UAV Navigation in depth: How to measure the quality of the datalink

Introduction.   A datalink is a means of transmitting information from one point to another. When the transmission is accomplished using a wireless means, the datalink may also be referred to as a 'radio link' or 'radio modem'.

In unmanned aviation, the datalink allows the transmission and reception of information between the autopilot in the aircraft and the Ground Control Station (GCS) as follows:

  • Uplink: commands from GCS to autopilot.

  • Downlink: telemetry information from autopilot to the GCS.

In order to measure link quality, Received Signal Strength Indicator (RSSI) and Signal-to-Noise Ratio (SNR) parameters may be used.

Sensitivity.   The signal received by the datalink must be of sufficient power (or signal strength) in order for the information to be received correctly by the datalink. This is a function of the datalink's 'sensitivity' and determines the weakest signal that can be successfully received. This value defines the performance of the receiver and is provided by the manufacturer in milliwatts (mW), or more commonly, in decibel-milliwatts (dBm).

 

Signal Noise

Source: Electronic Notes

RSSI.   RSSI is a relative value that measures the power of the received signal. Monitoring RSSI during the operation is critical in order to prevent a communication loss that could lead to accidents or endanger flight safety.

Signal strength is represented in dBm using a logarithmic scale. This is the power ratio in decibels (dB) of the measured power referenced to one milliwatt: 0 dBm is equal to 1mW. Positive values represent >1mW and negatives <1mW. The table below shows some reference signal strength values for a typical receiver.



Signal Strength

>-76 dBm

Excellent

-89 to -77 dBm

Very Good

-97 to -90 dBm

Good/Average

-103 to -98 dBm

Low

-112 to -104 dBm

Very Low

-113 dBm

Unlikely Connection

Source: Wikipedia. Reference values.

SNR.   In addition to RSSI, the SNR (sometimes also abbreviated to S/N) may also be considered when receiving information using datalinks. SNR compares the level of a desired signal to the level of background noise.

Signal Noise Equation

where P is average power.

This parameter may be also defined in dB:

Signal Noise Decibel

Signal Noise Logaritmic

Signal Noise dB

 

The higher the SNR, the better the signal quality. Datalinks require a minimum SNR to achieve good reception that typically depends on the type of modulation used.

UAV Navigation invests considerable time and effort into optimizing the quantity and structure of data that the system transmits via the datalink. The aim is to minimize the quantity so as not to overload the bandwidth available for a given datalink and therefore to make communications as robust and effective as possible. This is achieved, amongst other things, by sending equal-sized packets, which are transmitted in equal time periods. This ensures that datalinks can provide an almost constant data throughput without the need to use large 'buffers'.

Packet Statistics.   Whilst the UAV Navigation system has no direct control over the quality of the radio link, it does provide the operator with statistics about the number of packets which have been transmitted (uplink) and received (downlink).  This information is provided in a detailed report via the following window in Visionair:

UAV Navigation Data Flow

The report uses a color code to represent the quality of communications between the autopilot and the GCS:

  • Green: excellent communications. Over 66% packets correctly sent/received.

  • Yellow: workable communications. Between 33% and 66% packets correctly sent/received.

  • Red: poor communications. Less than 33% packets correctly sent/received.

Overcoming Data Loss.   Many datalink solutions implement several strategies to overcome data loss in the radio channel. One of these strategies is to 'buffer' the data into 'chunks', sending each chunk several times. This increases the chances of the data being correctly received at the other end, even if several attempts are required. Note that this process is handled by the datalink and is 'transparent' to the Visionair operator, i.e. the datalink actually delivers the data just once.

In light of the above, it is logically possible to have a good or even excellent percentage of uplink and/or downlink packets, whilst having a relatively poor radio signal quality.

The opposite scenario is also possible, for example if the datalink is not properly connected to the autopilot: in this case it would be possible to suffer huge data loss even though signal quality was excellent. Therefore, in order to ensure that the communications system as a whole is working within safe limits it is vital to monitor both the RSSI/SNR levels for the radio link quality, as well as the Visionair statistics as shown above.

Built-in RSSI Monitoring.   If the datalink chosen provides RSSI values, the UAV Navigation system is able to report this information and to display it via Visionair along with autopilot telemetry. This allows the operator to monitor both radio link quality (analog input value from the RSSI signal) plus the data statistics for packets in the uplink/downlink.

Flight Safety When Communications are Lost.   Flight safety is a primary concern for UAV Navigation; in case of a loss of communications the UAV Navigation system will automatically command SAFE mode, which means that the autopilot will control the aircraft to the previously set Safety Altitude. Once the Safety Altitude has been achieved, the autopilot will automatically switch to LAND mode, bringing the UAV safely back to the predetermined landing site. Other configurations are possible including, for example, the automatic deployment of a parachute.

For more information about how to select a good datalink, check out our Knowledge Base article: External Datalink Selection.

 

UAV Navigation: Cutting-edge Autopilots.