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PHANTOM UAV – Flight Group 005

 

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Flight Group 004 – Communication Range Tests

    Factors Affecting Signal Strength

        Signal Strength Around An Antenna

        Signal Strength And Polarisation

        Signal Strength And Distance

        Signal Strength And Line Of Sight

        Signal Strength And Reflections

        Signal Strength And Transmitter Power

    Telemetry Communication

    Control Communication

        Horizontally Polarised Control Signal

        Vertically Polarised Control Signal

Flight Plan

    Waypoint Display

    Command List

    Elevation Graph

Flight Review

    Horizontally Polarised Control Signal Flights

        Flight 1

        Flight 2

    Vertically Polarised Control Signal Flights

        Flight 3

        Flight 4

        Flight 5

        Telemetry Communication Performance

        Control Communication Performance

Flight Files

 

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Flight Group 005 – Communication Range Tests

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.

Factors Affecting Signal Strength

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.

Signal Strength Around An Antenna

DX7 Radiation Pattern

 

This diagram shows the signal strength radiating from a dipole antenna. The toroid (donut) shape represents the signal strength at the full range of angles around the antenna. The red lines are drawn from the antenna at several different angles to the edge of the toroid. The length of the red lines is proportional to the signal strength.

The greatest signal strength is perpendicular to the antenna. The signal strength drops as the receiver moves away from perpendicular. The signal is very weak in line with the antenna.

This also applies to a receiving antenna. The received signal strength is greatest when the receiver antenna is perpendicular to signal.

Signal Strength And Polarisation

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.

Signal Strength And Distance

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%.

Signal Strength And Line Of Sight

2.4GHz signals are effectively line of sight only. They do not bend around objects like buildings or hills.

Signal Strength And Reflections

Reflected Signal

 

A direct signal ‘A’ received by the aircraft can be interfered with by a signal ‘B’ reflected from the ground. Signal path ‘A’ is shorter than ‘B’ so the two signals can be out of phase. This causes the two signals to cancel each other out and cause a weak signal area in the sky.

At other areas the two signals can be in phase, so the signals add and create strong signal areas.

Reflections cause varying received signal strength as the aircraft moves through the sky.

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.

Signal Strength And Transmitter Power

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.

Telemetry Communication

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.

 

IMG_2443a

IMG_2445a   IMG_2448a

 

IMG_2444a

 

The 915MHz telemetry aircraft module is mounted beside the aircraft canopy with the antenna also vertical.

 

    IMG_2439a

Control Communication

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.

Horizontally Polarised Control Signal

For flights with horizontally polarised signals, the equipment was set as follows.

 

IMG_2436a

 

The two receiver antennas in the aircraft lie horizontally but at right angles to each other. As the aircraft changes heading, the signal strength in one will reduce while the other will increase.

 

The DX7 transmitter antenna also lies horizontally so that the horizontally polarised signal from the transmitter is received with the maximum gain by the receivers. The pilot holds the transmitter so that the antenna is perpendicular to a line between transmitter and the aircraft. This is also necessary to ensure maximum signal is transmitted in the direction of the aircraft.

Vertically Polarised Control Signal

For flights with vertically polarised signals, the equipment was set as follows.

 

2459a

 

One antenna in the aircraft is vertical and the other one is horizontal and inline with the fuselage. The vertical antenna is expected to receive the best signal when flying level.

 

The DX7 transmitter antenna sits vertically so the signal strength is the same in all directions in a plane perpendicular to the antenna.

Flight Plan

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.

Waypoint Display

. Flight Plan MP014

 

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.

Command List

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.

 

MP014 Command List

Elevation Graph

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.

 

Elevation Graph

Flight Review

The five flights are described here.

Horizontally Polarised Control Signal Flights

Flights 1 and 2 were both with horizontally polarised signals.

Flight 1

140714

Flight plan overlaid on telemetry log path.

Transmitter Voltage = 11.5V

Polarisation = Horizontal

 

Test 1 KMZ

Dataflash log in Google Maps.

 

The takeoff and flight to WP 3 (green) was first interrupted by a brief period in failsafe mode (blue) at the hill slope before continuing. The next loss of signal is at about 1.5km before again continuing. The signal was frequently lost and regained while trying to get to WP3 but eventually was lost for more than 20 seconds so the aircraft returned to the take off point (orange.)

Flight 2

MP014 Flight Plan & Tlog

Flight plan overlaid on telemetry log path.

Transmitter Voltage = 12.5V

Polarisation = Horizontal

 

test 2 KMZ

Dataflash log in Google Maps.

 

On this flight, with a higher transmitter voltage, the aircraft lost signal a few times but eventually did reach WP 3 and WP 4 (orange). On the return flight, however, with the aircraft pointing home, the signal was intermittent but was not lost for more than 20 seconds. It remained in auto mode and reached WP 5 before it was landed in manual mode. After getting past the hill slope the signal was uninterrupted. Curiously, the higher transmitter voltage seemed to be more susceptible to signal loss.

Vertically Polarised Control Signal Flights

Flights 3, 4 and 5 were all with vertically polarised signals.

Flight 3

Test 3

Flight plan overlaid on telemetry log path.

Transmitter Voltage = 12.1V

Polarisation = Vertical

 

tets 3 KMZ

Dataflash log in Google Maps.

 

The flight to WP 3 is uninterrupted (green) and just reached 2000m before losing signal for more than 20 seconds and returning to the take off point. It missed WP3 by about 60m

Flight 4

Test 4

Flight plan overlaid on telemetry log path.

Transmitter Voltage = 12.1V

Polarisation = Vertical

 

Test 4 KMZ

Dataflash log in Google Maps.

 

This is another flight under the same conditions as flight 3 but this time signal was lost a couple of times at the hill slope before continuing (red). This time it only reached 1750m before returning to the launch point. A video of this flight is in the Flight Files section below.

Flight 5

Test 5

Flight plan overlaid on telemetry log path.

Transmitter Voltage = 12.5V

Polarisation = Vertical

Test 5 KMZ

Dataflash log in Google Maps.

 

This flight was different to flights 3 and 4 because the transmitter battery was fully charged this time. I expected that this flight would reach a further distance but it turned around at only about 1650m before returning to the take off point. Clearly the control performance is not easily predictable at the limits of range.

Telemetry Communication Performance

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.

 

RSSI Graph

 

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.

RSSI Rx Error Graph

Control Communication Performance

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.

Flight Files

To download a file, right click on the filename and select ‘Save Target As …’

 

File Type

Filename

Flight Plan File

MP014

Display the waypoints and command list in ‘Mission Planner’

Dataflash Log Files

– Flight 1

– Flight 2

– Flight 3

– Flight 4

– Flight 5

 

2014-07-14 14-43-32.log

2014-07-14 16-24-17.log

2014-07-21 11-01-02.log

2014-07-21 11-34-19.log

2014-07-21 13-37-35.log

Display these flight paths in ‘Mission Planner’ software.

Dataflash .kmz Files

– Flight 1

– Flight 2

– Flight 3

– Flight 4

– Flight 5

 

2014-07-14 14-43-32.kmz

2014-07-14 16-24-17.kmz

2014-07-21 11-01-02.kmz

2014-07-21 11-34-19.kmz

2014-07-21 13-37-35.kmz

Display these flights in 3D in Google Maps.

Telemetry Log File

– Flight 1

– Flight 2

– Flight 3

– Flight 4

– Flight 5

 

2014-07-14 14-44-42.tlog

2014-07-14 16-25-19.tlog

2014-07-21 11-03-07.tlog

2014-07-21 11-37-30.tlog

2014-07-21 13-38-44.tlog

Display these flight paths in ‘Mission Planner’ software.

Onboard Video File

– Flight 1

– Flight 2

– Flight 3

– Flight 4

– Flight 5

 

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http://youtu.be/zoVInIhXk9U

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Click to play the YouTube video.

Observer Video File

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