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Global Positioning System over Fiber for Buoyant Cable Antennas Adil M. Karim, Stephen J. Stafford, and R. Benjamin Baker uoyant cable antennas (BCAs) are long, towed antennas used by submarines to receive RF signals. Deployed while the sub- marine is operating at depth, the cable is managed so that the internal antenna element or elements are positioned near the ocean surface. The received signal is sent to the submarine for processing over a transmission line within the cable. In this article, we examine the design of a fiber-optic signal trans- port link for a BCA that receives signals from the Global Positioning System (GPS). This signal and reception method is of significant interest because the resulting GPS solution can be fused with the submarine’s inertial navigation system to improve overall naviga- tion accuracy without requiring the ship to come to periscope depth. INTRODUCTION The evolution of network-centric warfare requires trailing length of cable is managed using an inboard that submarines have the ability to send and receive deployment system and passes through a seal that main- RF signals. Submarines have generally been forced to tains watertight integrity for the ship. The focus of this come to periscope depth and expose a mast antenna article is on the design of a fiber-optic network for trans- or deploy a buoy to use the electromagnetic spectrum. port of Global Positioning System (GPS) signals col- These actions increase detectability and the risk of colli- lected from multiple antenna elements within the BCA, sion. However, the development of buoyant cable anten- as shown in Fig. 1. The signals are transported from the nas (BCAs) has enabled a new mode of operation where receiving antennas to a shipboard location through a a submarine can remain at depth and still receive RF process known as antenna remoting. signals. This cable contains the active antenna element The diameter of the cable is constrained so as to or elements as well as the transmission line that carries reduce observability and mechanical risk. A typical the received signal to the submarine for processing. The BCA has an outer diameter of 0.65 in., including the JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 30, NUMBER 4 (2012) 309 A. KARIM, S. J. STAFFORD, AND R. B. BAKER GPS links Sea surface Antenna Buoyant ber-optic cable Figure 1. Submarine with deployed BCA for reception of GPS signals. outer jacket and any internal structure. The maximum satellites. The satellite constellation consists of at least length of any rigid section is 1.0 in., for compatibility 24 satellites in medium Earth orbit that transmit a vari- with inboard BCA handling equipment. These two fac- ety of direct sequence spread spectrum (DSSS) signal tors severely limit the size of the antenna elements and ranging codes, as shown in Fig. 2. supporting electronics. Performance is further limited A network of terrestrial control stations provides the by the ocean surface, which introduces attenuation for satellites with accurate time and position information. submerged or partially submerged antennas. In addition, This information is encoded in the signals transmitted the conductivity of seawater has a strong impact on the by the GPS satellites, enabling users on the ground to actual antenna pattern. extract the propagation time from the satellite to the The resulting disadvantaged nature of the antenna and the long length of the cable mean that transport in the electrical domain is not feasible at higher frequen- cies. Even with the use of electronic amplifiers, the sub- stantial propagation loss increases the noise figure of the system. Noise figure is a term that measures the degra- dation in signal-to-noise ratio (SNR) caused by a com- ponent or system. This increase in noise figure means that the receiver sensitivity is reduced and that many signals of interest will not be processed successfully. We show that for cables exceeding a certain length, the use of fiber optics for remoting of received GPS signals offers a distinct performance advantage. GPS OVERVIEW Historically, submarines have used low-frequency Long Range Navigation (LORAN) signals to determine their location and speed. However, this system has been replaced in recent years by the NAVSTAR GPS. This is a Global Navigation Satellite System (GNSS) that pro- vides worldwide radio coverage from a constellation of Figure 2. GPS satellite constellation in medium Earth orbit. 310 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 30, NUMBER 4 (2012) GPS OVER FIBER FOR BUOYANT CABLE ANTENNAS receiving antenna by demodulating the received signal. BCA is attenuation along the RF signal path. This This propagation time can be used to estimate a pseudo- includes propagation through the atmosphere, poten- range, an estimate of the distance from the antenna to tial propagation through seawater, antenna gain that is th an individual satellite. The pseudo-range, i, to the i limited by the BCA geometry, and, finally, propagation satellite is given by along the BCA transmission line to the GPS receiver. The GPS signal level for antennas at or slightly above i = |P – Pi| + cT , (1) the ocean surface is approximately –130 dBm, where dBm refers to the power ratio in decibels relative to th where P is the user’s position, Pi is the i satellite’s posi- 1 mW. Over the sampled bandwidth of the C/A signal tion, c is the speed of light, and T is the error contribu- (2.046 MHz), the integrated thermal noise is approxi- tion from the receiver’s clock. Equation 1 would be used mately –111 dBm. For an antenna gain of 0 dBi, this to determine the user’s position once sufficient pseudo- means that the SNR at the antenna is –19 dB. The term range information has been obtained. dBi refers to the gain of the antenna under consideration The GPS system transmits several DSSS ranging (in decibels) compared to that of an isotropic antenna, signals, the most important of which are the civilian which radiates uniformly in all directions. Although this Coarse Acquisition (C/A) signal centered at the L1 fre- number seems low, a significant amount of processing quency (1575.42 MHz) and the two military Precision gain (43 dB) can be achieved by despreading the DSSS P(Y) signals centered at the L1 and L2 (1227.60 MHz) signal. For a target sensitivity of –140 dBm (providing frequencies. The P refers to a precision signal with 10 dB of link margin), the SNR at the antenna is –29 dB. greater accuracy than the C/A signal, and P(Y) refers to This 10 dB of link margin can be used to compensate the encrypted version of this signal. For the purposes of for propagation losses or accommodate antenna detun- this article, the principal difference between the civil- ing caused by the ocean surface. A typical GPS receiver ian and military signals is their bandwidth. The C/A requires an SNR of 10 dB after despreading, which is signal has a bandwidth of 2.046 MHz, and the P(Y) equivalent to an SNR of –33 dB prior to despreading. signal has a bandwidth of 20.46 MHz. In addition to Comparing the SNR at the antenna with the required the ranging signal, each satellite transmits a navigation SNR, we see that the antenna and receive link can only message including clock corrections and almanac and degrade the SNR by 4 dB before the received signal ephemeris information. The clock corrections, almanac becomes unusable. The penalty caused to the SNR by information, and ephemeris information are required a component or subsystem is known as the noise figure, to synchronize the satellite atomic clocks with terres- expressed in decibels. On the basis of this calculation, trial receiver clocks. The almanac message contains the GPS BCA receive path must have a noise figure of coarse orbit information for all satellites in the constel- less than 4 dB. The numbers provided here are approxi- lation and can be used to determine which satellites mate values, but they do indicate that the BCA receive are within the receiver’s field of view. This reduces the link must have a low noise figure nearly equal to that of receiver search time. The ephemeris message contains conventional GPS receive links, which need to operate precise orbit information for the individual satellite. over only a short length of cable. This orbital information is required to minimize errors Receiving GPS signals over a BCA is further com- resulting from minor variations in the satellite’s orbit. plicated by the operating environment, shown in Fig. 3. The ephemeris data are valid for only a few hours and The antenna is presented with two obstacles: seawater must be updated periodically. A position is calculated washover and cable roll that can align the antennas through trilateration, which requires pseudo-ranges downward. Even mild antenna washover rapidly attenu- from at least four satellites in a nondegenerate geometry to solve 0 for the user’s four unknowns: latitude, longitude, altitude, and receiver clock error. Pseudo-range –0.2 measurements from additional satellites improve the geometry 1 –0.4 and allow integrity monitoring. Elevation (m) Wire Antenna elements –0.6 35 40 45 50 55 GPS SIGNAL RECEPTION Along-track range (m) IN BCAs The principal difficulty with Figure 3. BCA floating on ocean surface. Some antenna elements are submerged or par- GPS signal reception over the tially submerged. JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 30, NUMBER 4 (2012) 311 A. KARIM, S. J. STAFFORD, AND R. B. BAKER ates GPS signals—the attenuation of RF at L1 in seawa- equipment are a laser and photodetector. The laser is ter is more than 1000 dB/m.2 These impediments can a semiconductor-based component that generates an be overcome through redundancy.

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