Cube Sense V3 User manual

CubeSense V3
An integrated sun and nadir sensor module
User Manual

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Doc: User Manual
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Table of Contents
Document Approved By ..........................................................................................................2
List of Acronyms/Abbreviations.............................................................................................2
1. Introduction...................................................................................................................3
2. Getting Started..............................................................................................................4
2.1 Unpacking the CubeSense package...............................................................................................4
2.2 Before getting started..........................................................................................................................4
2.3 Ground Support Program (GSP).......................................................................................................4
2.4 Hardware setup ......................................................................................................................................6
3. Usage..............................................................................................................................8
3.1 Identification............................................................................................................................................8
3.2 Managing memory contents.............................................................................................................9
3.3 Doing detection................................................................................................................................... 10
3.4 Interpreting detection result .......................................................................................................... 12
3.5 Capturing and downloading images........................................................................................... 13
3.6 Typical use............................................................................................................................................. 15
4. Important Usage Considerations...............................................................................17
4.1 Detection thresholding..................................................................................................................... 17
4.2 Image exposure settings.................................................................................................................. 17
4.3 SRAM over-current protection ...................................................................................................... 18
4.4 Nadir Detection Adjustment........................................................................................................... 18
4.5 Sensor masking ................................................................................................................................... 18
5. Documentation History ..............................................................................................21

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Document Approved By
Document Approved By
Document
CubeSense V3 User Manual
Version
1.11
Date Modified
6 January 2020
Approved By:
Douw Steyn
Signature
List of Acronyms/Abbreviations
CMOS Complementary metal-oxide semiconductor
ADCS Attitude and Determination Control System
FPGA Field Programmable Gate Array
I2C Inter- Integrated Circuit
MCU Microcontroller Unit
OBC Onboard Computer
SRAM Static Random Access Memory
UART Universal Asynchronous Receiver/Transmitter
COTS Commercially Off-the-shelf
PCB Printed circuit board

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1. Introduction
The CubeSense module is an integrated sun or nadir sensor for CubeSat attitude sensing. It
makes use of a CMOS camera that is configured for either sun sensing or for horizon detection.
If configured as a sun sensor, a neutral density filter is included in the optics to ensure that
only the sun will be visible in the image. The camera has wide field-of-view optics (200 degrees)
for increased operating range.
The primary output of the sensor is the measured sun/nadir vector in the sensor’s coordinate
frame. The measured vectors are output as angles relative to the camera bore-sight. If the
CubeSense is configured as a nadir sensor then it can also be used as a camera to capture and
download 1024x1024 pixel greyscale images.
This document is only applicable to CubeSense V3.
The unit contains a variety of static sensitive devices. The appropriate electrostatic
protection measures must thus be implemented. The unit must never be handled
without proper grounding.
It is recommended that the unit be handled in a clean environment. A clean room of ISO
class 8 or better or an appropriate laminar flow workbench is recommended.
The unit should be kept free of moisture or liquids. Liquids and moisture could have
corrosive effects on the electronics and electronic joints which may lead to degradation
and loss of reliability of the circuits.
The unit must be handled with care and dropping or bumping the unit should be
completely avoided.
The camera lens should be kept clean and free of any dirt that may obstruct the images
captured by the camera. Dust should be removed with a cloth. If required, the lens may
be cleaned using ethanol and appropriate lens cleaning equipment, but unnecessary
cleaning of the lens should be avoided.
The optics are fitted with a dust cap which should be removed before flight.
The position of the lens relative to the image sensor is of extreme importance for
accurate detection. Any external force on the lens or lens holder should be
completely avoided.
Please read Section 4(Important Usage Considerations) very carefully.

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2. Getting Started
The Getting Started guide will show the simple steps to get CubeSense up and running.
CubeSense is provided with a simple test application to allow the user to gain experience with
the available functions as well as test the hardware.
2.1 Unpacking the CubeSense package
The received Peli-Case contains the following items:
•CubeSense unit(s)
•Support PCB
•UART-to-USB cable
•CubeSpace flash drive
The included items will allow the user to interface with a CubeSense unit without the need for
other hardware.
2.2 Before getting started
The following additional items are required before the user can get started with the
CubeSense:
•Multi-meter or oscilloscope
•Ground Support Program (see Section 2.3)
•Computer with an open USB port (running Windows 7 or later)
2.3 Ground Support Program (GSP)
2.3.1 What is the GSP?
The GSP allows the user to interface with a CubeSense via the UART connection. No additional
software or hardware (except the items mentioned in Section 2.2) is required, which means
that the CubeSense can act as a standalone module.
The software enables the user to request telemetry and to send telecommands from/to a
CubeSense.
2.3.2 Connecting to the UART cable
Follow the instructions below to verify the connection between CubeSupport and the UART
cable:
•Plug the UART-to-USB cable into the computer’s open USB port.
•The computer should detect the cable and install the drivers by itself.

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•After a couple of minutes, a message indicating the successful installation of the drivers
should appear.
•Browse to the folder containing the GSP and launch the application by running
CubeSenseV3_GSP_TestingApplication.exe.
•Select the COM port of the UART cable and click on Connect
Check the COM port assigned to the cable by browsing to Device Manager and
noting the number shown under Ports (COM & LPT) for USB Serial Port (COM x).
The number given by xwill be used to connect to CubeSense.
•The connection status should turn green if the connection was successful, as illustrated
below.
•Disconnect from the UART-to-USB cable by clicking on the Disconnect button.
The Health Check document will allow the user to become familiarized with the GSP interface
as well as perform the incoming health check to esure that the module arrives in full working
condition.

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2.4 Hardware setup
Figure 1 –CubeSense electrical connections
2.4.1 Power
The CubeSense module draws its power from the USB port if the Support PCB is used. If the
module is not used with the Support PCB, the power could also be provided by a stable lab
power supply with a 3.3V output. The current limit of the supply should be set to
approximately 300mA. Although the CubeSense circuitry has some protection, it is still good
practice to have this current limit. The standard connector populated on the CubeSense
module is a Samtec TFC-104-01-F-D. The 3V3 power is located at pin 1 as shown in Table 1.
Table 1 –Pinout of CubeSense Connector*
Pin
Name
Description
1
3.3V
Input voltage of 3.3V
2
SDA1
SDA line for I2C communication
3
SCL1
SCL line for I2C communication
4
GND
Ground pin (both ground pins must be used)
5
RX
RX line for UART communication
6
TX
TX line for UART communication
7
EN
Enable line that turns the CubeSense on or off (active high)
8
GND
Ground pin (both ground pins must be used)
1There are no pull-up resistors on the I2C bus. Any pull-up resistor should be implemented by the master.
2.4.2 Communication
There are multiple options for communicating with CubeSense. The primary communication
method for ground testing is UART. The location of the UART TX and RX pins are shown in
Table 1.

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For the ground tests:
•Plug the CubeSense module into the support PCB (header is marked as CubeStar)
•Plug the supplied UART-to-USB cable into the support PCB (the header is labelled as
“UART” on the PCB). Plug the UART-to-USB cable into the computer (if it is not plugged
in already).
•Open the GSP (if it is not open already).
•Connect to the appropriate COM port
•A successful connection should allow you to request the status and observe the
Runtime increasing as shown here:
•Refer to the Health Check document for further tests that can be performed.
•If the connection is not successful, check the following:
✓Verify that the CubeSense is plugged into the support PCB.
✓Verify that the UART connection cables are plugged in correctly.
In case of continuous connection issues, probe pin 1 (marked with line) of the
CubeSense/CubeStar header on the support PCB and verify that there are 3.3V present.
2.4.3 Enabling/Disabling
CubeSense has its own overcurrent protected switches which switch on/off the power that is
supplied to the board. At production, these switches can be configured to be default on or off.
If the support PCB is used the Enable line will be always on when power is applied. When
CubeSense is used without the CubeSupport board, the user can switch on/off CubeSense by
applying 3.3V to the CubeSense Enable pin. The Enable pin is active high. The location of this
pin is shown in Table 1.

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3. Usage
3.1 Identification
Each CubeSense module is programmed with a serial number, and details about its firmware.
To check these details about a CubeSense module, TLM no 0 and 1 can be requested. The
TLMs starts at address 128, so these addresses will be 128 and 129. The content of these TLM
frames are as follows:
Table 2 –Status TLM
Telemetry frame
ID
0
Name
Status
Frame length
(bytes)
8
Channels
Byte
No
Length
(bytes)
Channel
Data type
Detail
0
1
Node type
Unsigned 8-
bit
Identification of type of
CubeComponent Node
1
1
Interface
version
Unsigned 8-
bit
Interface definition version
2
1
Firmware
version
(major)
Unsigned 8-
bit
3
1
Firmware
version
(minor)
Unsigned 8-
bit
4
2
Runtime
(seconds)
Unsigned
16-bit
Number of seconds since
processor start-up
6
2
Runtime
(milliseconds)
Unsigned
16-bit
Number of milliseconds
(after the integer second)
since processor start-up
Byte 0: The node identification is used within CubeSpace ADCS bundles to identify which
modules are connected. CubeSense has Node Type = 2
Byte 1: The interface definition indicates which version of the software interface CubeSense
implements. This should match the ‘Node Definition’ version number that is being used.
Byte 2 & 3: Byte 2 is the major version and Byte 3 is the minor version of the CubeSense
module. If Byte 2 = 1 and Byte 3 = 0, the firmware version is V1.0
Byte 4-5: Bytes 4-5 forms an unsigned 16-bit integer that indicates the elapsed time since
start-up, in seconds.

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Byte 6-7: Bytes 6-7 forms an unsigned 16-bit integer that indicates the elapsed time since the
previous second, in milliseconds.
Table 3 –Serial number TLM
Telemetry frame ID
1
Name
Serial number
Frame length
(bytes)
2
Channels
Byte
No
Length
(bytes)
Channel
Data type
Detail
0
2
Serial
number
Unsigned
16-bit
Number that defines the
serial of CubeSense
Byte 0-5: Each CubeSense has a unique serial number.
3.2 Managing memory contents
The CubeSense memory module is divided into two halves, top and bottom. This is illustrated
in Figure 2.
2048 Kilobytes
Top - 1 MB
Used for:
-Image capture from camera
-Sun detection
-Nadir detection
Bot - 1 MB
Used for:
-Image capture from camera
SRAM
Figure 2 –SRAM layout
Each 1 MB (1048576-byte) halve can store one CubeSense image. The main functions
CubeSense can execute are:
1. Capture an image to one of the halves of the SRAM
2. Perform detection on the image stored in the top halve the SRAM
3. Download the image stored in one of the halves of the SRAM.
Only one of these functions can be executed at any given time. If the command is given to
execute multiple functions, they will be executed sequentially. It is important to note which
part of which SRAM will be influenced by a given instruction, since this will overwrite the

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contents that were stored in that position. It is the user’s responsibility to choose the correct
part of the SRAM to write to and to avoid overwriting data that should be stored for use at a
later stage.
3.3 Doing detection
Even though CubeSense has the capability to capture and download images, the main
application of the module is to do Sun or Nadir detection. While doing detection the user is
not required to download or manage images. When a detection command is received by
CubeSense, the module automatically captures a new image as soon as possible, executes the
applicable type of detection on that image, and saves the result in memory. There are two
ways of issuing a detection command:
1. Send TC 20 for a Capture and Detect:
TC 20 should be sent without any parameters
2. Request TLM 22 for the last detection result, and automatically trigger new
capture and detection:
When requesting the latest detection result through these TLMs, a new detection is
automatically scheduled. This is the method of operating CubeSense that requires
the least communications, and is the recommended method of operating the
module.
The result of a detection can be read by either reading the result of the previous detection
that was done (TLM 20) or reading the result while also scheduling a new detection (TLM 22)
as discussed above. All of these telemetries have the following structure:
Table 4 –Auto-trigger TLMs
Telemetry frame ID
20 and 22
Name
Detection result & Trigger
Frame length
(bytes)
6
Channels
Byte
No
Length
(bytes)
Channel
Data type
Detail
0
2

Signed
16-bit
angle in centi-degrees
(range = -100 to 100 degrees)
2
2

Signed
16-bit
angle in centi-degrees
(range = -100 to 100 degrees)
4
1
Capture
Result
Unsigned
8-bit
0 = start-up
1 = capture pending
2 = successfully captured
4 = camera timeout

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5 = SRAM overcurrent
5
1
Detection
result
Unsigned
8-bit
0 = start-up
1 = no detection scheduled
2 = detection pending
3 = Nadir error –too many
detected edges
4 = Nadir error –not enough
detected edges
5 = Nadir error –Bad fit
6 = Sun error –Sun not found
7 = Successful detection
The first step after reading this TLM is to check the detection and capture-results. The
description of each result is detailed in the table above. Only when the capture result is
“Successfully captured” and the detection result is “Successful detection” should the
result in bytes 0-3 be used as valid detection results.

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3.4 Interpreting detection result
The sensor’s axes are defined as follows:
Figure 3 –Camera axes definitions
The measured angles () that the sensor output are virtual angles that can be transformed
to angles or vectors that can be used in the satellite attitude determination. The direction
vector can be calculated using the following procedure:
First calculate and using the following formula:
󰇡
󰇢  
󰇛󰇜
Then, the direction vector is:


󰇭󰇛󰇜󰇛󰇜
󰇛󰇜󰇛󰇜
󰇛󰇜 󰇮
In finding the and detection values, the sensor makes use of a calibrated /“bore-sight”
location on the imaging sensor. This pixel location is the intersection of the optical axis of the
lens with the imaging sensor. This value is unique for every sensor and can be changed using
a “Set boresight” telecommand. After a reset, the boresight value reverts to the production
default.
This boresight value is programmed into the firmware of CubeSense at
time of production and is calibrated to each individual lens. It is advised
to not change this value.

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3.5 Capturing and downloading images
CubeSense provides the option of downloading an image that is located in any of the SRAM
locations. This can be done for images of the earth or of space, or to verify the exposure
settings of the sensor to tweak its performance. Images can be captured to the top or bottom
half of the SRAM. The module can therefor hold 2 images at any given time. It is however
important to note that doing detection will overwrite the top image in the SRAM.
TC 21 is used to capture an image to a specified SRAM location:
Table 5 –Capture Image TC
Telecommand ID
21
Name
Image capture
Parameters length
(bytes)
1
Fields
Offset
Length
Field
Data type
Detail
0
1
SRAM
Location
selection
Unsigned 8-
bit
0 = Top Half
1 = Bottom Half
After sending the command to capture the image, TLM 20 can be requested to see if the image
was successfully captured. Byte 4 (“Capture Result”) in this TLM can be used to read the status
of the capture.
Once the required image is captured, it will be stored as long as the CubeSense module is
powered, or it is overwritten by another image.
Downloading an image from CubeSense to an OBC or PC is a lengthy process (a few minutes
on highest resolution). It is important to note that while CubeSense is busy downloading an
image, no other image captures or detection can be done. Once another command is sent to
CubeSense, its electronics will reset into a new mode, and the progress of the download will
be lost. In such a case the download will have to be restarted. Because of this reason, it’s
advised to hold all downloads for eclipse, when no detection is possible.
A full resolution CubeSense image is 1024x1024 bytes. Smaller versions of this image can
however also be downloaded in cases where limited time is available for communication. To
set up a new image download, TC 64 is called:
Table 6 –Initialize image download TC
Telecommand ID
64
Name
Initialize image download
Parameters length
(bytes)
2

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Fields
Offset
Length
Field
Data type
Detail
0
1
SRAM
location
Unsigned 8-
bit
0 = Top
1 = Bot
1
1
Size
selection
Unsigned 8-
bit
0 = 1024x1024 (8192
frames)
1 = 512x512 (2048
frames)
2 = 256x256 (512
frames)
3 = 128 x 128 (128
frames)
4 = 64 x 64 (32 frames)
Images are downloaded in 128-byte packets. When TC 64 is received by CubeSense, the first
128 bytes of the image is loaded into CubeSense’s memory. Once this TC has been sent to
CubeSense, TLM 65 can be polled to check when the image frame has successfully been
loaded.
Table 7 –Image frame info TLM
Telemetry frame
ID
65
Name
Image frame info
Frame length
(bytes)
3
Channels
Offset
Length
Channel
Data type
Detail
0
2
Image frame
number
Unsigned
16-bit
Number of current frame
loaded into download
buffer
2
1
checksum
Unsigned 8-
bit
XOR checksum of frame
loaded into download
buffer
Once the frame number is set to 0, the image frame can be requested. To do this, TLM 64 can
be requested. If required, the integrity of the image frame can be verified by a XOR (Exclusively-
OR) of all of the bytes in the frame. This final value can be compared to Byte 2 of TLM 65 that
was read earlier. If the values do not match, the frame can be downloaded again by requesting
TLM 65 again.
Once the frame has been successfully downloaded, the next frame can be loaded into
CubeSense. This is done by sending TC 65. It is important to note that CubeSense expects
the next frame to be one more than the current frame. For example, if the current frame is
10, the next parameter sent with TC 65 should be 11. If this is not correct, CubeSense will
simply ignore the TC.
Table 8 –Advance Image download TC

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Telecommand ID
65
Name
Advance image download
Parameters length
(bytes)
2
Fields
Offset
Length
Field
Data type
Detail
0
2
Next frame
number
Unsigned
16-bit
Number of next frame to
be loaded
After this TC has been sent, TLM 65 can once more be polled until the ‘frame number’-channel
is equal to the appropriate image frame. These steps can be repeated until all frames have
been downloaded.
The data for an image with dimensions X by X can be packed into a bitmap in the following
manner:
Byte 1
Byte 2
Byte 3
.
128
.
X-2
X-1
X
X + 1
.
.
.
.
.
.
.
.
2X + 1
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
X*X
Figure 4 –Image bitmap example
3.6 Typical use
The way in which CubeSense is normally used is to periodically request the result of the last
detection, and to trigger the next detection.
The procedure for doing detection is outlined in Figure 5.

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Figure 5 –Flow diagram for typical use

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4. Important Usage Considerations
4.1 Detection thresholding
Sun/Nadir-sensors need to differentiate the sun/earth from the background of space in the
images that they take. In both cases this is done by comparing the brightness of the pixels in
the image to each other, in order to locate the sun and the edges of the earth. These
algorithms require a threshold and sensitivity value to execute. By setting the threshold high
enough, unwanted artefacts in the image taken by the camera are discarded, and only the
relevant celestial body is identified.
The threshold is set to a default value optimized for robust measurements. If the user wishes
to change the threshold values, telecommand 40 can be used. It is however recommended
that the default values be used.
4.2 Image exposure settings
The exposure times for the image sensors are key to their successful operation. The default
exposure settings for the sensors are set to values for robust operation and it is recommended
that the client do not alter these values. If, however the client wishes to use the Nadir sensor
as a greyscale imager, the exposure can be set by using telecommand 43. For correct detection,
the exposure should be set to acquire images similar to Figure 6.
Figure 6 –Images of earth (left) and sun (right) from orbit

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4.3 SRAM over-current protection
When high energy radiation causes a latch-up inside the SRAM, it will result in a high current
draw that may permanently damage the SRAM. CubeSense is fitted with a current sensor to
monitor the SRAM. In the case that the SRAM experiences an over-current, power to the SRAM
is turned off and an internal status flag is set in the CubeSense. Subsequent usage of the sensor
will result in an error response indicating an SRAM over-current. The “Power”-telemetry frame
(ID = 26) can be used to read the SRAM current or to check for SRAM over-current. After an
over-current has occurred, the user can clear the over-current flag, to re-enable the use of the
SRAM by using the “Clear SRAM overcurrent flag” Telecommand (ID = 11). In the case of
permanent SRAM failure, CubeSense will no longer be able to provide detections or image
downloads.
4.4 Nadir Detection Adjustment
Using a false Nadir detection in the satellite EKF can cause instability. The Nadir algorithm of
CubeSense utilizes simple logic to reject such false detections and displaying a Bad Fit error.
There are four adjustable parameters that can be used to reject false nadir detections –the
Max Deviation Percentage, Max Bad Edges, Max Radius and Min Radius parameters.
The Nadir sensor algorithm attempts to fit a circle to a number of detected edge points. The
Min- and Max Radius parameters (telecommand 15) can be used to specify the valid range of
the circle radius. This will depend on the height of the satellite in its orbit. The Max Deviation
Percentage (telecommand 14) is the tolerance that is allowed for all detected points to lie on
or close to the fitted circle. If there are more than Max Bad Edges such points, a detection error
is reported. Do not adjust any of these parameters without first consulting CubeSpace.
4.5 Sensor masking
The Nadir sensor may in some configurations have antennas or other deployable structures in
its field of view. For these cases, a mask on these images is required to avoid false detections.
The sun sensor’s detection algorithm does not require any masking to function, but in cases
where the object in its FOV is reflective, it may provide false detections. A mask should then
be placed over the reflective area to avoid this.
To mask an object, the coordinates of a square can be sent to CubeSense via telecommand
52. Every pixel that falls within this square will be ignored by the detection algorithms. To mask
more complex objects in the FOV, multiple squares (up to 5) can be defined. To find the
coordinates that should be entered, a full resolution bitmap image should be downloaded
from CubeSense via the GSP program. Open the image with Microsoft Paint and hold the
mouse over the top-left corner of the object to mask and note the minimum X and Y pixel
values as shown in Figure 7. Then hold the mouse over the bottom-right corner and note the

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maximum X and Y pixel values. These two coordinates define the masking square. Take great
care to ensure correct placement of these masking squares. Testing should also be performed
to verify correct masking. See the Node Definition for correct telemetry and telecommands to
perform masking.
Figure 7 –Object masking
When the CubeSense is used with the within a CubeADCS running CubeACP, the image will
first need to be transformed before reading the pixel coordinates. This is done by opening the
image in Microsoft Paint, rotating the image 180°, flipping the image horizontally and then,
reading the minimum X and Y pixel values at the top-left and the maximum X and Y at the
bottom-right. These steps are shown in the figure below.