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The GPS is capable of providing extremely accurate worldwide, 3 dimensional locational data. It has now become the method of choice for marking the location of wrecks and other interesting dive sites.
What Is GPS?
The Navigation Satellite Timing And Range Global Positioning System, or NAVSTAR GPS, is a satellite based radio-navigation system that is capable of providing extremely accurate worldwide, 24 hour, 3-dimensional locational data (latitude, longitude, and elevation). The system was launched in 1995 and is maintained by the US Department of Defence (DOD) as an accurate, all weather, navigation system. Though designed as a military system, it is freely available with certain restrictions to civilians for positioning.
How Your GPS Works
The GPS system consists of three parts. There are the satellites that transmit the position information, there are the ground stations that are used to control the satellites and update the information, and finally there is the receiver that you use. It is the receiver that collects data from the satellites and computes its location anywhere in the world based on information it gets from the satellites. There is a popular misconception that a GPS receiver somehow sends information to the satellites but this is not true, it only receives data. So, just how is it able to do compute its position?
Your GPS receiver uses an elaboration of a technique that is tried and true and used by navigators and surveyors for centuries. Basically you use a known set of locations to compute your current location by taking fixes on the known sites, not too dissimilar to taking transits. In the old days you took bearings (compass sightings) on existing locations and triangulated these on a chart to compute a fix on your location. Once you have a compass bearing you can draw a line through the known location and you know you are somewhere on that line. Do the same thing to a second point and the two lines will intersect. This is your position. If you try a third point it should intersect at the same place the other two lines intersect. Usually however, because of imprecise sightings, it intersects both lines at slightly different points thereby forming a small triangle or "cocked hat" as its commonly known. You are somewhere inside that triangle but you don't know exactly where. If the triangle is small enough you consider it good enough, otherwise you need to take another sighting. Accuracy is determined primarily on your ability to get and plot an accurate bearing as well as the geometry of the known sites available. This means that if the sites are very close together you will get poorer results than if they are at some angular distance apart. What you would really like were two sites that were 90 degrees apart for best accuracy.
 The GPS receiver uses a slightly different approach. It measures its distance from the satellites at a precise position and time and uses this information to compute a fix. It simply measures the distance by measuring the length of time a signal takes to arrive at the GPS and then based on knowing that the signal moves at the speed of light it can compute the distance based on the travel time.
Clearly to calculate this, the GPS must know the exact position of the satellite and the time the message was sent and it is precisely this data is beamed constantly to earth. Now, armed with the satellite location and the distance from the satellite we can expect that we are somewhere on a sphere that is described by the radius (distance) and centred at the satellite location.
By acquiring the same information from a second satellite we can compute a second sphere that cuts the first one at a plane. Now we know we are somewhere on the circle that is described by the intersection of the two spheres. If we acquire the same information from a third satellite we would notice that the new sphere would intersect the circle at only two points. If we know approximately where we are we can discard one of those points and we are left with our exact fix location in 3D space. Now, what would happen if we were to acquire the information from a fourth satellite? We should expect that it would show us to be at exactly the same point we just computed above. But what if it isn't? Before we can answer that question we need a little more background.
Precision timing
So how exactly does the GPS know the travel time so that it can compute the distance. The satellite sends the current time along with the message so the GPS can subtract its knowledge of the current time from the satellite time in the message (which is the time that the signal started its descent) and use this to compute the difference. For this to work the time in your GPS must be pretty accurate - to a precision of well under a microsecond. The satellite itself has an atomic clock to keep the time very precisely, but your unit is probably not big enough nor expensive enough to have an atomic clock built in, so your clock is likely to be in error! For this reason our assumptions about the distance calculation are likely to have considerable error and the fourth satellite fix will reveal this to us. However, if we assume the error is caused by an error in our clock then we can adjust our clock a little and re-compute all 4 fixes, continuing to do this iteratively until the error disappears! We will then have a good position fix and as a side effect we will also have the correct time to about 200 nanoseconds or so. One of the applications of GPS technology is to provide the correct time even when we don't care about our position.
Maintaining the fix means that we need to continuously recalculate the information based on the moving satellites. Once we have a number of fixes we can derive much more information than just location data. For example a GPS can compute the travel direction by comparing current location to previous location. Similarly the GPS can keep track of travel distance, average speed, record travel time and other valuable data.
In addition to the data already mentioned the unit uses Doppler data from the moving satellites, almanac data to figure out the approximate positions of all the satellites, and ephemeris data download directly from the satellite that can be used to compute its position in the sky.
The steps involved in calculating a position are:
- Sync with an available satellite and download the navigation information.
- Convert the messages to internal format for calculation. These include clock information, ionosphere data, and ephemeris (orbit) data.
- Calculate the exact satellite position. This will include both the elevation and azimuth data so we can apply troposphere modeling corrections that are dependent on how far above the horizon the satellite is.
- Calculate the pseudo range data and then correct for ionosphere and other modeling errors.
- Repeat these steps for each available satellite.
- Correct the satellites position for earth's rotation based on the time it takes for the signal traversal using the pseudo range data. (If the internal clock is close this can be done once, otherwise it will have to be repeated after the receiver position is computed.)
- Correct using differential data if available. (This may have to be done after the initial position is computed as part of the refinement step if the internal clock isn't accurate.) If the differential station is near the GPS receiver it will be able to skip the corrections for modeling errors since this is part of the correction data available.
- Calculate the initial receiver position.
- Convert the data based on whatever datum and grid system you have chosen and display the answer on the position page.
- Add in the leap seconds and time offset from UTC time to the computed time data and convert it for display.
- Refine the position based on additional satellites and the correct time to obtain a 3D fix and subsequently improve the fix based on choosing satellites with a better DOP,
The Satellites
In addition to the receiver there are about 24 satellites orbiting the earth. They are about 11,000 nautical miles high in carefully controlled orbits at a speed that means they will make a complete orbit twice a day. Each orbit takes 11 hours and 58 minutes, so like the stars they will seem to drift 4 minutes a day. The complete constellation consists of a minimum of 21 operational satellites and 3 working spares. Currently there are 27 total satellites in the sky and it is possible that there could be as many as 31 or 32. There are 6 different orbits with multiple satellites in each orbit as depicted in the drawing on the right. Each orbit is inclined 55 degree from the equator and thus there are no orbits that go directly over the poles, but certainly a great many orbits can be seen from the poles or anywhere else on the earth. The goal of the system is to always provide at least 4 satellites somewhere in the visible sky. In practice there are usually many more than this, sometimes as many as 12.
Each satellite contains a supply of fuel and small servo engines so that it can be moved in orbit to correct for positioning errors. With update control from the ground units it can maintain an essentially circular orbit around the earth. It also contains a receiver to get update information, a transmitter to send information to the GPS receiver, an antenna array to magnify the weak transmitter signal, several atomic clocks to accurately know the time, control hardware, and photoelectric cells to power everything.
The GPS satellite signal contains information used to identify the satellite, as well as provide position, timing, ranging data, satellite status and the updated ephemeris (orbital parameters). The satellites are identified by either the Space Vehicle Number (SVN) or the Pseudo random Code Number (PRN).
GPS satellites transmit on two L-Band frequencies: 1.57542 GHz (L1) and 1.22760 GHz (L2). The L1 signal has a sequence encoded on the carrier frequency by a 'spread spectrum' modulation technique which contains two codes, a precision (P) code and a Coarse/Acquisition (C/A) code. (The C/A code is also sometimes incorrectly referred to as 'Civilian/Acquisition' code.) The L2 carrier contains only P code which is encrypted for military and authorised civilian users. Most commercially available GPS receivers utilize the L1 signal and the C/A code. There are a few civilian receivers capable of utilising the L1 P code without actually decoding it.
Ground Stations
The satellites are controlled and monitored by full time land based sites under the control of the US Department of Defence (DOD). These are situated around the Equator and check the health of the satellites, check how close they are to their optimum orbits, check the clock accuracy, and send adjustments as needed. The land based sites are located a precisely known positions so that they can verify the operation of the satellites. A master control station updates the information component of the GPS signal with satellite ephemeris data and other messages to the users.
GPS Limitations
Though GPS can provide worldwide, 3D positions, 24 hours a day, in any type of weather, the system does have some limitations. First, there must be a relatively clear "line of sight" between the receiver's antenna and several orbiting satellites. Anything shielding the antenna from a satellite can potentially weaken the satellite's signal to such a degree that it becomes too difficult to make reliable positioning. As a rule of thumb, an obstruction that can block sunlight can effectively block GPS signals.
The receiver must receive signals from at least four satellites in order to be able to make reliable position measurements. In addition, these satellites must be in a favourable geometrical arrangement. The four satellites used by the receiver for positioning must be fairly spread apart. In areas with a relatively open view of the sky, this will almost always be the case because the GPS satellite constellation was strategically designed to provide at least four satellites with favourable geometry.
Differential GPS
A typical civilian GPS receiver provides 20 to 75m accuracy, depending on current status of selective availability, number of satellites available, and the geometry of those satellites. More sophisticated and expensive GPS receivers, can provide accuracies within a centimetre by using more than one GPS frequency. However, a typical civilian GPS receiver's accuracy can be improved to 5m or better through an additional process known as Differential GPS (DGPS).
DGPS employs a second receiver to compute corrections to the GPS satellite measurements. How are these corrections provided to your GPS receiver? There are a number of free and subscription services available to provide DGPS corrections. The U.S. Coast Guard and U.S. Army Corps of Engineers (and many foreign government departments as well) transmit DGPS corrections through marine beacon stations. These beacons operate in the 283.5 - 325.0 kHz frequency range and are free of charge. Your only cost to use this service is the purchase of a DGPS Beacon Receiver. This receiver is then coupled to your GPS receiver via a three-wire connection, which relays the corrections in a standard serial data format called 'RTCM SC-104.'
Typical Applications for the Diver
Until underwater navigation systems are developed, their main use will be for the location and position marking of wrecks. They are also extensively used by boats and shipping to navigate through narrow shipping channels, to dive sites or determining the most direct course from point A to point B.
Using several different screens they can inform the users of:
- Their location and current course
- Their position in relation to land, wrecks and other marks
- Their speed over ground (SOG)
- The direction and distance (as the crow flies) to a selected way point
- The predicted time it will take to arrive at their selected way point, given the current speed
- The distance users are off course i.e., their Cross Track Error (XTE).
- The track of the current (and previous) journey's
- Warning sounds when a way point is approached
- An emergency position fixing button for 'Man Over Board' (MOB) relocation.
Modern GPS's can store several hundred way points and include chart and chart data. Some even include electronic compasses.
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