by Tom Clancy
The backpack version of the SINCGARS jam-resistant frequency-hopping radio. Dismounted cavalry scouts might carry this type of radio.
OFFICIAL U.S. ARMY PHOTO
Revolutionary advances in electronics during the 1970s and ’80s left the U.S. Army with a collection of obsolete radio sets that were heavy, and hard to maintain, drew too much power, and put out too much heat. They were often incompatible with frequency bands and transmission modes used by the Navy and Air Force. Even worse, they were vulnerable to enemy interception and jamming. The Russians, with decades of experience in jamming Western radio broadcasts (Voice of America, Radio Free Europe, etc.), had made “radio-electronic combat” a key feature of their tactical doctrine.
In the early 1980s the U.S. Army specified the design for a new SINgle-Channel Ground and Airborne Radio System (SINCGARS). SINCGARS (which entered service in 1988) is a family of compact, lightweight, reliable, and secure FM radios that can use any of 2,320 different frequencies between 30 and 87.975 MHz in the VHF band. The Army plans to procure 150,000 SINCGARS radios from General Dynamics (San Diego, California) and ITT Aerospace (Fort Wayne, Indiana), with additional orders from the Marine Corps and various government agencies. The system resists jamming by using “frequency hopping”: The transmitter and receiver are synchronized to jump between widely spaced frequencies at very short intervals. To defeat the system, the enemy would have to radiate a tremendous amount of energy spread across a large slice of the electromagnetic spectrum. There are eight basic models:• AN/PRC-119-This is a backpack model, capable of being carried by a man.
• AN/VRC-87-A vehicle-mounted, short-range model.
• AN/VRC-88—A vehicle-mounted, short-range model that can be dismounted if desired by the crew.
• AN/VRC-89—A vehicle-mounted, long- and short-range transceiver model.
• AN/VRC-90—A vehicle-mounted, long-range transceiver model.
• AN/VRC-91—A vehicle-mounted, long-range model that has the option of being a dismountable short-range transceiver.
• AN/VRC-92—A vehicle-mounted, dual-channel (essentially two radios together), long-range model with retransmission (over separate radio net) capabilities.
• AN/ARC-201—The standard helicopter/aircraft transceiver model.
The AN/PRC-119 backpack portable model weighs about 22 lb/ 10 kg. The short-range models have a power output of about 5 watts, and a maximum range of between 2.5 to 5 miles/4 to 8 km. Long-range models have a power output up to 50 watts, and a maximum range of 5 to 22 miles/8 to 35 km. These relatively low-power output levels make it harder for an enemy to detect and locate the transmitter. All of the SINCGARS systems can handle voice, text, digital, and data communications (just hook up the desired transmission device), and even fax transmissions with the appropriate attachments. The latest models also have built-in cryptographic (scrambling) units for added security. During Desert Shield and Desert Storm, SINCGARS stood up to heat, dust, sandstorms, and bad weather with exceptional reliability. They should be the standard Army communication package well into the 21st century.
The NAVSTAR GPS System
To wage a successful war of maneuver, a commander must constantly know the answer to two questions:Where the hell is the enemy?
Where the hell am I?
Military history is filled with defeats that resulted after bold, aggressive flanking columns got lost in the woods, or stouthearted defenders dug in on the wrong hill. Ever since reasonably accurate terrain maps were first drawn up (around the beginning of the 18th century), armies have tried to teach junior officers the art of map reading and navigation. The results have often been disappointing. The advent of modern electronics has provided some limited advances with gyrocompasses and inertial navigation systems, though their high cost limited their usage. Early satellite navigation systems also had promise, though their cost, and the cost and size of receiver sets, made them unusable by most of the military. Something new was needed to provide exact position accuracy. That something was the NAVSTAR GPS system.
The NAVSTAR Global Positioning System (GPS) is a dramatic advance in navigation. It starts with an array of twenty-four satellites (called a constellation) 10,900 miles/17,600 kilometers up in an orbit inclined at an angle of 55° to the equator. At this altitude, it takes a satellite twelve hours to circle the earth. When all twenty-one primary satellites (plus three spares) have been launched, at least four will always be visible to a receiver virtually anywhere in the world. Each satellite carries a super-accurate “atomic clock” and a low-powered transmitter that broadcasts specially coded time signals and status messages on radio frequencies of 1227.6 and 1575.42 MHz. By correlating the signals from at least four satellites, and doing some fancy trigonometry, the computer in a small portable receiver can determine your location, altitude, speed, and time with great precision. Relatively inexpensive civilian GPS receivers are typically accurate to within 25 meters/82 feet of 3-D positional accuracy. And military receivers, which can decode a more accurate encrypted part of the GPS signal, called the P(Y) code, get much better performance. The original GPS specification demanded accuracy with a P(Y)-code receiver to 16 meters/52.5 feet, though positional accuracies of around 5 meters/16.4 feet are considered typical by military GPS users. As an aside, by linking several GPS receivers and a radio transmitter at a known (i.e., surveyed) geographic point, surveyors can determine positional accuracy down to plus or minus one centimeter/.4 inch! As an added bonus, since every GPS receiver automatically synchronizes itself to the super-accurate clocks on the satellites, combat units can now precisely coordinate their actions in time as well as space. Handheld military GPS receivers as small as 14 oz/397 grams are already available, with the ability to display information (in six languages!) about the exact position of the sun and phase of the moon on any given day.
Now you might ask, since GPS receivers are commercially available from many American, European, and Asian electronics firms, what prevents an enemy from buying and using off-the-shelf units to gain the same kinds of tactical advantages? The GPS system was designed to provide “selective availability” during a crisis or conflict. When the GPS satellites receive a special encoded command from Air Force ground controllers, they can start broadcasting less accurate data. Then, unless you have a military P(Y)-code receiver, and the proper cryptographic key for the day, you will only be able to determine your position within about 100 meters, rather than 25 meters. In fact, it is possible for the controllers to selectively degrade (say, to only a 1,000-meter/.61-mile accuracy) the non-P(Y) accuracy of the system on a local basis, such as over the Middle East during a time of conflict, if that is desired.
The Small Lightweight GPS Receiver, which enabled the Army to navigate the deserts of Iraq. The square bulge on the top of the case is the antenna, which can detect transmissions from five NAVSTAR satellites simultaneously. To receive the highly accurate P (Y) code, the device must be loaded with a “crypto key.” The command “Zeroize Cryptokeys” erases this secret information.
TRIMBLE NAVIGATION
So how does one make use of this amazing little bit of technology? Well, consider the following. The AN/PSN-10 (V) TRIMpack GPS receiver is the best-selling military GPS receiver in service today. Literally thousands of them were used in the Persian Gulf during Desert Shield and Desert Storm, with more than 18,000 units in service worldwide since then. A civilian version, called the Scout-M, is available for anyone who cares to use one for their 4WD or bass boat. Made by Trimble Navigation, the Centurion, the newest P(Y)-capable version, weighs 3.1 lb/1.4 kg, and is about the same size as a good pair of binoculars. The flat antenna is built into a nearly indestructible green plastic case. It has a removable power pack with NiCad rechargeable batteries (non-rechargeable Lithium batteries were used in earlier units). On the front panel there is a backlighted (suitable for use with the new AN/PVS-7 low-light goggles) four-line LCD display panel, a rotary selector switch for different operating modes, and two toggle switches
for changing the various options on the display.
The unit also has a serial data port that allows it to communicate with any compatible computer, digital system on a vehicle or aircraft, or even other GPS receivers. During Desert Shield the Army acquired 8,000 of these Small Lightweight GPS Receivers (SLGR—the troops call them “sluggers” for short) for about $3,600 each under an emergency procurement. The SLGR can be programmed with the locations of up to 1,089 “waypoints”; and by simply flipping the knob to R+A (Range and Azimuth), you can read out your current range and bearing from any three waypoints (such as an enemy position, a friendly base, or a logistics base). During Desert Shield, SLGRs were initially so scarce, and so vitally needed in the trackless desert, that many personnel tried to buy them directly from the manufacturer, using their own credit cards. Special Forces teams that operated deep inside Iraq, and a few pilots shot down in enemy territory, credited their survival to the precision of their GPS receivers, which enabled them to link up with friendly helicopters in exactly the right place at exactly the right time.
Using the SLGR is simple in the extreme (please note that I was using a non-P(Y)-code SLGR for the following) so much so that during Desert Shield, soldiers who got them just ripped open the packing containers, and were using them within half an hour. You start by turning the selector knob from the OFF position to Status and Setup (STS). As soon as the startup screen clears, the STS readout screen indicates the following data:
This means that the SLGR is not yet tracking any satellite vehicles (SVs). Thus, there is not a GPS position yet available, as shown by the GPS n/a indicator. The Battery used indicator tells us how much time has been logged since the last charging; and the INT antenna indicator shows that we are making use of the internal antenna, as opposed to an externally mounted one. After a minute or two you will begin to see the SVs counter begin to change. Once it reads:
you have a three-dimensional fix (two-dimensional fixes are possible with just three satellites) and can start to work. One of the first things that you might want to do is set up your preferences for using the SLGR. By clicking the horizontal L/R toggle switch, the
The Datum: WGS-84 indicator tells you that the SLGR is currently using the WGS-84 Merchich mathematical model of the earth to figure its coordinate readouts. Different maps use different earth models as their point of reference. Suppose that we wanted to take a walking tour of Washington, D.C. If we acquire al : 50,000 tactical map (Sheet 5561 I, Series V734, Edition 1-DMA, Alexandria) of the area from the Defense Mapping Agency (they are available through the National Geological Survey and NOAA), and we look at the legend of the map, we find that it conforms to the 1927 North American Datum. So, we click the L/R switch once, and the Datum indicator begins to blink. Now, using the vertical switch, we scroll through until we get to NAD-27, CONUS. Clicking the L/R switch twice now, we get to the Time: setting. This allows you to select either UTC (Universal Time Code—Greenwich Mean Time) or Local (your current time zone) as your clock readout. Leave them at UTC for the moment. Clicking the L/R switch again, we can select the readout units, in this case ENGLISH/DEGS (English and degree units). This done, we again use the L/R switch to set the Mode indicator to Degrees-Minutes-Seconds (other options include UTM and the military grid reference system MGRS, as well) and True North, so that it reads DMS/Tr. This done, you can now use the L/R and INC/DEC switches to get back to the basic STS screen. The settings that you have just made will now be the defaults for the system until you again change them.
Now, if you turn the rotary switch to POS, the receiver will display a time and position. A fix on the front steps of Union Station (I love to travel on trains!) on a Saturday afternoon (EST) might show as follows:
A quick review of the map shows that to be right in front of the station, just south of the Metro station. If we were to go back to the STS setting and reset to the Military Grid Reference System (MGR) setting, then go back to the POS setting with the selector knob, then the readout would look something like this:
If you had one of the maps drawn to this system, specifically sheet 18S, you would be able to see down to the square meter where the unit was telling you your position was. But let us assume that we want to use the GPS receiver to set up a tour for others to follow. You now select the Waypoint (WPT) setting for the selector knob, and you will see the following display:
By clicking the L/R switch to the left, you can make the
This done, we walk southwest across Constitution Avenue to the north steps of the National Air and Space Museum (one of my favorite places on earth!) on the north side of Independence Avenue. Taking another fix, we find the POS readout to be:
Another check of the map shows the receiver to be generating good fixes. We add the new position to the waypoint array, and the readout shows:
Our next waypoint fix point is the west steps of the Capitol building:
Note the sudden drop in altitude, though it still is within the accuracy tolerances (100 meters/328 feet). We move on and add waypoint fixes for the Washington Monument (east parking lot):
Moving west again, we walk up the reflecting pool, past the Vietnam Veterans Memorial (be sure to stop and see the black wall), and up to the steps of the Lincoln Memorial to get our next fix:
Continuing west again, we walk across the Memorial Bridge over the Potomac River to the entrance to Arlington Cemetery and our final waypoint:
With the six waypoints now stored, it is possible to use the stored positions to get actual guidance information for our return walk to Union Station. For example, if we switch to the R + A setting, and use the L/R and INC/DEC settings to display the data for waypoints AA (Union Station), AB (the Air and Space Museum), and AC (The U.S. Capitol), it should look something like this:
This shows us that we are 3.5 miles from our starting point (based on a great circle navigation plot—essentially as the crow flies) at a heading of 84° true (as opposed to magnetic). Now suppose that you want to use the SLGR to dynamically navigate you back to Union Station, on-the-fly as it were. To do this, turn the knob to the NAV mode setting. You should see the following:
This tells us that to navigate to waypoint AA (Union Station) we need to move along a heading of 084° true (roughly east along the mall). The velocity (vel) readout of 0MPH tells us that we are not moving yet, and the time to go (ttg) readout of * means that we have not yet started our trip home to the train station. As soon as we begin to walk (most humans walk at about 5 mph/8.2 kph), the SLGR will begin to calculate vel and ttg numbers for the readout (anything over 3 mph/5 kph will give the SLGR the necessary Doppler to begin calculating these readings). Almost immediately, the following readout should appear:
This tells us that if we maintain a straight line to the train station (unlikely in Washington traffic, but what the hell), and walk at our current speed all the way, we can get back in forty-two minutes. Now, the SLGR will continuously update these figures, and if we stop and look at something else, will continue to guide us home. Later, if we want to add other known places for waypoints, we can insert them via the toggle switches on the front panel of the SLGR. In addition, we can connect our SLGR to another, and dump the waypoints to anyone else with one of these clever little devices.
You might ask, what does all this have to do with tactical operations in the field? More than you might think actually. Consider the following story. Prior to the beginning of Desert Storm, special forces teams from the United States, Great Britain, and other Coalition Allies went into Kuwait and Iraq armed with their usual array of weapons, as well as some of the little SLGRs doing exactly what we have been doing, taking readings and
fixing waypoints. These found their way to the SLGRs of the cavalry officers in 2nd and 3rd ACRs, so that they could program their own SLGRs to guide them to the phase lines and road junctions that even the Bedouin nomads couldn’t find. One story has it that an Air Force officer, operating under diplomatic passport while it was still possible, flew to Baghdad in late August 1990 with nothing more than a briefcase containing an SLGR. Once there, he was driven to the U.S. Embassy, and went to the courtyard to sit on a particular bench to wait for the GPS constellation to fly overhead (there were only six satellites up at that time) and take a single fix. Once this had been done, he got up, went back to the airport, and flew home with that one waypoint in the memory of his SLGR. From that one firm geographic fix came all of the targeting coordinates for the Tomahawk cruise missile and F-117 Stealth Fighter targets that were hit early in the war. Later on, so important was GPS to the conduct of the war that the famous “Hail Mary” sweep into Iraq could not have taken place without it.
Now that the system has been pretty much completed, the Army and the other services are rushing to put GPS receivers onto virtually everything that moves. Tanks, helicopters, fighters, guided missiles, and even trucks are all being equipped with the new navigational tools. In late 1993, the largest GPS receiver procurement program running is the Portable, Lightweight GPS Receiver (PLGR-called a “plugger” by the troops) being procured from Rockwell International. These handheld receivers look like oversized portable calculators, with LCD displays and keyboards, and will be issued to infantry units, scout and special forces teams, and other units requiring GPS navigational capabilities. In addition, there are many other GPS-based programs being developed, all of which will be more accurate than the systems that they will replace. And maybe most important of all, the GPS system is being made available for all kinds of civil uses. Everything from civil surveying to blind airliner-landing systems are being tested. By the end of 1994, it is probable that GPS receivers tied to moving map displays (remember the one in James Bond’s Aston-Martin?) will be available as optional equipment on civilian automobiles. GPS may be the most exciting technology that the military has introduced in years. What makes it even more interesting is that it is something we all can use. Those who conceived it so many years ago deserve our thanks for this new kind of public utility, which finally tells us where we are, and how to get where we want to go.
