GPS FAQ

What connects to the vacant C2 location on the TF10 board?

A 0.22F supercap can be inserted to provide backup for the almanac information rather than using an external battery backup. Charging current is derived from the +5V supply so no other power connections are necessary when the supercap is fitted. The supercap may be fitted at the factory for approximately $US2.00 with a MOQ of 100. The recommended device is a Panasonic part number EECS0HD224H which is available from Farnell Electronics as order code 3036790 for around $AUD5.00 in single quantities. You can also check the Panasonic datasheet for more information on this device.

Can I alter which NMEA sentences are sent by the TF10?

Yes, refer to the query / rate control command that is on page 103 of the current TF GPS user's guide. This not only allows certain sentence types to be enabled or disabled but also specifies the rate at which they are sent. It is also possible to use the GPS in a polled mode where setences will only be sent upon request.

What is the maximum rate I can receive position and speed information at?

The TF10, TF30 and TF200 GPS modules can provide a new position and velocity at a maximum rate of 1Hz (once per second). The TF50 GPS/GLONASS receiver can provide this information at up to 5Hz.

What modes of output does the TF series provide?

The default output is NMEA at 4800bps 8-N-1. The modules also support SiRF binary mode but this mode is not recommended unless necessary for a particular application because NMEA provides the best compatbility with other GPS modules and is also more convenient during the debugging process. The SiRF binary mode does provide some additional commands and information not available in NMEA mode, and offers a more compact transmission format, but for most applications NMEA is the best choice.

What is the length of the GPS antenna lead?

All the Laipac GPS antennas have a five meter lead terminated with a male SMA connector. Some confusion was caused by an article in Silicon Chip magazine that incorrectly mentioned a two meter length.

Can the antenna magnet be removed?

The external antenna casing can be removed although this negates the weaterproof properties of the antenna, so other mounting arrangements will be required. It also voids the manufacturer warranty. The magnet is glued to the outer casing so removal can be tricky, although it is possible. Please contact us about alternative mounting arrangements for high volume applications.

What are recommended antenna mounting locations?

The best position for the antenna on a vehicle is to have it mounted in the center of the roof. The general rule is that it should have the best possible view of the sky at all times. Positions such as front or rear bumpers do not provide good performance, and mounting on a parcel shelf inside a window provides marginal performance because quite a bit of the view will be obscured by the roof. Fiberglass does not affect signal reception so on commercial vehicles mounting under fiberglass canopies and roof-top signs such as those on taxies provides good performance and also provides further shielding from the environment.

Why does the evaluation kit sometimes restart unexpectedly ?

This problem has only ever been reported with unregulated and unfiltered power supplies such as cheap plug-packs. Most plug-packs do work fine although some models seem to have very noisy outputs. The two alternatives are to replace the 1000uF filter capacitor with a larger value such as 4700uF or to use a cleaner power source. Also ensure the AC adapter is rated at 300mA or greater.

What are the pros and cons of using an SMA connector versus MCX ?

An SMA connector has a screw thread and is quite mechanically robust. An MCX connector is much smaller and is a push fit meaning it is typically only well suited for making internal connections. Because it is often not convenient to mount the GPS module in a position where the on-board SMA connector will protrude externally from the case it is quite common to use a GPS module with the smaller MCX connector internally with an MCX to SMA cable to run an external antenna connection. For prototyping and applications where the antenna is rarely disconnected the SMA modules prove quite convenient because the antenna may be connected directly to the board.

What is the best way to mount a GPS antenna on a building ?

Many customers have expressed the desire to mount a GPS antenna on the outside of a building for use as a time reference. While many GPS reference antennas are available and designed expressly for this purpose they can be quite costly, and using the Laipac GPS antenna is quite a good solution if you don't mind improvising a mount. One solution that works well is to use a standard TV antenna mast with a steel plate on which the antenna may be placed. As always a clear view of the sky works best so try and mount the antenna in a location where adjacent structures will have the minimum effect.

How do I extend the antenna length of my building mounted GPS system ?

Why won't my GPS acquire a position sitting on the workbench ?

While sometimes you can get a position inside a building when close to a window, the GPS antenna normally won't receive sufficient signal strength from the satellites to acquire from inside a building. It doesn't take much at all to obscure the high frequency and relatively low power GPS transmissions and a tin roof is more than enough to block the signals. Atmospheric conditions and satellite positions also affect reception, so you may find that sometimes you can get a position with an antenna mounted inside or another poor location, but at other times of day you do not. For testing new applications it is best to make a mount so that the antenna may be placed outside where it has a clear view of the sky, preferably on the roof. For testing of new GPS systems under dynamic conditions one technique that works well is to capture the NMEA information using a laptop and terminal program. The captured data can later be replayed into the system under development and this technique also provides 100% repeatability during the firmware / software debugging and testing process. The latest version of the GPSDiag software supports capturing of data to a file.

Can Laipac GPS modules run at 3.3V ?

The TF10 can only run at 5V but a similar model called the TF200 is capable of 3.3V operation. While CommLinx Solutions do not carry this model as a standard stock line we can obtain the modules with a fairly short lead time, please enquire for details and pricing. The TF200 does cost more, meaning for many applications the TF10 with level conversions is a better choice for 3.3V systems, but the TF200 does consume less power making it a worthy choice for systems where power consumption is of concern. The TF30 miniature GPS modules also operate at 3.3V and once in full production pricing is expected to drop to a similar level to the TF200 range.

What connector is recommended for use with the TF30 ?

The TF30 requires a 20-pin surface mount connector with 1mm lead spacing. The recommended part number is a Samtec FTM-110-03-F-DV-P. They have a web site at http://www.samtec.com with on-line ordering and further technical details on this series of connectors.

My GPS receiver is sending NMEA data but not finding satellites, what's wrong ?

This indicates your power and digital interface are probably working fine but the RF front-end or antenna is not operating. One common cause of this is using an active antenna without feeding power to the antenna power pin on the receiver. This supplies the DC power feed to the antenna's LNA (low-noise amplifier) and without power the gain of an active antenna will be poor. The next most common cause is that the antenna or its cable has been physically damaged, so the next step is to try an alternative antenna. Another cause is that the antenna connector on the receiver has been shorted while in operation, see below. Finally you could have a faulty receiver, although the failure rate of the Laipac GPS modules is very low.

I shorted the receiver antenna input while in operation and now it won't work.

If power is supplied to the antenna power input on the receiver and the antenna connection becomes shorted the current surge will damage the RF front-end of the receiver. This damage is not covered by warranty and requires replacement of the receiver module, user repair is not practical.

Is the BOOTSEL pin on the TF30 active high or low ?

For normal operation this pin should be in a high state, with the low state enabling an external boot for the software upgrade process. The pin has an internal pull-up resistor so it may be left disconnected for normal operation.

Why doesn't the TF30 have an antenna power pin ?

What are the physical mounting arrangements on the TF30 ?

Although the physical specification doesn't show it the TF30 has a full metal shield with mouting tabs protruding from the four outer corners. These are marginally under 1mm in length and around 0.2mm in width so on our PCB layouts we use a solder pad with a 1.2mm inside diameter tied to the PCB ground plane. With the new shielded version of the module it is marginally larger than the specification shows, so these four mounting holes should be placed at 40mm by 28mm across.

How do I convert between ECEF (earth-centered earth-fixed) and latitude/longitude ?

Please check the ECEF Datum Transformation notes that were provided courtesy of Laipac Technology. You will need to perform this conversion yourself when operating the TF GPS modules in SiRF binary mode.

Where do I find detailed technical information on GPS operation ?

The best place to find this information is from the U.S. Coast Guard that runs the Navstar GPS system. They have a page at http://www.navcen.uscg.gov/gps/geninfo/default.htm that contains the Navstar GPS User Equipment Introduction among many other useful documents.

How does the TIMEMARK pulse relate to the serial data received ?

The TIMEMARK pin provides pulses at 1Hz with the positive edge aligned to UTC +/- one microsecond. The pulses are 100ms in duration and always proceed the NMEA sentences containing information for that point in time. For instance if you're using the date & time from the $GPRMC sentence you will know that the time contained in the sentence relates to the previous positive edge of the TIMEMARK signal.

Where do I find detailed technical information on the Russian GLONASS system ?

This is available from the Coordinational Scientific Information Center of the Russian Space Forces. The english site is at http://www.rssi.ru/SFCSIC/english.html.