initial period of 5 minutes duration. There is no way to determine solely by independent observation if the length of the initial GPS operational period is a contributor to any integral GPS beacon's location performance deficit in this evaluation.

A good argument can be made that the ideal self-locating beacon would offer both the option of using an external GPS when that is advantageous and would also have an internal GPS for situations when that is an advantage. Shortly after the completion of these field tests ACR Electronics announced the upcoming availability of a PLB that offers this capability.

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