by
Darryl Phillips
The hottest topic in avionics these days is GPS. Prices are coming down and improvements are showing up all over the place. So this month's column is devoted to the how and why of the Global Positioning System, and it's competitor GLONASS.
GPS belongs to the Air Force, which belongs to the government, which belongs to us. So it's our system. It consists of a bunch of satellites (21 active, plus 3 spares in orbit) located 10,900 miles above the earth, circling every 12 hours. They're arranged in such a way that at least 5 active birds are visible at all times from anywhere on earth. Or at least should be visible, if you aren't flying in a canyon or making a turn.
Basically, GPS is a DME-DME-DME system. Each bird transmits on the same frequency, 1575.42 MHZ, and primarily just sends out time signals. Based on travel time, your equipment can determine distance to the satellite. By finding distance to three birds, latitude and longitude can be determined, with a fourth satellite altitude can be obtained as well. If your equipment can receive more than four GPS satellites at once, it is able to do various cross checks for improved reliability.
Perhaps the most misunderstood part of GPS is accuracy. We've all heard pilots tell that their GPS will bring them back to the same tiedown spot or other reference. Don't you believe it, sometimes it works, usually it doesn't. There is this thing called Selective Availability, or SA.
If the military would leave SA turned off (as they did during the Iraq war) the system would have pretty good accuracy. Typical error is 15-25 meters (50-80 feet) horizontal, and about 50% worse in the vertical direction. That is, altitude could be expected to be off by 75-120 feet without SA.
But the military says SA is needed to prevent our enemies from using GPS for precision missile guidance. One might wonder what enemies (a Somali warlord, perhaps?), or what missiles (rented vans work pretty well). Anyway, SA is on, and if the DoD has it's way SA will remain on.
What does SA do for accuracy? According to the Coast Guard, who are charged with disseminating GPS information to the public, accuracy with SA turned on is nominally 100 meters (330 feet) horizontal, and 156 meters (512 ft) vertical. That's nominal, or typical, or average error. But it's not maximum.
Maximum error (99.99% probability) is 300 meters (984 ft) horizontal, and 468 meters (1535 ft) altitude. A probability of 99.99 means that the error will be worse than the stated figure about one minute each week. But GPS equipment manufacturers talk of the military turning SA up well past the published maximums, apparently just to see if anyone is paying attention. Scud runners take note.
Hey, fellow aviators, we're talking 1500 ft vertical error here. Your GPS altitude may be indicating too high, or too low. Most days the error will be smaller but you can't count on it. And you certainly cannot use it to protect from altitude violations. If you depend on your GPS to keep you over (or under) various bodies of forbidden airspace, you'd better study up on this. Some GPS units use data from the encoder to help determine altitude, some don't. And encoder data is just pressure altitude which can be off 1000 feet anyway. So beware.
It wasn't always this bad. The first GPS birds were Block I satellites, and they didn't have SA. These have proven to be a hardy breed, and several Block I birds are still operating. The newer Block II units do have Selective Availability. During the war SA was turned off because the troops needed GPS to navigate amongst the sand dunes, and the only available GPS equipment was designed for the civilian market. To get the accuracy needed in the battlefield, the answer was simple, just turn SA off!
Coincidentally, that was about the time most of us were first trying GPS in aircraft. We found wonderful accuracy, and just assumed it could only get better. Unfortunately, after the war the military turned SA back on again.
However, various interests have come up with ways to restore the accuracy we need. The trucking industry uses a differential GPS developed by Magnavox called ACC-Q-POINT (formerly called Pinpoint). And the U.S. Coast Guard is working on DGPS in a big way.
What is DGPS? Suppose you put a GPS receiver at a surveyed location. You know exactly where it is, and you know where it SAYS it is. Subtract one from the other, and you know how much error exists at any instant. Relay that error (the difference) to a truck or ship or aircraft, and their equipment can add the difference to their GPS position, and read true position. Latitude, longitude, and altitude. Voila, Differential GPS. Magnavox relays the differential data on FM radio station subcarriers in the same way pocket pager signals are transmitted. The marine users get their differential data from Coast Guard low frequency (ADF) beacons.
Think about that for a minute. The United States government has a good system. Then they spend extra money to make it a bad system. Then they spend a LOT more money to make it a good system again. Got that?
The Coast Guard operates over 150 beacons on the Atlantic, Gulf, and Pacific coasts. Some of these beacons already have DGPS data, the rest soon will. The entire country will be ringed with high-accuracy DGPS 24 hours a day. Half the population of our nation lives in coastal counties, and strategic locations such as ports and shipyards and naval bases and even Washington DC are within range of a Coast Guard-operated DGPS. I hope no enemies find out about this.
Tell me again why we need Selective Availability?
GPS is actually two systems. The Standard Positioning Service (SPS) is what we civilians get to use. Even if SA were abolished SPS wouldn't be accurate enough for Category I approaches. But the other system on the GPS satellites is called Precise Positioning Service (PPS) and it is fully capable of providing accuracy for precision approaches today. It exists, it belongs to us, we paid for it already. But the military keeps PPS encrypted and highly classified.
The Russian system isn't classified. It's called GLONASS and works much the same as GPS. But it has higher accuracy and it has no SA, so full accuracy is available to everyone. At this time only part of the GLONASS constellation is in orbit, but progress is being made. At the annual GPS technical conference (ION GPS-93) held recently at Salt Lake City, Dr. Jacques Beser of 3S Navigation spoke of his recent visit to the GLONASS control center 50 km north of Moscow. There had been some disruption during the breakup of the Soviet Union, but they are again back in the launch business. In fact, they launch as many as eight birds at a time!
Instead of the originally planned 24 satellites (like our system) the Russians are now beginning to place two birds in each slot, exactly 180 degrees apart. These antipodal orbits allow each satellite to appear to set and almost immediately rise from the other horizon! Of course they don't really, each bird travels around the earth, but the receiver can be fooled into believing that the satellite took no time before reappearing. With 42 active birds, over a dozen will be in view at all times, permitting greatly increased reliability and accuracy far superior to GPS.
One of the papers presented at GPS-93 compared the measured accuracy of GPS and GLONASS. Let me put it this way. The GLONASS graph looks like a target that had been hit bull's-eye by one .22 caliber bullet. The GPS target is the same size, but appears to have been hit by a blast from a twelve gage shotgun! No question, GLONASS is the better system.
While the United States is pressing for GPS to become the worldwide navigation standard, many countries are resisting any system controlled by the Pentagon. Normally friendly countries such as Germany and the U.K. understand that their air commerce (and thus all their commerce) cannot be left in the hands of any foreign military power. It is possible that GLONASS might become the preferred worldwide system. At this time GLONASS is military oriented too, but Burt Rutan was suggesting at Oshkosh that this might change, and he may be right.
At GPS-93, there was even talk of a third system, built and operated by the worldwide aviation community. For now, it's just talk, but a very interesting idea.
In the near term, the most exciting system for aviation at GPS-93 was the Inmarsat DGPS plan. This is a very workable solution for the problems of SA and SPS. Inmarsat intends to put a DGPS "bent pipe" on a geostationary satellite always in view of North America. It will be on the GPS frequency, and to the receiver will look much like any other GPS bird. But the data it carries is the differential correction, obtained from monitoring stations scattered across the U.S. Since the Inmarsat signal can be picked up by the existing GPS antenna and receiver, only a software change is needed to turn any GPS into DGPS. Nifty. I found it interesting that the military didn't seem to offer any objection. Indeed, the brass seemed resigned to the fact that GPS is growing up and may not remain in military control forever.
Only 7% of the GPS market is aviation, and that is expected to decline to one percent within a few years as cars become equipped with GPS moving maps. Even so, about half the technical papers presented at GPS-93 related to flying, mostly to Air Traffic Control. There is no doubt that FAA is depending heavily on GPS to provide the navigation system of the future. The sad thing was the complete and total lack of any General Aviation presence. We are not being ignored, we simply don't exist. The fine, educated, capable people planning systems for the decades ahead think a 757 is an average size aircraft! Perhaps it shouldn't surprise us that there is no place for the Ercoupe or C-150, but there isn't even a place for a Citation or Lear! Where was AOPA? Where was EAA? Where was NBAA?
When the plans were made, where were you?