| 2.4 GHz. SSFH Tx/Rx RF link principles. |
| The purpose of this article is to show the data types that manufacturer's collect from their Rx/Tx systems in order to correctly monitor the state of the RF link and thereby correctly command failsafe activation of their Rx systems. |
| The amount of data that manufacturers offer to the aeromodeller varies. Below is an example of one manufacturer that gives particularly high levels of data. |
| Antenna fades. |
| An antenna fade represents a loss of data on one of the receiver's antennae. While active, the Rx is continually monitoring the RF link. When a loss of data is recognised by the Rx on one of it's antenna, the Rx increments the associated antenna fade counter by one, thus recording one antenna fade. In itself, an antenna fade is a minor event. Anything that blocks or interferes with the RF link, (the radio link between Tx an Rx), will cause an antenna fade. Typically, in flight, fades are caused by in flight reflection events, (so called reflected antenna fades), such as the model aircraft's engine and/or exhaust blocking line of sight between the Tx on the ground and the Rx in the aircraft. Should the RF blocking event be serious, the antenna fade counter increments very quickly and a second layer of protection against loss of signal, (RF Link), is engaged. |
| Frame Loss. |
| The second layer of protection against loss of RF link is the designer's strategy which recognises a Frame Loss has occurred. While the technology may vary slightly between manufacturers, a frame loss is typically counted if all attached receiver's antennae record simultaneous antenna fades. |
| Lockout or Hold. |
| The third layer of protection against loss of RF link is the Lockout or Hold. While strategy between different manufacturers will vary, a Lockout or Hold is registered at around forty plus contiguous frame losses.This equates typically, to a solid loss of radio contact, of about one second duration. |
| A Lockout or Hold is a serous event, and the aircraft is put into failsafe mode when this occurs, and will descend in a variety of ways depending on the Failsafe protocol in use. |
| This tiered protection, (viz antenna fades, frame losses, holds), allows the Rx system to precisely recognise the serious state of signal loss that must be present before the Rx's failsafe command is issued to the aircraft's servos. |
| When a Lockout occurs and failsafe mode is triggered, the Rx continues to look for a restored RF link. If the RF link is restored, indicating the RF link problem was transient in nature, then the hold is released and control is usually restored in one or two seconds, i.e. fast enough to save the aircraft usually. If the RF link is not restored, then Lockouts/Holds remain active, and control is not restored, and the aircraft descends in Failsafe mode. The failsafe mode used, is up to the pilot and his choice of radio equipment and is covered elsewhere in RCAerobase. |
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The strategy used to inititiate the failsafe protocol will vary with manufacturer. Given the seriousness of issuing the failsafe event however, it is likely to be similar to the above. i.e. other manufacturer's will likely design their strategy around a time period that they will allow for total RF loss, before issuing the failsafe. |
| The standard range check for most modern 2.4 GHz SSFH RC systems is thirty yards. This figure is typically much lower than is actually achievable in practice. We have seen good systems working perfectly in range check mode at 140 yards or more. This fact is very important as it allows the modeler to range test his equipment to the limit, and so check relatively, how well his equipment is working. This ability to check equipment thoroughly is very valuable to modelers and enables them to make a valid decision as to whether or not to get airborne. To optimise this process the Advanced Range Test protocol has been developed. While more involved, and requiring more work by the modeler, it can identify a weak Rx/Tx system which the standard range test mode might not. |
| Some manufacturers make the needed data available to the modeler via on board telemetry, and/or external devices that can be plugged into the aircraft post flight so that readings can be taken. While other manufacturers may not offer this degree of information to the modeler, it is worth pointing out that the Advanced Range Test can be carried out without it. See the links below. |
Related Article Links:
2.4 gHz. RF link problems 1. New equipment failure.
2.4 gHz. RF link problems 2. Tx/Rx RF link principles.
2.4 gHz. RF link problems 3. Advanced Range Test Protocol.
2.4 gHz. RF link Problems 4. Causes and Fixes
2.4 gHz. RF Link Problems 5. Failsafe Triggered
2.4 gHz. RF link Problems. 6. Failsafe Strategies.
2.4 gHz. RF Link Problems 7. Faultfinding.
2.4 gHz. RF Link problems 8. Failsafe Recovery Time.
Shows fundamental principals and limitations of 2.4 GHz equipment, along with installation recommendations: http://www.archeli.com.au/forums/showthread.php?t=136223
Comments
11/7/2016 Added:
Shows fundamental principals and limitations of 2.4 GHz equipment, along with installation recommendations: http://www.archeli.com.au/forums/showthread.php?t=136223