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Surveillance and communication formats (Fig. 13[b]) contain all of the fields of a surveil-lance format with an additional 56-bit mes-sage field, which permits simultaneous data-link and surveillance activities. The design of
The Lincoln Laboratory Journal, Volume 2, Number 3 (1989)
the Mode-Sdatalinkallows the connectionofup to four of the 56-bit message fields into a single message entity.
The communications formats (Fig. 13[c]) handle longer data-link messages. The use of communications formats provides greater link efficiency in two ways. First, since the message field is longer, fewer interrogations or replies are required for a given message. Second, the message transfers use the Extended Length Message (ELM) protocol. ELM permits a single reply to acknowledge up to 16 communication interrogations, which conserves channel time. A similar approach is used for downlink ELMs. The sixteen 80-bit message fields provide a message length of 1,280 bits; the design ofthe Mode-S data link allows the connection ofup to 32 of these messages.
Mode-S Data-Link Characteristics
The Mode-S data link offers characteristics
that are well suited to the needs of air traffic services. For instance, the association of the Mode-S data link with the surveillance function offers a number of operational benefits. Com-munication with an aircraft can be established based solely on surveillance detection. Thus a message can be sent to an otherwise unidenti-fied aircraft. This capability is important for safety services; e.g., warnings can be sent to an unidentified aircraft that is flying too low or that is heading into controlled airspace. Since the same address is used for both surveillance and communication, the possibility of sending a message to the wrong aircraft because of an error in cross-referencing the surveillance and communication identities is eliminated. A fur-ther operational benefit is that communications coverage is assured whenever surveillance coverage exists.
Because of the inherent characteristics of Mode-S sensors, the Mode-S link discourages both accidental and intentional jamming. With a Mode-S radar, coverage is restricted to the sensor's line of sight. This restriction limits not only the airspace in which the sensor can cover traffic, but also the area in which an interfering source can affect the sensor. A narrow antenna beam further limits the active area. Thus a single interfering source, if it is outside the sidelobe region of the sensor, will prevent operation in only a single antenna beamwidth. In the worst case, in which the interfering source is within the sidelobe region (i.e., within a distance of about five miles from the sensor), the source could prevent operation of that sen-sor in all directions. However, the interfering source would have little effect on the opera-tion of all other Mode-S sensors. Thus the distributed nature of Mode S makes it tolerant of interference.
Mode-S Experimental Facility
The Mode-S design was first validated at the Mode-S Experimental Facility (MODSEF), lo-cated at Lincoln Laboratory (Fig. 14). Initially used for link measurements and monopulse development, MODSEF was later upgraded to a fully functional Mode-S sensor. Validation at MODSEF, however, is not sufficient verification of Mode S because the site does not experience high traffic density, fruit, or ground reflections known as multipath.
Transportable Measurements Facility
The Transportable Measurements Facility (TMF) [12) was constructed for observation of the Mode-S sensor operation at FAAsites known to provide environmental difficulties. TMF (Fig. 15) includes its own antennas, tower, and equipment van that contains a transmitter, receiver, and digitizing and recording equip-ment. The antennas shown in the figure are an Airport Surveillance Radar-7 antenna with a monopulse beacon feed and a monopulse-capable antenna on loan from the United Kingdom. In operation, TMF transmits and then digitizes the received video pulses, which are recorded for subsequent playback and analysis at Lincoln Laboratory.
　
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