Main / Satellite
A bus is a common commercial satellite hardware platform, not a communications interface. CubeSatOriginal spec was for a cube that would fit into a 3-unit rectangular deployment box (Poly-Picosatellite Orbital Deployer) which sits inside the launch vehicle. On receiving a command from the launch vehicle, the cubesats are pushed out one at a time with a spring loaded mechanism. There was a 100 W-hour limit on stored chemical energy per cubesat (like propulsion system fuel), and at least 3 required locking mechanisms for propulsion activation. Certain magnetic field restrictions as well. The volume was 1-3 liters and mass 1-3 kg, with an expected mission length of around 3 months. Other requirements include center of gravity, construction materials and finish (i.e. anodized aluminum). Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does. Anodizing changes the microscopic texture of the surface and the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Orbit = 600-1200 km (LEO) All systems are powered off until after deployment. The Cubesat systems are Command and Data Handling (C&DH), RF Communication, Attitude Determine and Control (ADC). Deployables must wait 30 minutes after ejection to deploy, and signals must wait 45 minutes. TechnologiesGPSDOGPS disciplined oscillator is a high quality timing source used to keep a lot of systems on UTC. It is similar in basic architecture to a PLL. Provides time, a 1-PPS signal and a ref clock output (e.g. 10Mhz). Uses GPS antenna input to steer a stable oscillator such as quartz or rubidium. SpacewireSpaceWire's modulation and data formats generally follow the data strobe encoding - differential ended signaling (DS-DE) part of the IEEE Std 1355-1995. SpaceWire utilizes asynchronous communication and allows speeds between 2 Mbit/s and 400 Mbit/s, with initial signalling rate of 10Mbit/s. Uses LVDS. Spread-spectrumThe GPS system is a CDMA spread-spectrum service. The PRN refers to a unique PRN code that identifies a sat for CDMA nav systems; spreading codes for spread-spectrum interference minimization. GNSSWhen a GNSS position is computed, there are four unknowns being solved: latitude, longitude, height and receiver clock offset (often just called time). The solutions for each of the four unknowns are correlated to satellite positions in a complex way. Since satellites are above the antenna (none are below) there is a geometric bias. Therefore, geometric biases are present in the solutions and affect the computation of height. These biases are called DOPs (Dilution Of Precision). Radiation RobustnessNASA research on selecting parts: https://trs.jpl.nasa.gov/bitstream/handle/2014/51837/CL%2319-7471.pdf?sequence=1 Circular Polarization of AntennasGood write up here explaining left hand and right hand CP. https://antennatestlab.com/wp-content/uploads/2017/09/CP-Explained-Without-Math.pdf Traditional linear polarization antennas have a problem in satellite use due to rotation and movement across the horizon, requiring constant realignment. But you can benefit even if only one side uses CP. A good use case is cell phones, because you can keep a simple linear antenna on the handset. Another is GPS, with RHCP antennas on the satellite for downlink, and simpler designs generally on the receiver. But a RHCP receiver gets 3dB better signal strength. The CPRR is cross polarization rejection ratio which is just the difference between RHCP and LHCP gains. Ambiguity FunctionAn ambiguity function is a two-dimensional function of propagation delay and Doppler frequency. It represents the distortion of a returned pulse due to the receiver matched filter. A matched filter is obtained by correlating a known delayed signal, or template, with an unknown signal to detect the presence of the template in the unknown signal. Also called CAF, complex ambiguity function. Put another way in an IEEE abstract, calculation of the complex ambiguity function is viewed as the basis for joint estimation of the differential delay and differential frequency offset between two waveforms that contain a common component plus additive noise. In many applications, the required accuracy leads to a need for integration over long data sets that can become a challenge for near real-time digital processing. Keeping TimeAlthough we use the Gregorian calendar (jumped ahead of the Julian by 10 days to fix the yearly drift problem) the "Julian day" is used in some software for calculating elapsed days between events. A Julian year is 365.25 days, a Gregorian year is 365.2425, and a true tropical year is 365.2422 days. The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from day number 0 starting at noon on Monday, January 1, 4713 BC, Julian calendar (November 24, 4714 BC, in the Gregorian calendar). The significance of this date is three multi-year cycles started (which are: Indiction, Solar, and Lunar cycles) and which preceded any dates in recorded history. For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was 2 451 545. J2000 refers to the instant of 12:00 noon on Jan 1 2000 on the Gregorian calendar. The currently-used standard epoch "J2000" is defined by international agreement to be equivalent to: The Gregorian date January 1, 2000, at 12:00 TT (Terrestrial Time). The Julian date 2451545.0 TT (Terrestrial Time). January 1, 2000, 11:59:27.816 TAI (International Atomic Time). January 1, 2000, 11:58:55.816 UTC (Coordinated Universal Time). 630763148.816 GPST (GPS Time). TAI is a weighted average of the time kept by over 400 atomic clocks in over 50 national laboratories worldwide. The majority of the clocks involved are cesium clocks; the International System of Units (SI) definition of the second is based on cesium. GPS Time is kept with the atomic clocks on GPS sats and ground stations, and started at 0 on 1980-Jan-6 0:00. GeolocationAn AOI pass is defined by a vector of ephemerides which are the satellite's ECEF position and velocity values. You may do calculations on a fixed full set of ephemerides or on streaming ephemerides as they come in. These are used to synthesize a scenario and emitter waveform. The channel simulator uses an emitter location and satellite ephemerides to calculate things like slant range and Doppler effect to produce a correct Doppler and AWGN impacted waveform. If using a channel generator, interpolation is used with a signal's sample rate. You can emulate GPS with a 1 Hz ephemerides update. Glossary
Tactical Awareness Kit UsageCursor On Target InformationCoT's <event> base class describes the what, where, when of a single object "a-h-G-E-V-A-T-t" means "atoms::hostile::ground::equipment::vehicle::armored::tank::t72"
There are whole ton of these type flags, and here is a list:
The "time" attribute is a time stamp indicating when an event was generated. The format of time, start, and stale are in standard date format (ISO 8601): CCYY-MM-DDThh:mm:ss.ssZ (e.g., 2002-10-05T17:01:14.00Z), where the presence of fractional seconds (including the delimeter) is optional. Also, there is no constraint on the number of digits to use for fractional seconds. The following are all valid: 2002-10-05T18:00:23Z, 2002-10-05T18:00:23.12Z, 2002-10-05T18:00:23.123456Z A CoT object (<event>) describes only one object. There is no aggregation element defined, and there is no streaming model (where you can continue to get multiple CoT messages over a single TCP connection). This was done on purpose. If we allowed streaming, we'd have to require everyone to implement a framing mechanism. That goes against the CoT KISS philosophy. Instead, for a reliable transfer, we open a TCP connection, ?squirt? one CoT object, and close the connection. We don't advocate a persistent TCP connection for CoT message transfer because doing so requires a framing protocol (to delineate message boundaries) which TCP doesn't provide. We handle this by moving SA data via UDP, reserving TCP for things like targets, maydays, images. Industry TrendsSome mission operators would like to flip traditional industry practice when it comes to satellites. You usually pick an available bus and then customize payloads to fit onto the bus. But they want to unveil their big collection of payloads already designed and instead ask bus builders to create something that can handle all the payloads picked for a mission. Probably won't work despite how neat it sounds? The bus is the bigger, more complicated, more expensive part of the build. |