CSEL stands for Combat Survivor Evader Locator and names a US communications and location system for militarily isolated personnel. Its handheld combines military GPS reception, protected satellite data messages, direct voice radio and distress or locating signals. The system also includes ground stations and recovery coordination workstations: they receive reports and can reply over the two-way data path.
GPS: determining your own position
The GPS receiver calculates its position from the satellites' navigation signals. These signals travel from satellite to device. A calculated position helps the isolated person navigate; it reaches the recovery centre only through a separate radio transmission. GPS reception and outgoing communications therefore use different functions within the same handheld.
Terrain, buildings or vegetation can obstruct reception; radio interference can also affect positioning. A position report therefore depends on whether the position used corresponds to the person's present location. A position obtained before the last movement refers to the earlier location. Before another report, the person must consider whether a new position fix is available or only the last known position.
Protected data messages to the recovery centre
OTH means Over the Horizon. A CSEL data message using this path travels from the handheld through a communications satellite to a ground station. A military data connection then forwards it to the responsible recovery centre. A reply follows the reverse path. A recovery aircraft does not yet have to be within direct radio range of the isolated person for this connection. It requires a usable satellite path, an operating ground station and onward routing to the centre.
The radio combines position data with preset status messages. Reports of an injury or an enemy nearby give the recipient information that a coordinate alone cannot provide: an injury may hinder movement to a pickup, while enemy proximity may endanger the approach of recovery forces. The person must select a message whose meaning matches the actual situation. The centre can reply over the return path and coordinate further contact.
The isolated person reads an incoming reply to establish which information it confirms and which instruction it contains. An instruction for further coordination has a different meaning from a report that forces are actually on their way.
The OTH data path is encrypted. The property known as Low Probability of Exploitation aims to make hostile exploitation more difficult. Encryption concerns message content; radio emission remains a technical event that an adversary may be able to detect.
Direct contact and locating in the pickup area
LOS stands for Line of Sight. Direct voice radio provides contact with reachable recovery forces or other compatible handhelds. It requires a suitable radio path between the participating antennas; a terrain ridge can mask that path. Conventional CSEL LOS voice is unencrypted. Someone speaking their location, intended movement or other situation information over it also sends those details in intelligible speech to a possible unwanted listener.
TAC, Terminal Area Communications, adds another direct connection. With this upgrade, the isolated person and a suitably equipped air platform exchange protected text messages over LOS. Compatible handheld software and the corresponding interrogator in the aircraft must work together. TAC reaches that air platform directly; the protected OTH data path instead reaches the recovery centre through the satellite and ground network.
TAG, Terminal Area Guidance, supports the local approach. The aircraft interrogates the handheld and uses onboard equipment to obtain range and bearing to the isolated person. These measurements relate to the connection between the search platform and the handheld. They serve a different purpose from a GPS coordinate in a situation report: the crew receives direction and distance for its approach. This also requires the appropriate aircraft equipment and a handheld with the corresponding function.
Beacon and SARSAT: a locating target and an open distress alert
A beacon mode transmits a locating signal to which a suitably equipped search or recovery aircraft can respond. The conventional CSEL locating signal requires a compatible receiving and locating system. The signal gives the search platform a homing target; the person does not send a two-way situation report through it. The conventional beacon mode is unencrypted, as is conventional LOS voice.
The SARSAT function is a separate, unprotected distress-alert path. Alert and location data pass through the international Cospas-Sarsat system to ground stations and mission control centres, which forward them to the responsible search and rescue authorities. These recipients can include civilian rescue organisations. The conventional CSEL SARSAT path is one-way; the person does not use it for a protected dialogue with the military recovery centre.
Conventional distress beacons are intended to reach rescue authorities that do not have access to the protected CSEL military network. This use is primarily intended for friendly territory or a situation without an immediate threat of capture. Under hostile threat, a decision to use the open distress alert concerns both the recipients it can reach and the disclosure of a locating target. Switching from protected data to an open beacon therefore changes the type of contact and the detection risk.
Preparation and the user's own operating decisions
The handheld is prepared for its mission before use. A CSEL Planning Computer loads the required identification, communications and navigation data through a radio adapter. This prepares, for example, the person's identification, the radio connections to be used and planned waypoints. A powered radio with GPS reception can still contain unsuitable mission data. Preparation therefore includes checking the loaded data against the mission, available CSEL infrastructure and intended recovery platforms in the area of operations.
Preparation also includes checking the battery and state of charge, inspecting for visible damage and carrying out the specified functional tests on the radio actually carried. The person must also distinguish its modes and be able to find the status messages intended for their situation.
In isolation, use of an available, prepared CSEL radio starts with the person's situation: which position is available, which status message fits, and which planned communications path has a reachable counterpart? The next information comes from an actual incoming reply. If data contact fails, a usable GPS position can still support personal navigation. Planned alternatives must be assessed by their actual availability and protection. Open voice or a distress beacon may reach different recipients and disclose different information from the protected CSEL data path.