Controlled reception pattern antennas (CRPAs, pronounced “serpers”), adaptive antennas, null-steering antennas, beamforming antennas…
You’ve probably heard of at least one of those terms in any discussion around GPS anti-jam technology for defense.
Because they are all terms that describe essentially the same thing: a specialized antenna that helps protect GPS receivers from interference and jamming.
But what exactly are they? Where did they come from? How do they work? What comes next? Read on and find out.
A bit of history
Let’s go back to the Cold War era, at a time when Soviet and Western states were continuously battling for electronic warfare (EW) superiority. In the early to mid-Cold War, radar jamming was the name of the game. Soviet aircraft, such as the TU-16 Badger and its derivatives, carried a range of EW equipment, including some very high-power jammers designed to interfere with radar systems.
Fast forward to the latter years of the Cold War, and we reach the era when the U.S. was busy developing the exciting new GPS system. The Department of Defense (DoD) wanted to ensure that a robust and accurate global navigation system was available to the military, and so the Navigation System with Timing and Ranging (NAVSTAR) launched its first satellite in 1978, eventually becoming the fully operational GPS system by 1993.
Magnificent and ground-breaking though it was, it was recognized very early on that GPS relied on very low-power satellite transmissions, and would be vulnerable if someone tried to interfere with it. Given the prevalence of high-power jamming during the still-ongoing Cold War, there was concern that, if an adversary knew about GPS, they could easily render it useless in a given operational area.
And so it was that the CRPA came to the rescue.
Enter the CRPA
Once again, this GPS anti-jam technology finds its roots in the Cold War, and specifically in radar technology, where engineers developed clever ways to ensure their radars could continue to operate in the presence of jamming. Sidelobe cancellation (SLC) was a well-established technique in the radar community, where a received jamming signal could be “cancelled” by combining the outputs of more than one antenna in the right way.
So, it didn’t take long to adapt this radar anti-jam technology to the problem of GPS protection, and the CRPA was born. At this point I must declare a modicum of national pride, as the earliest operational GPS anti-jam unit that I know of was British. The Plessey PA 9800 GPS Anti Jam Unit was built at Roke Manor in 1984, and tested in the U.S. at the Yuma Proving Ground, Arizona, in 1985.
This pioneering technology could defeat up to three simultaneous jammers in the shown configuration, but was modular in construction, allowing further channels to be added for handling higher numbers of jammers. And all of this in 1984, in the UK, for a U.S. military navigation system that wasn’t even fully operational yet. Incredible.
From then until the present day, CRPAs have seen continual interest and development as the technology of choice to protect GPS from jamming. So how do they work?
Theory of operation
A CRPA is attractive, because it doesn’t require you to make any changes to the GPS receiver itself: It simply replaces the existing antenna. CRPAs are generally larger than typical GPS antennas, because they contain a number of antenna elements, and some associated electronics to do the clever stuff.
There’s nothing magical or mystical about the basics of CRPAs: It’s just standard theory from your favorite textbook on adaptive signal processing. But, as ever, the devil is in the detail — how to make them work well in practice is more involved. And as the technology is generally export-controlled, I shall leave out the important in-depth details.
CRPAs work by exploiting spatial diversity; that is, making use of the fact that the desired satellite signals, and the unwanted jamming signals, generally arrive from different directions. In simple terms, you create a spatial filter, one that removes signals that arrive from particular directions, whilst letting through signals from other directions. To achieve this, rather than use a single antenna, we use an array of antenna elements.
Let’s think in simple and intuitive terms about how this works. Take a look at Figure 3. Here we have a primary antenna P, and some auxiliary antennas A1, A2, and so on. A signal arriving from the direction shown impinges on antenna A2, and slightly later it arrives at A1, and later still it arrives at P. For the sake of argument, if the signal is a simple sine wave, you will then find that the output from each antenna is that same sine wave, but with a different phase shift depending on the spatial arrangement of the antennas.
Now, let’s consider what we call the “weights,” which are labeled as w1, w2 and so on. Each of the weights, in this case, is simply a phase shift that we can define. By careful choice of weights, we could choose to make each of the antenna outputs align perfectly in phase, and then, when we sum all the outputs together as shown, we end up with a bigger version of the input signal.
This is what we would like to achieve if the signal was a satellite. We “steer” maximum overall antenna gain towards that satellite. This is typically what is meant when we refer to “beamforming;” It means steering maximum antenna gain towards a satellite.
Conversely, we could also choose the weights to have the opposite effect: to minimize or completely cancel out the signal. This, of course, is what we would like to do if the signal was a jammer, and is referred to as “nulling” or “null-steering.”
How do we determine what those weights should be? Well, this is where your standard theory in adaptive signal processing comes in. Let’s say the objective is to minimize the jamming power out of the antenna. We can write the output power of the adaptive antenna as:
The average output power can be found by taking expectations:
Taking the minimum and rearranging this leads to the well-known Wiener equation:
This Wiener equation is the one to remember. It says that the optimum weights can be found by taking the inverse of the data covariance matrix, and multiplying it by the vector of cross correlations between the primary and auxiliary antennas. As in any adaptive signal processing problem, a simple way to solve the Weiner equation and get the weights might be to use your favorite gradient descent algorithm, such as least mean squares (LMS):
However, a solution using this approach does have its problems, for reasons beyond the scope of this article. The mathematics of beamforming are also bit more involved, so I’ll leave that out here.
Rather than the grossly simplified diagram used here, most decent CRPAs also use a more complex architecture based on space-time adaptive processing (STAP) or space-frequency adaptive processing (SFAP). This generally allows much higher levels of jammer cancellation against a wider range of threats.
To finish off this whirlwind section on CRPA basics, let’s see what some example antenna gain patterns might look like. In the figures below, the blue line represents the direction of arrival of a GNSS satellite signal, whilst the red lines indicate the direction of arrival of a jammer. In the first diagram we have a single jamming signal: the antenna gain pattern is a nice hemisphere, as we would generally like, but there is a nice deep null in the direction of the jammer. Moving on to the next diagram, we can see the effect of having three simultaneous jammers on the same CRPA: again we have nice deep nulls in the direction of each jammer, but we are starting to lose more of the sky, and we may start to lose the odd satellite as a consequence. Finally, we have an example of beamforming on a single satellite, whilst nulling out a jamming source.
Again, it’s beyond the scope of this article, but the layout of the antenna elements plays an enormously important part in the performance and behavior of the CRPA.
Operational Anti-Jam Units
With some images courtesy of my friends at Raytheon, let’s look at a few examples of deployed military CRPA hardware over the years.
The GAS-1 system entered service in the U.S. in 1997, as a replacement for the earlier AE-1 (1990 to 1996). The CRPA is composed of two parts: the antenna array, which is a seven-element layout, and the antenna electronics as a separate box. The GAS-1 was incredibly successful and became the de facto standard anti-jam technology, fitted to air and sea platforms around the world. Even today, 20 years after its launch, it continues to be fitted to many platforms.
By the late 1990s and early 2000s, the Navigation Warfare (NAVWAR) program was in full swing, and the military was looking for enhanced protection against evolving jamming threats. The U.S. initiated a program called Advanced Digital Antenna Production (ADAP). The ADAP product, launched in 2006, was a direct form-fit replacement for the analog GAS-1 system, and introduced a number of advanced features. Most notably, the ADAP simultaneously protects both the L1 and L2 frequency bands, and utilizes STAP processing to achieve high levels of wideband jammer cancellation.
In parallel with the ADAP development, the Digital Antenna Control Unit (DACU) was different in a number of ways. Firstly, it was a true beamforming solution, allowing simultaneous antenna beams to be steered toward satellites, whilst simultaneously nulling out jammers.
Secondly, it was tightly integrated with the GPS receiver, with the GPS receiver hardware located in the same unit.
Thirdly, the DACU was able to perform a number of other advanced functions, such as direction-finding of interference sources. Interestingly, the DACU was used to help locate the source of the interference at the notorious Newark airport jamming incident in 2009.
By the mid-2000s, CRPA electronics were pretty mature and well-understood. The electronics had been miniaturized, and pretty much everything was put onto a single chip. But the physical size of the antennas persisted as a problem for some platforms requiring low size, weight and power (SWAP).
The Landshield, launched in 2014, was a step-change in CRPA technology. Not just because it was a small and fully self-contained unit (about the size of a hockey puck), but because it was the world’s first CRPA to include true anti-spoofing capability.
Blurring the lines between military and civilian
Going back a few years, the military was heavily focused on CRPAs and anti-jam techniques in general. Military GPS receivers had been developed and deployed, and the question was how they could retrofit robustness to them. At the same time, the commercial world was heavily focused on mass-market GPS receivers — reducing cost, increasing performance — with little care about jamming.
If you’d talked to me five or six years ago, I would have said the military sector is 20 years ahead of the commercial sector in anti-jam technology, and the commercial sector is 20 years ahead of the military sector in receiver technology.
This assertion holds far less true these days; the lines of separation are much more blurred. The military is learning from the commercial world, embracing COTS, and developing new GNSS receivers. Conversely, civilian applications are now much more concerned with jamming, leading to the adoption of low-cost CRPAs in non-military applications.
The future of the CRPA
Where will CRPA technology go from here? We’ve already seen that the latest generation of CRPAs now performs anti-spoofing, as well as anti-jamming. But there is plenty more to see yet.
Although the core technology behind CRPAs is now mature, the trend for the future will be about “doing more with less.” CRPA technology will become more of a multi-function system. Military platforms need to cut down on the number of separate systems they install, and so CRPAs are likely to become multi-functional, performing situational awareness and signals intelligence.
As antenna technology progresses, we will likely see protected navigation solutions utilizing the same hardware as communication systems and radar systems, providing CESM and RESM functions, and being part of an integrated electronic warfare suite. And conformal antennas will see a resurgence of interest for complex and space-constrained platforms.
Watch this space.