ORIGINAL: rhklenke
ORIGINAL: tyrick69
With a DSSS systems, if the receiver is shadowed behind an engine, battery, carbon fuselage, etc.. it can actually mistake a reflected signal as the actual signal which would appear as a glitch, unintended input, or delayed input. FHSS systems are not really susceptible to reflected signals because by the time the reflected signal reaches the receiver, the transmitter and receiver have already hopped to a new frequency, thus the reflected signal is ignored. With the Spektrum system, the main and satellite receiver are always talking to one another and use the strongest signal that's being received by either the main or satellite thus reducing the potential for reflected signals to cause glitching because any direct signal will have more strength than a reflected signal.
Sorry, but that's
wrong - on a number of different issues. The main one being that multipath (which is what its called) does not result in ''glitches'' although if its bad enough, it can result in a packet being lost. Frequency hopping systems experience multipath as well (its a function of the frequency), its just that if multipath is bad enough to block the transmission, then when the system moves to another frequency, it changes the signal path enough (usually) that the transmission will get through.
Since DSMX is now ''frequency agile'' does that eliminate the need for the satellite RXs? Again, its not a function of what transmission mechanism you use, its a function of how good your receiver is... The chipset used in the Spektrum RX's (at least according to the info. that is out there - I haven't taken a Spektrum RX apart) was designed for short-range communication is USB-type devices, not long range radio control.
Bob
"Multipath propagation occurs when an RF signal takes different paths when propagating from a source (e.g., a radio NIC) to a destination node (e.g., access point). While the signal is en route, walls, chairs, desks, and other items get in the way and cause the signal to bounce in different directions. A portion of the signal may go directly to the destination, and another part may bounce from a chair to the ceiling, and then to the destination. As a result, some of the signal will encounter delay and travel longer paths to the receiver.
Multipath delay causes the information symbols represented in an 802.11 signal to overlap, which
confuses the receiver. This is often referred to as intersymbol interference (ISI). Because the shape of the signal conveys the information being transmitted,
the receiver will make mistakes when demodulating the signal's information. If the delays are great enough, bit errors in the packet will occur. The receiver won't be able to distinguish the symbols and interpret the corresponding bits correctly.
When multipath strikes in this way, the receiving station will detect the errors through 802.11's error checking process. The CRC (cyclic redundancy check) checksum will not compute correctly, indicating that there are errors in the packet. In response to bit errors, the receiving station will not send an 802.11 acknowledgement to the source. The source will then eventually retransmit the signal after regaining access to the medium.
Because of retransmissions, users will encounter lower throughput when multipath is significant. The reduction in throughput depends on the environment. As examples, 802.11 signals in homes and offices may encounter 50 nanoseconds multipath delay while a manufacturing plant could be as high as 300 nanoseconds. Based on these values, multipath isn't too much of a problem in homes and offices. Metal machinery and racks in a plant, however, provide a lot of reflective surfaces for RF signals to bounce from and take erratic paths. As a result, be wary of multipath problems in warehouses, processing plants, and other areas full of irregular, metal obstacles.
802.11b suffers the most
When comparing FHSS (frequency hopping spread spectrum), DSSS (direct sequence spread spectrum) and OFDM (orthogonal frequency division multiplexing), DSSS used by 802.11b networks is the most susceptible to multipath propagation. The frequency elements of a wider band signal will vary greatly in terms of reflectivity as they encounter obstacles in the facility.
FHSS uses relatively narrow channels (1 MHz) and changes transmit frequency often, making it difficult for multipath to occur. OFDM (used by 802.11a and 802.11g) transmits information on many narrow subchannels, which also reduces the impacts of multipath.
DSSS, however, transmits information continuously over a relatively wide channel, nearly 30MHz. This leaves enough room for lower frequency elements of the DSSS signal to reflect off obstacles much differently than the higher frequency elements of the signal. The differences in reflectivity will cause a wider range of signal paths. Thus, 802.11b systems are more susceptible to multipath delays."
Source:
http://www.wi-fiplanet.com/tutorials...AN-Problem.htm
Maybe glitch wasn't the right word to use but if the error correction and subsequent retransmission of the data results in a delay, this could be viewed as a glitch by some, certainly latency by others. The fact that I may have had some of the terminology wrong, doesn't negate the explanation for the use of satellite receivers. I also understand that this article is in reference to WLAN networks but the theory is the same.