UAV Backhaul / Reachback

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Unmanned Aerial Vehicle (UAV) Reachback / Back-haul

This application describes the use of the Metrodata AP3000 Access Processor to transport asymmetric data from the Ground Data Terminal (GDT) over fiber or ATM to the Mission Planning and Control Station (MPCS) as shown in the application diagram below.

UAV Backhaul over Fiber

UAV Systems

In 2001 Metrodata became involved with the implementation of Unmanned Aerial Vehicle (UAV) Systems, and specifically the characteristics of the data link that the payload enforces on the UAV System. Typically a UAV System will have the following elements:

  • Air Vehicle
  • Data Link
  • Payload
  • Mission Planning and Control Station (MPCS)

Ground Data Terminals Attract Hostile Fire

The Ground Data Terminal (GDT) is the ground portion of the data link. The GDT communicates with the UAV typically via microwave antenna that can provide Line-Of-Sight communications. Where this is not possible, satellite can be used.

In the past, the GDT would be co-located with the MPCS. Decisions can be made at the MPCS, as video, command, and telemetry data are all processed and displayed there. However, the RF emissions from the GDT can attract hostile fire.

So, where applicable the MPCS can be located remotely from the GDT, ensuring the safety of the Decision Makers and Controllers of the UAV. Typically this would be via Fiber Optic Cables, or if it was on an Air Base via the Base's ATM/Secure Network.

UAV System Distinctive Data Characteristics

The distinctive characteristics of a UAV System in terms of data communications are that the data rates are typically "odd", being asymmetric and not conforming to standard terrestrial telecommunications carrier data rates. This isn't a problem if you are co-locating the GDT and MPCS. However, if you need to locate the GDT remotely from the MPCS, then you still have to transport this asymmetric, odd data rate across a networks that support different and usually symmetric data rates (e.g. terrestrial Carrier data rates).

Metrodata UAV Applications

Metrodata support several applications for UAV Systems, where we can help solve problems such as:

  • Clocking at "Odd" UAV data rates - i.e. 10.671Mbps transported over an OC-3 ATM link
  • Supporting your exact data rates - i.e. 9.312Mbps receive 266.67Kbps transmit
  • Asymmetric operation - backhauled over symmetric fiber links

These applications are from real world problems that have been encountered and resolved. They might be slightly different from your application of a UAV system in which case Metrodata probably already has a solution for those problems as well. If you are having problems transporting the UAV data link over terrestrial or fiber networks, then please contact us and see if we can help.

There are 3 applications that Metrodata has recently been involved with:

  • Remotely locating the GDT from the MPCS via a fiber optic Link
  • Controlling the UAV from a central MPCS on an air base, where the GDT is located remotely at the edge of the base
  • Controlling the UAV via satellite and backhauling communications from the satellite teleport to the MPCS

Typically these will involve taking a serial satellite modem connection from the GDT and converting this to fiber or ATM cells. In terms of asymmetric operation, the best interfaces to use on the satellite modems are EIA530, High Speed Serial Interface (HSSI) and V.35 as these all support asymmetric clocking.

UAV Data Link

Metrodata offer UAV systems the flexibility and scalability to operate between 8kbps and 155.52Mbps in 1bps increments. For example, on our EIA530 module we can support 20 million different data rates, so we can match the exact data rates of the UAV data link.

Usually the uplink data rate is only a few kbps, simply providing flight-path control information to the UAV and commands to the UAV's payload. The downlink consists of 2 channels with the low data rate channel used to acknowledge commands to the UAV's payload, to acknowledge flight path control requests and to transmit information about the UAV. The high data rate channel usually transports either video or radar at data rates which are typically between 1Mbps and 10Mbps.

Several different elements combine here to give the odd data rate:

  • Availability of satellite/microwave may not be constant or fixed.
  • Capability of satellite modems used to take the RF from an antenna on the GDT
  • Atmospheric or geographic issues may affect the usable satellite bandwidth
  • Encryption overhead, FEC, MPEG framing on the video stream

One thing to consider is that to most data communications manufacturers 9.312Mbps is an "odd" data rate, but to Metrodata it is simply the maximum data rate that some satellite modems run at (for example the Comtech EFDATA SLM8650).

Remotely Locating the GDT from the MCPS via a fiber optic link

Fiber optic cable is widely used in the tactical environment, mainly due to the fact it does not suffer from EMI & RFI (electromagnetic and radio frequency interference), which can affect data integrity and cause corruption on electrical or radio based links.

Using the Access Processor we are able to drive the asymmetric data rate from the Ground Station Terminal up to 70 kilometres (dependent on fiber type). In this instance the data rates are 266.67kbps transmit (communication/payload controls signals) and 9.312Mbps receive (maximum bit-rate achievable on the SLM8650 satellite demodulator).

Controlling the UAV from a central MCPS on an air base, where the GDT is located remotely at the edge of the base

Within any air base, there will be a given transport infrastructure for data. Typically this is ATM, due to the ability of this protocol to transport different traffic/application/service types. The ATM network may be extended by using fiber optic cables, microwave radio etc. The ability to interconnect the UAV control system into the secure network on the air base is important. This requires a connection from the fiber optic devices to a secure ATM cell encrypter, which in turn connects into the ATM Switch. At this point, we connect the Access Processor to the Ground Station Terminal. The MPCS end of the link is identical.

In this application the data rates are very different. 266.67Kbps Transmit and 26.67Mbps receive. This can present a problem as the EIA530 satellite modems only operate up to 20Mbps, so instead we can use an EIA530 satellite modulator and a High Speed Serial Interface (HSSI) satellite demodulator.

As this means we are supporting 2 different physical interfaces on the satellite modems, we need to support two physical interfaces on the Access Processor. With its in-built uni-directional functionality it is possible to present this as a single physical HSSI interface to the MCPS at the far end of the link. This single interface would present both 26.67Mbps and a 266.67kbps data rates.

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