GG 710 - Remote Sensing in Submarine Environments Satellite Navigation

The first artificial satellite to circle Earth, the former Soviet Union’s Sputnik 01, was launched in October 1957. At the time those living in communist countries greeted this accomplishment with glowing enthusiasm; the rest of the world wasn’t as happy. The successful launch of Sputnik 01 and Sputnik 02 (less than one month later) initiated the “Space Race,” a contest to put the first person on the moon, that would occupy the attention and significant resources of the U.S. and the U.S.S.R. for over a decade. The U.S. entered into the Space Race within months of the Soviet launches by establishing the National Aeronautics and Space Administration (NASA) and launching its first satellite, Explorer 01, in February 1958 and the first telecommunications satellite, Score, in December 1958. As we all know, the U.S. ultimately won the Space Race by putting Neil Armstrong and Buzz Aldrin on the moon in July of 1969. In the process the scientists and engineers who competed in the Space Race helped establish the technology and framework for the present-day satellite network systems that we use for navigation at sea.

Transit Satellites The first satellite-based navigation system was developed at Johns Hopkins University in 1959, and the first successful launch of an operational transit satellite, TRANSIT 1B, occurred in the following year. Transit satellites operate in conjunction with land-based “tracking” stations that monitor the satellite as well as “injecting” stations that insert updated information describing orbital position into the satellite’s database. The function of the transit satellite is to transmit their updated position continuously at different frequencies. On a ship using a transit satellite system for navigation, a receiver determines the position of each satellite it tracks as a function of time. The receiver monitors each satellite signal looking for a change from positive to negative in Doppler shift (see text box on the following page for a brief description of the Doppler effect). This change in polarity indicates the time of an individual satellite’s closest approach to the ship. If the two-way travel time of the satellite signal is also known, it is possible to compute the distance from the satellite to the receiver. With multiple satellites (typically four or more), location can be narrowed down to an ~200m wide region.

Figure 1 - The steepness of the satellite curve at left is a function of the angular distance between the observer and the satellite – the steeper the curve the closer the satellite is (i.e., in this example, Sat1 would be at an elevation of ~45° while Sat2 would be at an elevation of ~80°, almost directly overhead). Doppler effect From Wikipedia, the free encyclopedia.

The Doppler effect, named after Christian Andreas Doppler, is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves that propagate in a wave medium, the velocity of the observer and the source are reckoned relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from either motion of the source or motion of the observer. Each of these effects is analyzed separately. Figure 2 - Sound waves emanating from an ambulance moving to the right. The perceived frequency is higher on the right, and lower on the left.

Doppler first proposed the effect in 1842 in the monograph Über das farbige Licht der Doppelsterne und einige andere Gestirne des Himmels (On the colored light of the binary star and other stars). The hypothesis was tested for sound waves by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. He confirmed that the sound's pitch was higher as the sound source approached him, and lower as the sound source receded from him. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler- Fizeau"). It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in your direction. Assume that balls travel with constant velocity. If the thrower is stationary, you will receive one ball every second. However, if he is moving towards you, you will receive balls more frequently than that because there will be less spacing between the balls. The converse is true if the person is moving away from you. So it is actually the wavelength that is affected; as a consequence, the perceived frequency is also affected.

The advantages of using a transit satellite system for navigation is that the approach is relatively inexpensive, and it can be used almost anywhere and at any time (although some continuous wave systems have difficulties operating at night). Unfortunately, transit satellite navigation systems have fairly low accuracy, largely because of atmospheric and ionospheric variations that affect estimates. Satellites at elevations outside of 20-70° above the horizon are typically not very useful and neither is it possible to use two or more satellites that are operating at the same frequency.

Global Positioning Satellites The Global Positioning System (GPS) began in 1973 as a replacement for the TRANSIT system; the first satellite was launched in 1978 and the full constellation of 24 satellites was completed in 1994. At its inception, GPS was known as Navigation Satellite Timing And Ranging (NAVSTAR). GPS satellites circle Earth in six orbital planes at a high altitude of ~12,000 miles (20,900 km), which allows four satellites to be visible from any point on Earth at any given time (Figure 3). The GPS satellites have 55° inclined circular orbits with a period of 12 hours. Each GPS satellite contains two rubidium and two cesium atomic clocks with the stability of 1 second in 300,000 years (rubidium) and 1 second in 160,000 years (cesium). These clocks are updated every day. This combination of overlapping high, repeated, rapid orbits and precise clocks allows the Global Positioning System to provide extremely accurate navigation: position within 16m, velocity to 0.1 m/sec, and time to within 100 nanoseconds. Figure 3 - The complete constellation of 24 GPS satellites. In order to complete approximately two orbits in a 24-hour period, the satellites are moving at a speed of ~7,000 mph. Figure from http://www.garmin.com/aboutGPS.

Each GPS spacecraft weighs 820 kg and is 17 feet across with the solar panels extended. They contain two Sun sensors, an Earth sensor, and three gyros for attitude determination along with 22 thrusters and four reaction wheels for attitude control. Each satellite is also equipped with backup batteries so that they don’t lose power in the event of a solar eclipse. All GPS satellites broadcast on the same two frequencies designated L1 and L2. The civilian population uses the L1 frequency of 1575.42 MHz in the UHF (ultra-high frequency) band. To allow each satellite to be identified, the transmitted signals are modulated by a unique code called a pseudorandom code. Each satellite also broadcasts ephemeris data, which tells the GPS receiver where each GPS satellite should be at any time throughout the day. Finally, each GPS satellite constantly transmits almanac data, which contains information about the status of the satellite (healthy or unhealthy), current date and time. This is the part of the signal that is essential for determining position. With four satellite signals received, we can solve for geocentric latitude and longitude, the elevation of the receiver and the time of reception. Like the TRANSIT satellite system, GPS data are susceptible to a number of sources of errors including delays introduced by transmitting a signal through the ionosphere and troposphere, reflections or blockage of signals because of high-standing features such as topography or buildings, errors in the receiver’s clock (these are typically not as accurate as the clocks in the satellites), and satellite geometry. The latter error results from tighter groupings of the satellites; typically better positional accuracy is achieved when the satellites are widely separated as opposed to being tightly clustered or arranged in a straight line. Initially, the GPS signal was dithered, basically modified to include a random noise component in the timing signal that introduced positional errors on the order of 50-100m, to prevent adversaries of the U.S. from using the system. However system users quickly developed a new technique, differential GPS, to remove this effect. Differential GPS involves using two receivers, one moving and one stationary. Because it remains at a known, fixed location, the stationary receiver is used to determine timing errors and broadcasts this information to the receiver in motion. In this way all sources of error whether from the atmosphere or a deliberately modified signal can be removed yielding a positional accuracy on the order of meters. Due to the development of differential GPS, the selective availability of GPS (dithering) was turned off in May 2000.