Ocean Navigator Communications Newsletter #9

The Future of GPS

March 26 was a busy day on both sides of the Atlantic. In the United States, the biennial Federal Radionavigation Plan (FRP) co-authored by the Departments of Defense and Transportation was released. While in Europe, the European Council of Transport Ministers voted to release 450 million euros to begin the implementation of the Galileo satellite-based navigation system.

The FRP is revised and published every two years. It spells out the administration's plans for all radio based navigation systems, including GPS, Loran-C, WAAS, DGPS and others. In addition to the maritime community, the aviation world and railroad industry are also keenly interested in what each report has to say. Of course lately, anybody involved with using GPS (and who isn't?) should also be aware of the report.

Of particular note within the report are significant dates and milestones planned for each system. For instance, Loran-C has been given yet another reprieve -- the FRP mandating that it continue to operate in the short-term while the Administration evaluates the long-term need for the continuation of the system. For most of us bluewater sailors, this doesn't give us cause to go out and celebrate as we've long ago made the move over to GPS. But then again, when did we ever need a substantial reason to find an excuse to go out and celebrate. However, many segments of the fishing and aviation communities still rely on Loran-C and are no doubt encouraged by the report's plans.

GPS will of course continue to operate as is, though two new improvements are planned. The first is the addition of a civilian code on a second frequency. Currently, GPS operates on two frequencies: 1575.42 MHz (called the L1 frequency) and 1227.60 MHz (L2 frequency). On the L1 frequency, there are two signals -- a coarse/acquisition (C/A) signal for everyday civilian use and a precise (P) signal that's encrypted for military use only, known as the P(Y) signal - the Y representing encryption. Don't ask why they chose Y for encryption and not E - I just don't know. The second or L2 frequency contains only the P(Y) military signal, leaving us civilian users with just one (C/A) signal on one (L1) frequency.

In 2008, the FRP calls for the addition of the civilian C/A signal to the second frequency, L2. This will give us, and our GPS receivers, the ability to hone in on two signals concurrently on two seperate frequencies. This allows the receiver to account for most atmospheric interference by comparing one signal to the other and calculating how they each propagated through the atmosphere on their trip from the GPS satellite down to earth. In short -- it means even greater accuracy will be available. We'll first start to get these signals sporadically in 2003 as new satellites are launched, but don't expect full-time coverage until 2008.

The second improvement planned for GPS is the addition of an entirely new frequency, 1176.45 MHz, called the L5 frequency. Before you ask, no I don't know what happened to L3 and L4. L5 is intended to meet "critical safety of life applications such as civil aviation", but of course we'll all benefit.

As for differential GPS, it will be around for a while. Born out of the need for increased accuracy in the maritime world -- mostly for deep draft ships operating in narrow channels, DGPS supplements the GPS signals with a terrestrially transmitted correction (or differential) signal to account for errors in the GPS derived position. So a DGPS receiver actually receives two signals - one from the satellites and then a correction signal from a nearby DGPS beacon on land. The FRP shows no end date in sight for the DGPS system. In fact, due mostly to lobbying by a national railroad group, the DGPS system has been undergoing an expansion into the interior to blanket the entire continental 48 states with differential signal beacons rather than just keeping them along the coasts. Some other countries also use DGPS for increased accuracy. . . .

Lately, DGPS has fallen somewhat out of favor with the recreational boating community. This is due mostly to the fact that the Department of Defense's intentionally injected errors (Selective Availability or SA) were removed as of May 1, 2000 leading to increased accuracy, and also to the fact that the Wide Area Augmentaion System (WAAS) signals are now available. WAAS is in theory similar to DGPS, with the significant exception that the beacon transmitting the correctional signals is on a satellite rather than on land. This correctional signal is able to reach nearly an entire hemisphere all at once. And since the WAAS signal can be received using a convential GPS antenna, there's no need for a separate DGPS whip antenna.

The Federal Aviation Administration (FAA) initiated the creation of the WAAS system largely because the Coast Guard's DGPS system doesn't work well for planes. Though the WAAS system isn't due to be operational until 2003, the signals are available for use (for non-safety appilcations) and they work quite well. The European EGNOS signals are compatible with the US WAAS signals, so your WAAS receiver can still pick up satellite differential signals while cruising the Med.

In summary, GPS is sticking around for a while and will be continuously improved with new frequencies and WAAS/EGNOS improvements. Next time, we'll take a look at the Europeans' decision to continue with Galileo - essentially a non-US based GPS system. One of the interesting things about Galileo is that it's motivated almost as much by politics as it is by technology. More to come on that.

- Dan Piltch
dpiltch@MarineComputer.com