72 SATELLITE NAVIGATION

287. SATELLITE NAVIGATION SYSTEMS

Today’s small-craft mariners can conduct long-distance ocean navigation by means of electronic satellite navigation systems with which they are able to obtain regular and accurate positions with great ease. By pushing a few buttons Lat. and Longitude can be displayed at almost any time required anywhere on the globe.

Why, then, is it necessary to learn position-fixing with a sextant, sets of tables and a knowledge of celestial navigation?  Simply because, in spite of modern technology, electronic aids sometimes do not work, particularly those used on small craft and under the severe conditions often experienced by such craft.  

Should the satellite receiver fail in mid-ocean, the mariner could be in serious trouble if unable to find and plot the vessel’s position, correct the course accordingly and proceed safely to the destination.

The term SATNAV is misleading because it conceals the fact that there are already two major satellite navigation systems in operation in the W. and four if you include the Russian equivalents. These systems work on different principles, and give very different results, so that it is important for small craft navigators to know which one they are buying.

When the first satellites were put into orbit in the late 1950s it was found that their orbits could be plotted by measuring the Doppler shift in the frequency of their transmitted signals as the satellite approached and receded from the receiving antenna. American scientists quickly saw that this effect could be exploited to create a truly accurate global positioning system.

It was this realisation which led to the satellite navigational system known as USNNSS (US Navy Navigation Satellite System) or TRANSIT. This system was developed to serve as a precision guidance aid !for the Polaris submarine fleet and became operational as such in Jan, 1964. It was later made available for commercial use by the release of its operational details in 1967, and one of the first commercial vessels to be fitted with NNSS receiving equipment was the British liner Queen Elizabeth 2.

The NNSS complex consists of a system of radio satellites circling the Earth every 107 minutes in fixed polar orbits at an altitude of 600 nautical miles.  Their paths intersect at the poles and are spaced to create a kind of orbital ‘cage ‘ around the globe, duplicating the familiar orange-segment pattern of surface meridian lines (fig. 72-1).

The orbital cage is stationary with reference to the Earth’s rotation so that, as the Earth turns, every point on its surface is carried under successive orbits and is within range of one or another satellite every 30 to 120 minutes depending on Lat..

Each satellite transmits continuously updated time and position data which enable the navigator equipped with NNSS receiving and computing equipment to plot a vessel’s own position with great precision. The system gives world-wide coverage at all times of the year.

Since it is not possible to predict the position of an artificial satellite accurately for a long period of time, each satellite is equipped with a magnetic memory capable of storing its own positional data for 12 hours or so.  These data are provided by a ground station, called an Injection Station, which transmits the data to the satellite each time the satellite makes a passage at or near the station. Before the 12 hour data are transmitted for storage in the satellite, the satellite’s memory of the previous 12 hours is erased.

The positional data are determined through the agency of a number of tracking stations at which Doppler data are measured and transmitted to a Computing Centre which analyses the data and computes up-dated information.  This is transmitted to the Injection Station for transmission to the satellites as they come into line of sight (see fig. 72-2).

In its time, TRANSIT was a breakthrough, especially for long-distance ocean navigation on large ships but in coastal waters and for small craft its value is very limited, for three reasons. Firstly, because there are so few satellites in the system, the average interval between fixes is about an hour, but can be as much as four hours. Secondly, although most receivers will automatically ignore satellite passes lower than 10° and higher than 75° above the horizon (which would give inaccurate fixes), fixes from satellites near these limits can still be several miles adrift.  Finally, because the accuracy of a USNNSS satellite fix is dependent upon the relative velocities of the satellite and the boat, results from a stationary yacht will be good but once the yacht is moving errors will increase because the yacht’s speed and compass inputs are rarely accurate enough to maintain high levels of position accuracy – errors from this cause alone can be between ½ and 1 mile.

These drawbacks have been overcome with the new Global Positioning System known as NAVSTAR GPS which is designed to provide accurate positional information in three dimensions: Lat., Longitude and altitude (the last clearly being of interest only to aircraft). The GPS system is described in some detail later in this Study.

288. BASIC SATELLITE THEORY

Before considering specific systems, it might be as well to review the characteristics of satellite orbits, and to consider the advantages and disadvantages which satellites have as compared with more commonplace navigational landmarks.

The path of a space craft may be any one of a number of what are sometimes referred to as conic sections (see fig. 72-3). The circle and the ellipse are closed curves called orbits, and when a space craft traverses an orbit it is referred to as a satellite. The parabola and the hyperbola are not closed curves and are referred to as trajectories, and when a space craft follows a trajectory it is usually referred to as a space probe.

Obviously it is satellites in orbit which are of interest to Earth bound navigators. The orbit of a particular satellite depends upon its injection velocity, that is, its velocity when it enters its orbit, and also its position relative to the Earth at the time of injection. If its injection velocity is equal to what is called the escape velocity the path will be a parabola. This precise balance is difficult to achieve and so in general the injection velocity will be either greater or less than escape velocity. If greater, the trajectory will be a hyperbola and the space craft will escape from the Earth; if less, the trajectory will be an ellipse.  Again the circular orbit is difficult to achieve precisely and so all satellites follow elliptical orbits.

The inclination is the angle between the Earth’s equatorial plane and the plane of the satellite orbit. Fig. 72-4 illustrates orbital inclinations of zero (for the equatorial orbit), 45 degrees, and 90 degrees (for a polar orbit). The final inclination desired partly determines the launching site chosen since in practice it is difficult to achieve an orbit which is less than the Lat. of the location of the launching site.

Artificial Earth satellites follow elliptical paths, in most cases the ellipse being due to the Earth not being a perfect sphere.  The closest point of approach to the Earth of an elliptical orbit is called the perigee and the furthest distance away is termed the apogee as illustrated in fig. 72-5(a).  When satellites are observed from the surface of the Earth specific terminology must again be used.  The direction vector of the satellite from the observer is called the azimuth and is quoted in degrees.  The angle between the satellite and the Earth’s surface tangent at any instant is termed the elevation and is also quoted in degrees [see fig. 72-5(b)).

The choice of an orbit for a particular navigation system depends on many factors of which the more obvious follow. There must be a line of sight between the satellite and anyone who wishes to communicate with it. The higher the satellite the greater is the area of the Earth’s surface which it can ‘see’.  In fact a satellite at an altitude of 22,000 miles (35.000 kilometres) can see about one third of the surface of the Earth, will have a period of about 24 hours and is called a synchronous satellite.  If its orbital plane lies in the plane of the equator and it traverses its orbit in the same direction as the Earth rotates, its G.P. will be stationary and it is then called a geo-stationary satellite. Only three such satellites should be required to cover the Earth’s surface completely. Orbital paths can be transferred to a Mercator chart as shown in fig. 72-6.  The inclination must be the same in both the northern and the southern hemispheres and corresponds to Lat.. The six orbits shown in fig. 72-6 are for Navstar GPS satellites with an inclination of 55 degrees.

Transit and Nova satellites used for the NNSS system are in polar orbits and therefore follow longitudinal paths.  The effect of the Earth’s atmosphere producing aerodynamic drag cannot be ignored since it causes orbital decay.  As time goes by, the dimensions of the ellipse decrease, the satellite comes closer and closer to the surface of the Earth in air of ever increasing density and eventually frictional heating is sufficient to destroy it.
Whatever type of system is used, a balance must be struck between economy, reliability, accuracy and expense. An expensive tracking system is required because the orbit of the satellite is changing continuously. The satellite itself must be able to receive and transmit signals and it needs a power source to do this, yet it must be as small in size and mass as possible. The greater the altitude of the satellite the greater is the amount of total energy required to get it into orbit at the required speed. On the other hand, the lower the satellite orbit the greater is its speed and hence accurate tracking is more difficult.

289. SATELLITE LAUNCHING AND GUIDANCE SYSTEMS

Satellites are basically non-powered, being dependent upon the initial velocity given by a launching rocket and usually a second stage rocket, both of which automatically separate and fall to Earth when their fuel is burned, while the satellite enters an orbit around the Earth. The satellite may be equipped with small rockets for purposes of manoeuvring or for attitude control.  It may also be Placed’ in orbit by a space shuttle vehicle.

Spacecraft or Space Vehicles (SV’s) are general terms which include artificial satellites, sounding rockets, space shuttles and space probes.

There are two main classifications of artificial satellite. A passive satellite transmits no radio signals and may be tracked either optically or with radar.

Radio communications signals may be ‘bounced’ off its surface. An active satellite sends out radio signals to make tracking easier and to transmit data from its instruments to Earth stations or other craft.

One other differentiation of satellites is by function: scientific or applications. A scientific satellite carries instruments to obtain scientific data on magnetic fields, space radiation, the Sun, other stars, planets, etc. Applications satellites have utilitarian tasks such as land survey, atmospheric observation (meteorology), radio and television communications, etc. The Navigation satellites with which we are principally concerned in these chapters are, of course, prime examples of applications satellites (see fig. 72-7).

The spacecraft at the forward end of a multi-staged vehicle is projected upward by reaction to the high-speed jet combustion gases produced in the rocket motor. Because the liquid propellants are being consumed and ejected from the rocket motor, the vehicle is continual] lightened and acceleration increases.

The method of mounting several rocket propulsion systems one on top of the other is called staging. The lowest, or first stage ignites and lifts the vehicle at increasing velocity until exhaustion of its propellants. At that point the stage drops off (thus lightening the weight of the launch vehicle) and a second stage is ignited. The second stage of lower thrust then begins to accelerate the launch vehicle and space craft, commencing at the velocity reached under first-stage power. Some space launch vehicles have three stages, the third and final stage being similar to the second.

The Earth orbital flight required by artificial satellites is achieved by launching vertically and then tilting the trajectory so that flight is parallel to the Earth’s surface at the time that orbital velocity at the required altitude is reached. At this precise point, the rocket engine is cut off. A satellite attached  to the final stage rocket is then in free fall about the Earth, the centrifugal pull on the satellite being equal to the Earth’s pull of gravity. At an altitude of 200 kilometres (125 miles) which is above the outer reaches of the atmosphere, aerodynamic drag is not present, and the satellite will (in theory) continue to orbit indefinitely.

During the launch phase, corrections to deviations in the planned flight path are made at once by operating small thrust motors on the launch vehicle, by deflection of the rocket exhaust jet. or by swinging one or more of the rocket engines in a gimbal mount. This launch guidance system, usually incorporated in the second stage booster, is tracked and interrogated by Earth radar. Computations are made in the ground control station, and guidance commands given to the vehicle.

In a typical launch the launch vehicle puts the satellite in an elliptical orbit whose apogee lies at the correct altitude. The satellite is then started spinning and tracked while it completes four to twelve orbits. A spinning body can maintain a nearly constant direction of axis of spin despite small disturbing forces, while a non-spinning body can be turned appreciably by such forces and thus lose its desired orientation. For this reason most (but not all) satellites use spin as a means for attitude control.

Artificial satellites require a local source of electrical power to operate the equipment they carry. Most of them utilise panels of solar cells, often in conjunction with storage batteries, although much experimental work has been done on the feasibility of nuclear power sources. Nickel-cadmium storage batteries used by satellites for standby power are suitable because they can be hermetically sealed, no gas being evolved during normal operation.

Radio communication to and from artificial satellites uses very high (microwave) frequencies. Two bands of frequencies, each 500 mega hertz wide, are used: 3.7 to 4.2 giga hertz for space-to-Earth and 5.925 to 6.425 giga hertz for earth-to-space communication.