The following is taken from "An Overview Of The Petite Amateur Navy Satellite (PANSAT) Project",
Spread spectrum modulation provides the advantages of low probability-of-intercept (LPI), low probability-of-detection (LPD), resistance to jamming and low probability-of-interference (to and from other users in the band) [Ref. 1] . The use of spread spectrum techniques offers several addi tional significant advantages, especially in military applications. Although not all of the following are applicable to PANSAT, additional advantages of spread spectrum include [Ref. 2, pp. 8-9]:
Code division multiple access (CDMA) capability, which enables selective addressing of communications and multiple user access to a single communications channel;
Low probability of recognition (LPR) signal, which enables the effective "hiding" of the signal in the noise, in the interference, or in other communications;
High resolution ranging, which is determined by the rate of the system. Very high range resolutions are available in spread systems, such as the Global Positioning System (GPS);
Interference rejection, which is intentionally putting the signal level around the ambient noise level, while still being able to successfully extract the signal.
In the past, the use of spread spectrum techniques has been limited mostly to military applications. By their nature, spread spectrum systems utilize more bandwidth than is required for the information being transmitted. This inherent inefficiency has kept spread spectrum from widespread use in the private sector where the bandwidth allocations are limited. Recently the paying public has expressed an increased interest in spread spectrum modulation and its intrinsic advantages, particularly con cerning low-power, high-density personal communication devices. [Ref. 3. Appendix A]
The PANSAT team initially attempted to complete the necessary documentation to utilize frequen cies in the military band. Due to the multitude of systems and signals already in residence and the extended time period required for approval, the Naval Electromagnetic Spectrum Center (NAVEMSCEN) suggested the use of the amateur band. Thus, the decision was made to pursue a HAM frequency through the American Radio Relay League (ARRL), which greatly simplified the requisition process while still meeting all of the original PANSAT design/engineering goals. [Ref. 2, p. 10]
With Direct Sequence Spread Spectrum (DSSS), the modulated carrier is expanded to the extent that it occupies a bandwidth that is much greater than the information bandwidth of the signal. This is accomplished by multiplying the information signal by a pseudo-noise (PN) spreading sequence which has a bit rate that is significantly greater than the information data rate. The resulting process spreads the signal energy over a wider portion of the frequency spectrum. The same PN sequence must then be used by the receiver to despread the signal. [Ref. 3, App. A]
The amount that the signal is spread is determined by the ratio of the bit rate of the spreading se quence divided by the data rate of the information signal. This ratio is also referred to as the process ing gain, which is denoted by Gp. The bandwidth of the spread signal is the product of the band width of the unspread signal and the processing gain. Each bit of the PN sequence is commonly referred to as a chip, so the bit rate of the sequence is correspondingly called the chip rate. For PANSAT, the chip rate is 1.25 Mcps and the data rate of the digital message is 9.842 kbps. This results in a processing gain of 127, which is equivalent to 21 decibels (dB). [Ref. 3, App. A]
As its name implies, a pseudo-noise sequence is a deterministic sequence that appears to be random. The most common PN sequence is a maximal length sequence, or M-sequence. The sequence is generated by using a binary linear feedback shift register. The PANSAT Communications subsystem utilizes a seven stage M-sequence generator, with taps at 7 and 1, which results in a maximum sequence length of 127. This number is derived by using the formula 2 n -1, with n=7 (27-1=128-1=127). [Ref. 3, App. A]
1. Payne, Robert Andrew Jr., Applications of the Petite Amateur Navy Satellite (PANSAT) , Master's Thesis, Naval Postgraduate School, Monterey, CA, Sep. 1992.
2. PANSAT Systems Analysis Group Systems Review, AA4831 Class Project, Spring 1994.
3. Huneke, Stephen P., The Design of a Digital Direct Sequence Spread Spectrum Demodula tor for the Petite Amateur Navy Satellite (PANSAT) , Master's Thesis, Naval Postgraduate School, Monterey, CA, Dec. 1994.
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