Onboard battery
Connecting to Ultimate-series kits
1pps accuracy measurement by David G0LRD
Comparison of SKM61 with SKM52 module
Eddie G3ZJO experiments with the GPS antenna location

The Skylab SKM61 is a high performance GPS module with built-in antenna, suitable for the Ultimate1Ultimate2 and Ultimate3/3S QRSS/WSPR kits. When used with the Ultimate QRSS/WSPR kits, the GPS module will provide accurate time-keeping, frequency calibration, and location information (for encoding the Maidenhead locator automatically into WSPR messages).

Compared to the previously supplied VK16E module, the SKM61 GPS receiver module has higher sensitivity and a much more accurate 1pps signal (10ns RMS accuracy, compared to 1us on the Sirf III-based VK16E module). The SKM61 GPS module uses the same chipset as the previously supplied SKM52 module, but is more convenient because it can use a 5V supply. Therefore it is highly compatible with the Ultimate-series kits. The SKM61 is slightly larger than the previously supplied SKM52 module, and the larger patch antenna provides better sensitivity (see comparison).

The following photos show the front and back of the module, and a side-by-side size comparison of the SKM61 (left module in photo) and the SKM52 (right module in photo). Please click the photos for full size versions.

CLICK HERE to read the SKM61 datasheet.

Main features, from the datasheet, are:

- Ultra high sensitivity: -165dBm
- Timing accuracy (PPS) 10nS RMS
- Extremely fast TTFF at low signal level
- Built in high gain LNA
- Low power consumption: Typical 23mA @ 3.3V
- NMEA-0183 compliant protocol or custom protocol
- Operating voltage: 3.3V to 5.5V
- Operating temperature range: -40 to 85℃
- Small form factor: 30x26x7.6mm
- RoHS compliant (Lead-free)

Notes on the onboard battery

There's a small battery on the SKM61 GPS module board as you can see in the photo, that is not mentioned in the datasheet. This is a rechargeable battery. When powered, the SKM61 GPS module charges this battery. When power is removed for a short time, voltage from this battery maintains satellite data in memory on the module. When you next power up the module, the satellite fix aquisition is faster because of the previously stored information. Do not worry if the voltage across the battery measures zero, and do not worry that the module is supplied wrapped in aluminium foil.

Notes on connecting the SKM61 GPS module to Ultimate-series kits

1) You need to connect 4 wires to the GPS: Vcc, Gnd, serial data and PPS. All can be connected directly to the Ultimate kit (it is suitable for use with Ultimate1, Ultimate2 and Ultimate3/3S kits). Note that the GPS "TX" output is connected to the Ultimate kit "RxD" input. Make sure of this! (do not connect GPS "RX" to Ultimate kit "RxD"!).

2) Pull-up resistors are necessary. You should connect a pull-up resistor from the kit's RxD input to the +5V connection, and another pull-up resistor from the kit's Pps input to the +5V connection. 4.7K resistor values are best (though anything in the range 4.7K to 10K should work). The pullup resistors are NECESSARY!

3) The SKM61 GPS module has a high sensitivity, but you may still want to look out for issues if you use it indoors and your building has a steel frame (Faraday cage behaviour) etc. GPS modules may also be disrupted by RF from the Ultimate kits. The suggestion is that it is worth connecting a few metres of screened cable between the Ultimate kit and the GPS Module, with the screen connected to Gnd at the Ultimate kit end of the cable only, and a supply decoupling capacitor at the GPS module end.

4) The default 9,600 baud rate settings of the Ultimate kits will work fine with the SKM61 GPS module. On the Ultimate3 kit use GPS Mode 2.

5) The GPS module must NOT be enclosed in a metal case, which will block the signal. Also, the GPS module should be sited with the antenna side pointing upwards. There must be a minimum 10mm gap between the bottom side of the GPS Module (component side) and any metal surface below the module. As much sky view as possible is best. If the Ultimate kit is kept below the plane of the GPS antenna, this helps to shield the antenna from any interference from the Ultimate kit. If necessary, the GPS module can be tilted slightly so that the Ultimate kit is below the plane of the antenna.


1pps accuracy measurement by David G0LRD

David G0LRD has measured the accuracy of the 1pps signal in comparison to his Rapco 1804M Frequency Reference. The jitter of the 1pps pulse had a 3.8ns standard deviation which seems to be well within the 10ns specification. The screenshots below show the 1pps signal over 600 samples, with the GPS antenna sited in a good strong signal location, and a poor location. In both cases the performance is superior to the formerly supplied SKM52 GPS module, which can probably be attributed solely to improved sensitivity due to the physically larger patch antenna of the SKM61 module.

Comparison with SKM52

From firmware version v3.06 of the Ultimate3/3S kit, there is a GPS information display during the wait period between transmission frames. The GPS information display shows 5 items:

1) GPS data validity (V = Invalid, A = Valid)
2) Type of fix (2D, 3D, None)
3) -f parameter: Number of satellites used in the fix solution calculation
4) -t parameter: Number of satelites being tracked
5) -s parameter: Average signal strength of the satellites being tracked

The following photograph (please click it for the full size image) is a side-by-side comparison between SKM52 and SKM61 GPS modules. The comparison uses two Ultimate3 kits. The top one is connected to an SKM52 module, the lower one is connected to an SKM61 module. The lower Ultimate3 kit is in the official QRP Labs case. The two GPS modules were sited outside but with limited sky view. Their location is only 30cm apart so essentially they can be considered to be at an identical location, for the purposes of comparison.

It can be seen that the lower Ultimate3 kit connected to the SKM61 has higher signal strength, tracks more satellites and uses more satellites in its fix computation (-s, -t and -f parameters respectively). Both GPS modules have a good fix, however, in what really is an inferior location for sky visibility. In my opinion both SKM52 and SKM61 modules are fantastic performers, but the SKM61 is more convenient because of the 5V supply, as well as somewhat more sensitive due to the larger patch antenna.

Eddie G3ZJO experiments with the GPS antenna location

Eddie investigated the variation in GPS module performance with the module placed in different locations relative to the U3, to investigate how proximity to the U3 affected the module. This experiment uses the older SKM52 GPS module but should be just as valid for the SKM61 module. His conclusion is interfering noise from the U3 is conducted to the GPS antenna, so that the proximity to the module is less important than having it electrically separated by 2 metres of screened cable. Read Eddie's PDF document (179KBytes) here.


The GPS Module is available from QRP Labs for $22.50. Click here.