These flights were done to gain an insight into the ranges of the two communication systems i.e. control and telemetry communication systems.
The plan is to fly to a distant waypoint and check how far the aircraft gets before it loses control communication. The control frequency is 2.4GHz frequency hopping and the telemetry system is 915Mz also frequency hopping.
Here is some background information about the characteristics of radio waves that affect the range of radio communication. This information applies to both control and telemetry radio communication.
The signal transmitted from a dipole antenna is polarised. If the antenna is vertical, the signal is vertically polarised. The signal is polarised at the angle of the transmitter. To receive the strongest signal, the receiver antenna must be at the same angle. A receiving antenna perpendicular to the polarisation of the signal will receive a very weak signal.
The further the receiver is from the transmitter, the weaker the signal is. The signal strength drops in proportion to the square of the distance. For example, if the distance doubles, the signal strength drops to 25%.
2.4GHz signals are effectively line of sight only. They do not bend around objects like buildings or hills.
Signals reflected from the ground also become partially horizontally polarised. If the source signal is horizontally polarised and the receiving antenna is horizontal, the reflected signal has a strong interference effect. However, if the source signal is vertically polarised and the receiving antenna is vertical, the reflected signal has less interfering effect.
The battery voltage of the transmitter affects the transmitted signal power which correspondingly affects the signal strength. The voltage of the LiPo battery in the DX7 transmitter can vary between 12.5V when fully charged to 11.1V when almost depleted.
The telemetry base station is mounted on a camera tripod. The laptop PC sits on a purpose built tray and is secured with Velcro strap. The tray attaches to two hooks attached to two of the legs. A 3m USB extension cable is connected to the PC and runs up the centre of a 2m long, 25mm diameter PVC tube. The 915MHz telemetry base module plugs into the end of the extension cable at the top of the tube. The telemetry module antenna can be set to 3.3m above the ground.
The 915MHz telemetry aircraft module is mounted beside the aircraft canopy with the antenna also vertical.
The aircraft is controlled by a Spektrum DX7 transmitter which uses a 2650mAh lithium polymer battery.
The aircraft receiver is an Orange DSM2 7 channel receiver with a satellite receiver.
For flights with horizontally polarised signals, the equipment was set as follows.
For flights with vertically polarised signals, the equipment was set as follows.
Five flights were done. The flight plan is the same for all flights. The plan was to fly to a waypoint 2km from the home point. If the aircraft flew out of range it would first enter failsafe mode one during which it would fly in a circle for up to 20 seconds. If the receiver in the plane regained signal within 20 seconds, the aircraft would return to auto mode and resume the waypoint course. However, if the aircraft was in failsafe mode one for more than 20 seconds, it would enter failsafe mode two and then execute a ‘Return To Launch’ command. This means it would abort the command list, fly in a straight line to the take off location and fly in a circle until control was taken over by the pilot.
The aircraft location is monitored during the flight using Mission Planner software on the PC. However, if the telemetry communication fails the position of the aircraft is not known until it flies back within range.
To know where the aircraft flies, the flight data on the aircraft can be downloaded and displayed after the flight is completed.
The course chosen was over open fields, forest and tracks that are closed off to vehicles. The flight was on a cold (13oC) weekday in July so very few people were expected to be in the area. Waypoints 3 and 4 are just over 2000m from the home point.
The takeoff altitude was set to 50m above the takeoff point and the flight altitude was 100m above the takeoff point. An RTL command is programmed after the waypoint at command 5. The plan is to take manual control and land after reaching waypoint 5.
The elevation graph is created in Mission Planner from the mission plan and the elevation data on Google Maps. The lines of sight have been edited onto the graph. It shows that the takeoff point elevation is 38m above sea level and the flight altitude is 138 m above sea level (100m above the take off altitude). The highest point on the ground is at about 70m .i.e. 32m above the takeoff point. This means that even over 2000m away, waypoints 3 and 4 are still in line of sight. Notice, too, that there is a hill face about 24m high and about halfway between the takeoff point and waypoint 3. This is potentially a source of reflections.
The five flights are described here.
Flights 1 and 2 were both with horizontally polarised signals.
Flights 3, 4 and 5 were all with vertically polarised signals.
An important observation of all the telemetry logs is that there were no breaks in the logs. Reliable communication was maintained all throughout all the flights which were up to 2250m from the telemetry receiver.
This is a graph of the telemetry RSSI (radio signal strength indication) during flight 1, the flight that reached the greatest distance from home.
The lowest signal strength of 40 was at the 5 minute mark.
This graph shows the received errors that were fixed (green) and the received errors that were unable to be fixed (red).
The total fixed errors was 296 and the total unfixed errors was 189. This is a low value for a flight of about seven minutes up to a distance of 2250m.
All the flights had uninterrupted control communication up to 800m from the transmitter where the hill slope seemed to cause significant interference. The vertically polarised signals seemed to be less affected by the reflections on the hill slope but not totally immune. The overall range is perfectly good for normal visible aircraft control independent of polarisation. A horizontally polarised signal seemed to have a longer range but was significantly less reliable at long range than a vertically polarised signal. Perhaps the primary and reflected signals were adding together to create a stronger signal at long range.
The best way to minimise problems with signal strength is to fly within a few hundred metres of the transmitter and far away from reflection sources like hills and buildings. With an autopilot on the aircraft, however, losing control signal does not mean the aircraft will crash. It simply flies itself back into transmitter range. An autopilot is an effective way of avoiding the multitude of implications resulting from an uncontrolled crash.
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