The pros and cons of multistatic radars

Multistatic radars have unique attributes, but they have been embraced by only a small number of nations and are likely to remain niche capabilities.

Most radars are ‘monostatic’: they use a single antenna. This antenna will transmit radio waves travelling at the speed of light, 299,274 kilometres-per-second. These radio waves hit targets in their path and are bounced back to the same antenna as echoes. 

By dividing the time for the wave’s round trip, the radar can determine a target’s distance from the antenna. If the target is moving away from the antenna, the frequency of the echo will be progressively lower than the original transmission. Conversely, the frequency will progressively increase
if the object is moving towards the antenna. 

This process mimics the doppler effect: the perception of a rise in tone of a police car’s siren as it approaches someone standing on the kerb, and the tone lowering as the car drives past. These principles are at the heart of most radars, civilian or military. 

Less common and less discussed are bistatic and multistatic radars used by a handful of militaries and the radar academic research community. 

Bistatic radars use two antennas; one to transmit (Tx) the radio waves and one to receive (Rx) , with the antennas typically spaced a set distance from each other. Multistatic radars take this a step further – one Tx transmitter is used along with several Rx antennas. These receive the echoes bouncing off a target caused by the original transmission. The Rx antennas are distributed across an area a set distance from the Tx antenna, and are networked to one another. 

Multistatic radars can capture echoes that might be reflected away from the Tx antenna – echoes that could be lost by a conventional monostatic radar. As we shall see this is especially advantageous against targets with a low radar cross-section (RCS). 

If the user has a Tx antenna and somewhere to position several Rx antennas, multistatic radar can be used on the ground, in the air, at sea, or even in space. Above all, as Dr Matthew Ritchie, a lecturer in electronic and electrical engineering and expert on multistatic radar at University College London (UCL) told ADBR, multistatic radars help the user collect “more data from more nodes (increasing) the probability of detection for certain targets like those with a low RCS while expanding the detection area.” 

UCL is a centre of excellence for this technology in collaboration with the University of Cape Town. Since 2008 the two institutions have trialled two prototype multistatic radars in South Africa – the NetRad and its successor, NextRad. 

NextRad is a dual L-band (1.215GHz to 1.4GHz) and X-band (8.5GHz to 10.68GHz) radar active since 2015. It is based in Simon’s Town, just outside Cape Town and home to South Africa’s largest naval base, and is used to evaluate the performance and feasibility of multistatic radars for naval and maritime surveillance.

COMPLEXITY
It would be unfair to say that multistatic radars are less complex than their monostatic cousins. The Tx and Rx antennas need to be spread over a large area with a precisely measured distance between them. This is imperative so that a target’s position relative to each Rx antenna can be correctly ascertained. The target’s position will be the point at which all the bearings from each of the Rx antennas meet. The radar will need a “high degree of coherence,” Dr Ritchie tells us – precision timing which needs to be governed by an accurate time source like an atomic clock. 

He says these clocks are a potential alternative to a GNSS (Global Navigation Satellite System) timing source transmitted from space, where GNSS signals can be vulnerable to jamming. The radar will need to know the precise moment a transmission was sent from the Tx antenna. If Rx antenna ‘A’ receives the reflected echo slightly before Rx antenna ‘B’, then the radar will correctly deduce that the target is nearer Rx antenna ‘A’. Dr Ritchie emphasises that timing is everything in radar: “There is a general rule-of-thumb that a nanosecond of error equates to approximately one foot (0.3 metres) of inaccuracy.”

ANTI-RADAR MISSILES
For ground-based radars, the feared weapons are Anti-Radiation Missile (ARMs) which exploit the radar’s own transmissions for targeting. They detect transmissions from a hostile radar which are then used to guide the missile towards the radar’s antenna. The missile detonates near the antenna destroying it and rendering the radar either temporarily or permanently inoperable. 

Having a radar’s Tx and Rx functions separated from one another means the Rx antenna will still function even if the Tx antenna is destroyed. The Rx antenna is passive as it does not emit, hence an ARM cannot use radar transmissions as a homing source to hit these antennas, although the Tx antenna may still be at risk. 

Also, newer ARMs such as Raytheon’s AGM-88F HCSM (High Speed Anti-Radiation Missile Control System Modification) and the Northrop Grumman AGM-88E AARGM (Advanced Anti-Radiation Guided Missile) have GNSS (Global Navigation Satellite System) apparatus which allows the satellite coordinates of the hostile radar to be programmed into the missile, freeing it from reliance on radar transmissions to target the antenna.

Multistatic radars with several combined Tx/Rx antennas networked to one another have inherent graceful degradation. One or two of these antennas can be destroyed with a partial loss of performance, but without jeopardising the whole ensemble. 

This brings obvious benefits concerning electronic attack. It can be relatively easy to jam a Tx antenna. By detecting the transmissions, their source can be located allowing for direction of jamming signals into the antenna. Multistatic radar’s Tx antenna may still be at risk from jamming, but, as Dr Ritchie said, “the advantage of Rx-only antennas is that you do not transmit from them, it is harder to locate these antennas and use electronic countermeasures against them.”

He said any hostile jamming performed against a multistatic radar will need to be omni-directional rather than directional if it is to hit the Rx and the Tx antennas. This will reduce the amount of jamming power that can be directed against all these antennas compared to that which could be directed against a single radar.

Multistatic radars can do without Tx antennas altogether. Passive multistatic radars work in two ways – they can use several Rx antennas spread across a defined area. These will detect disturbances to ambient electromagnetic radiation caused by flying objects. For instance, a passive multi-static radar could be tuned to detect disturbances to the propagation patterns of local VHF/UHF (30MHz to 3GHz) parts of the spectrum largely populated by television, cell phone, or radio traffic. By ascertaining the bearing of these disturbances from each Rx antenna and when they occur, the radar can compute the target’s location. 

A variation on this theme is to detect the VHF/UHF transmissions from an aircraft and use several Rx antennas over a defined area to detect these transmissions and triangulate the aircraft as all aircraft emit some form of radio emissions, typically from their communications and navigation equipment, altimeters, and weather radar. 

There are disadvantages to these approaches, however. Passive multistatic radars using existing ambient VHF/UHF transmissions depend on this radiation always being present. What happens if a radio or TV transmitter is taken offline for servicing, or transmissions are suspended because of technical problems? Secondly, local radiation levels may be low. Rx antennas would need to be very sensitive with a possible risk that these become susceptible to false alarms. Plus, military aircraft performing combat missions will minimise their transmissions to hamper their detection by passive radar. 

LOW RCS
Multistatic radars can potentially degrade the protection offered to an aircraft exploiting a low RCS design. Low RCS aircraft or weapons are designed to be difficult to detect by radars using specific frequencies looking at the target from certain angles. A low RCS design may be optimised to ensure the aircraft returns as few echoes as possible when the radar is pointed directly at the front of the airframe, although the aircraft may still scatter lots of echoes or ‘lobes’ in other directions away from the radar. 

Dr Ritchie noted that an asset of a multistatic radar is that you “can perceive the target from many different angles”. Having Rx antennas scattered around an area, positioned some distance from the Tx antenna means the former may capture echoes not returned towards the latter. This helps to reduce some advantages of low RCS design. 

JORN
Despite the clear attributes of multistatic radars, they have remained a niche capability because of several factors. 

Firstly, they are complex – these radars need precise timing systems and robust communications. They need complex software to synchronise their operations, interpret received echoes, and ascertain the position and behaviour of targets. Monostatic radars have the asset of their Tx and Rx elements being in one place. This makes them easier to install on an aircraft or ship. They also occupy comparatively less space than their multistatic cousins when deployed, and radars on ships or aircraft can be used while mobile. 

Multistatic radars could be used while mobile, but this would add complexity to their software. Thus, these radars have tended to be large, static affairs with their Tx and Rx antennas spread across hundreds or thousands of kilometres, and they are typically used to detect targets at long distances.

Perhaps the most famous operational multistatic radar is Australia’s Jindalee Operational Radar Network, better known as JORN. It consists of six Tx and Rx antennas located in the west, north, and centre of the country. These transmit on High Frequencies (HF) of 5MHz to 30MHz and, crucially, cannot penetrate the ionosphere, an atmospheric layer between 60km and 1,000km in altitude. 

HF transmissions are reflected towards the ground which allows the transmissions to circumvent the curvature of the Earth and detect air or surface targets thousands of kilometres away. Transmissions at higher frequencies would continue in a straight line through the ionosphere into space.

To put JORN’s capabilities into perspective, open sources say these radars have a range of between 1,000km and 3,000km, and JORN’s Tx and Rx antennas are arranged to provide coverage of Australia’s western, northwest, and northern air and maritime approaches. 

JORN is a collection of three bistatic radars. One is located in Queensland with a Tx antenna in Longreach and an Rx antenna at Stonehenge. A second radar is based in Western Australia with a Tx antenna in Leonara and an Rx antenna in Laverton. The third radar is in the Northern Territory with a Tx antenna at Harts Range and an Rx antenna at Mount Everard. 

JORN radars are controlled from RAAF Edinburgh, and it is likely these are networked to perform in a multistatic fashion. For example, the Tx antenna at Leonara could be used for target illumination with the Rx antennas in Laverton, Stonehenge, and Mount Everard receiving target echoes in a multistatic configuration. 

JORN’s radars are undergoing an enhancement via the Project AIR 2025 JORN Phase 6 effort which commenced with a contract award to BAE Systems Australia in March 2018. Richard Udall, BAE Systems’ JORN project director, told ADBR that the upgrade introduces “new technology and architectures that will improve performance (and) address obsolescence to extend service life beyond 2042, as well as increase flexibility.” 

It will add open software architecture to improve existing performances while supporting ongoing upgrades and the introduction of new capabilities. Udall told us that early releases of the JORN software have established the open architecture characteristics with the software’s “core signal processing capabilities” and operator interface aspects being completed this year. 

New hardware is being added across all JORN Tx and Rx sites. Mr. Udall says that this replaces legacy analogue hardware with new digital waveform generators for the radar’s transmissions. He says that timing and frequency precision for the transmitted signals will be improved with new sapphire crystal oscillators. Moreover, the radar’s high-power amplifiers will be replaced. 

New ionosondes – sensors which monitor the ionosphere, a prerequisite in determining how the radars will perform in prevailing atmospheric conditions – will be constructed and located at Murray Bridge in South Australia, Learmouth and Ajana in Western Australia, and on Horn Island in Queensland. While JORN’s Tx and Rx sites are in remote locations, Udall does not see this as a problem, saying the company’s “highly motivated, dedicated project team” will greatly assist the implementation of the upgrade and that, despite the challenges of the COVID-19 pandemic, “all operational performance measures have been met”.

OUTLOOK
Long term, it seems likely that multistatic radar will continue to be a niche capability, with Australia’s JORN leading the pack. These radars are attractive given their ability to provide some detection and tracking of low RCS targets, and are practical “if you have large geographical areas to cover”, said Dr Ritchie. “They make sense as a way of defending your territory if you are confident of the likely trajectory of possible threats.” 

Conversely, their large size and fixed nature restricts wider uptake.

The networking of deployed monostatic radars in theatre may instead become the order of the day, where the use of wideband communications to network deployed radars would provide detailed real-time detection and tracking. It may even be possible to configure some of these radars to act in an Rx-only mode, or where a single radar could transmit with other radars acting as receivers. This may give the benefits of a multistatic radar without needing bespoke apparatus. 

But there are caveats – complex software and communications would be needed to get the radars to work in this fashion, and such an approach would also be challenging when radars are using different frequencies and antenna designs. Perhaps a more practical alternative would be to use passive radars to provide multistatic coverage?

Multistatic radars are fascinating systems with unique characteristics. Nonetheless they are only likely to be a niche complement to existing conventional radars rather than an outright replacement. 

This article appeared in the March/April 2021 issue of ADBR.