The Return Of LORAN!

 I've written a bit about LORAN before. LORAN stands for LOng RAnge Navigation. it's not a great acronym, (technically an initialism.) LORAN came in many flavors: AT-LORAN, SS-LORAN, LORAN-A, LORAN-B, LORAN-C, LORAN-D, and LORAN-F. There was a reason they skipped the letter "E."  LORAN-F was an experimental standard Motorola was playing with while eLORAN was being deployed. Note the emphasis there on the past-tense. eLORAN or Enhanced LORAN, uses the same specifications as LORAN-C but adds a data channel on the transmitted signal. The roll out of eLORAN... the successor to LORAN-C just never happened. More here.

In 2008 George Bush Jr. threatened to shut down our existing LORAN-C system. Instead after much hand-wringing the administration of the LORAN system was transferred from the US Coast Guard (USCG) to the Dept. of Homeland Security. A year later, in November of 2009, the USCG announced that LORAN-C is no longer needed for maritime navigation.The Department of Homeland Security concurred and they terminated the transmission of all U.S. LORAN-C signals on February 8th, 2010. Many fishermen disagreed with the verdict but LORAN was gone.

Fast forward five years. The Federal Register (the official journal of the US federal government) began publishing articles about possible new eLoran implementations. [here] and [here] Back in 2009 numerous federal agencies, committees and business groups were advocating that we keep LORAN. GPS is great, but it has outages and is vulnerable to large scale long-duration failures. It's not a hardened system and there is no back up system. In 2009, the US National Space-Based PNT Executive Committee Advisory Board plainly suggested that "eLoran is the only credible and cost-effective option that can be declared operational in a timely fashion."  They argued that eLORAN had to be deployed because there was was no other possible answer. In another technical paper they went as far as to suggest we merge GPS and eLoran to create a more redundant, robust system. Obviously that never happened.

Several countries in Europe already moved to eLORAN. In the western world, it's the US alone which uses GPS only for maritime navigation. But the topic just keeps coming back. Then in March of 2015, The Federal register asked for public comment on eLORAN:

"The purpose of this notice is to seek comment from the public and industry regarding potential plans by the United States Government to implement an enhanced Long Range Navigation (eLoran) system as a complementary positioning, navigation, and timing (PNT) capability to the Global Positioning System (GPS). The positioning, navigation, and timing performance of eLoran will vary widely depending on the number of transmitters and monitor sites for corrections that are implemented."

Perhaps hope still remains...

GEE

In 1935, the British began developing GEE. It was developed by Robert Watson-Watt at the Air Ministry and introduced by the Royal Air Force (RAF). The idea of hyperbolic navigation known in the 1930s, but the first trials didn't happen until 1941.  Starting in 1942 these radio signals were used to guide the WWII carpet bombing of Germany. Germany began it's attempts at jamming the GEE system in 1943 but these were largely ineffectual. So what's hyperbolic navigation?

Hyperbolic navigation is a class of radio navigation systems based which determines position based on the  microsecond difference in timing between the reception of two signals.  While you need three points to triangulate a location in 3 dimensional space, the surface of the earth is effectively a 2 dimensional plane. But hyperbolic navigation requires the plotting all of the potential locations of the receiver for the measured delay. This produces a series of hyperbolic lines (a parabola) on a chart. Taking multiple measurements narrowed this down. GEE was the first such system. 

Imagine two terrestrial radio stations located 186 miles apart. At light speed (the speed radio signals travel) their signals can reach each other in exactly 1 millisecond. So Station A emits a pulse, this pulse is received at Station B and it triggers the transmission of another pulse 1 ms later. But a ship in motion at a third location will receive these at odd intervals because of the distance from each station. But knowing their interval, they can plot a circle around each transmitter, and at their points of intersection (parabolas) they can determine their possible location down to a set of two. Then they had to use traditional navigation to pick which of the two. The source of that problem... is time.

In order to measure the exact time it took for a signals to reach the receiver, the receiver must know the exact time the signal was transmitted. But reliable millisecond time-keeping wasn't possible in the 1930s. The most common clock to use with GEE was a crystal oscillator. But that drifts about 1 to 2 seconds in a month. That may sound trivial, but 2 seconds is a distance of 372 miles... that's the difference between Bermuda and the Carolina coast. That's a big deal. (This problem wasn't overcome until the advent of atomic clocks in the 1960s.) So instead of absolute time, differential time was used. Your distance from the two transmitters was unknown, but the difference between their signals was measurable on an oscillograph and the delay curves were available on a chart.

GEE signals were all sent on the same frequency which made it difficult to distinguish the original signal from the response signal. Chains of GEE stations were built in the UK, France and northern Germany. Ultimately it was replaced by VOR systems and LORAN. Some of the British GEE equipment was used in the later GEE-H system which operated at the 20-80 MHz range. The last GEE chain was shut down in 1970

The irony in all this is that hyperbolic navigation was originally developed by Germany. Meint Harms lectured on the topic as a masters student at Seefahrtschule Lübeck, a navigational school. After becoming a professor of Mathematics, Physics and Navigation at the Kaisertor in Lübeck he began to demonstrate models of the system which he patented in 1932.

How to Jam GPS

This is about as fun as breaking RFIDs.  There are  about 500 million GPS-dependent devices is the United States.Being tracked by GPS is creepy, but being rescued because of GPS is not creepy, and Apple using it to gauge traffic congestion is pretty useful. You can decided where your own personal comfort level is. 

So here's the good news, GPS satellites only broadcasting at a power of about 10 watts.  Being 12,000 miles they're quite easily overpowered, even by devices broadcasting just a couple watts.There have been several large-scale incidents of accidental GPS jamming already. In 2007, the Navy was conducting an exercise in communications failure. They jammed their own radio communications in the exercise and "accidentally" jammed the radio signals from GPS satellites over a large portion of San Diego. I'll quote New Scientist about the scope of the error:

"In the tower at the airport, air-traffic controllers peered at their monitors only to find that their system for tracking incoming planes was malfunctioning. At the Naval Medical Center, emergency pagers used for summoning doctors stopped working. Chaos threatened in the busy harbour, too, after the traffic-management system used for guiding boats failed. On the streets, people reaching for their cellphones found they had no signal and bank customers trying to withdraw cash from local ATMs were refused. Problems persisted for another 2 hours."

In all likelihood they did intend to jam GPS, they just didn't intend to do so over such a large area. they probably miscalculated how much power it would require... or how many civilian devices would be effected. It's so simple it can happen by accident. LightSquared, a wireless company operating in the US has had several problems with GPS interference that have been hampering their roll-out. [More here] All you have to do is broadcast noise on the same frequency as the GPS satellites. One jammer with a simple GPS jamming device could take out GPS over a few square miles if his signal were unobstructed.  FYI: This is a type of failure the LORAN system wasn't sensitive to.

There are dozens of different commerciall y available devices the smallest and cheapest of which  (pictured above) is made by the Hong Kong-based BrandoWorkshop. It's a basic GPS jammer that plugs into a car's cigarette lighter.  There were over 40 of them on ebay when I checked.

For the record the FCC has been quite clear in their legal opinion. Michele Ellison, Chief of the FCC's Enforcement Bureau stated the following back in February:

"While people who use jammers may think they are only silencing disruptive conversations or disabling unwanted GPS capabilities, they could also be preventing a scared teenager from calling 9-1-1, an elderly person from placing an urgent call to a doctor, or a rescue team from homing in on the location of a severely injured person. The price for one person's moment of peace or privacy, could be the safety and well-being of others,"

The NAVSYS Corporation (a military contractor) developed a GPS Jammer Detection and Location system called JLOC. It's run by  National Geospatial Intelligence Agency. Much like the way the FCC catches pirates with triangulation, JLOC uses a network of GPS receivers capable of detecting regions of unusually high signal levels and low signal-to-noise ratios. How precise it can be while GPS isn't working... they don't exactly say. More here.

Low-Frequency Navigation Stations

Your vocabulary word for the day is "Multilateration." Rather than mangle the defintion I am taking it directly from Wikipedia.

"The process of locating an object by accurately computing the time difference of arrival (TDOA) of a signal emitted from that object to three or more receivers. It also refers to the case of locating a receiver by measuring the TDOA of a signal transmitted from three or more synchronised transmitters."

It's like triangulation, except distances are determined by the time it takes a radio signal to travel a known distance. LORAN-C stands for LOng RAnge Navigation. LORAN-C is just one version of Loran. It was preceded (of course) By LORAN-A and LORAN-B. LORAN-D was used exclusively by the United States Airforce. LORAN-A transmitted on 1750 kHz, 1850 kHz, 1900 kHz and 1950 kHz. the System wasn't very accurate and LORAN-B was a phase comparison variation that tried to improve on the model. There was infact a LORAN-F system that Motorla experiemnted with. This was never launched. the migration to a digital "eLORAN" system has already begun. More here.

The present LORAN system is harshly effected by weather, and changes in solar radiation including sunrise and sunset. Most LORAN-C transmitters use mast radiators insulated from ground with heights between 190 and 220 meters. Some of these stations broadcast in excess of 1000 kW. They each broadcast a precisely timed 1Hz signal and all of them are perfectly synchronized to UTC within 100 nanoseconds.

A chain of LORAN-C radio navigation stations transmitting pulse-coded signals on the same frequency. Provides navigation location and timing services for air, land and marine users. The Loran-C system serves the 48 continental states, their coastal areas, and parts of Alaska. Each station has a Group Repetition Interval and has an ID number starting with GRI to indicate which chain they are a part of. Obviously in order to determine your location through multilateration you need at least 3 of these stations. That's why you'll see the same ID three or more times below. Inside the United States, the following stations broadcast:

GRI 7960 - Tok, AK
GRI 7960 - Narrow Cape, AK
GRI 7960 - Port Clarence, AK
GRI 7980 - Carolina Beach, NC
GRI 7980 - Raymondville, TX
GRI 7980 - Malone, MS
GRI 8290 - Gillette, WY
GRI 8290 - Baudette, MN
GRI 8290 - Havre, MT
GRI 8970 - Dana, IN
GRI 8970 - Malone, MS
GRI 9610 - Gillette, WY
GRI 9610 - Las Cruces, NM
GRI 9610 - Raymondville
GRI 9610 - Searchlight, NV
GRI 9940 - George, WA
GRI 9940 - Middletown, CA
GRI 9940 - Searchlight, NV
GRI 9960 - Nantucket Island, MA
GRI 9960 - Carolina Beach, NC
GRI 9960 - Caribou, ME
GRI 9990 - Attu Island, AK

Strangely in 2008 George Bush Jr. threatened to shut down the highly effective LORAN system. Instead after much hand-wringing the administration of the LORAN system was transferred from the Coast Guard to the Dept. of Homeland Security.

The First Radio Beacon

A radio beacon is non-directional transmitter that usually transmits a constant signal on a licensed radio frequency. Before the days of VOR, GPS, LORAN, beacons were used with direction finding equipment to find ones relative bearing to a known location. The beacon being the known location.

There are a few different kinds of radio beacons. There are amateur radio beacons that are used to test propagation of radio signals. At sea, there are marine beacons, though largely phased out. In aviation they use a beacon called Non-directional Beacon (NDB). These are used to help find airports.

In 1921 the first non-directional radio beacon (N.D.B.) went into service. It was operated by the Bureau of Lighthouses to assist marine navigation. A manual radio direction finder that was light enough for airborne use was developed in 1934 for these very NDBs. They were the core of navigational technology for decades.

By June 30, 1955, there were 173 NDBs operating in the United States. Since then a variety of much more dependable and accurate systems have been developed. The current plan is to decommission these beacons over the next few years as they are replaced with newer supposedly better technology. But at least today NDBs still pepper the landscape. These that still remain in service are kept as an emergency backup system in the case that these more "sophisticated" modern systems fail. Old obsolete technology is always more sturdy somehow...