RTL-SDR for 433MHz/70cm CubeSats – Telecommunications, Navigation & Electronics

Introduction

Recently I read a book about new rocket and satellite advancements by Ashlee Vance: “When the Heavens Went on Sale” (Ref.1). This got me very excited and I decided to see if I could receive current Amateur Radio or University Cube Satellites.

Equipment Setup

Fig.2 Rx Setup Vee Dipole + BPF + LNA + RTL-SDR

Fig.3 437MHz/70cm Noise Spectrum Analyzer

Figure 1 shows the equipment setup. I decided to copy the method I used to receive NOAA15/18/19 transmissions. Signals are received on a Vee dipole tuned to 437MHz and then pass directly to a BPF/LNA located at the back of the antenna. Fig.2 shows the Vee Dipole + BPF + LNA (Ref.2/3). This then feeds an RTL-SDR V5. Signals are recorded as IQ & WAV files using SDR#/GNURC, then decoded by several methods depending on the modulation: GNURC and Scicos.

Reception Planning

PD Km Path Distance	Lfs Free Space Loss dB	Rx Level dBm	SNR dB
500Km	139.2dB	+10=-129.2dBm +15=-124.2dBm +20=-119.2dBm	0.8dB 5.8dB 10.8dB
1000Km	145.3dB	+10=-135.3dBm +15=-130.3dBm +20=-125.3dBm	-5.3dB -0.3dB 4.7dB
1500Km	148.8dB	+10=-138.8dBm +15=-133.8dBm +20=-128.8dBm	-8.8dB -3.8dB 1.2dB
2000Km	151.3dB	+10=-141.3dBm +15=-136.3dBm +20=-131.3dBm	-11.3dB -6.3dB -1.3dB
2500Km	153.2dB	+10=-143.2dBm +15=-138.2dBm +20=-133.2dBm	-13.2dB -8.2dB -3.2dB
	32.44+20log(F_MHz) +20log(PD_Km)	TxPwr=10dBm TxPwr=15dBm TxPwr=20dBm

Fig.4 Path Loss & SNR (BPF/LNA Gain not Included)

Let us consider path parameters for various possible transmit powers (Ref.4). Figure 3 shows a scan of 437MHz showing an average receive noise level of -130dBm. Figure 4 shows Lfs Free Space Path Loss for different transmit levels of 10dBm, 15dBm and 20dBm. For a typical pass over slant range of 1000Km the SNR is only 4.7dB at 100mW. This is why I added an BPF/LNA of G=22dB. Not only does it have gain but also low pass filtering 380-480MHz. I find a BPF is essential with the RTL-SDR as it gets overloaded easily. The LNA gain is not included above.

EIRSAT-1 Cubesat Details

Fig.5 N2YO Satellite Data Base EIRSAT-1 10 Day Prediction (Ref.6)

In order to chose a possible Cubesat for reception, I checked a Wikipedia database for recent cubesat launches (Ref.5). The latest in the data base was EIRSAT-1 a 2U launched on Dec1st 2023. In order to track the LEO satellite, there are several excellent databases such as N2YO (Ref.6), Gpredict (Ref.7) and SatNogs (Ref.8).

EIRSAT-1 Reception

Fig.6 Gpredict EIRSAT-1 Showing Az/Elev/Slant Range/Polar Plot 22:18

Fig.7 SDR# Showing In Channel Burst Approx 22:18

Fig.8 Signal Received April 20th at 21:31 Showing GMSK Type Pulses

My first try was the April18th_2025_11:47 pass as shown in N2YO 10day prediction. I noticed 2 in-channel bursts during the reception window, but I didn’t have the recorder on, the bursts were only 1-2 secs long. Later on in the day at 22:18 I had both IQ/WAV recorders on and I received several pulses as shown in Figure 7. This looks like a GMSK burst! (Ref.9). However last night April 20th at 21:31, I received several strong pulses on frequency but 30min before EIRSAT-1 was due to pass over, so looks my reception earlier was a coincidence and not EIRSAT-1. So I will try to decode these signals and determine the source. More to come in next post.