GPS Tracking System Test Report

GPS Tracking System

Mitchell Knotts

Description and Use

                  The GPS Tracking System is crucial for us to successfully retrieve our payload after the flight. The GPS Tracking System will send the payloads location over a radio frequency (144.39MHz). We selected the components that make up the GPS Tracking System specifically to work on the Automatic Position Reporting System (APRS). This system is completely automatic, once the radio receivers, located all over the globe, receive our position packets from our radio transmitter, the APRS automatically uploads the position of our payload to the internet. Using any device that has internet capabilities, such as a smart phone, we can find our payloads location and track its progress.

Example of the APRS working on a Droid Razor

Components that make up the GPS Tracking System

Garmin GPS 18x LVC (GPS Reciever)

MicroTrak 300 v1.7 (Radio Transmitter)

The MicroTrak 300 and the GPS 18x get connected together  through a RS-232 connection.  We have been testing the system on a 9V battery, but when we launch our balloon, the GPS tracking system will be running on two 9V batteries wired in series so that our system will have 18V when it launches.  The system runs well on 8V to 24V. The GPS reciever will communicate with GPS satelites and determine the payloads location. Once the GPS has GPS lock, it will communicate with the MicroTrak over the RS-232 connection. The Microtrak will process that data, then transmit it over 144.39MHz once every two minutes. The radio recievers located on the ground will receive the transmission and the APRS will upload the position to the internet where we can view it.

In order to use the APRS we are required to carry a valid HAM (amateur) radio licence because the APRS utilizes amateur radio to retrieve the position packets. The license level required for this system is the lowets level which is technician.To attain this license we had to complete the Amateur Radio Technician License test. There are groups and clubs of people that use amateur radio frequencies as a hobby. These groups are certified to proctor the exam several times per year so that new enthusiasts will join in. This is how we got licensed, we all took the Technician test and once completed we were legal to transmit over HAM frequencies, this enabled us to utilize the APRS.

Testing

                  Testing began with configuring the MicroTrak 300. The MicroTrak has configuration software that we used to adjust the critical setting on the unit. These settings included call sign and interval between transmissions. These settings are extremely important; the call sign enables us to identify our payload over all the other units that are transmitting on the APRS. Our call sign is unique and specific to us. The interval between transmissions must be set to 120 seconds or greater by law. Amateur radio requires us to limit how often we send a packet to the system so that we do not bog down the system. To configure the MicroTrak we had to plug it into a computers RS-232 port using a null modem. The null modem simply is a crossover cable which switches pins 2 and 3 on the DB9 connection (this is only required when connecting to a computer).

Configuration Software for the MicroTrak

Once the MicroTrak was sucessfully configured we could connect the entire system together and power it up.

Assembled GPS Tracking System

The MicroTrak has an LED indicator on it that is very useful for determining if the system is functioning properly. The LED is a bi-color LED that can be either green or red. Once the system is powered up the LED will begin to flash green indicating that the system is searching for GPS position. Once the LED changes from flashing green to solid green this indicates that the system has GPS lock, meaning it has GPS signal and is ready to transmit.  Once the system has GPS lock it will begin to transmit over the radio frequency, this is indicated with the solid green LED flashing to red every 120 seconds. If the unit is functioning as described above the unit is working successfully.

One problem with this system is that it requires a line of sight to communicate with the receivers on the ground. We ran into trouble seeing our call sign on the APRS because we were too far away from the nearest receiver. The system will be fine once we launch the balloon because it will gain altitude and acquire line of sight with the receivers on the ground.

Our solution to this was to set up a ground station, the ground station was used for testing purposes only, it consisted of a radio receiver with LCD display.  Once the ground station was set up and tuned to 144.39 MHz we could see the packets from our GPS Tracking System on the LCD display.

To test that our GPS Tracking System was transmitting properly we walked around campus and watched the Lat and Long from our Position packets change on the ground station’s display.

 Ground Stations Display showing how Lat and Long changed as we walked around campus with our GPS Tracking Syatem

Integration into Payload

The GPS Tracking System is mounted inside the Payload box with a hole that allows the radio antenna to stick out the side of the payload. The GPS receiver is located on the top of the payload so that it has a clear view of the sky.