The importance of long-distance alignments in Roman surveying is increasingly being recognised. It has now been discovered that they were used in setting out the central sectors of the Antonine Wall, but — in contrast to Hadrian's Wall — it appears that they were employed to determine the locations of the military installations along the Wall rather than the line of its rampart and ditch. It also appears that the enigmatic enclosures and expansions which are attached to the rear of the rampart of the Wall only seem to occur in connection with these alignments. A careful analysis of possible explanations indicates that the Romans may initially have sought to set up a two-level alarm system across the central sectors of the Antonine Wall, the possible impacts of which upon the planning and design of the Wall are examined. The Supplementary Material available online (https://doi.org/10.1017/S0068113X18000284) contains a table of inter-visabilities between known military installations, and OS grid references for the installations.

In general, a Reference Station calculates differential corrections which are valid for that exact location (zero baseline) at that particular epoch (age of corrections zero). However, DGPS users may be located as far as 200 nm away from the Reference Station and some of the errors compensated for by the Reference Station vary with space, namely satellite ephemeris, tropospheric and ionospheric errors. Therefore, the corrections calculated at the Reference Station suffer certain accuracy degradation as the separation distance increases, because of a decreasing relevance of the Reference Station data to the user. The error growth with increasing distance to the beacon is accentuated by the inability of Reference Station and user to see the same satellites, commonly termed the lack of intervisibility. The error growth with distance is the most important factor determining DGPS accuracy, but surprisingly very little has been done to assess it. US official documents and IALA state that the achievable accuracy degrades at an approximate rate of 1 m for each 150 km (80 nm) distance from the broadcast site, but this value is based on a theoretical prediction, made back in 1993. To estimate the error growth with real data, 6 DGPS receivers were placed along the Portuguese coastline at approximately 50 nm intervals from Sagres Broadcast Station, in a South – North direction. This paper describes the results of the trial.

This paper proposes that pulsars can serve as beacons for the discovery of and communication with extraterrestrials. The motivation for the communication strategy proposed is discussed in detail, along with relevant astrophysical considerations. It is shown that millisecond pulsars have characteristics and a distribution in space that make it possible to envisage communication being targeted towards and away from habstars (as defined by Turnbull & Tarter) aligned with pulsars in a specified way. Lists of candidate habstars and their pulsar alignments are included for those wishing to conduct searches using the strategy described.