======================================================================== Visual Satellite Observing F A Q Chapter-03 What Are The Different Types Of Satellites? ======================================================================== ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + This FAQ chapter is "under construction". Some of the + + sections may be unwritten as yet. Other sections may + + contain out-of-date, unreviewed, or "starter" material. + + Yet other sections may be works in progress, partially + + written and reviewed. + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + In this chapter, the following sections are considered + + to be completed (written and reviewed): + + None + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ---- 3.0 What are the different types of satellites? ---- 3.1 Remote-sensing satellites ---- 3.2 Communications satellites ---- 3.3 Navigational satellites ---- 3.3.1 GPS (Global Positioning System) The GPS consists of 24 satellites, four in each of six orbital planes. The satellites operate in circular 20,200 km orbits at an inclination angle of 55 degrees and with a 12-hour period. Each satellite transmits a very precise time signal using atomic clocks. By measuring the time of arrivial (TOA) of the signal from 2-4 satellites, the user's position can be computed on the Earth's surface, in the air, or in space. The user also can get velocity and precise time information. ---- 3.3.1.1 GPS Signal Characteristics The satellites transmit on two L-band frequencies: L1 = 1575.42 MHz L2 = 1227.6 MHz Three pseudo-random noise (PRN) ranging codes are in use: Coarse/Acquisition code Precision code Y-??? code The Coarse/Acquisition (C/A) code has a 1.023 MHz chip rate, a period of one millisecond (ms), and is used primarily to acquire the P-code. The Precision (P) code has a 10.23 MHz rate, a period of seven days and is the principle navigation ranging code. The Y-code is used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated. The C/A code is available on the L1 frequency and the P-code is available on both L1 and L2. The various satellites all transmit on the same frequencies, L1 and L2, but with individual code assignments. Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference. Each satellite transmits a navigation message containing its orbital elements, clock behavior, system time, and status messages. In addition, an almanac is also provided, which gives the approximate data for each active satellite. This allows the user's receiver to find all satellites once the first has been acquired. ---- 3.3.1.2 GPS Time Transfer GPS is at the present time the most competent system for time transfer, the distribution of Precise Time and Time Interval (PTTI). The system uses time of arrival (TOA) measurements for the determination of user position. A precisely timed clock is not essential for the user, because time is obtained in addition to position by the measurement of TOA of FOUR satellites simultaneously in view. If altitude is known (i.e. for a surface user), then THREE satellites are sufficient. If time is being kept by a stable clock (say, since the last complete coverage), then TWO satellites in view are sufficient for a fix at known altitude. If the user is, in addition, stationary or has a known speed then, in principle, the position can be obtained by the observation of a complete pass of a SINGLE satellite. This could be called the "transit" mode, because the old TRANSIT system uses this method. In the case of GPS, however, the apparent motion of the satellite is much slower, requiring much more stability of the user clock. ---- 3.3.1.3 GPS Positioning Services GPS provides two levels of service, Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). SPS is available to all GPS users on a continuous, worldwide basis with no direct charge. SPS will be provided on the GPS L1 frequency which contains a coarse acquisition (C/A) code and a navigation data message. SPS provides a positioning accuracy of 100 meters (95 percent) horizontally, 156 meters (95 percent) vertically, and time transfer accuracy to UTC within 340 nanoseconds (95 percent). The position can to some extent be improved using DGPS (Differential GPS) corrections from ground stations. PPS is a highly accurate military positioning, velocity and timing service which will be available on a continuous, worldwide basis to users authorized by the USA. P(Y) code capable military user equipment provides a positioning accuracy of at least 22 meters (95 percent) horizontally, 27.7 meters vertically, and time transfer accuracy to UTC within 200 nanoseconds (95 percent). PPS is transmitted on the GPS L1 and L2 frequencies. PPS was designed primarily for USA military use and is denied to unauthorized users by the use of cryptography. ---- 3.3.1.4 GPS Selective Availability and Anti-Spoofing Selective Availability (SA), the denial of full accuracy, is accomplished by manipulating navigation message orbit data (epsilon) and/or satellite clock frequency (dither). Anti-spoofing (A-S) guards against fake transmissions of satellite data by encrypting the P-code to form the Y-code. The USER segment consists of antennas and receiver-processors that provide positioning, velocity and precise timing to the user. ---- 3.3.1.5 GPS System Time GPS system time is given by its Composite Clock (CC). The CC or "paper" clock consists of all operational Monitor Station and satellite frequency standards. GPS system time, in turn, is referenced to the Master Clock (MC) at the USNO and steered to UTC (USNO) from which system time will not deviate by more than one microsecond. The exact difference is contained in the navigation message in the form of two constants, A0 and A1, giving the time difference and rate of system time against UTC (USNO,MC). UTC (USNO) itself is kept very close to the international benchmark UTC (BIPM), and the exact difference, USNO vs. BIPM is available in near real time. The latest individual satellite measures are updated twice daily. The best current measure of the difference, UTC (USNO MC) - GPS is based on filtered and smoothed data over the past two days. USNO Series 4 provides 10 days of past time comparisons of USNO Master Clock minus individual GPS satellites. ---- 3.3.2 GLONASS (URAGAN/Hurricane) The Russian GLONASS constellation is composed of 24 satellites, eight in each of three orbital planes. The satellites operate in circular 19,100 km orbits at an inclination of 64.8 degrees and with an 11-hour, 15 minute period. Each satellite transmits on two L frequency groups. The L1 group is centered on 1609 MHz while the L2 group is centered on 1251 MHz. Each satellite transmits on a unique pair of frequencies. The GLONASS signals carry both a precise (P) code and a coarse/ acquisition (C/A) code. The P code is encrypted for military use while the C/A code is available for civilian use. The position given with the C/A code to civilian users is within ______ and slightly better than the GPS C/A code. ---- 3.3.3 TRANSIT TRANSIT was the first operational satellite navigation system (1967). Developed by the Johns Hopkins Applied Physics Laboratory, the system was intended as an navigation aid to missile submarines. The TRANSIT system allows the user to calculate position by measuring the doppler shift on the radio signals transmitted by the satellite on 150 MHz and 400 MHz. With the satellite ephemeris information transmitted in the signal, the user was able to calculate position to within a few hundred meters. The TRANSIT system has several drawbacks. First, the system is inherently two dimensional, and the user must know geodetic altitude (the earth is not a perfect sphere). In the receiver this is approximated with a mathematical model (WGS-72). Second, the velocity of the user must be taken into account to get the correct doppler shift. Third, mutual interference between satellites restricted the total number of satellites to five. Thus, a satellite would only be visible for limited period of time, and the time between fixes is 30-115 minutes(!), depending on the user's latitude. (The satellites travel in polar orbits, which gives frequent passages near the poles and few passages near the equator.) These drawbacks pretty much eliminated aviation applications and severely limited land-based applications. ---- 3.3.4 TSIKADA/NADEZDHA/PARUS Russian doppler measuring system similar to TRANSIT, still fully operational. Orbit... TSIKADA(Cricket)/NADEZDHA(Hope) is the civil navigation system. Four satellites are spaced in orbital planes 45 degrees apart. (Plane 11-14) transmits the navigation message with 50 bits/sec on 150.00 MHz and a continuous carrier on 400.00 MHz. Nadezhda satellites carry COSPAS-SARSAT transponders with a 1544.50 MHz downlink. This is an international system for location of distress beacons on 121.5, 243.0, and 406 MHz from ships and airplanes. PARUS(Sail) is the military system. Six satellites are spaced in orbital planes 30 degrees apart. (Plane 1-6) is sometimes called TSIKADA-M or only given a COSMOS number. Frequencies are different from the civil system. Unique frequency for each plane 149.91/399.76 - 150.03/400.08 MHz. The 150 MHz signals are very easy to receive on a scanner or VHF-receiver. For more information check "Hearsat-L". ---- 3.4 Research satellites ---- 3.5 Military satellites ---- 3.5.1 What does "ELINT" mean? ---- 3.6 Manned Spacecraft ---- 3.7 Inert Rocket Bodies and Boosters ---- 3.8 Space Debris ---- 3.9 Space Probes ---- 3.10 Amateur Radio Satellites ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in February 1998. ========================================================================