uas telemetryjsumey/uas270/n6-telemetry.pdf · uas270 avionic systems 11/19/2018 uas telemetry 1...

UAS270 Avionic Systems 11/19/2018

UAS Telemetry 1

UAS Telemetry

UAS270 Avionic Systems

F18

Objective

• primary objective: to downlink UAV status information data to remote pilot, mission supervisor, etc.

• secondary objective: provide uplink path for C&C from GCS

UAS270 Avionic Systems 2


UAS Telemetry 2

Example UAS with Telemetry

UAS270 Avionic Systems3

https://diydrones.com/profiles/blogs/kickstarter‐project‐for‐a‐smart‐pod‐uav‐pod

Telemetry Data

• flight mode

• rx signal strength (RSSI)• battery/fuel usage/status• GPS quality (# satellites, DOP)• UAS position, heading, speed (air, ground, vertical)

• flight path, waypoint status, home

• other:– command acknowledgement

– payload status


EagleTree Vector OSD

MinimOSD


UAS Telemetry 3

Telemetry Data Transfer

• data transfer is accomplished using "RS‐232 style" serial data transfer method– each ASCII character is represented by a unique 7‐bit binary code (next slide)

– then framed with start/stop/parity bits before being transferred 1‐bit‐at‐a‐time over a single, unidirectional connection

– at a particular baud rate (bits/sec) which both ends agree upon

• baud rate determines the bit time via



Hex Hi 0 1 2 3 4 5 6 7

Lo Bits 000 001 010 011 100 101 110 111

0 0000 NUL DLE 0 @ P ` p

1 0001 SOH DC1 ! 1 A Q a q

2 0010 STX DC2 ” 2 B R b r

3 0011 ETX DC3 # 3 C S c s

4 0100 EOT DC4 $ 4 D T d t

5 0101 ENQ NAK % 5 E U e u

6 0110 ACK SYN & 6 F V f v

7 0111 BEL ETB ’ 7 G W g w

8 1000 BS CAN ( 8 H X h x

9 1001 HT EM ) 9 I Y i y

A 1010 LF SUB * : J Z j z

B 1011 VT ESC + ; K [ k {

C 1100 FF FS , < L \ l |

D 1101 CR GS - = M ] m }

E 1110 SO RS . > N ^ n ~

F 1111 SI US / ? O _ o DEL

KEYNUL = NullSOH = Start Of HeaderSTX = Start of TransmissionETX = End of TransmissionEOT = End of TextENQ = EnquiryACK = AcknowledgeBEL = Audible BellBS = BackSpaceHT = Horizontal TabLF = Line FeedVT = Vertical TabFF = Form FeedCR = Carriage ReturnSO = Shift OutSI = Shift InDLE = Data Link EscapeDCx = Device Control xNAK = Negative AcknowledgeSYN = Synchronization char.ETB = End Transmission BlockCAN = CancelEM = End of MediumSUB = SubstituteESC = EscapeFS = Field SeparatorGS = Group SeparatorRS = Record SeparatorUS = Unit SeparatorDEL = Delete


UAS Telemetry 4

RS‐232 Data Transfer

• framing: each character transferred gets its own start bit and stop bit, optional parity bit– start bits are always 0, stop bits always 1

– parity used for error checking• may be even, odd, fixed‐at‐0, or unused

• data bits are sent/received lsb‐to‐msb

• characters may be sent back‐to‐back– lines idles at '1' when no data to transfer

• ex: 2 chars using 8 data, 1 stop, no parity:


RS‐232 Example

• what does the message Hello! look like over an RS‐232 style comm link?

???

• standard baud rates– 110, 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600, 115200, 128000, 256000

• so, how long will it take to transfer (a) each bit and (b) entire message at xxx baud?

???



UAS Telemetry 5

Two‐way Data Transfer

• normally, 1 wire or channel provides unidirectional data transfer from A to B

• simplest way to implement bidirectional data transfer is to have 2 wires/channels!– and a common ground…

• additional wires sometimes used for 'handshaking' signals


A B

Protocols

• when multiple data bytes are transferred over a comm link, they are often packaged into data packets according to one or more protocols

– ex: TCP/IP, SMTP, HTTP/HTTPS

• more robust error detection/correction is then handled by the protocol

– than simple parity which is only 50% accurate



UAS Telemetry 6

Data Packets

• aka "message", is the unit of transfer in a protocol

• typical data packet fields– packet ID/type number

– packet sequence number

– source address

– destination address

– payload (the actual data being transferred!)

– error check (usually at end)


• one such protocol of interest is MAVlink

– an open‐source protocol commonly used with modern UASs (also UGSs and USSs)

– between unmanned vehicle and GCS

– used in PixHawk, Mission Planner, QGroundControl

• packet layout (v1.0):



UAS Telemetry 7

MAVlink Packet DetailsField name Index (Bytes) Purpose

Start‐of‐frame 0 Denotes the start of frame transmission (v1.0: 0xFE)

Payload‐length 1 length of payload (n)

Packet sequence 2Each component counts up their send sequence. Allows for detection of packet loss.

System ID 3Identification of the SENDING system. Allows to differentiate different systems on the same network.

Component ID 4Identification of the SENDING component. Allows to differentiate different components of the same system, e.g. the IMU and the autopilot.

Message ID 5Identification of the message ‐ the id defines what the payload “means” and how it should be correctly decoded.

Payload 6 to (n+6) The data into the message, depends on the message id.

CRC (n+7) to (n+8)Checksum of the entire packet, excluding the packet start sign (LSB to MSB)


MAVlink Packet Application

• G.C. software– Mission Planner– QGroundControl– UgCS– Tower

• "terminal emulator"– RealTerm

• experiment:– custom software to capture, decode, and display MAVlink packet contents



UAS Telemetry 8

MAVlink Packet Capture: RealTerm

• straight ASCIIdisplay mode

• shows a lot of control (binary) characters

• not fit for human consumption!


MAVlink Packet Capture: RealTerm

• hex displaymode

• shows each byte received as 2 hex digits

• packet structure can be discernedMsgID=0, "HEARTBEAT"



UAS Telemetry 9

MAVlink Decoder Program

• written in 'C'

• approx. 200 lines of code

• payload contents extracted & interpreted


Telemetry Systems: External

• FMU‐based

– external solution via a direct, hardwired (RS‐232 or USB) link or a set of wireless telemetry transceivers

• 915 MHz: USA

• 433 MHz: Europe

– SiK Telemetry Radio• one popular open‐source implementation

• updated version at mRobotics.io


http://ardupilot.org/copter/docs/common‐sik‐telemetry‐radio.html


UAS Telemetry 10

Telemetry Systems: RC‐based

• RC provided

– an extension to 2.4GHz 1‐way C&C links to also downlink telemetry data over same link

• some RC system manufactures now provide telemetry features in their systems

– ex: Futaba, Graupner, FrSky, Spektrum

– sensors, datum points


https://www.spektrumrc.com/Technology/Telemetry.aspx

Futaba Telemetry

• via "T‐FHSS" receivers using SBUS2

– SBUS2 = "2‐way"

• sensors:

– GPS, voltage, current, temperature, altitudeRPM, airspeed


https://futabausa.com/products/telemetry‐sensors ensors


UAS Telemetry 11

FrSky Telemetry

• via receivers with "Smart Port"

• sensors:

– GPS, Vbatt, Ibatt, temperature, Variometer, airspeed, RPM,fuel flow


https://www.frsky‐rc.com/product‐category/sensors

Setting Up Telemetry: FrSky

• telemetry data display is supported by certain FrSky transmitters such as our Taranis X9D

• demo: Nate



UAS Telemetry 12

Flight Parameters• altitude

• attitude, AoA

• heading

• turn indicator

• engine RPM

• temperature: engine, OAT

• vertical speed (VS)

• airspeed (AS)

• groundspeed (GS)

• fuel supply

TelemetrySensors

• voltage• current• temperature• tachometer (RPM)• barometric pressure

(altitude)• gyros, accelerometers

(attitude)• pitot tube (airspeed)• GPS (position)• magnetometer (heading)


Voltage Sensing

• determines voltage of an unknown input by performing analog‐to‐digital (A/D) conversion on a "sample" of the input being monitored (VM) then scaling the binary result by an appropriate conversion factor (CF)

– block diagram: VM xCF ADC BM

• ex: monitoring V of a 6S LiPo using an 8‐bit ADC with VREF=5V



UAS Telemetry 13

Current Sensing

• determines amount of current (I) extracted from a battery by accumulating periodic instantaneous current measurements over many time intervals

• instantaneous current found by monitoring voltage drop across a very low value resistor and exploiting Ohm's Law

– but high power! (wattage)

• ex: monitoring up to 100A


Temperature Sensing

• modern solution is to use a solid‐state precision integrated‐circuit (IC) device

⁄

• then perform A/D conversions on VOUT


http://www.ti.com/lit/ds/symlink/lm35.pdf


UAS Telemetry 14

Tachometer Sensor

• an instrument that measures the rotational speed of an engine or motor, typically in revolutions per minute

– aka tach, rev‐counter, RPM gauge

so ⁄ ∙ ⁄ ⁄


Atmospheric Pressure

• atmospheric pressure / BP is the pressure in the Earth's atmosphere– i.e. "weight of the air"

• measured in force per unit of area– in Pascals, = 1 newton per sq. meter (1 Pa = 1 N/m2)– standard atmosphere (atm) = 101,325 Pa

• which also = 1013.32 mbar = 29.92 inHg = 14.7 psi

• atmospheric pressure is directly affected by altitude– and by temperature and humidity to lesser degrees

• pressure decreases 12 Pa per meter of altitude increase



UAS Telemetry 15

Altitude Sensing

• atmospheric pressure changes by12 Pa for each 1 m altitude change

• modern UAV solution is to use a silicon pressure sensor IC device

– Invensense ICP‐101xx BP & temp sensor IC• ±1 Pa accuracy or 8.5 cm!

• temperature sensor provides temperature stability

• perfect for UAV/drone applications


https://www.invensense.com/products/1‐axis/icp‐101xx

Airspeed Sensing

• measure of speed of movementthrough the air– typically in knots (kts, 1 kt = 1 nautical mph = 1.15 mph)

• method: use a pitot tube and compare ram pressure against static pressure

• pitot tube: a device that measures dynamic pressure ( )



UAS Telemetry 16

GPS

• system of satellites (24 active + 7 spare) used to geolocate a receiver's position– orbiting at 20,000 km altitude

• started in 1973 as Navstar by US DoD for military applications, currently operated by USAF (Space Command)

• requires direct visibility to minimum of 4 satellites– more accurate results with more satellites

• original accuracy of 5m (2000), improved to 30 cm (2018)


GPS Operation

• based on precise distance to satellites each having highly accurate atomic clocks

∙where speed is speed of light (3x108 m/s)


• and a mathematical process known as trilateration– to find intersection point of at least 3

satellites (for 2D fix, 4 for 3D fix)

– results in lat/lon/alt of the GPS receiver

– "How does GPS work?":https://www.youtube.com/watch?v=FU_pY2sTwTA


UAS Telemetry 17

GPS Accuracy

• dilution of precision (DOP)• GPS errors:

1. atmospheric refraction• which modifies (lengthens) signal path

2. ephemeris (satellite time & location)• caused by slight position and time synchronization errors of satellites; although they maintain accuracy to 1 ns!

3. multipathing• from "echos" off of ground structures/features

4. GDOP (geometric dilution of precision)• result of separation between satellites in view


https://www.developerfusion.com/article/4652/writing‐your‐own‐gps‐applications‐part‐2/2/

GPS Accuracy

• results of GPS errorsPDOP: 3D position DOP

– HDOP: horizontal DOP

– VDOP: vertical DOP

TDOP: time DOP

• lower DOP (better accuracy) achieved by:

–more visible satellite sources

– increased separation

• DOP value interpretation


Value Quality Rating

>20 Poor

10‐20 Fair

5‐10 Moderate

2‐5 Good

1‐2 Excellent

<1 Ideal


UAS Telemetry 18

GPS Receivers


• GPS receivers are available in both stand‐alone and component form factors

• component receivers attach to FMU via dedicated port

Magnetometer

• a device that measures/outputs its orientation with respect to the Earth's magnetic field

• produces "heading angle"like a compass (0‐359°)

• used in UAVs to stabilizeyaw heading

– sometimes in combinationwith GPS heading



UAS Telemetry 19

References

• MAVlink, https://en.wikipedia.org/wiki/MAVLink, Oct. 2018

• MAVLink Developer Guide, https://mavlink.io/en, Oct. 2018

• Pilot's Handbook of Aeronautical Knowledge, FAA

• "How GPS Works", https://www.maptoaster.com/maptoaster‐topo‐nz/articles/how‐gps‐works/how‐gps‐works.html, Nov. 2018

• "Trilateration vs Triangulation – How GPS Receivers Work", https://gisgeography.com/trilateration‐triangulation‐gps, Nov. 2018

• "GPS Accuracy: HDOP, PDOP, GDOP, Multipath & the Atmosphere", https://gisgeography.com/gps‐accuracy‐hdop‐pdop‐gdop‐multipath, Nov. 2018


uas telemetryjsumey/uas270/n6-telemetry.pdf · uas270 avionic systems 11/19/2018 uas telemetry 1...

Documents