DEVELOPMENT OF A BASELINE TELEMETRY SYSTEM FOR A. L. Eatchel R. Fevig

DEVELOPMENT OF A BASELINE TELEMETRY SYSTEM FOR A. L. Eatchel R. Fevig
DEVELOPMENT OF A BASELINE TELEMETRY SYSTEM FOR
THE CUBESAT PROGRAM AT THE UNIVERSITY OF ARIZONA
A. L. Eatchel
R. Fevig
Graduate Students
C. Cooper, J. Gruenenfelder, J. Wallace
Undergraduate Students
U. Fink, L. C. Schooley, A. Hudor
Advisors
The University of Arizona
ABSTRACT
A telemetry system has been developed at the University of Arizona to serve as a baseline for future
CubeSat designs. Two satellites are scheduled for launch in November of 2002. One features a
beacon that operates autonomously of all but the power system and can independently deploy the
antennas. The other will test the performance of new semiconductor devices in low earth orbit.
Sensors will monitor voltages, currents (from which attitude and tumble rate can be derived),
received signal strength and a distribution of temperatures. The CubeSat’s architecture, operating
system, sensors, telemetry format and link budget are discussed.
KEY WORDS
Telemetry System, CubeSat, Pico Satellite, Amateur Radio Satellite.
INTRODUCTION
The CubeSat program was initiated by Cal Poly and Stanford Universities to involve students in the
development of small satellites of ~ 1kg mass to perform experiments of limited scope at greatly
reduced cost. Students at the University of Arizona have joined this national program and are
presently constructing our first set of three. These satellites fit well into launch vehicles carrying
multiple payloads, allowing costs to be shared internationally by private enterprise, government
agencies and educational institutions. The first two are scheduled for launch from an SS-18 launch
vehicle in Baikonur, Kazakhstan in November of 2002. Our primary objective is to produce a
baseline design upon which further developments can be made. The main purpose of the first
satellite is to formulate a bus for future science experiments. The second satellite will carry an
experiment which will provide radiation dose and dose rate information as well as threshold voltage
values from which the performance of developmental semiconductor devices can be determined.
FUNCTIONAL DESCRIPTION OF THE SYSTEM
The architecture of our satellites is shown in Figure 1. Note that one satellite has a PC board with the
semiconductor device experiment on it and the other replaces the experiment board with a beacon
and corresponding antenna.
2 m Dipole
70 cm Loop
70 cm Dipole
Beacon
146 MHz
Receiver
437 MHz
Transceiver
Multiplexor
Half Duplex
1200 bps
Modem
PIC
Microcontroller
UART
I2C Buss
68 KBytes
FRAM
Real Time
Clock
Power
Electronics
Experiments
24 Channels
A/D
Sensors
Figure 1: Architecture of the CubeSat
All data handling, communications and control functions are performed by a PIC microcontroller
(PIC16F877). Communication with non-volatile memory (Ferroelectric RAM or FRAM) and the
realtime clock (RTC) on the controller PC board is via an I2C bus. Communication with the PC
board containing the experiment and A/D converters on the power PC BOARD where the sensor
conditioning electronics are found is also via the I2C bus. The bus operates in standard-mode with a
data transfer rate of 100 kbps and 7-bit addressing. An on-board UART of the PIC communicates
with the modem at a data transfer rate of 1200 bps. Communication with the modem is half duplex.
The modem generates mark and space tones at 1200 Hz and 2200 Hz.
Last minute hardware changes required us to incorporate COTS radios manufactured by SpaceQuest,
Ltd. We are using a TR-433 transceiver for the 70 cm band and a RX-145S receiver for the 2 m
band. Both operate in AFSK mode. Receive audio from the two are analog multiplexed along with
accompanying received signal strength indicators (RSSI) and carrier detect (CD) signals to provide
squelching for the modem. The beacon also operates in the 70 cm band.
The antenna system is comprised of a 70 cm dipole for transmission and reception, a 2 m dipole for
receive only, and a 70 cm loop for beacon transmissions. Figure 2 depicts the mounting of these
antennas on an external view of a developmental model of the satellite. The flexible antennas are
deployed by means of a Nichrome (tm) wire powered through an FET switch circuit that melts a
nylon tie-down. A wired OR configuration of the wireharness allows access by both the beacon and
the controller to all antenna FET switch inputs. This eliminates the need for (and possible failure of)
antenna fuse switching components. Also shown in the figure is the placement of the power board,
controller board and the radio board (top to bottom) along with the interconnecting wireharness.
Figure 2 – Component placement for the 10 cm CubeSat
Power supply electronics perform the functions of battery charge regulation, generation and
regulation of 7.5 VDC and 5.0 VDC from four AA sized Ni-Cd cells and six solar panels that
recharge the batteries. The satellite temporarily powers down when the battery voltages drop below
~ 4.0 V in order to allow the solar panels to adequately recharge the batteries for normal operation.
Following power-down, the satellite undergoes a re-initialization sequence to place the satellite in a
default operating mode. Antenna deployment will also be attempted unless a flag has been
previously set in the FRAM to indicate that valid communications have been received.
THE BEACON
The beacon was produced by Rincon Research Corporation and provides a redundant means of
relaying sensor data in analog form. These can be compared with the digital forms at processing
time. The beacon operates autonomous of all other satellite systems except for the power and the
sensor electronics. Besides the main controller, it has the capability of deploying each of the
antennas. The main controller is given the first try at five minutes after separation from the launch
platform. The beacon makes its attempt at deployment 45 minutes following separation.
Component minimization was achieved using highly integrated devices to handle complex functions
such as a single chip transmitter in which no external power amplification is required and a single
chip anti-fuse programmable gate array (AFPGA) instead of many discrete logic devices. The low
part count made it possible to design the PC board with all components on one side which was
critical considering the space limitations. A low power consumption of 150 mW was achieved using
a 10 dBm transmitter. This power level is adequate due to the narrowband 5 kHz channel and the
slow sample rate. Phase modulation was deemed best given these conditions.
For the receiving ground station, a 10 Hz bandwidth is adequate to retrieve the information and will
severely restrict the noise power in the received signal. This is equivalent to a large processing gain
of 30 dB over a conventional 20 kHz wide FM signal. The ground station uses an AR5000 receiver
with a variable IF bandwidth from 3 kHz to 220 kHz. A 25 kHz bandwidth is adequate to receive the
carrier and sidebands given the Doppler shift during a pass. The IF output of the receiver, centered at
~ 10 MHz is buffered and digitized at ~ 20 MHz by an ICE-slimPIC (Innovative Computer
Engineering) PCMCIA card, then decimated by a Texas Instruments Graychip GC4014. The
resulting data is analyzed by performing ten FFTs per second and demodulating the received signal
in the frequency domain using Midas2K software. Peak detection functions locate the carrier and
sidebands. The deviation of the sidebands from the carrier is then used to determine the voltage
present at the voltage to frequency converter. A script logs this information to a file where individual
sensor values can be calculated.
Referring to Figure 3, the main subsystems are power regulation, control logic, sensor selection,
modulation circuitry, and the transmitter. The beacon comes on automatically once the batteries have
charged enough to provide a minimum of 4.5 VDC at the battery terminals. Unless commanded off,
it will transmit indefinitely at a 50% duty cycle (30 seconds on, 30 seconds off) to provide a quiet
window for the secondary receiver to receive on its nearby frequency. The beacon transmitter
provides a stable carrier for Doppler tracking. Sensor readings are time-division-multiplexed analog
voltages. A Morse code ID is present at the end of every frame. Control of these functions and
telemetry frame formatting are provided by the AFPGA. A complete frame as illustrated in Figure 4
consists of eight long sensor samples of two seconds each and 24 short ones of 0.25 seconds each.
These values are DC outputs from the sensor signal conditioning electronics. The long sample times
are to allow for additional time to observe how the solar panel currents change over short intervals.
Sensor outputs are fed into a voltage-to-frequency converter through an analog multiplexor. The
voltage to frequency converter produces a square wave which is then smoothed by a switched
capacitor low pass tracking filter. The switched-capacitor-filter produces unwanted high frequency
components due to the switching action that can be seen as steps in its sinusoidal output. This minor
effect is eliminated by a low pass RC filter placed immediately before the modulation input to the
transmitter.
SENSORS
Four types of sensors are used to measure received signal strength, voltage, current and temperature.
Sensor values are scaled to take advantage of the full dynamic range of the A/D. Received signal
strength indicators are positive slope linear outputs of energy level detection circuits at the IF stages
of the 2 m and 70 cm receivers. Simple resistor combinations provide the scaling for voltage and
current senses. Temperatures are measured with a commercially available YSI 44203 Thermilinear
Network with a linear range temperature range of –30oC to +50oC.
+5 VDC
Antenna
Deployment
Control
Analog
Multiplexers
Beacon Subsystem
Voltage
Regulators
AFPGA
Linear
Power
Regulation
Control Logic
Gate Array
P
70 cm
loop
antenna
t < t*
f
V
S
e
n
s
o
r
s
P
t*
M
U
X
t
VFC
1
Unity
Gain
Buffer
f
RC
SCF
Voltage to
frequency
converter
Switched
Final
Capacitor
Smoothing
Low Pass
Filter
Tracking Filter
V
V
t*
t > t*
t
TX
Transmitter
t
t*
Figure 3 -Beacon subsystems block diagram
Figure 4 -Beacon telemetry frame
Table 1 provides a summary of our measurements. Sampling periods listed are those for day 1 of the
default mode of operation in which whole orbit data is recorded. In realtime operation, the sensors
are read once per second.
Table 1: Summary of sensor measurements
Quantity
6
6
2
1
1
1
1
Sensor Channels
Solar cell currents
Solar panel temperatures
Frame temperatures
Battery temperature
Transmitter sink temperature
Controller temperature
Power supply recharge current
Sampling Interval (seconds)
10
70
70
70
70
70
70
Table 1: Summary of sensor measurements (continued)
1
1
1
1
1
5 V power supply current
7.5 V power supply current
Battery current
Battery voltage
Received signal strength indicator
70
70
70
70
70
OPERATIONAL CHARACTERISTICS
Immediately following deployment, the satellites enter a default mode of operation that can be
modified in response to commands from the ground. These modes are described as follows.
Realtime mode
Data is transmitted as it is collected. A block of 24 sensor readings is transmitted in a half second
followed by a sleep period of a half second to allow for command reception. This cycle is repeated
over a 15 minute interval unless interrupted by reception of a command. When the 15 minute time
limit expires, the operating system (OS) will switch to Whole-Orbit mode.
Whole-Orbit mode
Data is gathered according to a timing schedule. The sensor read table will be checked once each
second and if a sensor is due to be read, the OS will read it and store the data in FRAM. The default
read schedule is calculated to allow two orbits of data to be stored for later transmission.
Default mode
This mode is initiated at the time of deployment and after any hardware reset. The default mode will
relay information about the operating status of the satellite including a full set of sensor data over a
two orbit period according to the default sensor read schedule. The OS begins on day 1 in WholeOrbit mode allowing for battery recharge and sensor data collection. On day 2, the OS will transmit
the stored sensor data to the ground station in 5600 byte blocks with a half second pause between
them to allow for command reception. After all data is transmitted, the OS will sleep for 90 minutes,
approximately one whole orbit. On day 3, the OS enters a real-time transmission mode with a one
minute transmission followed by a five minute sleep cycle. During the transmit cycle, sensor data is
read directly from the sensors and immediately transmitted to the ground station.
Available by command is the ability to read and write test patterns to FRAM, manage the system
clock, change the sensor read schedule and downlink sensor values. Space limitations will not
permit further discussion of the operational modes and capabilities of the satellite.
COMMUNICATIONS PROTOCOL
Non-standard formatting of transmitted data was used for several reasons. Significant noise and
interference from neighboring satellites operating in band and other sources are expected. We felt
that repeated transmission of very small packets would be our best approach. Since the controller
handles communications as well as control functions, we decided to format for minimal packet size
to reduce data handling. Following is our basic packet design.
Uplink transmission format
Header (2 bytes)
Command (1 byte)
Command modifier (2 bytes)
Checksum (2 bytes)
Header information includes a password and satellite ID code. The two byte command modifier
(argument) contains necessary information to implement the command such as the set time for the
RTC or a bit pattern to write to or read from the FRAM for memory tests.
Downlink transmission format
Header (7 bytes)
Format specification (1 byte)
Data (variable)
Checksum (2 bytes)
The header contains a six byte call sign and a satellite ID code. The format specification stipulates
the packet size since the data-stream is of selectable length. The total number of data bytes in a
packet can range from 0 to 255. Typically 32 bytes of data would be sent at once which is the default
value. Data block sizes can be altered via commands. Having control over the packet size will allow
us to respond to differing conditions of the link. Data is comprised of sensor values and operational
status information such as system flags. The data being transmitted depends on the mode of
operation and any commands recently sent as explained previously. The checksum is calculated with
a CRC16 polynomial using the same algorithm as the ZOO protocol.
GROUND STATION
Hardware description
With the exception of the modem which we built to match that of the satellite, all ground station
components are commercially available. Table 2 lists the equipment used at our site.
Table 2 . Specification of ground station components
Item
2 m antenna
70 cm antenna
LNA – mast mounted
2 m linear amplifier
70 cm linear amplifier
Transceiver
Antenna controller
Manufacturer
M2 Antenna Sys.
M2 Antenna Sys.
ICOM America
Mirage
TE Systems
ICOM America
ZL2AMD
Model
2MCP22
436CP42U/G
AG-35 LNA
B-5016G
4452G
IC-910H
UNI TRAC 2000
Feature
G = 12.25 dBdc
G = 16.8 dBdc
G •G%IURPWR0+]
Output power = 160 W
Output power = 180 W
Sensitivity = -124 dBm
Tracking and Doppler tuning
Ground station software
To facilitate the custom communications protocol and hardware design of the satellite, original
ground station code was developed. The program is 4125 lines at this writing. It was written in Java,
using Sun’s JDK 1.3.0_03 IDE and performs the following functions:
•
•
•
Acquire incoming telemetry packets from a serial port
Process, display, and store incoming data as appropriate
Allow the operator to compose and send command packets
The code was designed using a standard model-view-controller modular structure. Functions of the
program are handled through a GUI whose appearance is depicted in Figure 5. At the upper left is a
Figure 5– Screen-view illustrating the GUI employed on the ground station PC
Command Center window wherein commands are displayed along with a checkbox used to select
them. If a command requires one or more arguments, another window will pop up as the command
is selected. The operator enters arguments as decimal integers. The command is then added to a
transmission queue. The box at the bottom of the Command Center window displays the commands
currently in the queue. Hitting the send button will transmit the queue.
The Satellite Status window in the upper right features two tabs. The tab selected in the screenshot
displays the latest received values for all 23 sensors. The other tab displays a log of commands sent,
as well as the satellite’s responses to inquiries not related to sensor readings. The Transmission
Status window is in the lower right corner. It displays the header information, time sent or received,
and the checksum for all incoming and outgoing packets. The arrow buttons at the bottom allow the
user to browse the packet history. Bytes Sent and Bytes Received windows are in the lower left
corner. These windows simply display the incoming and outgoing bytes in hex and/or ASCII
formats. In the extreme lower right is the Console Log window which displays status information for
the program itself. Any errors in initialization including port contention during acquisition are
displayed here as well as the load progress for various source files and components.
LINK BUDGET
Our primary concern for the downlink budget was the expected signal level and SNR at the output of
the ground station’s mast mounted LNA which would then provide sufficient gain at a low enough
noise figure to overcome receiver mismatch and cable losses. We measured ~ 300 mW of
unmodulated carrier power output from our 70 cm satellite transmitter. Worst case values for
relevant system parameters are listed in Table 3.
Table 3: Summary of link budget parameters
Parameter
Transmit power
Transmit antenna gain
Receive antenna gain
LNA gain
LNA noise figure
Carrier wavelength
Acquisition range
Transmission bandwidth
VSWR for each antenna
Pointing loss factor
Polarization loss factor
Atmospheric attenuation loss factor
Transmission line loss for a 150 ft cable
Overall system noise temperature (receiver not included)
Value
24.8
1
16.8
15
1
68.7
2500
15
1.5:1
1
3
0
2.55
301.7
Units
dBm
dB
dB
dB
dB
cm
km
kHz
dB
dB
dB
dB
K
Link budgets for the 2 m and 70 cm uplinks were also calculated using the proper values for
wavelength and a transmitter power of 180 W at 70 cm and 160 W at 2 m. Table 4 summarizes the
results for each of the satellite frequencies. The SNR was calculated at the receiver terminal for the
uplink.
Achievement of the SNR value for the beacon is a result of the use of a sliding filter of 10 Hz BW.
Since the beacon produces a 10 mW EIRP (5 mW of which is in the carrier), the irradiance at 2500
km is 6.35 x10-17 Wm-2. An omni-directional antenna with 0.0375 m2 of capture area and a 3 dB
polarization loss will deliver 1.2x10-18 watts (-149 dBm) to the terminal. The noise power in a 10 Hz
bandwidth of a 290 K system is -164 dBm, giving a 15 dB SNR for the carrier and a 12 dB SNR for
the sidebands. Link closure just above the horizon under worst case conditions for the main
transmitter and beacon appears certain.
Table 4: Results of link budget calculations
Frequency
SNR
Signal strength at receiver
(MHz)
(dB)
(dBm)
436.870 down
39.48
-92.56
436.870 up
53.21
-78.83
145.835 up
62.23
-69.82
436.825 down
12.00*
-152.24
* Resulting from a10 Hz BW sliding filter for the beacon
** Achieved through computational filtering
Receiver sensitivity
(dBm) for 12 dB SINAD
-124
-110
-120
< -164**
CONCLUSION
At the time of this writing, some testing remains to be completed. The primary communication and
operating systems appear to be working properly. We have found that for high noise levels and weak
signal conditions, it may be necessary to send back-to-back repeats of a command in order to achieve
synchronization. Full integration of our radiation effects experiment has not been completed so we
do not yet know how well it will work.
As stated in the introduction, our main objective is to develop a bus that will be useful for a variety
of science projects. We also want to involve other amateur radio operators from dispersed
geographic locations in ground station operations. With minor modifications, we will be able to
adopt the standard communication protocols commonly used in the amateur bands. We expect the
complexity of future projects to increase significantly. The primary controller will be fully occupied
with running those experiments and processing data. Communication, guidance and power
electronics will probably need their own controllers. FPGAs are good candidates for this purpose.
We will also likely move to a higher frequency requiring changes to our radios and are looking into
the use of patch antennas to avoid the issue of deployment. In addition, we found that AFSK may not
be the best choice of modulation schemes because of the large proportion of FM receivers that
incorporate pre-emphasis and de-emphasis circuitry. As a result, we were forced to add additional
circuits to our modem to balance the mark and space tones so that we could recover our data. This
will present difficulties if we are to involve other amateurs in future projects. Therefore, we will be
examining the possible use of standard FSK, QPSK or other alternatives.
REFERENCES
1. Cardin, F., Telemetry Systems Design, 1st Ed., Artech House, Norwood, MA, 1995
2. Horan, S., Introduction to PCM Telemetering Systems, 2nd Ed., CRC Press, Boca Raton, FL, 2002
3. Northrop, G., M., “Aids for the Gross Design of Satellite Communication Systems”, IEEE
Transactions on Communication Technology, vol. com-14, no. 1, February, 1966, pp. 46-56
4. Stanford University, Space Systems Development Laboratory, <URL http://ssdl.stanford.edu/>
5. University of Arizona, CubeSat, <URL http://CubeSat.arizona.edu/>
6. Parker, M., CEO Rincon Research, private communications
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