I recently had the chance to test out the X90-OPUS GPS receiver. Essentially, the X90 receiver is designed for collecting static GPS occupations (when operating in real time is not available or needed) and easily submitting observations to OPUS or PPP services to be processed for a solution or for your own post-processing. It’s simple to operate and quite capable, but the kicker was the price: $1,620 USD.

There was lot of temptation to get blinded by the price. I had to keep in mind the reasons why we were looking at affordable static boxes in the first place: we are doing a lot of static, OPUS, and PPP for various types of control densification, participating in height modernization, bluebooking, and OPUS-DB. I also had in mind having crews run this onsite while they are doing other work, for a check on real-time work, and for densification of our control networks. We can put a prism under this receiver as well. So I did a few days of tests trying it on some projects.

Hardware

The unit itself weighs only around three pounds and has a good-sized antenna with ground plane inside the housing (I have had bad experiences with dinky little antennas). It holds one (easy to find online) rechargeable lithium ion battery, but it also has a port to connect an external battery for longer campaigns. I was able to do a three-hour static session on just the internal battery with no problems. 

This receiver has a very simple control panel with easy-to-see icons. The inside is definitely not cheaply built; it feels and looks just as solid (if not more) than some high-end rovers I’ve tried. This receiver has a dual-frequency 24-channel board (an OEM version of the same board we have in one of our rovers) and also supports the new L2C signals. And with 4GB of internal memory, it is a professional piece of equipment for static work. 

Use

The receiver is very straightforward to set up and get going. Once you are level over your point, simply turn on the receiver, and within a couple of minutes it will begin collecting at five-second intervals (which is the default setting but can be changed by the user). 

It would be great to pair this box with a fixed-height rod, but if you use a standard tripod the included software will calculate the slope if you add the appropriate code, and there is even a nice “hold-a-pole” adapter. 

Downloading the X90 and sending the data to OPUS is simple with the included software. That is a key feature—keep it simple and easy to deploy and reduce the steps—and reduce operator errors.

Software

I don’t care how inexpensive something is or what gee-whiz folks claim a box has, if I have to compromise workflows with workarounds it could end up costing us more in the long run. So, the really big surprise about the package was the outstanding software utilities provided. It was like they anticipated our workflows for static. There’s no need to install any additional COGO program or post-processing software, just the disk of utilities provided. Plug in the receiver via USB and mount the device just like a digital camera or thumb drive, and it automatically shows a spreadsheet-style list of all the GPS sessions recorded. 

There is also a utility provided that does a QC check on the files and preps them by sampling at 15Hz (default for OPUS). The QC check is based on TEQC, the powerful (but command line) suite of GNSS utilities available via UNAVCO (facility.unavco.org/software/teqc/). If you’ve never used TEQC you should definitely try it; it can help debug funky sessions and Rinex files.  

Once you see the list of sessions, check the ones you want processed and click on the OPUS icon. The software will take you to the NGS OPUS page for loading files. Just load your file, input your GPS measure up height, and put in an email address you want the results delivered to.

Results

Solutions are usually processed and delivered quickly, come in an easy-to-read layout, and include state plane coordinates, NAD83-2011 latitude and longitude, ellipsoid height and orthometric height computed using the newer 12A geoid model. If you do multiple OPUS sessions you can compare the results in another utility and see the standard statistics for evaluating uncertainty and precision. Uploads to PPP services are quite simple as well.

In comparing several three-hour static sessions I did, the results were excellent (at least what we’d expect for OPUS in our region, as OPUS can vary slightly around the country). The OPUS results were very precise, being within 9mm of each other vertically. Longer sessions would bring that precision even closer as OPUS accepts occupations as long as 48 hours. 

The unit is rugged enough to be left out in the elements. On any box, I look for weakness in the seal of the battery compartment, but this one looks pretty good. We left it out in heavy northwest rain all day, and the battery compartment stayed completely dry.

A downside for the X90 is no GLONASS or L5 tracking. Yes, OPUS accepts GPS only, but up here in the northwest OPUS does not work well by the Canadian border or out on the coast; we are on the extremities of the CORS network geometry that OPUS uses and can’t use OPUS in every instance. Plus, we have a lot of canopy, and GLONASS can come in handy for post-processing and for uploads to the various PPP services (like NRCAN, RTX, AUSPOS and more); at least one of these PPP services can also process GLONASS. But with careful planning, having GPS only will work just fine. In most other parts of the country the issues we run into might not be as much of a problem.
The manufacturer, iGAGE (a surveying equipment dealer), had this box manufactured to their specifications and specifically for static and OPUS, so do not expect an internal radio, ethernet, or other scalable features. iGAGE sells a range of products that offer other features if they are something you require. 

The X90 comes in a pelican case with a charger, battery, cables, manual, and software—everything you need for a static or OPUS session except a tripod, a good book, and a bag of Cheetos. I picture this unit being able to help smaller outfits or surveyors who are leery of stepping into the GPS world of surveying but without the larger overhead usually associated with high-end GNSS. 
The low cost was a consideration, but what won us was the ease of use and it fitting into our workflows. Because OPUS has no user fees, processing is easy and free. Static observations could easily be sent to OPUS in the field with a laptop as long as it has an internet connection. And with minimal processing time, a field crew could begin conventional surveying production quickly based on the newly known points. 

With many local government agencies requiring stricter adherence to published coordinate systems and with the ability to localize to other coordinate systems, the X90 looks like a great way to go.