Marine Differential Corrections Egypt Navigates into the DGPS Era
Bruce Butler "Marine Differential Corrections Egypt Navigates into the DGPS Era".
GPS World. FindArticles.com. 17 Dec, 2009. http://findarticles.com/p/articles/mi_m0BPW/is_7_11/ai_n27562314/
The Suez Canal has made Egypt a hotbed for marine travel and trade, with thousands of ships passing through
the area each year. In 1997, the Egypt Ports and Lighthouse Authority moved toward upgrading navigation
systems throughout the region by implementing a differential GPS network that provides complete coverage of
Egypt -- for centuries it has been a Mediterranean center for maritime commerce and travel. With the
completion of the Suez Canal in 1869, the country became even more important for vessels and people traveling
the world's oceans. In the not-too-distant past, though, simply navigating across the Mediterranean Sea to find
the canal could prove challenging.
Almost 200 kilometers in length, the Suez Canal is the longest canal in the world without locks. It provides a
6,000-mile shortcut for ships traveling from London to Bombay. Nearly 10 percent of all global sea trade passes
through the canal, including oil from the Gulf, cars from Japan, coal and iron from Australia, and wheat from
North America. The Egyptian government collects more than $2 billion in tolls annually.
Recently, Egypt entered a new age in marine navigation, when the Egypt Ports and Lighthouse Authority (PLA)
implemented its own marine differential GPS (DGPS) radiobeacon service, covering thousands of kilometers
and spanning two coastlines. Today, ships can navigate the approaches to the canal, as well as the rest of
Egypt's coastlines, with much greater accuracy than ever before.
COVERING THE COASTLINES
PLA awarded the contract for the Egypt Marine DGPS network in 1997 to Beacon Co., an Egyptian firm
specializing in the engineering, installation, and maintenance of marine aids-to-navigation in Egypt and the
Middle East. Beacon Co., of which author Ramadan Mohammed All is owner and president, subcontracted the
Canadian systems integration firm CANAC/Microtel to provide the system design, equipment integration,
training, and installation support for the project. That company's maritime group, which MacDonald Dettwiler
(MDA) acquired in 1997, was also selected to implement the Canadian Coast Guard's DGPS network. Author
Brace Butler joined CANAC/Microtel just after the award of the Egypt DGPS contract and was put in charge of
the engineering effort.
Even before the Egypt contact award, though, two engineers from CANAC/Microtel traveled to Egypt to work
with Beacon to determine site eligibility. They visited a number of potential locations selected by PLA to
perform standard site surveys, analyzing them based on the following criteria:
* proximity to the ocean;
* an unobstructed view of the sky, preferably horizon to horizon;
* existing PLA real estate for the various antennas and site buildings;
* access to electrical power and telephone lines;
* good ground conductivity; and
* an acceptable level of radio interference.
We ultimately chose six radiobeacon sites that provide total and overlapping coverage of Egypt's Mediterranean
and Red Sea coastlines. Figure 1 and Table 1 show these locations along with the estimated 75 [micro]V/m
(microvolt per meter) field strength contours for each site. Because most commercial DGPS beacon receivers
can lock onto beacon signals at much lower strengths (10[micro]V is typical), this site arrangement ensures
virtually complete dual coverage along both coastlines except at the ends of the network.
The DGPS network consists of six Control Stations, each one a reference station and radiobeacon broadcast site
with integrity monitoring and communications links. Along the Mediterranean, three sites (Mersa Matruh,
Alexandria, and Port Said) provide coverage from Egypt's western border with Libya east to Israel. The three
southern sites (Ras Umm Sid, Ras Gharib, and Quseir) provide coverage from the northern end of the Gulf of
Suez south to Egypt's border with Sudan. Port Said and Ras Gharib together also provide full and overlapping
coverage of the Suez Canal and the oil fields in the Gulf of Suez. In addition, a central Control Monitor in
Alexandria regularly and automatically polls each Control Station using communications links to obtain up-to-
date site status information. If an alarm condition occurs at a site, its Control Station can also independently
contact the Control Monitor at any time.
Onsite Systems. Because a DGPS radiobeacon station is a key aid-to-navigation, it must be designed with high
reliability to minimize downtime. These computer-based systems are expected to ran for years without
interruption of service. We therefore based our system hardware choices on the proven architecture developed
for the Canadian Coast Guard. Figure 2 shows a system block diagram, with each DGPS rack consisting of the
* dual-redundant GPS reference stations (L1, upgradeable to L1/L2);
* dual-redundant integrity monitors;
* dual-redundant industrial computers;
* color monitor, keyboard and trackball;
* serial multiplexer;
* programmable logic controller;
* high-frequency (HF) communications equipment (transceiver, modem, and ALE);
* radio-frequency (RF) cross-over switch; and
* telephone modem.
We selected each component for operation from 24 VDC that allows the use of a 24-hour standby telecom-
grade DC backup power system. The central Control Monitor in Alexandria, colocated with a Control Station, is
a standard personal computer (PC) with the same communications equipment as a Control Station.
In addition, high reliability and real-time performance requires an operating system and application software of
a much higher caliber than is found in the typical office environment. The software we chose is currently used
at more than 30 DGPS sites around the world- in Canada, Egypt, Holland, and the Persian Gulf- where it has
demonstrated the ability to meet DGPS network availability requirements, even in the presence of hardware
Ensuring Safely. This ability to detect equipment failures and switch to backup units without interrupting the
DGPS broadcast is critical. As shown in Figure 2, all essential components have redundant units running in hot-
standby mode. These devices are wired to provide point-to-point redundancy, which offers the best possible
fault-tolerant operation. What does this fault tolerance mean to typical DGPS users out on the water in a boat or
other vessel? It means that they will not experience an interruption or degradation in the DGPS corrections
being broadcast even if the site experiences a failure of one reference station and one integrity monitor and one
computer and one transmitter, regardless of whether the failed components are the active or backup units.
In addition, the Control Monitor at Alexandria automatically polls each remote station several times per day to
obtain up-to-date status information and ensure continued smooth operations. The system operator, who lives
onsite at Alexandria, is notified of any site problems with audible and visual indicators. We therefore knew that
the communications method we chose would be a critical link within the network. The nearest site, Port Said, is
only 230 kilometers away, but Quseir is almost 700 kilometers distant. Because communications using
telephone lines may not be reliable or cost-effective, we opted for a digital high-frequency (HF) radio link as
the primary communications mode. Existing telephone lines and standard telephone modems provide backup
communications in the event that HF communications are not possible.
This digital radio system provides up to 2,400 bits per second of data transfer in a half-duplex mode. HF
communications are notorious for their variability based on the time of day, weather, sunspot activity, and other
ionospheric conditions, so we conducted a frequency study to determine a set of HF frequencies that would
provide at least one good HF path under most expected situations, including the upcoming solar maximum. We
selected six widely spaced frequencies in the 3-15 MHz band and have thus far had encouraging results, with
good connections being made to each site on average at least twice per day. To make these communications
possible, each Control Station has a dedicated directional HF antenna, installed to point toward Alexandria, with
the Control Monitor setup there using an omnidirectional antenna. All antennas are broadband, to accommodate
the wide frequency range (see Table 2).
Each DGPS radiobeacon station has four GPS antennas -- one for each reference station and one for each
integrity monitor. The mounting locations for these antennas varies from site to site, but they are generally
mounted on top of a pair of 5- or 10-meter-high towers. These towers are rigid to minimize movement, even in
high winds, and provide a clear view of the sky from horizon to horizon. To ensure that the differential
corrections generated by the reference stations are accurate, we must also know the positions of the antennas to
within 10 centimeters 2drms (twice distance root mean square). Therefore, once we had installed them, an
outside surveying firm was hired to survey each antenna location. The reason we used a pair of antenna towers
was to ensure that should lightning strike one tower or should a tower structure fail or get damaged by some
means, only 50 percent of the antennas would be damaged, allowing the site to continue broadcasting
Accuracies and Antennas. Using all of this equipment, each reference station generates corrections by taking the
difference between the measured pseudorange to each tracked satellite and the expected range to each satellite,
given the known position of the reference station's GPS antenna and each satellite's position. It then outputs
these corrections in serial format (RTCM SC-104 Type 9 messages) and minimum shift key (MSK) modulates
them into the 283.5-325.0 kHz marine radiobeacon frequency band. The integrity monitor also receives the
differential corrections broadcast by the site, computes a DGPS position fix, then compares that fix to the
known position of its GPS antenna. Significant position errors, and other problems detected with the broadcast
signal, result in a station alarm. This self-monitoring feature ensures that the station only broadcasts good
The reference stations are 12-channel, C/A-code, single-frequency (L1) receivers that can measure
pseudoranges to an accuracy of approximately 25 centimeters. Because differential corrections are only
broadcast to the users for satellites that are above the mask angle (typically 10 degrees above the horizon), the
12 parallel receiver channels allow the reference station to track a rising satellite that is below the mask angle.
When the satellite elevation reaches the mask angle, the reference station is already prepared to generate
corrections. Although single-frequency reference stations were specified by PLA in its system requirements, the
units we deployed can have their firmware field upgraded, which allows them to track the P-code and carrier
phase on L2 as well. This can improve the pseudorange accuracy to 10-20 centimeters, providing the capability
for a future upgrade that will improve system accuracy.
Topping It Off. An essential component of station operation is obviously the radiobeacon transmitter, which
sends out the differential correction data. We therefore selected the same nondirectional radiobeacon
transmitters as used at the U.S. Coast Guard DGPS radiobeacon sites.
Beacon Co. engineers and technicians installed all of the civil works (the buildings, antenna towers, equipment
trailers, and so forth) and also designed and installed the power distribution at each site, which provides for 24
hours of battery operation in the unlikely event of a main power failure. With this work complete, we then spent
roughly 30 days traveling to each site, performing the final inspections, power-up tests, software configuration,
antenna tuning, and communications tests. Following are some of the high points from our grand tour, both
during installation and the later systems tests.
Alexandria. Our first stop was Alexandria or 'El Askandariya', home to both a Control Station and the network's
central Control Monitor. Both systems are colocated in an existing PLA facility at the western edge of the city,
about two blocks from the ocean. This site is currently used as a marine HF communications center and has a
marine radiobeacon. This radiobeacon is part of a chain of transmitters that provides offshore ships with a
direction-finding service. Each chain of three beacons transmits an unmodulated carrier for two minutes every
six minutes. This allows a ship's navigator to create up to three lines of position, which is sufficient to determine
an approximate position. (These radiobeacons will be decommissioned sometime after year 2000, once the
maritime community has had sufficient notice of the new DGPS service.)
The only real problem we encountered at this site was some minor interference from the existing radiobeacon,
which operates in the same frequency band as our DGPS system. When tuning up our antenna coupler, we had
to wait until the radiobeacon was in its four-minute quiescent phase. When our transmitter was off, the
radiobeacon still induced sufficient RF current into our antenna coupler to provide a painful reminder of what
not to touch (and as a bonus, the Egyptians learned a couple of new English phrases!).
Mersa Matrub. Situated a little more than half way between Alexandria and Egypt's western border with Libya,
this city is a pleasant oasis at the edge of the desert. The DGPS site here sits in one half of an existing PLA
compound, tight next to an Egyptian naval base. The navy personnel next door obviously weren't too pleased
whenever we peeked over the wail into their compound - I guess pointing their guns in our general direction
was a not-so-subtle hint! It was all taken in the spirit of good fun, as we finished our work at this site, which
now provides the first DGPS radiobeacon signal for maritime traffic traveling east along the African coastline
toward Alexandria and the Suez Canal.
Putt Sald Located at the northern end of the Suez Canal, Port Said often sees many large ships anchored
offshore, awaiting permission to travel through the canal. The DGPS Control Station here is also in an existing
PLA compound, which also houses an HF communications facility, a radiobeacon, and the 35-meter-high Port
Said lighthouse. Situated on the shoreline, this site has a wet sandy soil that is highly conductive and provides
an excellent grounding for the radiobeacon antenna.
Ras umm Sid. Getting here is a full day's drive from Suez, south along the coast of the Sinai Peninsula through
some of the most arid yet picturesque territory. Ras Umm Sid (Ras is Arabic for point) is situated at the
southern tip of the Sinai Peninsula, next to the town of Sharm El Sheik, one of the premiere tourist destinations
for scuba divers. The DGPS site is colocated with an existing PLA lighthouse, on a rocky outcropping 20
meters above the ocean (and overlooking one of the best dive sites around).
We mounted the GPS and HF antennas on pole masts at the top of the lighthouse, with the radiobeacon mast
antenna situated just north of the lighthouse compound. Because of the hard rocky ground, it was impossible to
bury the radiobeacon antenna's counterpoise. To overcome this problem, we installed extra ground radials,
brought in additional fill to cover the radials, and ran a ground wire from the center of the counterpoise down
the cliff face to a ground rod submerged in the ocean. Of course, we simply had to don the snorkeling gear and
do a proper "inspection" of the ground rod.
We also discovered that when the HF transmitter was keyed, the electric field coming off the HF antenna near
field was strong enough to trigger the main contactor in the lighthouse, thus turning on the lighthouse beacon
light. Fortunately, this proved a simple job for our engineers to fix.
Ras Gharib. Traveling to Ras Gharib from Ras Umm Sid requires driving all the way back up the coast to Suez,
then down the western coast of the Red Sea another couple of hours. Gharib is an oil town, with a number of
refineries in the area. Our site lies just south of town, right on the shore of the Red Sea. An existing PLA
lighthouse and HF station provided the necessary infrastructure.
There's always a very hot wind blowing at this site, requiring some additional guy wires on the GPS antenna
towers. (As you can imagine, a moving reference antenna is not a good thing at a DGPS broadcast station.) The
oil refineries also release a great deal of sulfur dioxide into the air. After being installed for only a few months,
all connectors had begun to turn black from corrosion. We therefore implemented extra corrosion abatement
procedures, including adding connector seals and a more aggressive preventative maintenance schedule for the
first year of operation.
Quseir. Another day's drive south from Ras Gharib through the desert with temperatures regularly exceeding
45[degrees]C brings us to Quseir, the southern-most radiobeacon site. Quseir is a fairly unremarkable town but
is the crossroads for those traveling overland to Luxor. PLA built a brand new compound just north of town for
the DGPS equipment and for housing PLA personnel. Building the compound and installing the DGPS
equipment, though, was briefly delayed, as the Egyptian military determined it first needed to sweep the area for
land mines. Interesting, how that was never mentioned when our engineers performed the original site survey
more than a year earlier.
This site's DGPS broadcast covers the central portion of the Red Sea south to Egypt's border with Sudan. We
installed a 500-watt transmitter here, which requires an antenna 50 percent taller than at the other sites.
Fortunately, this proved no more difficult to install than the other sites' antennas.
I Can't Hear You! During this project's design phase, we recognized the potential for interference between the
radiobeacon transmitter and the HF radio at each site. Because of the limited land available at some sites, we
needed to install the radiobeacon and HF antennas in moderately close proximity. We were thus concerned that
the presence of a strong signal in the 283.5-325-kHz band could affect the performance of the HF receivers,
which operate in the 3-30-MHz band.
We observed this interference problem when we powered up the first site for testing. The two antennas'
proximity and the strong radiobeacon signal combined to desensitize the HF receiver, resulting in poor HF
communications. To alleviate this problem, we installed a custom low-pass filter on the output of each
radiobeacon transmitter, between the transmitter and the antenna coupler. This filter was specially designed for
this project, with greater than 100-dB rejection in the 2-30-MHz band, yet minimal loss in the DGPS frequency
band. These filters have significantly improved HF communications performance, without any noticeable
effects on the radiobeacon signal.
Each DGPS broadcast site had to pass two important functional tests to gain operational status and acceptance.
A range test verified that each site's transmission signal was strong enough to provide the necessary area
coverage for marine users. An accuracy test verified that the differential corrections being broadcast by each
site would allow a remote user to compute an accurate DGPS position.
Range and Coverage. For each site, we had to demonstrate a field strength of at least 75 [micro]V/m at a
specific range over water. Quseir, which has a 500-watt transmitter, had a range requirement of approximately
260 nautical miles. For the five other sites, which have 200-watt transmitters, the range requirements were
approximately 120-150 nautical miles (depending on the antenna mast chosen). We made two different types of
measurements to verify range and coverage -- single-point measurements made at adjacent DGPS sites and
continuous measurements at sea throughout the coverage area.
As mentioned earlier, overlapping coverage (and thus redundancy) is provided by situating the sites so that they
are roughly on the 75 [micro]V/m contour of an adjacent site. This allowed us to use one of the integrity
monitors at each site to measure the field strength of an adjacent site's broadcast. Because an integrity monitor
is really just a GPS receiver and a DGPS beacon receiver integrated together into a single unit, all we had to do
was temporarily retune an integrity monitor to the adjacent site's broadcast frequency. Because the beacon
receiver portion of the monitor provided a direct measurement of field strength, these measurements were easy
To measure field strength at sea, we temporarily installed an integrity monitor on board PLA's vessel Aida and
took it on a sea cruise paralleling both coastlines at the appropriate ranges. The equipment continually logged
position and field strength, and we only had to retune the integrity monitor as the vessel passed from one
radiobeacon coverage area to the next. We found that all sites provided a signal with a field strength in excess
of 75 [micro]V/m at the required ranges.
Accuracy. For each site, we also needed to demonstrate that, using the site's broadcast differential corrections, a
user's DGPS position fix would have less than 5 meters (2drms) position error. This requirement had to hold all
the way out to the measured 75 [micro]V/m contour for that site. To quantify the accuracy of a position fix, it is
necessary to compare it with a reference position (truth) that is known to a higher level of accuracy. Normally,
this would be done by setting up a remote DGPS receiver on a survey monument or other known position,
taking many readings, and comparing the averaged position fixes against the reference position.
Rather than trying to locate existing survey monuments, which may have been nonexistent or in the wrong chart
datum, we came up with a much simpler solution. Because each radiobeacon site already has four surveyed
GPS antennas, and most sites have at least one adjacent site roughly on their 75 [micro]V/m contour, we could
simply perform the following test:
* To test the accuracy of site X's differential corrections, go to its adjacent site Y.
* At site Y, reprogram the integrity monitor to receive corrections on site X's frequency.
* Switch the integrity monitor into DGPS mode and record DGPS positions and field strength readings.
* Once sufficient data have been collected, compute the 2drms error value for the position fixes.
The accuracy pass/fail criteria are simply: If the distance between sites is greater than the contracted radiation
range, the field strength measured is greater than or equal to 75 [micro]V/m, and the 2drms position error is less
than 5 meters, then site X passes the accuracy test.
Figure 3 shows the position scatter plot for a broadcast from the Ras Umm Sid site, which is on the southern tip
of the Sinai Peninsula. Data were collected at the Quseir site, which is 190 kilometers south across the Red Sea.
The integrity monitor logged positions every five seconds over a two-hour period. The position accuracy of this
data set is 1.8 meters (2drms).
One should note that under good conditions, each set of corrections is no less than about four seconds "old"
when it arrives at a beacon receiver. This age is primarily due to the low data transmission rate of 200 bits per
second. We chose the five-second logging interval to ensure that each position fix recorded was based on a
different set of corrections. This helps ensure that each position fix is statistically independent, which permits
proper statistical analysis.
A SUCCESSFUL IMPLEMENTATION
All sites passed the accuracy test, each providing corrections allowing a DGPS position accuracy better than 5
meters (2drms). Some sites had accuracy measurements below 2 meters, while others were between 4 and 5
meters. Because each site has identical reference stations and transmitters, the differences were due to the time
of day that the tests were performed, local electrical and RF noise, and other site-specific factors.
It is important to note that we used a high-quality DGPS receiver to perform these accuracy tests. Users with
inexpensive, less-accurate DGPS receivers will probably not be able to obtain this level of accuracy. We did
discover, though, that locating radiobeacon sites on or near the 75 [micro]V/m contours of adjacent sites has
provided dual coverage (or better) through-out most of Egypt's coastlines. In fact, as we traveled the length and
breadth of Egypt by vehicle, completing site installations and testing, we found many locations where we could
lock on to two or three radiobeacons. Just south of the city of Suez (the southern end of the Suez Canal), we
could lock on to four different radiobeacons (Alexandria, Port Said, Ras Umm Sid, and Ras Gharib) and obtain
good DGPS position fixes with an inexpensive DGPS receiver.
As a result of this project, vessels traveling the Suez Canal and other waterways of Egypt can now navigate with
much more confidence. The effort also allowed engineers from two very different regions to work together
toward a common goal, forging new friendships and gaining a better understanding of each other's culture. The
entire experience proved very rewarding for all involved.
Author Bruce Butler would like to acknowledge Beacon Co. engineers Alaa Abdel Fattah Ibrahim and Adel
Mohamed Aly for their friendship and ongoing technical work on this project. All employees of Beacon Co. are
thanked for their outstanding hospitality. Shokran! PLA is acknowledged for providing space on its vessel Aida
to make the field strength measurements at sea. Both authors would like to thank Mr. Bryon Bird for his
In 1997, Beacon Co. of Egypt and the Maritime Systems Business Unit of MacDonald Dettwiler, (formerly the Maritime
Information Systems Group of CANAC/Microtel), were awarded a contract to provide a complete turnkey National DGPS
system for the Egyptian Ports & Lighthouse Authority. This contract consists of the engineering, procurement, integration,
factory testing, delivery, configuration, and site-testing of the Egyptian Marine DGPS.
The system consists of one control monitor station linked to six DGPS control stations via an HF radio network. Backup
access to the networks is provided by dialing from standard telephone lines.
Each DGPS control station broadcasts DGPS corrections on a standard marine radio beacon frequency as a supplement
to the standard GPS signals. The corrections enable the Egyptian and International Maritime communities to determine
their positions to better than 5 meters accuracy, a substantial improvement over the 100-meter accuracy provided by
commercially available GPS technologies.
The DGPS system implements a fault-tolerant architecture with dual redundancy in all key equipment. If any equipment
were to fail, the system architecture would ensure continued operation. All irregularities in operatio are reported
immediately to the control monitor.
Egypt Marine DGPS Station Data
SITE MACHINE NAME Station ID Range RS IDs Freq Baud
(KM) (KHz) (Bps)
Port Said 1
Port Said 321 324 442,443 290.0 200
Port Said 2
Alexandria 320 278 440,441 284.0 200
Mersa Matrouh 324 278 448,449 307.0 200
Ras Umm Sid 322 234 444,445 293.5 200
Ras Gharib 1
Ras Gharib 323 278 446,447 298.0 200
Ras Gharib 2
Quseir 325 482 450,451 314.5 200
A Differential Global Positioning System (DGPS) provides improved accuracy by using a reference GPS receiver
(Reference Station RS) located at an accurately surveyed point, By comparing the coordinates of the known location with
that predicted by the GPS, satellite range corrections can be determined and broadcast in real-time by a radio link to
nearby users who use the corrections to significantly improve their positional calculations,
This overcomes most of the errors that are inherent in SPS, mainly the satellite clock and Ephemeris errors, as well as
other factors that contribute to errors in GPS measurements. These factors include ionospheric noise, satellite clock error,
and ephemeris errors.
A primary marine DGPS system monitors the corrections and contains backup facilities to provide consistent and reliable
DGPS operates in all areas and all weather conditions. The corrections can be used to accurately produce other marine
navigation information such as speed, heading, distances to waypoints etc. Further, a marine DGPO, user system
requires little modification to shipborne equipment and can use existing radio beacon transmission stations. Generally
DGPS shore stations provide "free" transmission of data to ship borne equipment in an internationally accepted format
known as RTCM SC104.
The essential function of the DGPS broadcast system is to continuously broadcast to the users the pseudo-range
correction factors of all GPS satellites in view and provide quality analysis of this broadcast data in real-time.
Reference stations with precise known locations can calculate the errors associated with each satellite and advise the
users of the corrections necessary to reduce the errors in the location calculation. The accuracy of users' GPS receivers
equipped with differential correction receivers is improved to less than 10 meters error. The FTMB system provides the
hardware and software for error reducing signal transmission for the use of vessels and other navigators.