Deployment Switches
The idea of a Remove Before Flight (RBF), Apply Before Flight (ABF), or deployment switches is that when it is removed or applied, it either opens or closes a circuit. This circuit causes a signal, which causes the satellite to "know" whether it is in space. On a fully assembled satellite, the RBF is clearly marked, typically with a red tag. The launch provider will remove the tag just before launch, to either open or close the circuit.
This is important for CTS-SAT-1 because we need to make sure that the antenna is not deployed too soon after launch. In order to do this, the satellite is equipped with Apply Before Flight (ABF) switches on the EPS, Remove Before Flight (RBF) switches on the custom of the shelf (COTS) solar panel, and deployment switches, located on the outside of the satellite and implemented electrically with the EPS.
Photo credit: https://x.com/pa3weg/status/1059950348083912709
When the deployment switches are pressed, the EPS "thinks" that the satellite is not deployed. When the switches are not pressed, the EPS "thinks" that the satellite is deployed.
When the ABF connector is present, the EPS will be on and the OBC will get power. When the ABF is absent, the EPS will be off, and the OBC will not get power.
The RBF connector is located on the solar panels, and connected to the OBC. When the OBC turns on, it checks the GPIO pin for the signal telling it whether the RBF has been removed. If the RBF has been removed, the OBC will begin a 30 minute countdown to deploy the antenna.
As we assemble the satellite, we will make the following connections: * The ABF connector to the ABF pin on the EPS * The connector will be attached with epoxy * The deployment switches to the SEP_SW connector on the EPS * RBF on the Solar Panel to the RBF connector on OBC (this is a GPIO input)
Once the satellite is assembled, as we are packaging it, we will use a case or some other 3D printed object to ensure the deployment switches are pressed during shipping. The deployment switches will not be pressed for a briefly as it is placed in launcher, but will be pressed again for launch. The RBF connector will be removed after the satellite is placed in the launcher. When the satellite is deployed, the deployment switches will not be pressed, the RBF will have been removed, and the ABF will have been applied, therefore the satellite will "know" that it is in space.
The deployment switches are model D2F-L3-D3 from Omron Electronics. They have three terminals, COM (common ground), NO (normally open), and NC (normally closed).
When the satellite is deployed, the switch is closed electrically and not pressed physically. Therefore, the switch is configured to be normally closed (NC). This means that we will connect the SEP_SW connector on the EPS to NC. There will be a connection between NC and COM. The NO terminals on the switches will be unused.
To have redundancy in the deployment sensing, we have four deployment switches. The terminals of these switches will be connected to the EPS connector J8. J8 has 6 pins: !SEP1, !SEP2, !SEP3, and three ground (GND) terminals. !SEP3 will be unused in our satellite. Two switches will be connected to the !SEP1 pin, and two will be connected to !SEP2.
The RBF on the solar panel is implemented with one connector on the back side and two connectors on the front side. Each of the front side connectors works as one RBF pin. The RBF on the solar panel uses a normally open (NO) configuration. The RBF tag shorts the NO RBF pin with the COM pin. CTS-SAT-1 will only use one of these pins. We will use one of the general purpose LEDs on the OBC to display the state of the RBF pin so that we can see the countdown to antenna deployment.
The RBF connects to the OBC through the connector on the back side of the solar panel and connector J15 on the OBC. The pins for RBF 2 on the solar panel connector will not be used. The COM RBF1 on the solar panel connector will be connected to the ground pin on J15, which is pin 2. The NO RBF1 on the solar panel connector will be connected to the RBF1 pin on J15, which is pin 3.
On our OBC, we can make the RBF pin active low enable by removing R81 and R82, or active high by removing R79 and R80. Active high means that the pin gets a signal when it is at a high voltage. Active low means that the pin gets a signal when it is at low voltage. In our case, high voltage is 3.3V and low voltage is 0V.
How does this work? Say we remove R81 and R82. When the voltage is low at the RBF pin, there is a complete circuit, therefore, current flows and the RBF pin gets a signal. When the voltage is high at the RBF pin, no current flows, so the pin does not get a signal.
We decided on an active low enable, so we will not populate R81 or R82 when we bring up the OBC. Active low is more typically used in this application. One potential reason for this is that it is easier to manage which voltage domain the signal uses - a "high" voltage could be 3.3V or 5V while a low is 0V.