Date: 20 Nov 96 05:48:29 From: Ed Hahn <firstname.lastname@example.org> Organization: The MITRE Corporation, McLean, Va. References: 1 2 3 4
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email@example.com (Scott Odle) writes: > >For the TCAS-III (in project) : The TCAS can generate manoeuvering in both > >vertical and horizontal plan. > > There will be no TCAS III. For whatever reason it will be designated TCAS IV. The reason it is designated TCAS IV is that the method for determining the correct horizontal resolution manuever will be entirely different than initially envisioned in TCAS III. One of the results of TCAS II experience has been that the directional antenna used by the TCAS processor to assign a bearing to a received transponder reply is not accurate enough to generate an accurate horizontal position, and thus a safe horizontal resolution. TCAS IV will use additional position information encoded on an air-to-air data link to generate the bearing information, so the accuracy of the directional antenna will not be a factor. In order to compare and contrast the two, the decision was made to refer to the new system as TCAS IV, to prevent confusion between technologies. In summary: TCAS I: Uses a directional antenna to view Mode A, C, or S transponders on other aircraft to generate a situation display and "Traffic Advisory" (TA) for nearby targets. This TA is used to help pilots visually locate nearby co-altitude traffic (Mode C) or unknown altitude traffic (Mode A). TCAS II: Uses a directional antenna to view Mode A, C, or S transponders on other aircraft to generate a situation display and a TA for nearby targets. For target aircraft with Mode C or S transponders, the TCAS display can generate a "Resolution Advisory" (RA), which commanded vertical manuever (climb/descent) to avoid nearby co-altitude traffic. For target aircraft with Mode S transponders *AND* TCAS II equipment, RAs will be coordinated between aircraft (e.g. the two TCAS processors will cooperatively agree to send one aircraft in a climb and the other in a descent.) Note: aircraft equipped with TCAS II must have Mode S transponders installed. TCAS III: Attempts to use the TCAS directional antenna to assign a bearing to other aircraft, and thus be able to generate a horizontal manuever (e.g. turn left or right). Judged by the industry to be unfeasible due to limitations in the accuracy of the TCAS directional antennas. The directional antennas were judged not to be accurate enough to generate an accurate horizontal-plane position, and thus an accurate horizontal resolution. TCAS IV: Uses additional information encoded by the target aircraft in the transponder reply (i.e. target encodes it's own position into the transponder signal) to generate a horizontal resolution to an RA. Obviously, this requires the target aircraft to have some data link capability at a minimum. In addition, some reliable source of position (e.g. GPS) is needed on the target aircraft in order for it to be encoded. Mode A: A transponder which can encode a number into the reply signal. This code is a four digit octal number XXXX, with each digit having the value 0-7. The famous "1200" VFR transponder code is an example of a Mode A code. Mode C: A transponder which can encode its altitude into the reply signal. This code is known as the "Grey Code", and it encodes 100 ft. increments into 12 bits. Note that Mode C transponders can also encode Mode A, and that ground radar typically alternates which information it asks for on successive sweeps. Mode S: A transponder which can be selectively interrogated (hence Mode S = Select), which can also encode additional information into the data stream. This transponder essentially gives a basic data link capability, which in TCAS II is used to coordinate RA manuevers. TCAS IV could use Mode S data link capability to encode position information into TCAS replies. TCAS IV development is still underway, but it is not likely to be fielded in the next year or so, as there are still technical and institutional issues to resolve. Also, new trends in data link such as Automatic Dependent Surveillance - Broadcast (ADS-B) have popped up recently, and have pointed out a need to re-evaluate whether a data link system dedicated to collision avoidance such as TCAS IV should be incorporated into a more generic system of air-to-air data link for additional applications. These issues are being worked by the government and industry in groups such as RTCA. Note that I am writing this posting to provide information only. This post does not intend to endorse the merit of any particular solution for collision avoidance or other application. Any errors in the above are mine. Hope this helps, ed -------- Ed Hahn | firstname.lastname@example.org | (703) 883-5988 -------- The above comment reflects the opinions of the author, and does not constitute endorsement or implied warranty by the MITRE Corporation. Really, I wouldn't kid you about a thing like this.