Let’s assume you somehow came into possession of a Raspberry Pi (rpi) 4 and want to configure it for something useful.
After writing the raspbian image you discover though that you have no suitable cables nor converters for the micro HDMI port.

If you do have a USB to serial TTL converter though, you can just configure rpi over the serial port.

In order to do that we first have to enable the first serial port on the rpi. To do that you have to mount the micro SD card with the raspbian image on your computer and add these lines to enable the serial console to config.txt:

sudo mkdir /mnt/tmp
sudo mount /dev/sdz1 /mnt/tmp
sudo echo "enable_uart=1" >> /mnt/tmp/config.txt
sudo umount /mnt/tmp

… where you have to replace sdz with actual device name of your SD card (you can find it from the output of dmesg for example).

Now connect the USB to serial cable like this:

• ground -> pin 6 (GND)
• rx -> pin 8 (TXD1)
• tx -> pin 10 (RXD1)
• +5V will remain unconnected

… which looks something like this:

You should be able to access the terminal using something like this:

picocom -b 115200 /dev/ttyUSB5

Where you have to replace ttyUSB5 with whatever device for the USB to serial converter on your system (which you can again find out from the output of dmesg).

Back in 2014 I wrote a post on RTK-GPS using Navspark NS-RAW boards. Back then I promised to write a follow-up once I have done some testing. I didn’t quite think it would take so long but the reason behind the delay is that I never quite got it working as well as I expected and tried various things over the years which took time, and I lost interest couple of times in between.

A couple of people have asked me how it worked, so finally I decided that enough is enough and I’ll just describe how far I got.

TLDR version: NS-RAW in seems to perform significantly worse than the u-blox of the same era. Even if it had performed at a similar level, single frequency GNSS in general just seems to be unsuitable for reliable ground based robot navigation. It might work at some times of the day where the satellite visibility and geometry happens to be just right and you are testing in a large open field, have good antennas etc. but for most realistic cases it will not be reliably usable.

Setup

Navspark has this to say about the required conditions to be met for a stable RTK FIX:

NS-HP or NS-HP-GL are single frequency RTK receivers. For reliable consistent single-frequency RTK operation, they require 12 or more usable satellite signals above 15 degree elevation angle with SNR no less than 38dB/Hz.

NS-RAW is single-constellation so I’m limited to only GPS. Looking at the GNSS radar tells me 5-9 GPS satellites would be visible from my location at different times of the day (with 15 degree elevation). So 12 satellites would be impossible here. Other sources say that you need 7 satellites min. to get a RTK FIX solution (and min. 5 to hold it). That would have been a bit more realistic requirement, and besides, an earlier u-blox LEA-6T test got a FIX solution just fine so I decided to go on with the test.

Here’s what my test ground looks like:

The plot has 20m high forest in the N & NE directions. Other sides have better visibility but all the sides have at least one row of 20-30m trees + some sporadic buildings and trees all over the place.

BS1 is the original base station that was described in the previous post on the topic. It uses Satmar FME marine active GPS antenna. I have added a 15cm wide ground plane below it (a paint can lid). The location of BS1 is somewhat unfortunate since it is basically surrounded by some trees from all the sides. When I originally chose the location for the base station I didn’t have good access to the roof of the main building so I chose a spot that was furthest away from the house, so that it wouldn’t block satellites from that side. Here’s the picture of the location:

BS2 is the new base station at the top of the roof. It’s about 8m higher than BS1 and hence has better satellite visibility. Tri-Band GPS/Galileo, Beidou, GLONASS Active Antenna is used. It looks like this:

“Rover” is me walking around with a laptop and NS-RAW with an antenna setup identical to BS1.
Everything is connected over WIFI.

Best case – localization between two base stations

Using BS2 as base station and BS1 as rover in the Kinetic mode should give us a good idea of what is the best that can be achieved with this hardware on my plot. Even though BS2 doesn’t have good sky visibility it’s better than a real rover will ever have since it’s located on a pole that is a couple of meters higher than the rover will ever be, and has less sky blocked by the house than a rover normally would.

I finally got RTK working rather well in this scenario and it has FIX most of the time. The location stays in a ~5 cm^2 area for the 8h session:

I was cheating a bit though – for localizing static points I use Integer Ambiquity Res mode FIX and hold. This mode takes past measurements more into account when calculating the current location than Instantaneous or Continuous would, which makes position mode stable but would also make it return the wrong position for longer if the rover was actually moving. I also used demo5 ver. of RTKLIB by rtklibexplorer which got more FIX solutions than the mainline RTKLIB.

Driveway test

Next I walked up and down the driveway as a rover a couple of times to see if I can get comparable results to my u-blox test at the same location in 2013.

The results were not bad but worse than the earlier u-blox LEA-6T test:

There were lots of slips, large residuals and ambiguity validation failures in the RTKLIB logs. Most of these errors are said to be commonly caused by not having clear enough view of the sky and the consequent multipath errors.
This seem likely since the rover certainly didn’t have a great view of the sky from 2 sides and things seemed to get worse with stronger winds (when trees had leaves and these were moving a lot).

With that said, the driveway basically has the best visibility in my yard that a real rover could ever have, so this is the most optimistic-realistic scenario. Even though the common satellite count is really low and we seem to be in FLOAT mode half of the time, the points are reasonably close (~20-30cm) and it would be usable for real navigation if this precision were repeatable 24/7 all year round. This test was done in January so no leaves on the trees which probably reduced errors a bit and made it even more of a best-case scenario.

Rover circle

Now for a more realistic test – walking in a circle with 20m diameter in the middle of the yard in mid-summer when trees have leaves.

This looks rather bad, only 2.1% of the points have FIX solution. The FLOAT solution was reasonably precise for 3 circles and then suddenly wandered through the circle. We see that most of these points have a lower confidence than FLOAT points on the actual circle so we could filter those out on that basis, but we also see a single FIX solution point in that path which is a couple of meters away from the actual location.
This test was done with integer ambiquity res. set to continuous.

What next?

I have put the navigation project on hold for now. I expect L5 capable chips to be much more common in the next 2 years which along with new Galileo satellite launches should considerably improve the precision of navigation at my location (NE Europe). L5 should be much less prone to multipath problems that seemed one of the main things throwing my rover signal off. Broadcom 47755 is the first mass consumer L1/L5 chip and others will surely follow.

Finally here’s a list of relatively cheap alternatives to NS-RAW that are available right now and would be rather interesting to try in similar situation but not interesting enough for me to actually buy the test hardware:

• My original tests in 2013 seemed to get much better results with LEA-6T so u-blox LEA-8T that is available for ~75$ per module might give better results. You would also be able to try out various hacks and configuration tweaks described in the awesome rtklibexplorer blog.
• Navspark NS-HP series (NS-HP-GL, NS-HP-BD) might also give somewhat better results than NS-RAW since these have multi-constellation support. Having the ability to use Glonass satellites in addition to GPS would almost double the average number of visible satellites in my location. These chips also do RTK on board so you don’t have to have a separate computer for running RTKLIB which will save you both weight and power. They also claim significant improvements in environments like mine in the newer firmware versions.
• u-blox M8P based boards might also be interesting, these are multi constellation (GPS, BD, Glonass) L1 chips that do RTK calculations on board so you only have to provide a communication link. Such boards are for example Tiny RTK from Drotek (~215€) or the one from CSG (~239€).
• Drone operators seem to like Emlid Reach. Under the hood it’s a combo of u-blox M8T + Intel Edison for processing and RTKLIB with much friendlier frontends than bare RTKLIB has. It seems that some of their customers are happy with the results and others seem to almost never get a FIX solution.

After having read lots of forums of users trying to use these cheap single frequency RTK solutions, my conclusion so far is that it seems to require intimate knowledge of almost endless GNSS related stuff, a very good sky visibility, living in a place where satellite geometry is just right for large part of the time, good antennas and specific weather conditions. In short it doesn’t seem to be feasible for generic rover navigation that requires constant precision in the dm range in all corners of the yard.

Just some quick notes on using 5V esp8266 relay module. Manufacturers page seems to be this even though terminal and pin positioning on the pictures is a bit different.

For just ~2.4€ it will give you the ability to switch electrical loads over Wi-Fi 🙂

There are actually 2 basic designs floating around on Aliexpress. The one that I’m talking about here is where esp8266 module plugs into the mainboard through 8 pin connector like this:

The other one integrates esp8266 on the relay board directly.

From the shipping perspective this modular design is a bit unfortunate since cheap/free shipping from Aliexpress will usually arrive in normal soft envelopes that just have 1 layer of bubble wrap as a padding.

As a result all 3 boards that I ordered arrived looking like this:

Nevertheless they actually worked fine. I was probably just lucky though and I kind of like modular things more in principle.

Interesting thing about this modular design though is that the relay is actually controlled by the STC 15f104W MCU on the main board which is programmed to turn the relay on when it sees “\xa0\x01\x01\xa2” on its UART port and off when it sees “\xa0\x01\x00\xa1“.
That UART is accessible either through the 4 PINs between the screw terminals or input from the esp8266. So you can actually run it without the esp8266 and control the relay directly over UART PINs or even reprogram the 15f104w.

ESP8266 board comes flashed with esp8266 AT firmware which allows esp8266 to be controlled using AT commands on the serial port.

By default the module comes in AP mode which means that it will show up as an Wifi access point that you can connect to. I wasn’t really interested in that so I reconfigured it to connect as a client to my main wifi network.
To reconfigure ESP8266 you have to connect UART over the 4 pins between the screw terminals using an USB to UART TTL cable like this:

I just happened to have the 5V power supply also connected but it’s not really required at this step, just for reconfiguring power from USB should be enough.

This can be done like this:

$picocom -b 115200 --omap crcrlf /dev/ttyUSB5
AT+CWMODE=1
AT+RST,
AT+CWJAP="myssid","mypasswd"
AT+CIPMUX=1
AT+CIPSERVER=1,8080
AT+CIFSR
AT+IPR=9600

After this, communications will switch from 115200 to 9600 baud so you will lose the link and would need to switch picocom to 9600 baud to do anything further. But anything else isn’t really required anyway for our simple use case, so just disconnect picocom with CTRL+A CTRL+X.

AT+CIFSR command should have printed out what is the IP of the module. In my case it looked like this:

AT+CIFSR
+CIFSR:STAIP,"192.168.17.40"
+CIFSR:STAMAC,"2c:3a:e8:4f:30:02"
 
OK

Now you can just create a TCP connection to that IP at port 8080 and send the relay commands that I mentioned before. ESP8266 will pass these to the STC 15f104W MCU which in turn will control the relay.

As an example here’s a trivial python script that will connect to the relay and turn the relay on and off in an endless loop in 4s intervals:

import sys
import time
import socket
 
IP = "192.168.17.40"
PORT = 8080
RELAY_OPEN_CMD = "\xa0\x01\x01\xa2"
RELAY_CLOSE_CMD = "\xa0\x01\x00\xa1"
SLEEP_S = 4
 
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((IP, PORT))
 
while 1:
    s.send(RELAY_OPEN_CMD)
    time.sleep(SLEEP_S)
    s.send(RELAY_CLOSE_CMD)
    time.sleep(SLEEP_S)

And here’s how the result looks like:

As mentioned in my last post about cheap Chinese wireless temperature/humidity sensors, one of the three devices that I ordered was broken. It did show correct values on the local screen but no messages were seen over the radio link.

I decided to open it up to see if there’s anything obviously wrong inside. Here’s how the internals looked like:

The 2 working sensors both had the blue radio module shown on the left and the non-working sensor had the white module shown on the right. Since I had RTL-SDR I decided to see if the broken module sends out any signal at all. I found a very detailed blog post describing the steps for doing it.

Here’s what my signals looked like:

The topmost signal is from the broken radio module as measured ~30cm from the antenna. The next one is the signal from the replacement radio module from the same distance and the last 2 signals are from the two other sensors which were located much further from the antenna.

As we can see the broken signal is rather unstable and can’t hold stable high.
It looks even more interesting when zoomed in:

Here’s how the sensor looks with the replacement radio module:

The fixed sensor works rather well, the signal is clear from ~20m away through 2 concrete floors.

I was kind of missing the fun temperature charts that I had at my previous home so I decided to get some sensors for the new house too. Using 1-wire network like I did previously didn’t seem reasonable this time around since my current house has 3 floors and cabling it in a way that wouldn’t be ugly would have taken too much effort.

So instead I decided to find some cheap wireless temperature sensors. It turns out you can get a reasonably looking temperature+humidity sensor with a local display and 433 Mhz radio for just 7€ a piece (with free shipping). You can also buy these sensors in a cheap kit with a central screen that shows values from all the senors plus some additional stuff like clock and sunrise times.

I wasn’t really interested in the central screen, so I just bought 3 sensors.

These sensors display humidity and temperature values on the screen and send out these measurements once a minute. When the sensor sends the message it blinks the red led in the front panel which might be a bit annoying if you place it somewhere where you can see it at night. I would probably tape it over or solder it off under these circumstances.
A switch inside the battery compartment allows you to select the channel which makes it possible to differentiate one sensor from another. The switch has 3 positions so it seems that with the normal central screen you are probably limited to 3 sensors. Technically this channel just changes a value in the message so it’s not a radio channel or something like that.

From the protocol perspective there’s also a 8 bit field called random id(rid) which is initialized when the sensor starts. Using the rid field alone or in conjunction with the channel value allows you to use far more sensors than 3. Since rid isn’t under your direct control it might take a bit of fiddling to find a free ID with a larger number of sensors and at some point you will probably have too many collisions in the air but I imagine using 10-20 of these sensors wouldn’t be a problem.

Since I didn’t buy the central screen I still needed a way to actually listen for the measurements that these sensors broadcast. It turns out cheap RTLSDR compatible DVB-T receivers can be used for that purpose with the RTL 433 application.
So I bought the receiver USB dongle with an external antenna and remote from ebay for ~10€.

Once RTL 433 is installed all you need to do to listen for the measurements is to run the rtl_433 command:

# rtl_433 -R 3 -F json
{"time" : "2016-02-13 23:10:19", "model" : "Prologue sensor", "id" : 5, "rid" : 55, "channel" : 2, "battery" : "OK", "button" : 0, "temperature_C" : 10.000000, "humidity" : 52}
{"time" : "2016-02-13 23:10:24", "model" : "Prologue sensor", "id" : 5, "rid" : 170, "channel" : 3, "battery" : "OK", "button" : 0, "temperature_C" : 23.400000, "humidity" : 29}
{"time" : "2016-02-13 23:10:33", "model" : "Prologue sensor", "id" : 5, "rid" : 51, "channel" : 1, "battery" : "OK", "button" : 0, "temperature_C" : 19.800000, "humidity" : 39}

This output can be easily interfaced to whatever system you use for home automation. My graphs look like this:

My receiver is located on the second floor and the furthest sensor is about 20 meters away through 2 concrete floors and there don’t seem to be any communication problems so far.

One of the sensors came with a faulty radio that I had to replace, but more on that in a separate posting later.