The MIL-STD-810G GPS/GNSS antennas include multi-standard GPS L1, Galileo E1 and GLONASS options and are designed for environmental performance according to the MIL-STD-810G standard.
The antennas are available in passive and active versions and provide coverage from 1,597 MHz to 1,607 MHz. The MIL-STD-810G GPS/GNSS antennas feature linear polarization for cross-polarized isolation, nominal gain options of -3 dBic and 10 dBic, and SMA mounts.
Southwest Antennas has introduced a high-performance GPS and GLONASS active L1/L2 patch antenna for high-accuracy location service, timing and navigation applications.
Part #1065-042 covers both the L1 and L2 bands, supporting military, commercial and industrial applications. For military users, the antenna supports the GPS P/Y code with +/-20-MHz bandwidth, allowing for increased accuracy, jam resistance and encryption for authorized military applications.
Photo: Southwest Antennas
The antenna’s built-in low-noise amplifier (LNA) and filters give it a total system active gain of +28 dB and out-of-band rejection of >50 dB (+50 MHz / -35 MHz of L1, +35 MHz / -45 MHz of L2). The specifications allow the antenna to operate in contested and congested radio frequency (RF) environments.
“Our goal is to empower radio operators who require high-accuracy GNSS solutions with more choices for deployment and mounting,” said Benjamin Culver, president and co-owner of Southwest Antennas. “Adding onto our existing line of GPS antennas and gooseneck mounting options, users now have more freedom of choice in antenna placement to help overcome reception issues in challenging environments.”
The low-profile radome allows the antenna to be easily tucked into pockets or modular lightweight load-carrying equipment (MOLLE) pouches and mounted on body armor, vests, rucksacks or other tactical gear. The custom black chrome SMA(f) RF connector ensures the antenna is waterproof, while allowing users to fully customize antenna placement on their gear and select their own cable type and length to suit their preference for mounting location away from their receiver.
For additional flexibility in mounting locations, part #1065-042 features a magnetic mount integrated flush into the antenna’s radome, allowing it to be secured and removed quickly from any ferromagnetic surface. This extends the antenna’s operational capabilities through the ability to remotely locate the antenna away from the attached radio system to enhance satellite acquisition speed and signal strength when operating in environments with poor sky views.
Applications include:
SAASM GPS, GLONASS, GNSS receivers, and other precision navigation receivers
Manpack and handheld radios, dismounted soldier-level communications
Small form-factor radios
Low-profile vehicle mounts and unmanned ground vehicles
Lockheed Martin Space has released the GPS-III satellite antenna phase-center data, as well as the group delay and inter-signal correction data, for SVN-76 and SVN-77 as measured at the factory, announced the U.S. Coast Guard’s Civil GPS Service Interface Committee (CGSIC).
The phase center and inter-signal bias data included in this new release provide additional information that supplements the antenna gain pattern data previously available.
Note that the GPS III satellite vehicles (SVNs) also broadcast the Inter-Signal Corrections (ISCs) in the various LNAV/CNAV messages in accordance with all the external IS/ICDs. The value that is being broadcast by the on-orbit constellation is not the factory measured ISCs but the ISCs estimated on-orbit by the Stanford Research Institute (SRI).
Optical Zonu has introduced the ZonuSkyShot GPS tester, designed for quick testing during the critical installation phase of an antenna at a new site build or small cell integration.
The compact tester is designed for integrating one of Optical Zonu’s GPS solutions, but is equally capable of working as a neutral testing device.
Fig. 1. Screenshot of ZonuSkyshot software output. (Screenshot: Optical Zonu)
The ZonuSkyShot is a compact GPS receiver that detects the presence of a GPS signal, indicated on the top-panel LED. The receiver can be accessed at the USB port on the base unit, allowing the user to see the available satellites by using the app provided with the system and available at the Optical Zonu website.
The receiver can simultaneously track up to 16 satellites while searching for new ones. Because of this, a problem can be found and mitigated when a GPS antenna is installed, rather than when hardware is being integrating further down the line. Close-out of projects can be indicated with with screenshots of satellite visibility via the micro-USB port to a laptop.
The app provides:
RF GPS signal presence
GPS antenna functionality
Optical transmitter functionality
Fiber connectivity
Optical receiver functionality
Pre-orders are now being accepted for the kit, which includes the handheld device with power supply, carrying case, jumpers and SMA cable.
In the battle for reliable positioning and timing, the U.S. Army is engaged in a multitude of activities, including mounted and dismounted A-PNT (assured position, navigation and timing) systems, anti-jam technology and pseudolites.
The idea is simple: Take some GPS satellites, and put them on or near the ground. Now you have a navigation system where you have full control over the locations and power of the transmissions. You can ensure that the transmissions reach places that GPS normally struggles with, such as deep urban canyons, forests and valleys.
You can turn up the transmit power, so they are much harder to jam than spaceborne GPS signals. These pseudo-satellites, commonly referred to as pseudolites, have seen steady interest over the years for a variety of applications.
Now the U.S. Army is pursuing the use of pseudolites as part of its initiative to maintain operation in GPS-denied environments.
Pseudolite Basics
There are various types, and use-cases, of pseudolites. In this column we’ll consider the direct-ranging pseudolite, which can be simply considered as a ground-based GPS satellite. If we deploy several pseudolites on the ground, we can imagine that a normal GPS receiver would be able to receive the GPS-standard transmissions and derive a position, just as we would from the space-based satellite transmissions.
The fact that the pseudolites are ground-based introduces us to the first consideration: The locations of the transmitters are no longer described by orbital parameters. Instead of calculating the position of satellites, we need to describe the location of the pseudolites in geographical terms, perhaps with a fixed position described in Earth-centered, Earth-fixed (ECEF) coordinates.
The transmitted navigation data message, which would normally contain almanac and ephemeris information, may now need to contain the geographical position of the pseudolite. Not a problem, but our GPS receivers will need a software upgrade to be able to handle this situation.
The deployment of the pseudolites themselves poses an interesting problem. Imagine a military scenario, where the army is deployed to a region of interest. Navigation warfare is taking place, and GPS is frequently jammed in the region.
High-power pseudolites are deployed to allow the army to navigate despite the jamming, using the same standard-issue GPS receivers that soldiers are familiar with.
The first problem is, having placed your pseudolites in position, how do you know where they are?
You might choose to place your pseudolites at locations that have previously been surveyed, so you know where they are in advance. But this isn’t likely, particularly if you’ve just moved your troops into an unfamiliar area. You might also want to move the pseudolites regularly, as the army moves to new ground. So the pseudolites need to determine their own position, and the easiest way for at pseudolite to determine its own position is with GPS, of course.
Isn’t this a bit incestuous? If we’re using pseudolites because GPS is jammed, how does the pseudolite get its position? This is why military pseudolites will typically be fitted with some form of anti-jam technology, such as a controlled radiation pattern antenna. This allows the pseudolite to receive GPS satellite signals in the presence of jamming, determine its own position, and transmit that as part of its own navigation message.
So, now that we can get pseudolite locations, the next consideration is: Where should pseudolites be placed?
A-DOP-ting a Good Layout
If you know about GNSS, you’ll be familiar with the concept of dilution of precision (DOP). This is essentially a measure of how accurate your position estimate is likely to be, due to the geometry of the satellites: a good wide spread of satellite positions gives us better accuracy.
Figure 1. Poor satellite geometry, resulting in high DOP. (Image: Michael Jones)Figure 2. Good satellite geometry, resulting in low DOP. (Image: Michael Jones)
The DOP can be easily calculated by forming a covariance matrix of the geometry, expressed in an appropriate coordinate frame. If (xn, yn, zn) denotes the position of the nth pseudolite, and (x, y, z) the position of the receiver, we can express the unit vectors from the receiver location to the pseudolite location:
We then form a matrix of these unit vectors:
Finally, we form the covariance matrix from which we can extract the DOP values:
From the elements of this matrix we can determine the various DOP metrics. Let’s concentrate on horizontal DOP (HDOP), given by:
When positioning using GPS satellites, we are blessed with a Walker constellation that generally gives us a nice spread of satellite locations (unless we’re in an urban canyon). On the battlefield, using pseudolites, we do not have the same luxury.
Let’s consider a scenario: a conflict in Helmand province, Afghanistan. An operating base is established at Camp Shorabak, where a pseudolite is operating, and three further pseudolites are deployed in the field. This is shown in figure Figure 3.
Figure 3. Scenario with four pseudolites. (Image: Michael Jones)
Taking a look at Figure 4, we can see what this means for HDOP. The regions shaded green represent locations where our HDOP is less than 2.5, and the red areas represent an HDOP greater than 50.
Soldier #1 is surrounded by the four pseudolites, which is a pretty nice arrangement: We get an HDOP of around 2.4. But if we now consider soldier #2, located a bit further out, we get a very different picture.
Here we have an HDOP of 64, which is fairly terrible. It’s not really that surprising looking at the geometry — to soldier #2 the pseudolites all appear in a similar direction. Soldier #2 cannot expect to achieve good positional accuracy in this arrangement.
Figure 4. HDOP for the Afghanistan scenario. (Image: Michael Jones)
So getting a good geometric spread of ground-based pseudolite locations could be a bit of a challenge, especially if the operating area is constantly moving and changing. The next thing to think about is getting enough height.
Getting the Height Right
When we perform positioning using GPS, we typically track several satellites, which have a range of elevations. Many GPS receivers will choose to ignore the satellites at low elevations, such as those within 5 degrees of horizontal, because those satellites are generally the least reliable. They may be partially obscured, and subject to more noise and fading.
Ground-based pseudolites all have very low elevations by definition. Unless the terrain is perfectly flat and smooth, pseudolites quickly become obscured. Even with flat ground, pseudolite signals will disappear behind the horizon after a few kilometers.
Let’s go back to our Afghanistan scenario again. This time, instead of looking at DOP, let’s look at the geographical coverage of our four pseudolites. Here we’ll assume that our user, the soldier, is 2 meters (m) high, and the pseudolite antennas are mounted at a height of 20m above the ground. That’s pretty high — the army will need to erect some masts.
Figure 5 shows what we get. The green areas are locations where our soldier can see all four pseudolites; yellow three, orange two, and red one. At all other locations, no pseudolite signals can be seen at all. You can quickly see that the range isn’t great — terrain, even small undulations in the ground, is a line-of-sight killer. Add some buildings and trees and the situation gets worse. Reduce the height of our pseudolites below 20m, and the situation gets worse. Soldier #1 can receive three pseudolite signals, but soldier #2 has no hope in this case.
Figure 5. Pseudolite visibility at 20m antenna height. (Image: Michael Jones)
Let’s raise the height of the antennas to a fairly crazy 100m above ground (Figure 6). As expected, we get much better coverage, but soldier #2 still has a problem. To get good signal coverage over any sizable area, you really do need to get those antennas as high as possible.
Figure 6. Pseudolite visibility at 100-m antenna height. (Image: Michael Jones)
Augmenting GPS
Often, we don’t want to rely on pseudolite signals alone. If GPS is available, we clearly want to make use of it, and so we want to use a mixture of both GPS satellites and pseudolites. Consider working in a region of sporadic GPS reception, such as an urban environment or forest. We can usually receive a couple of good GPS satellites, but we also need a couple of pseudolites to help us get a complete navigation solution.
Coming back to one of our original objectives, which is to avoid redesigning the GPS receiver hardware, we need to make sure that our receivers can receive and process both GPS satellite signals and pseudolite signals simultaneously. To achieve this, we can decide to make our pseudolites transmit GPS-standard signals, and make use of unassigned spreading codes to essentially create new satellites in the constellation.
But we quickly run into a problem. GPS satellites are always a distance of around 20,000 kilometers away, and the received signal strength is also fairly constant: around –158.5 dBW. This is a very small signal, as we all know, sitting well below the noise floor. When we suddenly bring high-power pseudolites into the mix, we have quite a nasty problem to deal with.
Near, Far, Wherever You Are
Let’s say, for argument’s sake, we have a pseudolite transmitting with a power of 1 watt. Conducting a basic link budget analysis gives us the plot below and suggests that, at a distance of 10 km from the pseudolite, we can expect to receive the signal at around –112 dBW. This is way above our GPS satellite signal level, but might be manageable by a receiver. Now consider a receiver at a distance of 100 m from the transmitter: we receive a power of –72 dBW, which is huge.
In our quest to augment GPS and make it more robust, we have in fact created a GPS jammer, and achieved exactly the opposite. As with any radio communications link, the received power is extremely sensitive to the distance (varying with the square of distance). In pseudolite terminology, this is known as the near/far problem.
Figure 7. Theoretical received power for a 1-W pseudolite, under ideal conditions. (Figure: Michael Jones)
The near/far problem has given engineers headaches for quite some time. Essentially, the problem comes down to: How can our GPS receivers handle such a massive dynamic range of expected signals? Especially if our objective is to avoid modifying the GPS receiver hardware, if at all possible.
How can a receiver handle the high power of a close-up pseudolite, which is to all intents a jammer, whilst simultaneously receiving the tiny GPS satellite signals from space? Various solutions have been proposed over the years, but one of the current favorite techniques involves pulsing the pseudolite signal.
The idea, then, is to only turn on the pseudolite periodically, essentially applying a duty cycle to the transmission. If a pseudolite isn’t transmitting, it can’t interfere with the normal GPS signals. There are a couple of things to take into consideration here:
What should the pulse duty cycle be, to enable both satellites and pseudolites to be tracked?
How does the GPS receiver behave when presented with alternating large and small signals?
A mathematical analysis of duty cycle effects is beyond the scope of this column, but consider Figure 8 for a qualitative view. Here we have two pseudolites operating alongside GPS satellites. The duty cycle chosen here is for the pseudolite to be operational for 10% of a 1 millisecond integration period. This gives enough time, when the pseudolite is not transmitting, for the low-level GPS satellites to be tracked.
The second pseudolite, which is closer and therefore higher power, transmits for a further 10% slot after the first pseudolite. You can see that each additional pseudolite eats into the time available for tracking GPS satellites, and degrades the signal-to-noise ratio. There are some tricks you can play, such as transmitting multiple pseudolites at the same time if you know they will be similar power levels, but it can get complicated.
Figure 8. Received power versus time, for a pulsed pseudolite scenario. (Figure: Michael Jones)
The Importance of Gain Control
How the receiver copes with the large differences in received power level depends largely on the design of the RF front-end in the receiver. Most GPS receivers will have a certain amount of automatic gain control (AGC), which is a feedback loop designed to keep power levels constant. Many GPS receivers, though, simply aren’t designed with enough AGC to handle pseudolite-level signals (think GPS jammers again).
Military receivers, though, tend to have greater RF handling capabilities, and more bits in the ADC, so are better-suited to the situation. It is then a question of making sure the AGC loop responds in an appropriate time, compared to the duty cycle of pulses.
Figure 9 illustrates a slow AGC response, which is not particularly suitable. Compare this with Figure 10, where we have a fast AGC response, quickly adapting to the switches in power level. A receiver with this characteristic will be better able to track both pseudolite and satellite signals.
Figure 9. Pulsed pseudolites with slow AGC response (in red). (Figure: Michael Jones)Figure 10. Pulsed pseudolites with fast AGC response (in red). (Figure: Michael Jones)
Airborne Pseudolites
If you’ve read this far, you’ll now know that the main problems with ground-based pseudolites are lack of good geometry, signal blocking by terrain, and the horrendous near/far issues. Wouldn’t it be nice if we could raise the pseudolites to a really high altitude, and all these problems would go away? Wait, that’s the GPS satellite constellation!
Ok, let’s not put them that far up. But how about carrying pseudolites on high-altitude airborne platforms instead? Great idea, and that’s why this is a current thread of defense activity in various countries. High-altitude long-endurance (HALE) or HAPS (high-altitude pseudo-satellite; the clue is in the name) unmanned platforms can be used to carry pseudolites at high altitude.
This solution can provide excellent coverage, the pseudolites can be repositioned as necessary, and the near/far problem is also far less pronounced.
I leave you once again with our Afghanistan scenario, from the point of view of a high-altitude airship at 18,000 meters.
Figure 11. High-altitude platform, potentially carrying a pseudolite at 18,000 m. (Image: Michael Jones)
In my April column, I introduced the basic concepts behind GPS anti-jam technology, along with a bit of history around its evolution. I knew this was a popular topic, but I didn’t anticipate the enormous amount of positive correspondence I’ve received since, including many inquiries about where to buy this technology and who is entitled to have it.
So this month we return to the controlled reception pattern antenna (CRPA) topic, to look specifically at the major suppliers of GNSS anti-jam technology in a bid to help you select the best fit for your requirements.
As mentioned in April, CRPAs can trace their roots back to military radar developments in the 1970s and 1980s. It’s no surprise, then, that the main players in the CRPA market tend to be large defense primes. But there are many smaller companies, universities and research institutions that also play in the CRPA arena these days.
What about export?
When GNSS jamming was a little-known military problem, the situation was simple: anti-jam was a military technology for military applications only. Later, as GPS evolved into a dual-use technology, critical infrastructure and civilian applications brought a new demand for anti-jam in non-military domains.
Confusion then abounded about who exactly is entitled to make use of anti-jam technology. There are two distinct factors here: security classification, and export control. Let’s clear these up.
Security classification is simple: If a product is classified, it is only available to customers who hold the appropriate level of security clearance. Usually it is the performance and vulnerabilities of a product that would attract a classified status. As you might expect for in-service military products, the military would not wish everyone to know the performance and weaknesses of its deployed technology. This is why many datasheets for CRPAs omit performance information.
The second issue is export control. This, of course, varies by country. In the U.S., a CRPA developed towards a defense program is likely to have International Traffic in Arms Regulations (ITAR) restrictions attached to it. In Canada, CRPAs are subject to the Controlled Goods Program. In the UK, CRPAs sit on the “dual-use” export control list, which recognizes that CRPAs have both military and non-military application. An export license is usually required.
Before I go any further, a little disclaimer: I am not making any product recommendations in this article. There are many things to consider when choosing anti-jam technology, and you should always consult a navigation warfare expert and carry out appropriate evaluations prior to choosing a product. You should also seek guidance from your own government regarding any restrictions on export or import.
With that out of the way, let’s look at the offerings of a few suppliers. This is by no means a complete list, but I did manage to catch up with a few of the major players to ask them about their anti-jam technology offerings.
NovAtel
I spoke with Peter Soar, business development manager, Military and Defence, at NovAtel about NovAtel’s offerings.
Peter Soar: “The GAJT-710 series are retrofittable GPS anti-jam products that combine a seven-element controlled reception pattern antenna (CRPA) and the antenna electronics in a single unit. The GAJT-AE-N is a GPS anti-jam antenna electronics system that supports a separated four-element antenna.”
Photo: NovAtel
Photo: NovAtel
Photo: NovAtel
Main features: “All three products protect the GPS L1 and L2 bands simultaneously, and are suitable for military (SAASM) receivers as well as open-signal receivers, normal civil receivers and ‘survey grade’ receivers. The wideband design means that the units are ready for M-code. In the GAJT-710, there are seven antenna elements for up to six independent nulls on both frequencies, and the GAJT-AE-N supports four antenna elements, for up to three independent nulls. All products use space-frequency adaptive processing for increased degrees of freedom. System messages provide an indication of jamming presence, even when the nulling is defeating the jamming.”
Intended market: “GAJT-710ML is optimized for land use, while GAJT-710MS is used for maritime and littoral applications. Both types are currently in use on mobile platforms and fixed installations. The GAJT-AE-N is optimized for smaller platforms such as unmanned air vehicles, and is currently in use on a variety of platforms. GAJT products have been shipped to customers in 16 countries to date.”
Example customers: “The GAJT-700ML (a predecessor to the 710ML) was selected for trials by the Canadian Army through the Build in Canada Innovation Program, with exercises performed on the Artillery Observation Post Vehicle (LAV III OPV). Both GAJT variants were selected for field testing by the U.S. Army Communication-Electronics Research Development and Engineering Center (CERDEC) through the U.S. Army Rapid Innovation Fund. The United States Naval Observatory (USNO) selected the GAJT-710ML to satisfy a requirement at sites throughout the Department of Defense Information Network (DoDIN). The GAJT-AE-N is deployed on the Schiebel Camcopter S-100, and was also selected for testing on the M777C1 Howitzer by the Canadian Army.”
Situation with regards to export: “All GAJTs are designed and built in Canada. As such, they are subject to the Controlled Goods Program of Canada, but they are free from ITAR for non-U.S. customers.”
Raytheon UK
Some Raytheon products were mentioned briefly in the April column; I caught up with Alan Wright, business development executive, Force Protection, to get the latest information.
Alan Wright: “Raytheon UK offers a range of anti-jamming products ranging from high-performance products with multiple-element CRPAs to low size, weight and power products. Our current product lines utilize either analog or digital technologies to suit specific end-user requirements.”
Product
Image
Key Features
GAS-1
Analog technology, 7 antenna elements, switchable L1/L2 protection, minimal quiescent time delay, nulling, J/N, M-code signal bandwidth, AE/antenna integrated variant, fiber optic output variant.
Digital technology, 5 antenna elements, simultaneous L1/L2 protection, low size, weight & power, STAP, nulling, J/N, direction finding, anti-spoof, jamming flag, M-code signal bandwidth.
Landshield
Digital technology, integrated 4-element antenna, simultaneous L1/L2 protection, low size, weight and power, STAP, nulling, J/N, direction finding, anti-spoof, jamming flag, M-code signal bandwidth, switched antenna variant.
MiniGAS
Analog technology, integrated 4-element antenna, simultaneous L1/L2 protection or L1 with L2 passthrough, low size, weight and power, minimal quiescent time delay, nulling, jamming flag.
MicroGAS
Analog technology, integrated 2-element antenna, simultaneous L1/L2 protection, very low size, weight and power, minimal quiescent time delay, nulling.
Intended market: “With over 25 years’ experience, Raytheon UK is a world leader in the development, production and supply of GPS Anti-Jamming (GPS-AJ) systems to the majority of the world’s military forces (including the U.S. DoD and UK MOD), with solutions developed and certified for air, maritime and land applications. Raytheon UK has designed and manufactured in excess of 10,000 GPS anti-jam units for the worldwide market.”
Situation with regards to export: “GAS-1, ADAP and SAS are subject to U.S. ITAR restrictions. Landshield, MiniGAS and MicroGAS are free from ITAR and subject to UK export control.”
Rockwell Collins
I spoke with Al Simon, business development for navigation products/solutions, to get the latest on Rockwell Collins’ offerings. Rockwell’s portfolio includes some CRPA products aimed specifically at weapons. Al kindly provided the following table to summarize:
Product
Image
Platform
Key Features
Integrated GPS Anti-Jam System (IGAS)
Weapons (Embedded)
GPS receiver + AJ, nulling and beamforming, spatial, 20 in3, <2 lbs, up to 4 RF antenna inputs, 90+ dB J/S performance *, GPS (simultaneous L1 & L2), path to M-code
Strategic Anti-Jam Beamforming Receiver (SABR)
Weapons (Embedded)
GPS receiver + AJ, nulling and beamforming, STAP, 46 in3, <3 lbs, up to 7 RF antenna inputs, 120+ dB J/S performance*, GPS (simultaneous L1 & L2), path to M-code
NavStorm+
Weapons
Nulling, spatial, 6.9 in3, <.6 lbs, up to 5 RF antenna inputs, 20,000 G shock, 90+ dB J/S performance*, GPS (simultaneous L1 & L2), path to M-code
NavFire
Weapons
Nulling, spatial, 2 in3, <.2 lbs, 1 or 2 RF antenna inputs, 25,000 G shock, 85+ dB J/S performance*, GPS (L1 or L2), path to M-code
DIGAR-200
Airborne, Maritime, Ground
Nulling and beamforming, spatial, 218 in3, <11 lbs, up to 7 RF antenna inputs, 110+ dB J/S performance*, GPS (simultaneous L1 & L2), path to M-code
DIGAR-300
Airborne, Maritime, Ground
Nulling and beamforming, STAP/SFAP, 69 in3, <5 lbs, up to 7 RF antenna inputs, 125+ dB J/S performance *, GPS (simultaneous L1 & L2), path to M-code
Small Platform AJ (Pre-Production)
Ground, Airborne
Nulling and beamforming, STAP/SFAP, 45 in3, <3 lbs, up to 7 RF antenna inputs, 95+ dB J/S performance*, GPS (simultaneous L1 & L2), path to M-code
STAP (Space Time Adaptive Processing); SFAP (Space Frequency Adaptive Processing)
* Beamsteering mode. Actual performance is classified
Situation with regards to export: All listed products are unclassified, but are subject to U.S. ITAR restrictions.
Roke Manor Research
This column wouldn’t be complete without a few words on my own organization. Roke has been developing anti-jam CRPAs since the 1980s, but rarely offers its own products. Typically Roke develops bespoke anti-jam and anti-spoof technology for other defense organizations, including for some products already listed above. Examples of bespoke developments for more specialist markets include Gincan and the Helium antenna.
Photo: Roke
Photo: Gincan
Main features: Both these products are aimed at the commercial civilian market, but do also have defense interest. The Gincan is a very basic low-cost CRPA, with just two antenna elements. The Helium is a conical spiral design, using four antenna elements, and is primarily aimed at protecting GNSS in critical infrastructure. The Helium has excellent low-elevation performance. Both antennas feature very low latency, making them particularly suitable for timing receivers.
Intended market: The Gincan is primarily aimed at providing a basic level of anti-jam capability to the automotive mass market, including cars and trucks, but also has been adopted by some lightweight UAV platforms. The Helium is aimed directly at timing receivers for critical infrastructure, including mobile base stations, digital TV networks, stock exchange and financial institutions, and power and utility grids.
Example customers: Gincan has been delivered to 42 countries, with a mixture of commercial, defense and national security customers. Helium is a relatively new product, and is being trialed on infrastructure in two countries.
Situation with regards to export: Both products are unclassified and suitable for commercial use. They are subject to UK export control as dual-use items, and are ITAR-free.
Others
There are many other suppliers of CRPA technology — unfortunately, too many to cover in this column. Mayflower Communications offer a good range of CRPA products in the form of their NavGuard range. Some other suppliers include Cobham Antenna Systems, BAE Systems Rokar, Thales, Harris Corporation, L-3 Interstate Electronics and Lockheed Martin. I encourage you to contact these companies for the latest information if you are contemplating a CRPA product. If you’re a CRPA supplier and I’ve missed you, please feel free to post a link to your products in the comments section below.
So, that was a bit of a whirlwind tour through some of the products currently around. CRPAs come in all shapes and sizes, and they all have their own particular characteristics and subtleties.
I conclude by reiterating my earlier point. Always conduct a threat analysis, seek the help of a navigation warfare expert if necessary, and properly evaluate your choices. Happy choosing!
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.
Figure 1: TU-16 Badger, an important Soviet electronic warfare platform during the Cold War (Photo: Wikipedia)
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.”
Figure 3. Adaptive antenna basics.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:
Figure: Michael Jones
The average output power can be found by taking expectations:
Figure: Michael Jones
Taking the minimum and rearranging this leads to the well-known Wiener equation:
Figure: Michael Jones
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):
Figure: Michael Jones
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.
Figure 4. Illustrative beam patterns of a CRPA antenna in the presence of jamming. (Figure: Michael Jones)Figure 4: Illustrative beam patterns of a CRPA antenna in the presence of jamming (Figure: Michael Jones)
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.
Figure 5. GAS-1 CRPA. (Photo: Raytheon)
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.
Figure 6. ADAP Digital CRPA. (Photo: Raytheon)
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.
Figure 8. Landshield Advanced CRPA with Anti-Spoof Technology. (Photo: Raytheon)
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.
