SECURITY SYSTEMS AND METHODS RELATING TO TRAVELLING VEHICLES Field of the Invention The present invention relates to security systems and methods, and more particularly, to security methods for identifying, at the moment of verification, the situation of a threat from a land vehicle, a flying aircraft or a sea-going vessel. The invention also concerns a method for verifying the identity of the driver of the vehicle, the pilot flying the aircraft or the captain of a sea-going vessel, by comparison with the information stored in a control center, e.g., a pre-flight schedule. Background of the Invention Access control, both on the ground and in the air, has become a high priority issue dealt with by many, with a view to improving existing security systems and methods. While basically, the use of technological means for assisting in determination of authorization to enter a protected premises or air space is known, methods of employing same for remote identification and authentication are more problematic. This is amplified many-fold where identification and authorization to access ground and/or air space should be carried out on-line and without delay, e.g., in the case of an aircraft approaching the protected airspace. Disclosure of the Invention It is therefore a broad object of the present invention is to provide a method for identifying the situation of a threat to a protected ground and/or air space by an approaching vehicle, aircraft or sea-going vessel. It is a further object of the present invention to provide a method for verifying the identification of the driver of the vehicle, the pilot actually flying the aircraft and/or a captain of a sea-going vessel. It is a still further object of the present invention to protect a ground and/or air space by reducing the danger that a threatened driver, pilot or captain will unintentionally divulge the code granting permission to access the protected space. It is a further object of the invention to provide a method for the protection of ground, airspace and/or off-shore sea limits against unauthorized intruders, without the persons threatening the driver, pilot or captain being aware that their scheme has been exposed. The term "threat" as used herein is meant to define a situation where the entry or arrival of a ground, air vehicle or sea-going vessel at its destination may endanger the protected space. In accordance with the invention, there is therefore provided a method for identifying, at the moment of verification, the situation of a threat to a protected ground, airspace and/or sea limits by an approaching ground vehicle or aircraft, said method comprising having an authorized driver/pilot/captain select first and second different PIN codes, said first PIN code representing a first state in which the vehicle, aircraft or sea- vessel is not considered to present a threat and a second state in which the vehicle, aircraft or sea-vessel is considered to present a threat; entering said first and second PIN codes into a smart card for generating a One Time Indicia (OTI) for each of said PIN codes; disclosing the state associated with each of said first and second OTI codes to at least one control center; whereby, upon said control center receiving an OTI code, the control center obtains positive identification of the driver/pilot/captain and the degree of the threat that said vehicle, aircraft or sea-going vessel presents. Brief Description of the Drawing The invention will now be described in connection with certain preferred embodiments, with reference to the following illustrative figure, so that it may be more fully understood. With specific reference now to the figure in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the drawing: Fig. 1 is a flow chart illustrating the identification, according to the present invention, of the situation of a threat to a protected ground, airspace and/or sea limits by an approaching ground vehicle, aircraft or sea-going vessel. Detailed Description of Preferred Embodiments The method according to the present invention for identifying, at the moment of verification, the situation of a threat to a protected ground, airspace or sea limits by an approaching ground vehicle, aircraft or sea-going vessel, is applicable to both protected ground, air spaces and sea limits, with obvious variations to match the different situations. The method is based on the use of a smart card, perse known in its basic form. Such cards generate and display a unique, one-time code produced by an algorithm, in combination with a specific encryption key. Augmented by an internal clock, the algorithm can generate a new code every predetermined period of time for the programmed life of the card. Only the associated software can determine which code is valid at a specific instance, for each of a group of issued cards. Some such cards also have audio outputs in addition to the visual display of the generated changing codes. While, as discussed hereinbefore, the invention is applicable to ground, air and sea security systems, the following embodiment will relate to aviation, as the method according to the present invention is envisioned to be more widely used for air security. According to the invention, smart cards will be distributed, e.g., by an air control center (ACC) to any pilot, i.e., pilots who are employees of regular commercial air carriers, private pilots, pilots for private companies and institutions and military pilots. Each smart card carries a unique readable card number, which number is registered at the ACC together with other identification information concerning the holder of the specific card, for example, the name and address of the holder; name of the holder's employer, e.g., the name of the airline; ordinary assigned route of travel, and the like. Other types of information, which is personally known only to the pilot, may also be recorded at the ACC for extra identification verification of the pilot possessing that specific smart card. Such information may be utilized in cases of doubt. In addition to the above background data concerning the holder of a specific numbered smart card, the pilot enters two distinct personal identification number (PIN) codes into the card: a first code, representing a first state or situation in which the pilot and/or the aircraft is not under any threat; and a second code, representing a second state or situation in which the pilot and/or the aircraft is under a threat or constitutes a threat. These two, unique, secret codes are remembered by the pilot. When one of these codes is entered into the smart card, the latter will generate a display of numbers and/or letters which will then be transmitted, by any known means, to the ACC, to be decoded to determine the degree of threat presented by the aircraft. The algorithm clocks of the smart card and of the ACC are synchronized at all times. As stated above, since the encoding of the PIN codes are time-based, so is the decoding. Thus, a further degree of security can be obtained by the ACC in determining that the approximate time a One Time Indicia (OTI) code was entered into the smart card and sent, corresponds to the time that the OTI was received and decoded. Obviously, the two secret PIN codes could remain in force for a predetermined period of time and be changed periodically, or they could be entered into the system before every flight, as a prerequisite for obtaining flight permission. Upon the arrival of the aircraft at the identification point (box 2), e.g., 120 or 180 miles from destination, the ACC controller calls the aircraft for identification (box 4). The pilot transmits the smart card number and an OTI code instantly generated by the smart card and advantageously displayed on the card, to the ACC (box 6) and the latter enters it into the secured code system (SCS) (box 8). The SCS verifies the identification code (box 10). If verified (box 12), the pilot is given permission to enter FIR (box 14), however, if the SCS determines a threat, FIR entrance is not given (box 16). In case of doubt, or for further verification, the SCS controller may request that the pilot provide further personal verification details, as described above (box 18). Communication between the pilot in the aircraft and the ACC is advantageously effected by audio transmission, preferably by the pilot's voice reading the displayed indicia, i.e., numbers, letters and/or symbols into the microphone. Alternatively, the displayed indicia is "read" automatically by the smart card incorporating audio output into the microphone. According to a further embodiment of the invention, when a higher level of security is required, the smart cards may possess a biometric capability whereby the actuation thereof is possible only if it will identify a biometric feature of the holder, e.g., the holder's fingerprint and/or voice. It can thus be realized that the above-described method not only discretely informs the ACC of the state of threat of the aircraft, but it also verifies the situation by positively identifying the pilot possessing the smart card which generates the OTI code. It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.