Military Tracking Antennas

Manufacturers of tracking antenna systems for military and defense applications, including auto-tracking and airborne tracking antennas for tactical ISR, video and data communications
Overview Military Tracking Antennas
By Dr Thomas Withington Last updated: February 11th, 2024

Space communications involve many moving parts: The Earth is rotating at 901.7 knots (1,670 kilometres-per-hour) anti-clockwise and the Earth spins on an axis of about 23.5 degrees. To communicate from Earth to a satellite, it is imperative to know where both the planet and spacecraft are at any moment.

Precision is important because radio signals move in a straight line. Thus the antenna on Earth and the one in space must each be pointed in the same direction to maintain the radio link for as long as possible.

If radio communications are being made between Earth and a geostationary spacecraft, this process is relatively straightforward. A geostationary object appears to be continuously in the same position above a specific location on Earth. Geostationary orbits make it relatively easy to aim an antenna on Earth towards the spacecraft as the latter’s position remains constant. Moreover, geostationary spacecraft orbit along Earth’s equator, further helping predictability. All that is needed to ascertain the spacecraft’s position relative to the antenna on Earth is some spherical geometry.

Types of Orbit

Matters get more complex as far as communications between antennas on Earth and spacecraft in different orbits are concerned.

Low Earth Orbit (LEO) satellites

Low Earth Orbit (LEO) satellites typically fly at altitudes between 86.4 nautical miles/nm (160 kilometres/km) and 540nm (1,000km). LEO satellites can reach speeds of 15,162 knots (28,080km/h) translating into a single orbit every 90 minutes. Furthermore, LEO spacecraft are not restricted to equatorial orbits. As they are not geostationary, they are continually moving relative to an observer on Earth. This movement means the satellite’s position in the sky must be anticipated, and the antenna steered to maintain contact, as the spacecraft crosses the horizon.

Medium Earth Orbit (MEO) Satellites

Medium Earth Orbit (MEO) satellites fly at altitudes between those of LEO spacecraft and their geostationary counterparts; 1,080nm (2,000km) and 19,324nm (35,786km). Like LEO satellites, GEO spacecraft are not confined to equatorial orbits, although some MEO satellites do follow geostationary trajectories.

Sun-Synchronous Orbits

Polar- and sun-synchronous orbits pass within 30 degrees of the poles at altitudes between 108nm (200km) and 540nm. Solar synchronous orbits are classified as a type of polar orbit but spacecraft following these paths remain in the same spot relative to the sun. This behaviour means that satellites in sun synchronous orbits will continually revisit the same point on Earth at a set time every day. Sun-synchronous satellites fly at altitudes between 324nm (600km) and 432nm (800km).

Tracking the Satellites

TTS 2.0 Mobile Tracking Antenna System by Triad RF

TTS 2.0 Mobile Tracking Antenna System by Triad RF

Apart from spacecraft in geo-synchronous orbits, and those MEO spacecraft following similar paths, tracking satellites can be challenging. The speeds of some spacecraft, like LEO satellites moving at 15,162 knots, mean they may only be ‘visible’ to a SATCOM antenna for a short amount of time. During this period, the link between the SATCOM terminal and the satellite must be maintained.

Predictive Programming

To ensure a continual and uninterrupted link between the terminal and the satellite, it is necessary for the antenna to ‘follow’ the spacecraft as it moves across the sky. An operator can manually move the antenna to keep a line-of-sight with the satellite. However, this method depends on the tracking skill of the operator and may be unreliable. Alternatively, the antenna can be steered using predictive programming. The programming will inform the antenna of the satellite’s expected movements on any given day. The antenna can be programmed to start its motion when the desired satellite is expected to be in its field-of-view.

Monopulse Tracking

Monopulse tracking is another method. Using three beams transmitted from the satellite, one beam provides details on the satellite’s azimuth in relation to the terminal and another the satellite’s elevation. The third signal, known as the sum beam, gives an additional reference point. The terminal compares phase and amplitude errors in these beams compared to what they would be if the antenna was aligned. Corrections are then made to the antenna’s positioning to maintain the link.

Step-Tracking

The step-in tracking approach has the antenna initially searching for the satellite signal. Once a strong beam is found, the antenna fine-tunes its movement to ensure a proper alignment. Step-tracking has slightly less accuracy than monopulse tracking but is advantageous from a cost perspective.

Electronic Steering

The advent of Active Electronically Scanned Array (AESA) antennas for SATCOM is likely to improve tracking accuracy yet further. These antennas can electronically steer their transmissions to ensure a sharper and more robust connection to the satellite. At present, AESA SATCOM antennas are used in the air domain, but their proliferation for land- and sea-based applications may grow in the future. AESA antenna acquisition costs are expected to reduce as they proliferate.