Planning cameras, loudspeakers and radio links on site

The question: where should the masts go?

Anyone securing a festival, a construction site or a property with mobile security technology faces the same task: where do the masts go, where does the control room go, and how do the images get there without cables?

A MOVETOS SET consists of masts with camera, lighting and loudspeaker, independent of mains power and network cabling. A good layout meets three goals at once:

1. Coverage: cameras see the required share of the area, and loudspeakers reach the required share of visitors.
2. Little hardware: as few masts and cameras as possible.
3. Stable radio links: every mast reaches the control room over clear radio links with enough capacity.

These goals pull in different directions: a mast ideal for its camera may be hidden behind a building for the radio link. That is why it pays to calculate the layout.

What matters

Covering areas and entrances

Entrances, the stage and walkways carry most of the activity. Three fixed planning rules apply:

• One mast per entrance. Its camera sees the gate from no more than 25 m away.
• The entry area lies within the counting zone. This means the area within 25 m of the gate; at least half of it must fall inside the cone in which a camera reliably counts people. Otherwise the density at the gate is only estimated from neighbouring values.
• The stage gets two cameras facing the audience.

A simple distinction applies to cameras: at a mast height of 5 m, a camera reliably counts people up to about 35 m and keeps the area “in view” up to about 100 m. IEC 62676-4 provides the matching resolution levels (DORI).

Sight lines and buildings

No camera sees, and no loudspeaker is heard, behind a building. Building footprints and heights can be taken from OpenStreetMap; low walls under 2 m do not disturb the radio link. Anything not shown on a map – stage, containers, tents, high fences, vegetation – has to be added by the planner.

Public address coverage

A 100 W loudspeaker produces 128 dB at 1 m; the level falls by 6 dB for each doubling of distance (ISO 9613-2). For intelligible announcements, the standards for emergency sound systems call for at least 75 dB and at least 10 dB above ambient noise – with a safety reserve, this gives a range of around 60 m at most. Speech intelligibility is measured by the STI under IEC 60268-16, with a target of at least 0.5. Take care where two loudspeakers of similar level arrive more than 50 ms apart (a path difference of about 17 m): an audible echo results there (Haas 1951).

Radio links without cables

Radio links connect mast to mast up to the control room. Four rules apply:

Rule	Value	Why
Length of a link	at least 50 m, at most 1 km	Property of the antennas; practical limit despite a calculated reserve
Clear corridor	about 2 m at 100 m, 3 m at 500 m	Around 60 % of the Fresnel zone must be clear (ITU-R P.526)
Cameras per link	at most 6	Hence at most 3 masts with 2 cameras each in a row
Fade margin	12 dB above receiver sensitivity	Practical range 10–15 dB (ITU-R P.530)

A link grazing a façade does not count as clear. Where the direct path is blocked, a bridge module helps – a telescopic mast carrying only the radio link, which relays the connection around the corner of a building.

Control room

The organiser usually specifies the control room location. If it is open, it is best placed outside the event area, where every mast can be reached directly or via a few relay stations, and close to an entrance for the trailer.

Budget

When the number of masts is limited, the aim becomes the best coverage with what is available – and stating honestly what is missing.

How to calculate the best layout

Each part of the problem has a name in the literature:

• Coverage is a maximal covering location problem (Church & ReVelle 1974): choose sites so that as much of the area as possible is served.
• Seeing around obstacles is a variant of the art gallery problem (Chvátal 1975): how many guards does a room need so that every corner is seen?
• The radio network is a Steiner tree: the shortest network that connects all masts to the control room and may add intermediate points along the way (Lin & Xue 1999).

Taken together, this is known as connected facility location: opening sites and at the same time joining them into one connected network. All three problems are computationally hard. At the scale of an event – a few hundred candidate sites, a few thousand check points – the siting part can nevertheless be solved exactly.

The tool for this is integer optimisation: each candidate site is a yes/no decision, every rule (one camera per entrance, spacing between masts, coverage target, budget) is a constraint, and the aim is the cheapest solution that satisfies them all. A by-product is a lower bound: the statement “it cannot be done with fewer than N masts”.

A quick greedy method, which always takes the site with the largest gain, provably reaches at least about 63 % of the optimum (Nemhauser, Wolsey & Fisher 1978) – fine for a preview, not for the final result.

For the radio network, the method of Kou, Markowsky and Berman (1981) connects the masts via shortest paths; the result is guaranteed to be at most twice as long as the best possible network. The network is then checked for links carrying more than 6 cameras and, where necessary, split into several routes to the control room.

How MOVETOS OPS does it

The MOVETOS OPS platform applies these methods in its layout proposal.

1. The client’s site plan as the basis: the client’s drawing can be uploaded as a PDF and placed on the map using three distinctive points. It then replaces the street map in the technical plan. Obstacles shown only in the drawing are traced and count like a building for radio and sight lines.
2. Inputs: area, entrances, stages and walkways; the assistant takes the walkways from the map. Then the desired coverage levels for cameras and announcements.
3. Instant proposal and exact calculation: the map shows a proposal straight away; in the background the server calculates the layout exactly, including building heights, and replaces it after a few seconds. The explanation states the lower bound (“at least N masts”), the intelligibility of announcements, echo risks, the load on each radio link (“80 m · 4 cameras”) and any masts without a backup link.
4. Budget: anyone who fixes the number of masts sees a table with target, recommendation and value within budget for each level. The security concept states any shortfall clearly, with both figures.
5. “Not observed” made visible: during operation, the situation map shows places far from any camera hatched in grey as “not observed” – the map claims nothing there. A restricted area that no camera sees is marked as “not monitored”.

The proposal is a proposal. Masts can be moved, radio links set by hand where vegetation or a fence interferes, and each location is confirmed on a phone during the site walk. The calculation knows its limits: building heights come from the map, the terrain is assumed to be flat, and diffraction and reflections are not modelled. A person makes the decision.

Planning checklist

• Obtain the client’s site plan and place it on the map.
• Enter the area, entrances, stages and walkways.
• Add obstacles missing from every map: stage, containers, tents, fences, vegetation.
• Plan one mast per entrance; check that the entry area lies within the counting zone.
• Set coverage targets for cameras and announcements; estimate noise levels realistically.
• Agree the control room location with the organiser – including vehicle access.
• Check the radio links: 50 m to 1 km, a clear corridor, no more than 6 cameras per link.
• Enter the budget and state any gaps openly in the security concept.
• Confirm each location on site; set disturbed links by hand.
• During operation, watch for “not observed” areas and cover them with staff where needed.